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Model 409 Autocorrelator - Spectra

1. High Experiment Reflector Side View Beam path shown for a particular frequency component of the pusle Prism 2 Higher Frequency Blue Lower Frequency Red Prism 1 Direction in which to translate Prism 1 to add more positive GVD Top View Dispersion shown Figure D 1 Using two prisms to compensate for positive GVD This setup allows the higher frequencies blue to catch up with the lower frequencies red This 15 not intuitively obvious since it appears that the higher frequencies actually travel a longer path length than the lower fre quencies However it is the second derivative of the path with respect to wavelength d P dl that determines the sign of the GVD Table D 1 and Table D 2 provide dispersion values at 800 nm for optical material and grating prism pairs The dispersion D is expressed in units fs cm of path length Prism Pair Compensation Table D 1 Positive Dispersion Values 00 nm Material D ffs cm Fused Silica 300 BK 7 450 Ti sapphire 580 SF 10 1590 Table D 2 Negative Dispersion Values 800 nm System D fs cm SF 10 Brewster Prism pair double pass 80 2 BK 7 Brewster Prism pair double pass 12 8 Grating pair 400 lines cm 1500 30 incidence angle double pass Grating pair 1000 lines cm 10 000 30 incidence angle double pass The reason for double passing the prisms 15 to maintain the spatial profile of the beam If onl
2. Pp Pp LL LLL sl c LL LU IA Mis LOIN PUPPY L Dil ME HH pecie DA GN o li MOE EIA DN A E Y LLLI HP LL UT TETTI LIH LE Silili ILA Mini _ AIA AA STA L LL DON LLL Uil LLL A L 1 HET TE pl System Alignment Figure C 8 Precursor to mode locking a pulse ps as seen through an autocorrelator on an oscilloscope Model 409 Autocorrelator C 14 Figure C 9 An asymmetrical fs pulse is displayed when the large block is installed Finding the Autocorrelation Pulse Using a White Card 30 Remove the light shield from the UV filter and place a white card in 31 front of the filter Find the autocorrelation spot between the frequency doubled spots Figure C 10 illustrates this step Rotate the crystal assembly until one of the two frequency doubled bright blue beam spots appears on the card Note the position of the crystal angle adjustment wheel Continue to rotate the crystal until the second bright blue spot appears and again note the position of the knob Set the control wheel midway between these two positions and very slightly dither the wheel around this point A faint fluorescent autocorrelated spot the size of a pencil dot should appear 1t will blink as you pass through the autocorrelation position t will be hard to see you might have to darken the room in order to see it Once you see
3. Indoor use Altitude up to 2000 m Temperatures 10 C to 40 C Maximum relative humidity 80 non condensing for temperatures up to 31 Mains supply voltage do not exceed 10 of the nominal voltage Insulation category II Pollution degree 2 This equipment has been tested and found to comply with the limits for a Class A digital device pursuant to Part 15 of the FCC Rules These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment This equip ment generates uses and can radiate radio frequency energy and if not installed and used in accordance with the instruction manual may cause harmful interference to radio communications Operation of this equipment in a residential area 1s likely to cause harmful interference in which case the user will be required to correct the interference at his own expense Modifications to the laser system not expressly approved by Spectra Physics could void your right to operate the equipment Table of Contents daa aa ee meet dla iii Environmental V CE Electrical Equipment V CE Environmental 5 lt 5 V FOCHegulall liS ssi seg tes eer ante eer dpa dd de udo t
4. Model 409 Autocorrelator Time Calibration At 60 Hz each path length scan delay 1s equivalent to approximately Table A 1 Scan Time Block Delay Large 80 ps Medium 15 ps Thin ps 72 x 1 revolution 15 _ 6 67 ms scan 360 30 revolutions scan 100 ps delay scan 15 ps delay 6 67 ms ms of sweep time scan The scan completes within a 72 window of rotation An approximate cali bration of the oscilloscope can be determined by calculating the time required for each scan to be completed The equations above show how to calculate for pulse width using a 60 Hz motor Substitute 25 in the place of 30 for revolutions if you are using a 50 Hz system Selecting the appropriate oscilloscope sweep time allows you to display all or part of the autocorrelation signal A variable delay trigger operating at a repetition rate synchronized to the rotation of the quartz block provides accurate triggering of the oscilloscope time base relative to the arrival of the autocorrelation signal This allows you to position the output trace on the oscilloscope Calibrating the oscilloscope time base in terms of delay per sweep length can be accomplished using either the method described above provided the correct rotating quartz block is mounted or by using the calibration etalon and the procedure described in Chapter 5 Operation The calibration etalon is a piece of fused silica of known optical delay that can be in
5. is a thumb wheel that protrudes from the optics side of the autocorrelator just above the base plate and 15 used to rotate the nonlinear crystal This rotation sets the crystal to the phase matching angle for the wavelength used and produces the autocorrelation signal when the pulses in both beams are overlapped and properly focused in the crystal The tension screw applies just enough friction to keep the wheel from turn ing accidentally yet allows it to be rotated when intended HRR tilt adjust used to align reflected beam 1 parallel to the center plate When properly adjusted beams 1 and 2 spatially overlap in the non linear crystal The adjustment is accessed through the top of the cover via 4 7 Model 409 Autocorrelator 4 8 an access hole covered by a plastic plug This adjustment 15 used to opti mize the autocorrelation signal A tension setscrew on the side of the mount Figure 4 1 supplies friction to hold this adjustment in place loosen it only 1f the tilt adjust 15 too tight to turn Changing the tension on the tilt adjust can affect the autocorrelator alignment slightly HRR tilt adjust used to align reflected beam 2 parallel to the center plate When properly adjusted beams 1 and 2 spatially overlap in the non linear crystal This adjustment 1s accessed only by removing the cover and should be used only when the position of beam 2 15 adjusted during a sys tem alignment A tension setscrew on the side of the moun
6. 5 The exit beam spots on the large block should appear as shown in Fig ure C 4 1 e they should be equally distanced from the corresponding input beam As the block 15 rotated by hand each beam should approach the edge then stop about the same distance from the edge and return to its origi nal position If this is true continue with Step 6 If this 1s not true the autocorrelator needs realignment Proceed to Alignment below System Alignment 6 The reflected beam spots should be centered on the lens as shown in Figure C 5 If this 15 true continue with Step 7 If 1t 1s otherwise the autocorrelator needs realignment Proceed to Alignment below 7 Place a piece of transparent tape across the front of the UV filter If the two beam spots shown are not centered on the filter hole adjust M so that they are a If the beam spots are not vertically centered loosen the clamping nut on the mount Figure C 6 and swivel M to properly position the beam spots on the filter Tighten the nut b If the beam spots are not horizontally centered loosen the jam nut at the base of the mount and adjust the tilt setscrew to properly position the beam spots on the filter Tighten the nut c Remove the tape 8 Adjust the crystal angle to try to find an autocorrelation pulse Refer to Chapter 5 Setup and Operation Start up Procedure for help on finding a pulse If after following those instructions you can not find a pu
7. A Alignment Iris Opening Lens Lockdown Setscrew Tension Setscrew Input Height Adjust e ae 2 Alignment 4 Target d AM LAL N MA AZ Rear Control Side 7 MWYWY w 9 Front Input Side Filter Release Setscrew uv Filter Light Shield Crystal Release Setscrew Non linear Crystal Mount Tension Screw Crystal Angle Adjust Bottom Plate Figure C 1 Model 409 Controls Indicators and Connectors 3 C 4 Using an IR viewer if necessary check the alignment of the internal beams In a well aligned system the beam spots should fall on the beam splitter block and lens as shown in Figure C 3 Figure C 4 and Figure C 5 If the spots are properly positioned on these optics skip to Step 7 If they are not continue with Step 4 The retro reflected beams should be parallel to the center mounting plate and should appear on the beam splitter on either side of the input beam spot as shown in Figure C 3 If both of the reflected beam spots are in line with each other but are offset from the input beam adjust the rear height of the autocorrelator to move them in line with the input beam If only one beam is offset adjust the tilt of the HRR associated with that beam to bring it in line with the other two You may have to use a combination of these two procedures If you can get the spots aligned in this manner refer to the Note below then continue wit
8. and a fast oscilloscope to sample the output b If mode locked pulses are evident continue with Verifying the Correct Block 1s Installed c If no pulses are evident refer to your laser manual for instructions on how to coax pulses from your laser and on how to mode lock it When mode locked pulses are evident return to Step 3 under Start up Procedure If after following those instructions for set ting the crystal angle you still cannot find an autocorrelation trace continue with Verifying the Correct Block is Installed Verifying the Correct Block is Installed The Model 409 system comes with three sizes of rotating blocks with matching etalons to cover the 65 ps to 30 fs range Using the correct block ensures the highest pulse width resolution display possible 2 Refer to Table 5 1 and verify the correct block is installed Table 5 1 Block Sizes Pulse Width Block Size 1ps x 65 ps Large 0 2 ps x 5 ps Medium 80 fs x 500 ps Thin 30 fs x 80 ps Thin with prism compensation If you need to change the block and etalon refer to Changing the Block and Etalon at the end of this chapter for instructions then return here Always change the etalon when you change the block If pulses are evident once the correct block and etalon are installed replace the cover make sure the etalon 1s not bumped into the beam path and return to Step 3 in the Start up Procedure If no pulses are evident
9. it and it is stable remove the card Card Frequency doubled Beam 1 Spot Crystal Movement Autocorrelation Spot Non linear Crystal Frequency doubled Beam 2 Spot Figure C 10 The frequency doubled retro reflected beams and auto correlation spots on a white card System Alignment a If no frequency doubled spots are seen the laser 15 not pulsing Verify the laser is pulsing An ultrafast photodiode such as the ET2000 can be used or if you have a fs system a grating can be used to spread the 1 order spectrum If no pulses are present no spreading will result b If the frequency doubled spots are seen but the autocorrelation spot cannot be found either the lens is improperly focused or the front end beam alignment is not adjusted properly Refer to the early part of this alignment procedure c If all three spots are evident but you cannot get them to display on the oscilloscope refer to the troubleshooting section in Chapter 7 to identify the problem If after following the instructions there you still obtain pulses call your Spectra Physics representative Verifying Autocorrelation 32 While watching the oscilloscope block beams 1 and 2 near HRR and HRR Figure C 1 one at a time to determine if you are viewing a true autocorrelation pulse or ambient light or just one of the two fre quency doubled beams from the crystal If the signal disappears when only one of the two beams 1s blocked t
10. 560 640 nm Optic 15 mm 690 810 nm Photomultiplier tube Hamamatsu R928 HA PCB assembly Model 409 Autocorrelator User s Manual G0020 001 G0020 000 G0149 000 0449 5680 G0151 000 G0377 000 0424 2401 0424 2402 0424 25215 0451 0960 0424 25225 0424 6631 0424 6632 0445 6430 G0050 010 G0150 000 G0072 009 G0072 010 5 08 0091 0448 4310 0000 231A Chapter 8 Warranty Customer Service At Spectra Physics we take pride in the durability of our products We place considerable emphasis on controlled manufacturing methods and quality control throughout the manufacturing process nevertheless even the finest precision instruments will need occasional service We feel our instruments have excellent service records compared to competitive prod ucts and we hope to demonstrate in the long run that we provide above average service to our customers not only in providing the best equipment for the money but in addition service facilities that get your instrument repaired and back to you as soon as possible Spectra Physics maintains major service centers in the United States Europe and Japan Additionally there are field service offices in major United States cities When calling for service inside the United States dial our toll free number 1 800 456 2552 To phone for service in other coun tries refer to the Service Centers listing located at the end of this section Order replacement parts directly from Spectra P
11. B A simple formula for calculating the broadening of a transform limited Gaussian pulse by dispersive elements 15 2 2 B t ty 68 Dy L 1 where t 15 the input pulse width in femtoseconds is a dispersion value normalized for a given length and wavelength Table D 1 gives val ues for different materials at 800 nm Table D 2 gives values for some neg ative dispersion setups such as prisms and grating pairs for compensation at 800 nm Using these values B 15 calculated directly Consequently know ing the input pulse width and B B t We define S as L B 2 0 1 02 0304 05 06 0 7 08 09 1 0 11 12 13 14 15 16 1 7 18 1 9 2 0 S Normalized Units Figure D 2 Broadening Curve D 4 When using this equation and graph it 1s important to remember that the values of D are wavelength sensitive For example for BK 7 material the difference from 800 nm to 880 nm 15 17 Therefore it 1s important to use the correct value of D for the operational wavelength Also if there are several materials present the values for dispersion must be added before calculating B For example Prism Pair Compensation Dyronlarn Panta Pamtam BI This provides a simple means for calculating the spacing between prisms necessary for compensation Example 1 Calculating pulse width as measured by a Model 409 without pre compensation Assume the 800 nm pu
12. against the flat sur face The setscrew should be tight but not too tight Replace the cover and turn on the autocorrelator Verify the Model 409 1s still properly aligned to the incoming laser beam When properly adjusted the input beam 15 centered on the input iris and the alignment beam 15 centered the alignment target If this is not the case refer to the Setup Procedure at the beginning of this chapter for instructions This completes the installation of the block and etalon You should not have to realign anything when you replace these components Changing the UV Filter 5 16 To span the 550 to 1600 nm wavelength range of the Model 409 three UV filters are used Table 5 6 lists the wavelength range for each optic with its color code for easy identification The green filter for the 680 to 1080 nm Tsunami range is shipped standard with the autocorrelator The other two are optional black 15 for use with a dye laser and light blue green for use with the Opal laser Select the filter for the laser wavelength range being measured Table 5 6 UV Filters Filter Color For Input Wavelength Regions Black 550 to 680 nm Opt Green 680 to 1080 nm Std Light Blue green 1080 to 1600 nm Opt Setup and Operation Danger Laser Radiation To change filters 1 2 Turn off the autocorrelator Loosen the four Phillips screws around the bottom of the unit and remove the two screws on top of the unit th
13. back along path parallel to its incoming path so that the input beam spot and the reflected Beam spot appear on the block as shown in Figure C 4 and on the main beam splitter as shown in Figure C 3 13 Adjust the tilt of M to place reflected Beam about 8 mm to the out side away from the center mounting plate of the input beam then continue to adjust it until the distance between the input and the two reflected beams are the same on both the block and the main beam splitter Adjust the tilt angle of the M by loosening the jam nut at its base and adjusting the setscrew Retighten the jam nut when done 14 Tighten the M mounting bolt when the alignment is complete then verify the alignment of Steps 11 through 13 was not disturbed when the bolt was tightened 15 Before we move on verify the input beam 15 still centered on the align ment iris as well as on the main beam splitter and M The input and retro reflections of both beams 1 and 2 as seen on the block should approach clipping the glass edge and then reverse direction as the block 1s rotated The position of the beam spots should appear on the beam splitter block and lens as shown in Figure C 3 Figure C 4 and Figure C 5 If this 1s not the case repeat this entire alignment proce dure until they are If you still cannot get the beam spots to align prop erly call your Spectra Physics representative Aligning M4 16 Remove the nonlinear crystal assembly by unscrew
14. beam is Provide GVD compensation using prisms or gratings The autocorrelator is too great The amount of GVD from the source to the autocorrelator is different than that from the source to the experiment ok Refer to Appendix D for information regarding GVD compensation and on using prism pairs Verify the GVD is the same in both legs The Model 409 contains about 3 8 mm of fused silica Refer to Appendix D for information regarding GVD com pensation and on using prism pairs 200 to 1200 Vdc Chassis Ground Figure 7 1 Position of Pin 11 on PMT Socket 7 3 Model 409 Autocorrelator Replacement Parts 7 4 Crystal Movement Non linear Crystal Figure 7 2 The frequency doubled retro reflected beams and autocor relation spots on a white card Table 7 1 Replacement Parts Card Frequency doubled Beam 1 Spot Autocorrelation Spot Frequency doubled Beam 2 Spot Description Part Number Beam splitter broadband Beam splitter 25 4 fs substrate Routing mirrors Thick Block Medium Block Thin Block Thick Etalon Medium Etalon Thin Etalon uv Filter 280 320 nm Model 409 01 uv Filter 345 405 nm Model 409 06 uv Filter assembly Filter assembly 1000 1600 nm Mounted Filter 345 405 nm Crystal assembly KDP 540 640 nm Crystal assembly Li lodate 700 800 nm Crystal assembly BBO 700 1100 nm Mirror 430 810 nm Prism retro reflecting Optic 15 mm
15. check the fuse or change the voltage setting slide the window to the side Pull out on the FUSE PULL lever to remove the fuse To change the board setting use needle nose pliers to grasp the board and remove it from its seat rotate 1t and or flip it over so your volt age 1s selected for view in the window and reinsert it b Plug the power cord into the unit s power receptacle then plug the cord into your facility power source 3 Attach a BNC cable between the SIG OUT connector on the control panel and the high impedance 1 vertical channel on an oscillo scope Attach another cable between the TRIG OUT connector on the control panel and the external trigger input on the oscilloscope 4 Turn on the autocorrelator and oscilloscope and set the oscilloscope to 200 mV div for the vertical amplifier and 5 ms div for the time base Also set the time base to EXTernal trigger 5 Verify the autocorrelator amplifier 15 operating correctly by varying the GAIN control As gain 15 increased the trace on the oscilloscope forms a sawtooth wave form This is normal If this does not happen check that the cable 15 correctly connected to the oscilloscope and that the oscillo scope setting 15 correct Step 4 above A discussion of prism pair compensation for group velocity dispersion or GVD is found in Appendix D Setup and Operation 6 10 Verify the autocorrelator DELAY control works properly by turning this knob Do
16. continue with Verifying the Correct Filter 15 Installed Model 409 Autocorrelator 5 10 Verifying the Correct Filter is Installed Three UV filters are supplied to cover the 350 to 1600 nm range of measur able input wavelengths Each filter 15 color coded and covers a wavelength region that ensures the best autocorrelation signal possible for amplifica tion by the photomultiplier 3 Refer to Table Table 5 2 and verify the correct UV filter 1s installed for the laser wavelength used Table 5 2 UV Filters Filter Color For Input Wavelength Regions Black 550 to 680 nm Opt Green 680 to 1080 nm Std Light Blue 1080 to 1600 nm Opt If you do not need to change the filter skip to Adjusting the Focus of the Lens below If the UV filter needs to be changed refer to Changing the UV Filter at the end of this chapter for instructions then return here At this point the filter should be changed and the lens refocused Return to Step 3 in the Start up Procedure to adjust the crystal angle for the new wavelength and to find the autocorrelation trace Adjusting the Focus of the Lens If the wavelength being measured 15 quite different from that measured last time the focal point for beams 1 and 2 might be shifted enough to position it outside the nonlinear crystal Adjust the position of the lens to return the focal point to a place inside the crystal 4 Remove the nonlinear crystal assembly by rem
17. eee es cones eases REOR eh ewe ewe ated 6 1 Notes on the Cleaning of Laser Optics 6 1 EdguipmientHedulled 3 42 as aired wee el PIRA DR SUBE IS a 6 1 Cleaning Solutions Required 0 0c ccc tee E eee ene eens 6 2 Standard Cleaning Procedures 6 2 General Procedures for Cleaning Optics 6 3 Chapter 7 Service and 7 1 Troubleshoothg Guide dad EA a hi Wo Se emt BP CT og Nh PE ee 7 1 Replacement PablS us auudod acie DA dq x os 204 Bs Le aa AAA 7 4 Chapter 8 Customer Service eere 8 1 th ccd I A ES IS ie ae 8 1 Return of the Instrument for Repair 8 2 Service b p dee dd o ee Ao 8 3 Appendix A The Scanning A 1 TiMe Galibrafioni vida ere BS A 4 Appendix B Signal Interpretation Dye Lasers B 1 Appendix C System 1 C 1 Alignment Procedure Cautions llle hrs C 2 Verifying the Correct Components are Installed C 2 Verifying Beam Alignment Rh C 3 eau C 8 Appendix D Prism Pair D 1 Compe
18. intensity possible given the requirements of the application and especially during alignment Expand the beam whenever possible to reduce beam intensity Avoid blocking the output beam or its reflection with any part of your body Establish a controlled access area for laser operation Limit access to those trained in laser safety principles Maintain a high ambient light level in the laser operation area so the eye s pupil remains constricted reducing the possibility of damage Post prominent warning signs near the laser operation area Figure 2 1 Set up experiments so the laser beam 15 either above or below eye level Provide enclosures for beam paths whenever possible Set up shields to prevent unnecessary specular reflections Set up an energy absorbing target to capture the laser beam preventing unnecessary reflections or scattering Figure 2 2 CDANGER gt VISIBLE AND OR INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT POWER WAVELENGTH S AND PULSE WIDTH DEPEND ON PUMP OPTIONS AND LASER CONFIGURATION VISIBLE AND OR INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION POWER WAVELENGTH AND PULSE WIDTH DEPEND ON PUMP OPTIONS AND LASER CONFIGURATION CLASS IV LASER PRODUCT SEE MANUAL 0451 8080 Figure 2 1 These standard safety warning labels would be appropri ate for use as entry warning signs EN 60825
19. of the oscilloscope It should be set to 5 ms div Setup and Operation Table 5 3 Etalon Delay Time Etalon Delay Time for a Single Pass ms Large 20 ps Medium 1 5 ps Thin 300 fs Determining the Delay The following formula is used to determine the delay At Af D Where n defraction index of fused silica taken from Figure 5 7 D the thickness of the calibration etalon from Table 5 4 and C the speed of light 1 0200 1 0150 1 0100 1 0050 1 0000 0 9950 0 9900 0 9850 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 Wavelength nm Figure 5 7 Calibration Etalon Correction Factor Relative to Delay at 800 nm Table 5 4 Etalon Thickness Etalon Thickness Large 13 15 mm 0 52 in Medium 5 0 mm 0 20 in Thin 1 0 mm 0 04 in Model 409 Autocorrelator If the etalon is positioned so it intercepts both the input and return beam paths in addition to doubling the pulse delay time it allows observation of pulse artifacts that are off to the side and normally out of view Interpreting the Signal Unlike a dye laser the Tsunami laser outputs an easily measured sech pulse as does the Opal system Figure 5 8 illustrates the relationship of pulse width to the autocorrelated pulse shape for a Tsunami or Opal sech pulse It is transform limited and generally exhibits the shortest possible pulse width In additi
20. of the shipping container If there 15 any major damage holes in the con tainer crushing etc insist that a representative of the carrier be present when you unpack the contents Carefully inspect your autocorrelator as you unpack it If any damage 1s evident such as dents or scratches on the cover or broken knobs immedi ately notify the carrier and your Spectra Physics sales representative Keep the shipping container If you file a damage claim you may need it to demonstrate that the damage occurred as a result of shipping If you need to return the autocorrelator for service the specially designed container assures adequate protection Included with the autocorrelator is this manual a packing slip listing all the parts shipped and an accessory kit containing the following items e Medium and thin blocks e Medium and thin etalons e One or two optional uv filters 1f ordered in a wooden optics kit Opal light blue 1080 1600 nm optional Dye laser black 550 680 nm optional Tsunami green 680 1080 nm standard installed e 2 Table clamps e 2BNC cables 1 2 m e 1 US power cord 2 m e European German power cord 2m You will need to supply e A high impedance 1 input oscilloscope XV Chapter 1 Introduction Introduction The Spectra Physics Model 409 scanning autocorrelator 15 a device for measuring the duration of ultrashort pulses from mode locked femtosecond fs and picosecond ps laser
21. oscilosScODG oio ai ocaeca A t eS ed dE seid ead C 13 Figure C 9 An asymmetrical fs pulse is displayed when the large block is installed C 14 Figure C 10 The frequency doubled retro reflected beams and autocorrelation spots WANG CAN PPP 14 Figure C 11 Typical pulses using the medium block shown off center C 17 Figure C 12 0 90 270 and 180 fs pulses as shown on the oscilloscope 18 Figure D 1 Using two prisms to compensate for positive GVD D 2 Figure D 2 Broadening Curve D 4 Model 409 Autocorrelator List of Tables Table 2 1 Table 3 1 Table 3 2 EgbelTranslatiOfiSz crepas re elet Re eee a ee 2 3 A O O 3 5 Etalon Delay UB s E oo ox ees Res Rd entes ed tii i 3 6 Table 3 3 Model 409 Specifications 3 8 Table 4 1 Calibration x dus coe tete 4 4 Tabled BIOK SIZES 2454 Bo da ds deti EA 4 5 Table As UV FINES est oaths ets euh Gs dorem be is ohare ak Se Ge RES a 4 6 Table S i BIOEK SIZBS cicli ibi cud D Bak RE E eae eh 5 9 Table 5 2 UV EMOS Cad E e Re E aod EE as we eee ee 5 10 Table 5 9 Etalon Delay TIME xoa ass RD RIED bu ara UPPER d
22. recenter it then repeat this entire step Position the block as shown in Figure C 2 Beam should now pass through the center of main beam splitter reflect off M then pass through the block exiting it about 3 mm from the upper outer edge of the top surface as shown in Figure C 4 Dither the block manually between 45 and 80 degrees Beam should move toward the upper outer edge stop and then return to its original position If 1t does skip to Step 6 If 1t does not move as described use in Allen wrench to loosen M mounting bolt Figure C 6 suf ficiently so the mount can be rotated with moderate finger pressure Then rotate M so Beam just grazes the outer edge of the block as the block 15 rotated Leave the bolt loose and continue with Step 6 Adjust the tilt of HRR to reflect Beam back along a path parallel to its incoming path so that the input beam spot and the reflected Beam spot appear on the block as shown in Figure C 4 and on the main beam splitter as shown in Figure C 3 Adjust the tilt of M to place reflected Beam about 8 mm to the inside toward the center mounting plate of the input beam then continue to adjust 1t until the distance between the input and the reflected beam is the same on both the block and the main beam splitter Adjust the tilt angle of M by loosening the mounting bolt 1f not already loose then loosening the jam nut at the base of the mount and adjusting the setscrew
23. slight tilt of the mir ror head when a jam nut at the base of the mount is loosened and the set screw adjusted Do not loosen this mount unless specifically told to do so in the System Alignment section of Appendix C Calibration etalon a thin crystal that when inserted into beam 2 pro vides a fixed optical delay of the autocorrelation signal for calibrating the oscilloscope time base There are three etalons each used with a corre sponding rotating block Use the large etalon with the large block etc The single pass delay times for the etalons are shown in Table 4 1 Please note that the etalon can be moved into both the input and return paths of beam 2 at the same time and thus double the above delay times This will impact your time base calibration 4 3 Model 409 Autocorrelator Table 4 1 Calibration Etalons Etalon Delay Time Single Pass Large 20 ps Medium 1 5 ps Thin 300 fs The arm supporting the etalon extends through the top of the autocorrelator cover for easy adjustment see Figure 4 1 Position the arm toward the out side of the unit when the etalon 15 not in use The etalon 15 changed by removing the etalon arm the shoulder mounting screw then removing the etalon holder two Allen cap screws Always block the input beam and turn off the autocorrelator to stop the spinning block when changing etal ons Be careful when removing and replacing the autocorrelator cover The eta lon lever can be ea
24. systems The measured pulse 15 displayed on a standard high impedance oscilloscope for real time viewing This com pact unit contains only three moving parts a rotating block of fused silica for changing the relative optical path length of the two internal beam paths an etalon that can be moved in and out of one of these beam paths to pro vide a known delay for calibration and a doubling crystal that is rotated to phase match the two beams and create the auto correlation signal The Model 409 is capable of operating over several wavelength ranges and by changing rotating blocks and the calibration etalon can be used to mea sure pulse widths from 60 ps to lt 40 fs In short it provides you with instantaneous feedback of your laser performance and allows you to make meaningful adjustments of your operational parameters Features of the Model 409 Autocorrelator Some of the features of the Model 409 autocorrelator are e Easy to use e Instant feedback of pulsed performance e Picosecond and femtosecond resolution depending on which rotating block 15 installed e Measures pulse widths from 60 ps to lt 40 fs e High sensitivity e Broadband optical components 690 1080 nm for Tsunami 1 1 2 6 mm for OPAL e Hollow retro prisms and ultra thin optics incorporated where possible to minimize the effects of GVD pulse broadening e Compact size e Durability 1 1 Model 409 Autocorrelator Accessory Kit Patent 1 2 Inclu
25. up suddenly or they do not show up at all 26 Increase the gain on the autocorrelator so a trace 1s seen on the oscillo scope noise on the baseline Figure C 8a 27 Adjust the fine PHASE control on the Tsunami Model 3955 to maxi mize the amplitude of the pulse Figure C 8 b d small adjustments to the bi fi ps or prism pair fs may also be neces sary Turn the appropriate micrometer control turn at a time 28 Adjust the prism dispersion compensation control fs until a pulse occurs Figure C 8f or the GTI POSITION control ps until a pulse begins to appear Figure C 8 e f 29 Slightly adjust the angular adjustment knobs on the AOM mount to maximize the amplitude of the autocorrelator trace Repeat Steps 27 28 and 29 until the pulse locks The amplitude will dras tically increase and the pulse will be well defined at this point If the auto correlator gain 15 too high and the signal 1s saturated Figure C 8g lower the gain on the autocorrelator and or lower the beam input power until a clean pulse 1s evident Finally increase the oscilloscope sweep speed to broaden the pulse for viewing and measuring Figure C 8h Note that a fs pulse displayed using the large block is asymmetrical as shown in Figure If you have obtained a pulse proceed to Verifying Autocorrelation If you cannot find a pulse proceed with Step 30 A AS _ i o me tl cfc LLL LL E LLLI A _ ewe _ e 905
26. with the PMT Refer to the trouble shooting guide in Chapter 6 for information on replacing the PMT When this is fixed return to the very beginning of the Start up Proce dure If you still fail to obtain an autocorrelation pulse and you have performed all the steps 1n this procedure you might need to realign the autocorrelator Refer to Appendix C System Align ment Testing for Autocorrelation The following procedure is a simple test to verify the pulse shown on the oscilloscope is a true autocorrelation trace It is also a quick test to see 1f an optical alignment is warranted 1 Lower room lighting especially 1f you are using fluorescent lighting to minimize possible background noise The Test 2 Move the etalon into the beam s by pushing the lever toward the cen ter of the autocorrelator If the pulse shown on the oscilloscope is a true autocorrelation pulse it will shift when the etalon delays the pulse If it shifts continue with Calibrating the Oscilloscope If it does not shift repeat Step 3 of the Start up Procedure at the begin ning of this chapter then repeat this test Calibrating the Oscilloscope When the calibration etalon 15 moved into the beam path the etalon delays the pulse according to Table 5 3 and shifts the pulse on the oscilloscope screen The amount of shift permits us to calibrate the oscilloscope for this amount of ps or fs of delay per ms of sweep time Be sure to note the sweep time
27. with the beam path in any way and only unblock 1t during alignment Protect yourself with appropriate eyewear at all times First verify the autocorrelator voltage and fuse setting matches that of your facility voltage see Chapter 4 Installation and Alignment Setting up the Autocorrelator Confirm all connectors for continuity When the autocor relator 15 turned on the power switch glows Symptom No autocorrelator signal Possible Causes Corrective Action Crystal is not set to the correct Place a white card in front of the uv filter then rotate the crystal assembly phase matching angle for the until one of the two frequency doubled bright blue beam spots appears on wavelength in use the card Note the position of the crystal adjustment wheel Continue to rotate the crystal until the second bright blue spot appears and again note the position of the knob By setting the control wheel midway between these two positions and dithering the wheel around this point a faint fluorescent autocorrelated spot the size of a pencil dot should appear blink It will be hard to see you may have to darken the room in order to see it Once you see it remove the card and try to find the pulse on the oscilloscope Figure 7 1 illustrates the idea 7 1 Model 409 Autocorrelator Symptom No autocorrelator signal Possible Causes Laser is not emitting pulses or is not mode locked Misaligned lens Poor input bea
28. 1 ANSI 4 3 10 1 Figure 2 2 Folded Metal Beam Target Laser Safety Label Translations For safety the following translations are provided regarding the labels shown in Figure 2 1 for non English speaking personnel Table 2 1 Label Translations Label French German Spanish Dutch Left Right Attention Rayonnement Laser Visible et Invisible en Cas D Ouverture et lorsque la securite est neutralisee expo sition dangereuse de l oeil ou de la peau au rayonnement direct ou diffus Puissance et longueurs D onde dependant de la configuration et de la puissance de pompe Laser de Classe 4 Rayonnement Laser Visible et Invisible en Cas D Ouverture et lorsque la securite est neutrali see exposition dangereuse de l oeil ou de la peau au rayon nement direct ou diffus Puis sance et longueurs D onde dependant de la configuration et de la puissance de pompe Laser de Classe 4 Vorsicht Austritt von sichtbarer un unsichtbarer Laserstrahl ung wenn Abdeckung geoffnet und Sicherhetisschalter uber bruckt Bestrahlung von Auge oder Haute durch direkte oder Streustrahlung vermeiden Leistung Wellenlange und Pulsbreite sind abhangig von Pumpquelle und Laserkonfigu ration Laserklasse 4 Austritt von sichtbarer un unsichtbarer Laserstrahlung wenn Abdeckung geoffnet und Sicherhetisschalter uberbruckt Bestrahlung von Auge oder Haute durch direkte oder Streustrahlung vermeiden Leistung Wellenlange
29. 1s _ 6 67 ms scan 360 30 revolutions scan 100 ps delay scan 15 ps delay 6 67 ms ms of sweep time scan The scan completes within a 72 window of rotation An approximate cali bration of the oscilloscope can be determined by calculating the time required for each scan to be completed The equations above show how to calculate for pulse width using a 60 Hz motor Substitute 25 in the place of 30 for revolutions if you are using a 50 Hz system 3 5 Model 409 Autocorrelator Time Calibration Selecting the appropriate oscilloscope sweep time allows you to display all or part of the autocorrelation signal variable delay trigger operating at a repetition rate synchronized to the rotation of the quartz block provides accurate triggering of the oscilloscope time base relative to the arrival of the autocorrelation signal This allows you to position the output trace on the oscilloscope Calibrating the oscilloscope time base in terms of delay per sweep length 1s accomplished using either the method described above provided the cor rect rotating quartz block is mounted or by using the calibration etalon and the procedure described in Chapter 5 Operation The calibration etalon 15 a piece of fused silica of known optical delay that is inserted into one or both beams of the M HRR beam path When insert ed into both beams the delay 1s doubled For a quick estimate of the delay use the delay times shown in Ta
30. 7 U S Department of Labor 200 Constitution Avenue N W Room N3647 Washington DC 20210 Tel 202 693 1999 A Guide for Control of Laser Hazards 4th Edition Publication 0165 American Conference of Governmental and Industrial Hygienists ACGIH 1330 Kemper Meadow Drive Cincinnati OH 45240 Tel 513 742 2020 Internet www acgih org home htm Laser Institute of America 13501 Ingenuity Drive Suite 128 Orlando FL 32826 Tel 800 345 2737 Internet www laserinstitute org Compliance Engineering 70 Codman Hill Road Boxborough MA 01719 Tel 978 635 8580 International Electrotechnical Commission Journal of the European Communities EN60825 1 TR3 Ed 1 0 Laser Safety Measurement and Instrumentation IEC 309 Plug Outlet and Socket Coupler for Industrial Uses Tel 41 22 919 0211 Fax 41 22 919 0300 Internet http ftp 1ec c h Cenelec European Committee for Electrotechnical Standardization Central Secretariat rue de Stassart 35 B 1050 Brussels Document Center 1504 Industrial Way Unit 9 Belmont CA 94002 4044 Tel 415 591 7600 Laser Safety Equipment and Training Laser Safety Guide Laser Institute of America 12424 Research Parkway Suite 125 Orlando FL 32826 Tel 407 380 1553 Laser Focus World Buyer s Guide Laser Focus World Penwell Publishing 10 Tara Blvd 5 Floor Nashua NH 03062 Tel 603 891 0123 Lasers and Optronics Buyer s Guide Lasers and Optronics Gordon
31. Model 409 Autocorrelator User s Manual Spectra Physics The Solid State Laser Company 1335 Terra Bella Avenue Mountain View CA 94043 Part Number 0000 231A Rev B April 2002 Preface This manual contains the information necessary to safely install align operate maintain and service your Model 409 autocorrelator The Model 409 15 designed for use with the Spectra Physics Tsunami laser and OPAL optical parametric oscillator OPO which are Class IV laser devices These units emit laser radiation that can permanently damage eyes and skin The Laser Safety section contains information about these haz ards and offers suggestions on how to safeguard against them To minimize the risk of injury or expensive repairs be sure to read this chapter and care fully follow these instructions The introductory chapter contains a brief description of the Model 409 and how it compliments the Tsunami family of products The middle chapters describe the Model 409 controls and guide you through its installation alignment and operation The last part of the manual covers maintenance and service and includes a replacement parts list and a list of world wide Spectra Physics service centers you can call if you need help Appendices are included for those who wish more information on how autocorrelation Works The Maintenance section contains information you need to keep your Model 409 clean and operational on a day to day basis w
32. Publications 301 Gibraltar Drive P O Box 650 Morris Plains NJ 07950 0650 Tel 973 292 5100 Photonics Spectra Buyer s Guide Photonics Spectra Laurin Publications Berkshire Common PO Box 4949 Pittsfield MA 01202 4949 Tel 413 499 0514 2 5 Model 409 Autocorrelator 2 6 Chapter 3 Autocorrelator Description The Autocorrelation Technique Measurement of Ultrashort Pulses The autocorrelation technique 1s the most common method used for deter mining pulse width characteristics for fs and ps pulsed laser systems Using the principle that the speed of light within a given medium is constant the autocorrelator translates differences in optical path length into time for use as an oscilloscope scanning time base The basic optical configuration 1s similar to that of a Michelson interferom eter An incoming pulse train 1s split into two beams of equal intensity and an adjustable optical delay 15 imparted to one or both beams The two beams are then recombined within a nonlinear crystal for second harmonic generation SHG The efficiency of the second harmonic generation result ing from the interaction of the two beams 15 proportional to the degree of pulse overlap within the crystal Monitoring the intensity of this second harmonic uv generation as a function of relative delay between the recom bining overlapping pulses produces a correlation function directly related to pulse width Two types of autocorrelation con
33. Retighten the jam nut when done Tighten the M mounting bolt when the alignment 15 complete then verify the alignment of Steps 5 through 7 was not disturbed when the bolt was tightened Retro reflected Beam is now reflected off the bottom of the main beam splitter and routed toward the lens where it should appear on the lens as shown in Figure C 5 9 10 11 If necessary adjust the main beam splitter by rotating 1t so that Beam 15 positioned on the lens as shown in Figure C 5 The distance from the center mounting plate to the beam spot on the lens should be the same as 1t was from the center mounting plate to the spot on the main beam splitter The beam splitter should not require a tilt adjustment Release the beam splitter mounting bolt Figure C 6 sufficiently so the mount can be rotated with moderate finger pressure Reposition the block as shown in Figure C 2 Dither the block manually between 43 and 80 degrees Beam should move toward the lower outer edge stop and then return to its original position If 1t does skip to Step 12 If 1t does not move as described use a in Allen wrench to loosen the M mounting bolt Figure C 6 sufficiently so the mount can be rotated with moderate finger pressure Then rotate M so Beam just grazes the outer edge of the block as the block is rotated Leave the bolt loose and continue with Step 12 C 9 Model 409 Autocorrelator 12 Adjust the tilt of HRR to reflect Beam
34. When products manufactured by others are used in conjunction with Spectra Physics equipment this warranty 1s extended only to the equipment manufactured by Spectra Physics Spectra Physics will provide at its expense all parts and labor and one way return shipping of the defective part or instrument 1f required This warranty does not apply to equipment or components that upon inspection by Spectra Physics discloses to be defective or unworkable due to abuse mishandling misuse alteration negligence improper installa tion unauthorized modification damage in transit or other causes beyond Spectra Physics control The above warranty 1s valid for units purchased and used in the United States only Products with foreign destinations are subject to a warranty surcharge Return of the Instrument for Repair Contact your nearest Spectra Physics field sales office service center or local distributor for shipping instructions or an on site service appointment You are responsible for one way shipment of the defective part or instru ment to Spectra Physics We encourage you to use the original packing box to secure the Model 409 during shipment If the shipping box has been lost or destroyed we recom mend you order a new one Spectra Physics will only return instruments in Spectra Physics containers Service Centers Customer Service Benelux Telephone 31 40 265 99 59 France Telephone 33 1 69 18 63 10 Germany
35. a 5 13 Table 5 47 Etalon Thilekle S8 sur du he Uo Rind RE Word ex dar es qoe edu HORE She CEN Sw UP ERES EIS ES 5 13 Table 5 5 BOCK SIZES a i 5 15 Table 5 6 UV FRIOS osas RTT AS tala 5 16 Table 7 1 Replacement Parts 7 4 Table Ad Sed ae as oe oe es es Re ee Be es aah ne Se ee A 4 Table A 2 Etalon Delay Times aaa af Ale ee A 5 Table BIOCK SIZES creare Othe a C 3 Table C 3 BIOCK SIZES decian risa eres da ni St C 16 Table D 1 Positive Dispersion Values 9 800 D 3 Table D 2 Negative Dispersion Values 800 nm Danger Laser Radiation Danger Danger Warning Warning ESD Caution Don t Touch Eyewear Required V Y Y Y Y S NUS A Warning Conventions The following warnings are used throughout this manual to draw your attention to situations or procedures that require extra attention They warn of hazards to your health damage to equipment sensitive procedures and exceptional circumstances All messages are set apart by a thin line above and below the text as shown here Laser radiation is present Condition or action may present a hazard to personal safety Condition or action may present an electrical hazard to persona
36. al is asymmetric the autocorrelator 1s either misaligned relative to the input beam or its internal optics are misaligned XeAt 0 5 lt x lt 0 7 Actual Figure B 4 Nearly transform limited pulse Shape is uncertain B 3 Model 409 Autocorrelator B 4 Appendix C A Danger Danger Laser Radiation Note d Caution System Alignment The autocorrelator has no cover interlock and will continue to operate when the cover 1s removed Be extremely careful whenever the cover is removed and avoid contact with high voltage terminals and components Its electrical circuits operate at lethal voltage and current levels Only properly trained individuals should be allowed to align the autocorrela tor The cover of the Model 409 blocks a path of laser radiation in a plane parallel to the center mounting plate Exercise extreme caution when the cover is removed and while moving about this plane Always use eye protection appropriate for the laser wavelength being measured In all but exceptional cases the following alignment procedure is not nec essary the unit was properly aligned at the factory and the optics securely mounted Unless you are sure the system was dropped or misaligned use the procedures in Chapter 5 Setup and Operation to set up the autocorr elator for normal operation Normal operation includes placing the autocor relator on the table hooking up cables to the oscilloscope setting up a beam p
37. an optic Wash your hands thoroughly with liquid detergent Body oils and contaminants can render otherwise fastidious cleaning practices useless Always use clean powderless and lint free latex finger cots or gloves when handling optics Remember not to touch any contaminating surface while wearing gloves if you scratch that itch you will transfer oils and acids onto the Optics Use filtered dry nitrogen canned air or a rubber squeeze bulb to blow dust or lint from the optic surface before cleaning it with solvent per manent damage can occur if dust scratches the glass or mirror coating Use spectroscopic grade solvents Since cleaning simply dilutes contamination to the limit set by solvent impurities solvents must be pure as possible Use solvents sparingly and leave as little on the surface as possible As any solvent evapo rates it leaves impurities behind in proportion to its volume Store methanol and acetone in small glass bottles These solvents collect moisture during prolonged exposure to air Avoid storing methanol and acetone in bottles where a large volume of air 15 trapped above the solvent Use Kodak Lens Cleaning Paper or equivalent photographic clean ing tissue to clean optics Use each piece of lens tissue only once dirty tissue merely redistrib utes contamination it does not remove it Do not use lens tissue designated for cleaning eye glasses Such tissue contains silicones These molecul
38. and Export Countries Spectra Physics GmbH Guerickeweg 7 D 6429 Darmstadt Telephone 49 06151 708 0 Fax 49 06151 79102 Japan East Spectra Physics KK East Regional Office Daiwa Nakameguro Building 4 6 Nakameguro Meguro ku Tokyo 153 Telephone 81 3 3794 5511 Fax 81 3 3794 5510 Japan West Spectra Physics KK West Regional Office Nishi honmachi Solar Building 3 1 43 Nishi honmachi Nishi ku Osaka 550 0005 Telephone 81 6 4390 6770 Fax 81 6 4390 2760 e mail niwamuro O splasers co p United Kingdom Telephone 44 1442 258100 United States and Export Countries Spectra Physics 1330 Terra Bella Avenue Mountain View CA 94043 Telephone 800 456 2552 Service or 800 SPL LASER Sales or 800 775 5273 Sales or 650 961 2550 Operator Fax 650 964 3584 e mail service O splasers com sales O splasers com Internet www spectra physics com And all European and Middle Eastern countries not included on this list all non European or Middle Eastern countries not included on this list 8 3 Model 409 Autocorrelator Appendix A The Scanning Mechanism The scanning mechanism of the Model 409 is shown in Figure A 1 The length of the two beam paths are changed by passing both of them through a rotating block of fused silica The two beams enter the block at comple mentary angles with respect to the normal of the block surface As the block rotates the angle of inci
39. aph if necessary c Tighten the lockdown slide setscrew when you are done then ver ify the pulses did not move or diminish when the screw was tight ened Adjusting the Alignment Beam Splitter If 1 the beam alignment is correct according to the instructions in Verify ing Beam Alignment earlier in this chapter and 11 you are able to obtain an autocorrelation pulse but 111 the alignment beam is not centered on the alignment target the alignment beam splitter must be realigned If this 1s not the case and the beam 15 centered in the target skip to Step 46 The alignment beam splitter 1s aligned at the factory and should never require realignment and this procedure is provided in the event the beam splitter is mistakenly adjusted Make the following adjustments only after verifying the autocorrelator 1s aligned and showing a true autocorrelation 43 Remove the alignment beam splitter cover 2 screws 44 While viewing the alignment target adjust the three alignment screws around the beam splitter until the alignment beam 15 centered on the target 45 Replace the beam splitter cover 46 Replace the autocorrelator cover Be careful not to bump the etalon lever and move it into the beam This completes the autocorrelator alignment procedure Appendix D Prism Pair Compensation Compensation Required to Properly Measure Ultrashort Pulses Because the pulses produced by lasers such as the Tsunami can be extremel
40. au E wats Dod oda i m EE V Warning Conventions s c eed mv A era m ice xi Standard UNITS sodes EO pO EXE dex Rh ee o a ed EON ae ig qe p ded des xiii Unpacking and Inspection XV Chapter 1 introduction adas RE xo e 1 1 INTFOGUETNOR PIC Ii O IS ES E ES 1 1 Features of the Model 409 Autocorrelator 1 1 ACCOSSODAISII ia ans 1 2 mcn PEEL TEES 1 2 Chapter 2 Laser Safety a uuo RR o EE io EAE DU ie oe os 2 1 Precautions for the Safe Operation of Class 111 and IV High Power Lasers and Accessories 2 1 Label ansias iii ts dud Data irat eer biie 2 3 Sources for Additional Information o oooooooor hn 2 4 Laser Safety Standards ciliegi b bae 2 4 Equipment and FralDllig adeb acier oa adele es et leleine 2 5 Chapter 3 Autocorrelator Description 3 1 The Autocorrelation Technique 3 1 Measurement of Ultrashort Pulses 3 1 The Spectra Physics Model 409 3 3 ThescanningiMechanism cria te ar Dreh de da oe toe Ec o Mo ia 3 3 Signal ntenpretalblOL sss b rss AA AA ario 3 4 OscilloScoDpe DISDIay ne ince dot a ade d RA pr S
41. ble 3 2 below However because the delay is also affected by the wavelength being measured if you need to be more precise use the calibration correction factor listed in Figure 3 6 to deter mine the exact delay for calibrating the oscilloscope for the wavelength in use Table 3 2 Etalon Delay Time Etalon Delay Time Single Double Pass Large 20 40 ps Medium 1 5 3 ps Thin 300 600 fs Note insertion of the etalon into the optical path of the autocorrelator also allows you to examine an additional 40 ps or 600 fs into the wings of the pulse Additional information about alignment and use of the calibration etalon is provided in Chapter 5 1 0200 1 0100 1 0050 1 0000 0 9950 9 9900 0 9850 1 1 1 1 1 1 1 1 1 1 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 Wavelength nm Figure 3 6 Correction Factor of Calibration Etalons Relative to Delay at 800 nm Autocorrelator Description M Mirror HRR Hollow Retro reflector Xtal Non linear Crystal PMT Photomultiplier Tube Calibration _ Etalon IA Mo HRR Rotating Block i Thin Block Shown Y Alignment Beam Splitter Main Alignment Iris Splitter Y OD Lens Xtal r 4H ic M3 Alignment Target Figure 3 7 The Model 409 Autocorrelator Schematic 3 7 Model 409 Autocorrelator Specifications Table 3 3 Model 409 Speci
42. block scan range and the auto correlation pulse should still be visible C 15 Model 409 Autocorrelator 36 Center the autocorrelation pulse 1t will be the only one that moves on the squarish pulse by iterating adjustments of the HRR slide position and tilt controls When it 1s centered the beam path lengths are equal a Translate the HRR assembly in its slide mount until the peak 15 centered Slightly loosen the lockdown slide setscrew Figure C 1 on the side of the HRR mount if the slide position translation screw is dif ficult to rotate A movement of 0 15 mm equals 1 ps shift Move ment to the rear shifts the trace in the same direction 1t would move if the calibration etalon was inserted in the beam b Optimize the signal on the oscilloscope by adjusting the tilt on Carefully tighten the lockdown slide setscrew Setting the System for the Pulses You are Measuring If you have a ps only system and have obtained a true autocorrelation signal skip to Optimizing the Pulse If you have a true autocorrelation signal and a fs system or 1f you have a ps configured fs ps system that might be reconfigured later to mea sure fs pulses perform this step 37 Use the chart below to determine the block type for the pulses you ulti mately intend to measure If you need to change to the medium or thin block refer to Changing the Block and Etalon at the end of Chapter 5 for instructions However do no
43. ce is the true peak of the actual pulse The full width half maximum FWHM point is measured at one half its full height as shown and the width of the actual pulse 15 0 65 of the measured displayed autocorrelation pulse width Note that the autocorrelation function must always be symmetric If the output signal is asymmetric the autocorrelator 1s either misaligned relative to the input beam or its internal optics are misaligned If you are measuring the output of an older dye laser please refer to Appendix B for information specific to pulses generated by these systems Autocorrelator Description FWHM At lo At 0 65At Actual Figure 3 5 Transform limited Sech Pulse The information offered here and in Chapter 5 Setup and Operation 15 meant for day to day use of the autocorrelator Appendix contains math ematical models to explain the scan mechanism of the Model 409 For even more detailed information on autocorrelation refer to A J De Maria et al Picosecond Laser Pulses IEEE Vol 57 No 1 p 2 Jan 1969 Oscilloscope Display Rotation of the quartz block is accomplished by means of an ac synchro nous motor which spins the block at a rate of 30 rotations per second RPS for a 60 Hz source or 25 RPS for a 50 Hz source At 60 Hz each path length scan delay is equivalent to approximately Table 3 1 Scan Time Block Delay Large 80 ps Medium 15 ps Thin 3 ps 72 x 1 revolution
44. cedure outlined in Appendix C 4 1 Model 409 Autocorrelator HRR Rotating Fused Calibration Etalon Lever HRR Tilt Adjust Silica Block Tension Setscrew ail oA Lockdown Slide Setscrew HRR Slide Position Adjust M 2 HRRo Tilt Adjustment Tension Setscrew Photomultiplier Tube PMT Covered n Tension Setscrew TY RR Height Adjust RNY El ye E e ZZZ NL ear Control Side 09 Filler Release Setscrew uv Filter Light Shield Crystal Release Setscrew Non linear Crystal Mount Tension Screw Crystal Angle Adjust Figure 4 1 Model 409 Components Controls Indicators and Connectors LAA Etalon La My Main Beam Splitter Alignment Beam Splitter Under cover Adjustment Lever Alignment lris Opening Lens Lockdown Setscrew Tension Setscrew Input Height Adjust Mg Alignment Target Base Plate Front Input Side Bottom Plate Alignment target a reflective target in the top of the base plate on the input side It is used as a guide to align the autocorrelator to the input beam When properly aligned the input beam 1s centered on the input iris and the alignment beam is centered on the target Main beam splitter splits the input beam 50 50 and routes the two beams onto separate equal length paths to and from hollow retro reflectors HRR and HRR Figure 4 2 The re
45. ded with the autocorrelator is this manual a packing slip listing all the parts shipped and an accessory kit containing the following items Medium and thin blocks Medium and thin etalons One or two optional uv filters 1f ordered in a wooden optics kit Opal light blue 1080 1600 nm optional Dye laser black 550 680 nm optional Tsunami green 680 1080 nm standard installed 2 Table clamps 2 BNC cables 1 2 m US power cord 2 m 1 European German power cord 2 m You will need to supply A high impedance 1 input oscilloscope 4 406 542 Chapter 2 Laser Safety For your safety please read this section of the manual carefully before installing or operating your laser accessory Danger The Spectra Physics Model 409 autocorrelator may be used with Class Laser Radiation III and IV High Power Lasers whose beams are by definition safety and fire hazards Take precautions to prevent accidental exposure to both direct and reflected beams Diffuse as well as specular beam reflec tions can cause severe eye or skin damage X Danger The protective housing of this product should always be in place during Laser Radiation normal operation Removal of the protective housing may expose the user to unnecessary radiation and should be done only in accordance with specific instructions given in this manual Precautions for the Safe Operation of Class Ill and IV High Power Lasers and Accessories Danger S
46. dence that each arm makes with the surface of the block 15 varied and due to the effects of Snell s law a change in opti cal path lengths results The well known expression of Snell s law is n sin 0 n sin Where 0 the angle of incidence the angle of refraction and n and are the refractive indices of the first and second medium Double Pass E x E gt d N 4 4 s 7 s 7 s d s x 2 EY 2 N 2 s 2 N s N x Y H gt i Rotating gt s Block Figure A 1 Scanning Mechanism Model 409 The actual variation in effective optical path length of a single beam as a function of the normal incident angle to the block 1s given by the expres SION AL 6 _ sin 0 cos0 1 n Where 1 the path length variation relative to normal incidence for a single beam A 1 Model 409 Autocorrelator A 2 n the block index of refraction and f the thickness of the block Since the angles at which the two beams enter the block are complimentary 1 e as one angle 15 increasing the other 15 decreasing the expression for the relative change in optical path length between the two arms of the auto correlator 15 given by the difference between the expressions for each one individually bag E sin 0 cos0 An Where L the path length variation relative to nor
47. e Tsunami may be actually shorter than that 1ndi cated by direct measurement Consequently GVD must be compensated for when using this or any autocorrelator if an exact measurement 15 required Since the GVD of optical material 1s generally positive for the wavelengths produced by the Tsunami laser introducing negative GVD into the beam path compensates for the broadening effect produced by this material By using prism pairs grating pairs or a Gires Tournois Interferometer GTI negative GVD can be introduced into a system The prism pair provides the easiest lowest loss means for compensating for positive GVD A simple setup using two high index prisms made of SF 10 15 all that 15 necessary Figure D 1 shows the layout top and side views for an easily built pre compensation unit The laser pulse travels through the first prism where different frequency components are spread in space Then the broad D 1 Model 409 Autocorrelator ened pulse travels through the second prism strikes a high reflector and reflects back along its original path with one exception The high reflector is slightly tilted in the plane perpendicular to the spectral spreading and causes the pulse to travel back through the prisms at a slightly different ver tical height After the beam returns through the first prism it is picked off by a mirror and directed to the autocorrelator and or the experiment Prism 1 Prism 2 Pick off Mirror
48. e of the mount Figure 5 2 just enough so the wheel can turn yet you can still feel a drag on it Ge INS A A IIS Normal _ IRENE A A EE Si Oa Ld HM Saturated Figure 5 1 Normal pulse vs a PMT saturated Pulse with Wings 5 3 Model 409 Autocorrelator HRR Rotating Fused Calibration Etalon Lever HRR Tilt Adjust Silica BI Tension Setscrew Lockdown Slide Setscrew HRRo Slide Position Adjust Tension Setscrew Photomultiplier Tube Tension Setscrew an Height Adjust ES p Sil A HRR gt df HRR Tilt Adjustment I D S S Covered SS NS 2 ock Etalon M1 Mo Main Beam Splitter Alignment Beam Splitter Under cover Adjustment Lever Alignment lris Opening Lens Lockdown Setscrew Tension Setscrew Input Height Adjust Ms Alignment E mmm ZIA Base Plate IZ Rear Control Side RA Front Input Side Filter Release Setscrew uv Filter Light Shield Crystal Release Setscrew Non linear Crystal Mount Tension Screw Crystal Angle Adjust Figure 5 2 Model 409 Controls Indicators and Connectors Bottom Plate 11 Open the iris to the size of the beam diameter and repeat Steps 9 and 10 if necessary if the beam moved off the center of the opening Aligning on the Alignment Target 12 Using one of
49. e of these optics first then test the results This will usually prove satisfactory None of the optics need to be removed for cleaning clean them in place Several optics notably the beam splitters and the medium and thin blocks and etalons are extremely fragile because of their thin nature It doesn t take much pressure to chip or crack them A ways hold the block by the assembly base Accidental damage 15 not covered by your war ranty 1 Block the incoming laser beam 2 Usea squeeze bulb dry nitrogen or canned air to clean away any dust or grit before cleaning optics with solvent Stop at this point if the optic looks clean enough 3 Usea tissue in a hemostat to clean the optic a Fold a piece of tissue in half repeatedly until you have a pad about cm 0 5 1n square and clamp it in a plastic hemostat Figure 6 1 While folding do not touch the surface of the tissue that will contact the Optic or you will contaminate the solvent Figure 6 1 Lens Tissue Folded for Cleaning 6 3 Model 409 Autocorrelator 6 4 b If required cut the paper with a solvent cleaned tool to allow access to the optic c Saturate the tissue with acetone or methanol shake off the excess resaturate and shake again Do not allow the tissue to remain saturated excess acetone or methanol can run down the sides of optics such as a beam splitter and dissolve the cement holding it in its holder This cement can later migrate on
50. e ratio of the actual pulse width to the width of the autocorrelation trace 1s a function of the pulse shape e The pulse shape can vary between two extremes dependent upon the operating parameters of the laser system For the Tsunami and Opal systems we can dismiss the second factor for unlike dye laser systems these systems output an easily measured sech pulse Figure B 1 FWHM At lo At 0 65At Actual Figure B 1 Transform limited sech Pulse from a Tsunami Laser However for users of dyes systems 1t 1s not quite this simple Figure B 2 and Figure B 3 illustrate the relationship of pulse width to pulse shape Fig ure B 2 illustrates an autocorrelation trace corresponding to a Gaussian shaped input pulse Gaussian pulses are generally observed when the laser is operating in a nontransform limited mode and the pulses are no longer than the optimum width achievable These longer nontransform limited pulses exhibit the characteristic coherence spike which is a result of E fields adding coherently and not a result of autocorrelation of the intensity envelope Consequently the peak corresponding to the coherence spike cannot be taken as the true peak and the pulse height 1s deter mined from B 1 Model 409 Autocorrelator the plateau beneath the coherence spike 1 The full width half maximum FWHM point is thus measured as shown and the ratio of actual pulse width to autocorrelation width for a Gaussian shap
51. ed pulse 1s 0 7 A Figure B 2 Nontransform limited Gaussian Pulse The other pulse shape extreme is the single sided exponential pulse shown in Figure B 3 Single sided exponential pulses are observed during trans form limited operation and generally exhibit the shortest possible pulse width In the case of a transform limited pulse the peak of the autocorrela tion trace is the peak of the pulse Consequently the FWHM point is measured from its full height as shown In the case of a single sided expo nential pulse the ratio of actual pulse width to autocorrelation width is 0 5 At 0 5 Actual Figure B 3 Transform limited Single sided Exponential Pulse Interpretation of the autocorrelation trace FWHM 15 fairly clear in the two extreme cases pulses that are far from transformed limited Gaussian and pulses that are exactly transform limited no coherence spike The ambi guities in interpretation can result however from the presence of a coher ence spike when the pulses are almost transform limited The difficulty lies B 2 Signal Interpretation Dye Lasers in deciding where the actual peak of an almost transform limited pulse 15 so that a measurement of the FWHM point can be made Figure B 4 illus trates an intermediate case for which pulse shape and pulse width determi nation become a matter of individual judgement Note that the autocorrelation function must always be symmetric If the output sign
52. en carefully lift the cover off it can be a tight fit Note remove the cover only after the autocorrelator has been aligned to the incoming beam and the base plate has been clamped to the table This will prevent the alignment from getting jarred when the cover 15 removed or replaced The cover of the Model 409 blocks a path of laser radiation in a plane parallel to the center mounting plate Exercise extreme caution when the cover 1s off and when moving about this plane and always use eye pro tection appropriate for the laser wavelength being measured 10 11 Slide the light shield off the filter and set 1t aside Shine a light on the filter element to see which one is installed Loosen the filter release setscrew on top exposed corner of the holder Figure 3 1 and pull the filter out Replace the filter with the correct one from the optics kit refer to Table 4 6 Push the filter all the way in then tighten the release set screw Place the filter just removed into the optics kit Replace the light shield Readjust the lens to properly focus the two beams in the crystal The lens should not have to move more than 3 mm to focus the beams Refer to Adjusting the Focus of the Lens earlier in this chapter Replace the cover Verify the Model 409 15 still properly aligned with the incoming laser beam When properly adjusted the input beam 15 centered on the input 115 and the alignment beam 15 centered on the alignm
53. ength as a Function of Angle 3 Figure A 3 Pulse Position and Overlap as the Block Rotates 3 Figure A 4 Calibration Etalon Correction Factor Relative to Delay at 800 nm A 5 Figure B 1 Transform limited sech Pulse from a Tsunami Laser B 1 Figure B 2 Nontransform limited Gaussian Pulse B 2 Figure B 3 Transform limited Single sided Exponential Pulse B 2 Figure B 4 Nearly transform limited pulse Shape is uncertain B 3 Figure C 1 Model 409 Controls Indicators and Connectors C 4 Figure C 2 The Model 409 Components and Optical Path C 5 Figure C 3 Beam Splitter Showing Input and Retro reflected Beam Spots C 5 Figure C 4 Input and output beam position on the rotating block C 6 Figure C 5 Correct position of reflected beams on C 6 Figure C 6 The four clamping screws for the mirrors and Main Beam Splitter C 7 Figure C 7 sech Autocorrelation pulse bounded by the squarish pulses from the frequency doubled beams C 11 Figure C 8 Precursor to mode locking a pulse ps as seen through an autocorrelator Oman
54. ent target If this 15 not the case refer to Setup Procedure at the beginning of this chapter for instructions Turn on the autocorrelator This completes the installation of the UV filter Model 409 Autocorrelator Chapter 6 Maintenance The condition of the laboratory environment the amount of time you use the autocorrelator and the amount of time the cover 15 off affects your peri odic maintenance schedule Do not allow smoking in the laboratory the optics stay clean longer Con densation due to excessive humidity can also contaminate optical surfaces The cleaner the environment the slower the rate of contamination If the cover is left in place there 1s little you must do day to day to main tain the autocorrelator All controls required for day to day operation except when you need to exchange the rotating block and calibration eta lon are accessible from the outside When you finally do need to clean the optics follow the procedures below Notes on the Cleaning of Laser Optics Equipment Required Laser optics are made by vacuum depositing microthin layers of materials of varying indices of refraction onto glass or quartz substrates If the sur face 1s scratched to a depth as shallow as 0 01 mm 0 0004 in the operat ing efficiency of the optical coating can be reduced significantly and the coating can degrade Because the optics in the autocorrelator are outside the laser cavity the per formance
55. es bind themselves to the optic coat ings and can cause permanent damage Also do not use cotton swabs e g Q Tips Solvents dissolve the glue used to fasten the cotton to the stick resulting in contaminated coatings Only use photographic lens tissue to clean optical components Maintenance General Procedures for Cleaning Optics Caution Caution Caution W With the exception of the doubling crystal DO NOT remove the optics from their mounts for cleaning doing so will require a complete realign ment of the system Optics can and should be cleaned in place If the cover 15 kept on the unit little if any cleaning is required Only clean optics when you need to Clean only the optics that need to be cleaned If you bump or otherwise jar an optic or drip acetone on an optic you can cause more harm than good In addition the medium and thin blocks and the beam splitter are very thin and are easily broken Also note that because these optics are not inside the laser cavity they are much less susceptible to signal loss than those in the laser The autocorrelator is quite well sealed if the cover is kept on the unit there should be little chance of dirt and dust getting inside When you do feel you need to clean the optics you will probably only have to clean those optics that have a surface facing upward the main beam splitter the lens mirrors M and Mz and perhaps the rotating block Clean the top surfac
56. es the trace on the oscilloscope move from side to side when the DELAY knob 15 adjusted If this does not happen check that the cable 15 correctly connected to the oscilloscope and that the oscilloscope set ting 1s correct Step 4 above Close the alignment iris by pushing the lever to one side If the power of the laser beam to be measured 15 greater than 50 mw use an uncoated beam splitter to pick off part of 1t and direct it toward the autocorrelator A retro reflection from any beam in the setup can also be used as an input to reduce the amount of glass in the measure ment Make sure the picked off beam is parallel to the table Note it is not advisable to use the alignment iris as a beam attenuator if exact pulse width measurement is desired If the beam profile 15 not symmet rical the measurement can vary depending on the part of the beam measured If power greater than 50 mW 15 measured the photomultiplier tube PMT will saturate and the signal output will become distorted The pulse will widen and wings will appear Figure 5 1 shows a normal pulse and a typical saturated signal Reposition the autocorrelator on the table to horizontally align its alignment iris to the picked off beam Adjust the height of the autocorrelator so the input beam is centered on the input alignment iris Adjust the height by turning the knurled wheel on the front leg If the wheel 1s too tight to turn loosen the tension setscrew on the sid
57. etup section of Chapter 5 Loosen the 4 Phillips screws around the bottom of the autocorrelator and remove the 2 screws on top of the unit then carefully lift the cover off it can be a tight fit The cover of the Model 409 blocks a path of laser radiation in a plane parallel to the center mounting plate Reduce laser power and exercise extreme caution when the cover 15 off and when moving about this plane and always use eye protection appropriate for the wavelength being measured System Alignment Verify the correct block and etalon are installed for the pulse width being measured see Table C 1 If the wrong set 1s installed refer to Chapter 5 Setup and Operation Changing the Block and Etalon for instructions on changing these two items Table C 1 Block Sizes Pulse Width Block Size 1 65 ps Large 0 2 ps lt x lt 5 ps Medium 80 fs lt x lt 500 ps Thin 30 fs lt x lt 80 ps Thin with prism compensation Verify the correct UV filter 1s installed for the measured wavelength To span the 550 to 1600 nm wavelength range three UV filters are used and they are listed in Table C 2 Each filter has a unique color and 15 easily identified The 680 to 1080 nm green filter 15 shipped standard with the autocorrelator The other two are optional black for use with a dye laser and light blue green for use with the Opal laser Select the filter that 1s appropriate for the input wavelength being mea
58. f the autocorrelator Changing the Block and Etalon Caution Table 5 5 lists the three available block sizes and the pulse widths for which they are designed Select the appropriate block size for the pulse width you will be measuring then select the corresponding etalon Always change the rotating block and calibration etalon at the same time Remem ber to place the optics that were removed in the optics kit to prevent dam age The optics are both fragile and expensive Always wear clean powderless latex gloves when handling optics Table 5 5 Block Sizes Pulse Width Block Size 1ps x 65 ps Large 0 2 ps lt x lt 5 ps Medium 80 fs lt x lt 500 ps Thin 30 fs lt x lt 80 ps Thin with prism compensation 1 Turnoffthe autocorrelator 2 Loosen the four Phillips screws around the bottom of the unit and remove the two screws on top of the unit Then move the etalon lever toward the center of the unit and carefully lift the cover off it can be a tight fit Note remove the cover only after the autocorrelator has been aligned to the incoming beam and the base plate has been clamped to the table This will prevent the alignment from getting jarred when the cover 1s removed or replaced The cover of the Model 409 blocks a path of laser radiation in a plane parallel to the center mounting plate Exercise extreme caution when the cover 1s off and when moving about this plane and always use eye pro tection appropr
59. fications Optical Pulse Width Resolution Scan Range 60 fs 1 ps 200 fs 5 ps 1 25 ps Scan Rate Scan Linearity Wavelength Coverage Minimum Input Pulse Repetition Rate Required Input Polarization Input Power Electrical Voltage Current Mechanical Weight 3 ps 15 ps 75 ps 25 30 Hz 2 690 nm 1 1 um or 550 nm to 1 6 um by changing uv filters 10 kHz Vertical 4 mW to 40 mW 30 mW typ 220 110 Vac 50 60 Hz 10 0 250 A 7 4 kg 16 25 lbs The different rotating blocks provide three different scan ranges Represents the lower limit to resolution based upon pulse broadening caused by group velocity dispersion effects at 500 nm With external dispersion compensation the lower limit is lt 30 fs Over the central 5096 of scan range Can be external from 1 1 to 1 6 um by changing filter and beam splitter Outline Drawing inches millimeters Dimensions in Figure 3 8 Outline Drawing 3 8 10 50 to 12 00 Optical Input E 4 90 to 6 50 C 124 to 165 Chapter 4 Danger Danger Laser Radiation Components Controls Indicators and Connectors This section illustrates show the external and internal components of the Model 409 autocorrelator and explains their functions and use The autocorrelator has no cover interlock and will continue to operate when the cover 15 removed Be extremely careful whenever the cover 15 removed and avoid contact w
60. figurations are possible In the first type an interferometric autocorrelation Figure 3 1 the two beams are recom bined in a collinear fashion one on top of the other This configuration results in an autocorrelation signal sitting on top of a constant dc back ground The background 15 produced by uv generation resulting from the portions of the scan during which the pulses are not overlapped In the sec ond scheme a background free autocorrelation Figure 3 2 the two beams are displaced from a common optical axis and recombined in a noncol linear fashion In this configuration the background is eliminated because uv 1s generated only at the point where the two beams intersect 1 e the phase matching conditions are correct 3 1 Model 409 Autocorrelator Beam Splitter Photomultiplier Tube uv Filter 1 Non linear Crystal A EB Lens lt gt Adjustable Mirror Delay Incoming Beam i i he Mirror Figure 3 1 Interferometric Collinear Autocorrelation Photomultiplier Tube LA uv Filter Frequency doubled Autocorrelation n Non linear Signal z Crystal Retro Prism Beam Splitter GU CUTE lt l lt Adjustable ncoming Beam Delay Retro Prism Figure 3 2 Background free Non collinear Autocorrelation 3 2 Autocorrelator Description The Spectra Physics Model 409 Autocorrelator The Spectra Physics Model 409 scanning autoco
61. fused silica By their thin nature the medium and thin blocks are very fragile and care must be taken when handling or cleaning them or when moving about the cavity at any time Always block the incoming beam and turn off the autocorrelator when changing blocks Controls Indicators and Connections Table 4 2 Block Sizes Pulse Width Block Size 1 lt x lt 65 ps Large 0 2 ps lt x lt 5 ps Medium 80 fs lt x lt 500 ps Thin 30 fs lt x 80 ps Thin with prism compensation Hollow retro reflectors HRR and HRR reflect the split beams back through the rotating block to the main beam splitter and provide an offset that prevents reverse coupling into the input beam In lieu of prisms the hollow retro reflectors eliminate glass from the beam paths and thus min imize the pulse broadening effects of group velocity dispersion GVD Lens focuses reflected beams and 2 from the main beam splitter into the nonlinear crystal for efficient autocorrelation signal generation The lens can be moved up and down in the holder to compensate for change in beam waist position due to change in wavelength Made from thin mm thick BBO crystal it allows transmission of wavelengths up to 1 6 mm and minimizes the pulse broadening effects of GVD A single lockdown set screw holds the lens in place Figure 4 1 Nonlinear crystal frequency doubles the two reflected beams and gen erates an autocorrelation signal when 1 it 1s se
62. h Step 5 If you cannot position the spots properly on the three optics the auto correlator needs alignment Proceed to Alignment below System Alignment HRR M Mirror HRR Hollow Retro reflector Xtal Non linear Crystal 1 PMT Photomultiplier Tube Etalon HRR2 Rotating Block Thin Block Shown Alignment Beam Splitter P AN Input pep 1 I I 1 1 1 1 I 1 1 1 1 I X lt N Main Alignment Y Beam Y Iris Splitter c _ gt Lens UV Filter Xtal 0 4 8 n Ci M3 Alignment Target Figure C 2 The Model 409 Components and Optical Path Center Mounting Plate Beam Splitter Beam 1 Retro reflection from HRR Beam 2 Retro reflection from HRRo Input Beam Figure C 3 Beam Splitter Showing Input and Retro reflected Beam Spots Model 409 Autocorrelator To HRR Beamy Rotating Fused Silica Block Figure C 4 Input and output beam position on the rotating block Center Plate A Lens Mounting Bracket Lens Figure C 5 Correct position of reflected beams on lens lation pulse but the alignment beam is no longer centered on the align ment target the alignment beam splitter must be realigned Refer to Adjusting the Alignment Beam Splitter at the end of this chapter for Note d If after performing Steps 5 through 8 you are able to find an autocorre instructions
63. he pulse on the screen 15 from the other frequency doubled beam If the signal does not disappear when either of the beams are blocked there is ambient light getting into the PMT reduce the light in the room If the signal disappears in both instances and the oscilloscope shows a flat trace it is a true autocorrelated signal If you cannot verify an autocorrelation pulse repeat this alignment procedure or refer to Chapter 7 Service and Repair Troubleshoot ing if it appears there might be something else wrong with the unit If you still cannot get a proper signal call your Spectra Physics service representative Centering the Pulse in the Scan 35 34 33 To center the pulse in the scan equalize the lengths of the autocorrela tor beam paths by adjusting the position of HRR The location of the pulse can be determined by creating a very long input pulse gt 100 ps FWHM or by increasing the gain until a sharp drop off on the edge of the signal 1s observed Minimize room lighting or turn it off and set autocorrelator gain so that the pulse appears maximized yet not saturated Set the oscilloscope time base to 1 ms then widen the pulse by a Adjusting the GTI for a ps system or b Adjusting the prism compensation control for a fs system For the large block you should see a 80 to 100 ps squarish pulse with ripple on top It will be similar to one of the pulses shown in Figure C 8c d or e This 1s the large
64. he oscillo scope for each of the frequency doubled beams Midway between these pulses is the sech autocorrelation pulse Figure C 7 If you find the pulse skip to Verifying Autocorrelation Otherwise continue with Step 24 nm a Figure 7 sech Autocorrelation pulse bounded by the squarish pulses from the frequency doubled beams C 11 Model 409 Autocorrelator 12 24 If you can see squarish pulses but cannot find the autocorrelation pulse there might be too much light noise in the room Reduce the ambient light as much as possible Note that too much input beam power the GAIN set too high the wrong UV filter installed or a poorly focused lens will also cause this problem 25 If pulses are still not evident adjust the position of HRR using the slide position adjust screw until you see a nice large signal Once a sig nal 1s obtained iterate adjustments of the HRR slide position and tilt controls to maintain and enhance the signal If the slide is difficult to move loosen the lockdown setscrew just enough to be able to adjust the slide position If it 1s set too loose the slide will also become loose and will be poorly positioned Steps 26 through 29 apply to fs and ps Tsunami systems in particular but other systems will react similarly when similar controls are adjusted The precursor indication shown in Figure C 8 a e does not exist for fs sys tems Fs pulses either show
65. hereas Service and Repair 15 intended to help you guide your Spectra Physics field ser vice engineer to the source of any problems Do not attempt repairs your self while the unit is still under warranty Instead report all problems to Spectra Physics for warranty repair Every effort has been made to ensure that the information in this manual 1s accurate All information in this document is subject to change without notice Spectra Physics makes no representation or warranty either express or implied with respect to this document In no event will Spectra Physics be liable for any direct indirect special incidental or consequential damages resulting from any defects 1n this documentation Finally 1f you encounter any difficulty with the content or style of this manual please let us know The last page 15 a form to aid in bringing such problems to our attention Thank you for your purchase of Spectra Physics instruments CE Environmental Specifications CE Electrical Equipment Requirements For information regarding the equipment needed to provide the electrical service listed under Service Requirements at the end of Chapter 3 please refer to specification EN 309 Plug Outlet and Socket Couplers for Indus trial Uses listed in the official Journal of the European Communities Environmental Specifications FCC Regulations The environmental conditions under which the laser system will function are listed below
66. hose specified herein may result in hazardous radiation Follow the instructions contained in this manual for safe operation of your autocorrelator At all times during installation operation maintenance or service of this laser accessory avoid all unnecessary exposure to laser or collateral radiation that exceeds the accessible emission limits listed in Performance Standards for Laser Products United States Code of Fed eral Regulations 21 CFR1040 10 d The Model 409 autocorrelator contains electrical circuits operating at lethal voltage and current levels Be extremely careful whenever the cover is removed Avoid contact with high voltage terminals and com ponents Danger While the Model 409 autocorrelator cover 15 removed be extremely careful to avoid exposure to laser or collateral radiation Danger Laser Radiation Any electronic product radiation except laser radiation emitted by a laser product as result of or necessary for the operation of a laser incorporated into that product 2 3 Model 409 Autocorrelator Sources for Additional Information The following are some sources for additional information on laser safety standards safety equipment and training Laser Safety Standards 2 4 Safe Use of Lasers Z136 1 1993 American National Standards Institute ANSI 11 West 42 Street New York NY 10036 Tel 212 642 4900 Occupational Safety and Health Administration Publication 8 1
67. hysics For ordering or shipping instructions or for assistance of any kind contact your nearest sales office or service center You will need your instrument model and serial number available when you call Service data or shipping instructions will be promptly supplied This warranty supplements the warranty contained in the specific sales order In the event of a conflict between documents the terms and condi tions of the sales order shall prevail The Model 409 1s protected by a 12 month warranty All mechanical and optical parts and assemblies are unconditionally warranted to be free of defects in workmanship and material for one 1 year following delivery of the equipment to the F O B point Liability under this warranty is limited to repairing replacing or giving credit for the purchase price of any part of the equipment that proves defec tive during the warranty period provided prior authorization for such return has been given by an authorized representative of Spectra Physics Warranty repairs or a replacement unit is warranted only for the remaining unexpired portion of the original warranty period applicable to the repaired or replaced equipment This warranty does not apply to any component not manufactured by Spec tra Physics When products manufactured by others are included in Spec tra Physics equipment the original manufacturer s warranty 15 extended to 8 1 Model 409 Autocorrelator Spectra Physics customers
68. iate for the laser wavelength being measured 3 Rotate the block by hand until the mounting setscrew in the base is exposed Use an in Allen wrench or hex driver to loosen the screw then holding onto the assembly base slide the block assembly off the motor shaft and place it in the optics kit 5 15 Model 409 Autocorrelator Caution Caution Be very careful when you are near the block The blocks are fragile optics but the medium and thin blocks are extremely fragile because of their thin nature it doesn t take much pressure to chip or crack them Always hold the block by the assembly base Replace the etalon while the block assembly 15 out of the way Extend the etalon holder out as far as possible and remove the two in Allen cap head screws holding the etalon assembly to the lever Carefully remove the assembly Again be very careful when removing and replacing the etalon espe cially the medium and thin etalons These are extremely fragile optics because of their thin nature it doesn t take much pressure to chip or crack them A ways hold the etalon by the metal holder Install the new etalon from the optics kit and replace the two screws Do not overtighten Place the etalon just removed into the optics kit Note the orientation of the flat side of the motor shaft and again hold ing onto the assembly base slide the new block from the optics kit onto the shaft so the clamping setscrew will press
69. ick off adjusting the nonlinear crystal changing the rotating block etalon and UV filter when necessary testing for a true autocorrelation sig nal and measuring the pulse 99 of the time it is because the nonlinear crystal 1s set to an incorrect angle that an autocorrelation trace cannot be found Also if there was a large change in wavelength since the autocorrelator was last used the lens might require slight readjustment to refocus Beam and Beam into the thin nonlinear crystal The lack of an autocorrelator signal 15 rarely due to a misaligned autocorrelator Do not attempt to realign the internal components of the autocorrelator unless 1 you cannot get an autocorrelation trace even after following the instructions in Chapter 5 11 you are certain the system has been tampered with or 111 the unit has been dropped C 1 Model 409 Autocorrelator Alignment Procedure Cautions Danger Laser Radiation These procedures may require you to change the rotating block etalon and or UV filter while using the laser at high power For safety block the input beam every time you interfere with the internal beam paths in any way and unblock 1t only during alignment Protect your self with appropriate eyewear at all times Prior to aligning the Model 409 please heed the following Verify the Model 409 15 properly set up and aligned to the input beam and that beam power has been reduced to lt 30 mW as outlined in the Setu
70. ing the tension mounting screw and lifting it off the pivot pin 17 Remove the filter light shield and place a piece of transparent tape over the input to the UV filter 18 Adjust mirror M to center the two beams on the UV filter opening seen behind the tape They may both fit inside the circle or they may not The next few steps will place them where they should be At this point loosen the M mounting bolt sufficiently so the mount can be rotated with moderate finger pressure then rotate M so the two beam spots are vertically cen tered on the filter opening If necessary adjust the tilt angle of M by loosening the jam nut at its base and adjusting the setscrew Retighten the jam nut and mounting bolt when done This 15 a rough setting The fine adjustments will be made when the autocorrelation adjustments are made 19 Remove the tape and replace the light shield Focusing the Lens 20 Using a 4 in Allen driver loosen the lens clamping screw just enough so the lens can be moved up and down by hand but does not fall out C 10 System Alignment 21 Hold a white card directly over the crystal pivot pin and move the lens up and down until the two beams merge into one spot on the card You should not have to move the lens more than about 3 mm A small tilt correction on HRR might be necessary to place one beam on top of the other 22 Remove the card and replace the crystal assembly Do not tighten the tension mounti
71. it by holding onto its base 5 1 Model 409 Autocorrelator Input power greater than 50 mW to the Model 409 will saturate the photo multiplier tube causing the output signal to be limited or unusable If high laser power 15 required use a beam splitter to pick off a sample of the beam for measurement or use an attenuator In the procedure below we refer to the beam input side as the front of the autocorrelator and the control panel side as the rear Place Unit on the Table and Hook Up the Cables 1 Place the autocorrelator on the table near the point you plan to pick off a sample of the laser beam Allow room for any collimating lens or prism pair compensation you wish to use Place an oscilloscope close by The autocorrelator controls should be accessible to you and the inter connecting BNC cables should be able to reach between the control panel and the oscilloscope The power cord for the autocorrelator 1s about 2 m long 2 Plug the autocorrelator power cord to line power a Verify that the voltage select pc board 1s properly installed before plugging in the power cord The voltage select pc board is located inside the power receptacle on the rear control panel and the voltage setting 1s visible through the small sliding window Figure 4 3 Before operating the unit the first time verify this voltage setting matches your facility volt age Also verify a fuse 15 installed The fuse 15 rated at 0 25 A To
72. ith high voltage terminals and components Its electrical circuits operate at lethal voltage and current levels Only properly trained individuals should be allowed to install and align the autocorrelator The cover of the Model 409 blocks a path of laser radiation in a plane parallel to the center mounting plate Exercise extreme caution when the cover is removed and while moving about this plane Always use eye protection appropriate for the laser wavelength being measured The Model 409 scanning autocorrelator consists of the following compo nents refer to Figure 4 1 They are listed in each section in the order in which they encounter the input beam Alignment iris provides a simple mechanism for aligning the autocorre lator to the input beam It also provides a quick means to attenuate the input beam to acceptable power levels 50 mW as a short term fix For long term we recommend using a beam pick off or a neutral density filter instead The single lever adjusts the iris size Alignment beam splitter picks off a few percent of the input beam and directs it toward the alignment target in the base see Alignment target The beam splitter is covered and should never require alignment or clean ing However if it ever does it is aligned by adjusting the three spring loaded screws around the circumference Remove the two screws below the input aperture to take the cover off Align this component only when fol lowing the pro
73. ition of the wheel Set the wheel midway between these two positions and dither it around this point faint fluorescent autocorrelation spot the size of a pencil dot should appear it will blink t will be hard to see and you may have to darken the room in order to see it Figure 5 6 When you hind the autocorrelation dot remove the card and the pulse should be visible on the oscilloscope If the autocorrelation trace 15 evident on the oscilloscope replace the cover but do not bump the adjustment wheel or the etalon lever Then skip to Testing for Autocorrelation c HRR M Mirror HRR Hollow Retro reflector Xtal Non linear Crystal Y PMT Photomultiplier Tube Calibration Etalon _ Rotating Block Thin Block Shown 1 2 N Alignment Beam Splitter RS A fo Input My lt gt Main Alignment Beam Iris Splitter Y lt I gt Lens Ple Xtal aM d L 3 M3 Alignment Target Figure 5 5 The Model 409 Components and Optical Path Model 409 Autocorrelator Card el Frequency doubled Beam 1 Spot Crystal Movement Autocorrelation Spot Non linear Crystal Frequency doubled Beam 2 Spot Figure 5 6 The frequency doubled retro reflected beams and autocor relation spots on a white card c If the autocorrelation trace is not evident on the oscilloscope there might be a problem
74. l safety Condition or action may cause damage to equipment Action may cause electrostatic discharge and cause damage to equip ment Condition or action may cause poor performance or error Text describes exceptional circumstances or makes a special refer ence Do not touch Appropriate laser safety eyewear should be worn during this opera tion Refer to the manual before operating or using this device xi Standard Units The following units abbreviations and prefixes are used in this Spectra Physics manual Quantity Unit Abbreviation mass kilogram Kg length meter time second S frequency hertz Hz force newton N energy joule J power watt W electric current ampere A electric charge coulomb C electric potential volt V resistance ohm O inductance henry H magnetic flux weber Wb magnetic flux density tesla T luminous intensity candela cd temperature celcius C pressure pascal Pa capacitance farad F angle radian rad Prefixes tera 107 deci 10 d nano 10 n giga 10 centi 107 pico 107 p mega 10 M mill 10 m femto 105 f kilo 10 k micro 109 qu atto 10 xiii Unpacking and Inspection Unpacking Your Autocorrelator Accessory Kit Your Model 409 autocorrelator was packed with great care and its con tainer was inspected prior to shipment it left Spectra Physics in good con dition Upon receiving your autocorrelator immediately inspect the outside
75. lse refer to the troubleshooting guide in Chapter 7 Symptom No Autocorrelation Signal Faulty Electronics to try to hind the problem If this fails to solve the problem call your Spectra Physics representative Mo My Main Beam Splitter PC Board Figure C 6 The four clamping screws for the mirrors and Main Beam Splitter C 7 Model 409 Autocorrelator Alignment Caution C 8 For the front end beam alignment portion of this procedure a non pulsing low power laser can be used such as a HeNe laser However a pulsing ps laser will be required later in order to generate an autocorrelation signal A fs laser can be used but aligning the autocorrelator is more difficult This section assumes you have followed the instructions under Verifying the Correct Components are Installed and Verifying Beam Alignment above that the unit 1s aligned to the input beam and the measured input beam power is 50 mW l Install the large block and etalon if they are not already resident Refer to Changing the Block and Etalon at the end of Chapter 4 for instructions With the large block installed we can 1 verify the crystal is at the correct angle 11 verify the pulse displayed 15 a true autocor relation pulse and 111 easily center the pulse in the scan range The latter becomes increasingly difficult 1f not impossible with the medium or thin blocks Verifying the Main Beam Splitter is Installed C
76. lse at the output coupler surface of a Tsunami laser is 55 fs long and transform limited It passes through 1 9 cm of fused silica before exiting the Tsunami and another 0 25 cm of BK 7 glass and 0 26 cm of fused silica 1n the Model 409 Pantai Pa 4 300 1 9 300 0 26 450 0 25 760 fs Therefore S 760 fs 55 fs 0 251 Then looking at our normalized curve Figure D 2 S 0 251 and 1 22 1 1 22 t 67 fs gt out Example 2 Calculating the prism spacing necessary for pre compensating the Model 409 Since dispersion 15 additive it 15 only necessary to make the total disper sion equal to zero to eliminate all broadening effects This allows a direct calculation of the required prism spacing without finding the actual broad ening Again start with a 55 fs transform limited 800 nm pulse going through 2 16 cm of fused silica and 0 25 cm of BK 7 Also assume the use of an SF 10 prism pair pre compensator where the beam passes through a total of 2 mm of prism tip per pass or 8 mm total The GVD for all parts of the sys tem and the length for everything but the prism spacing are known The length can be calculated by setting total GVD 0 Dacorlattor Panta Patas Pasta DI 300 22 16 450 0 25 0 8 1590 L 80 2 0 Therefore L 25 3 10 in Note the spacing L 1s the distance between the two tips of a prism in a dou ble pass configurati
77. m quality Retro reflector HRH1 tilt is out of adjustment One beam path is longer than the other fs systems The wrong uv filter is installed for the wavelength used Faulty electronics Corrective Action Using a high speed photodiode verify the laser is emitting pulses and is mode locked Maximize the uv output from the crystal by moving the lens up and down slightly in its mount Place a white card in front of the photomultiplier tube case to observe the uv spot If a large change in wavelength is made the lens might need adjustment to refocus it in the crystal Refer to subsections Aligning and Focusing the Lens in Appendix C Check the power collimation and mode quality of the input beam Improve if necessary Slowly swivel the crystal mount through its range At each step rotate HRR tilt adjustment through a turn range about its original position while watch ing for a signal on the oscilloscope For gross maladjustment remove the cover and use a business card to stop beam 1 and 2 as they approach the crystal Tilt HRR to position the beams in the same horizontal plane then repeat the procedure listed above Adjust the HRR slide position Refer to Step 42 under subheading Optimiz ing the Pulse in Appendix C for instructions on aligning HRR for equal beam path length with fs pulses present Loosen the filter release setscrew on top corner of the filter holder Figure 4 1 and push the filte
78. mal incidence for a single beam f the thickness of the block n the block index of refraction q the angle of incidence for beam path 1 and f the angle of incidence for beam path 2 The factor of 2 1s due to each beam path making two passes through the quartz block A graph of the change in path length as a function of incident angle 1s shown in Figure A 2 where each beam path 15 taken separately and their respective difference 15 shown As you can see when both beams pass through the quartz block the variation 1n path length of one beam relative to the other 15 nearly linear over an angle of rotation of approximately 72 degrees The sequence of pulse position and overlap for each beam path 15 illus trated in Figure A 3 At the beginning of the scan path A 15 at a minimum and path B 1s at a maximum As the block rotates the pulses move together at a constant relative rate At the point where the angle of incidence for path A and B is the same the pulses overlap As the block continues to rotate the pulses move apart completing the scan The Scanning Mechanism Both Legs Traveling Through Block o A 0 6 Kan B 0 o 4 Le ES A 0 B 0 60 50 40 30 20 10 0 co 80 40 50 60 70 4 L Figure A 2 Variation in Path Length as a Function of Angle n us A B P L gt B A s B A P L gt B A x B A c PL gt B Figure A 3 Pulse Position and Overlap as the Block Rotates A 3
79. ng screw too much it supplies fric tion to keep the crystal from moving on its own yet allows it to be moved manually for angle adjustment Finding the Autocorrelation Pulse Using the Oscilloscope A mode locked pulsing laser 15 required for the rest of this alignment Frequency doubling occurs when the angle of the nonlinear crystal 1s prop erly set to match the phase of the beam wavelength Autocorrelation occurs at an angle midway between the two frequency doubling angles when a pulse from Beam arrives coincidentally with a pulse from Beam and the two beams are focused into the nonlinear crystal The amplitude of the autocorrelation signal 15 maximized when the two beams have maximum overlap in the crystal It 1s difficult to observe the central UV spot since both the block and the crystal must be at the proper angular position simultaneously The follow ing procedures simplify this Because the PMT is more sensitive than the eye we will first try finding the autocorrelation position in Steps 23 through 29 by observing the PMT out put on the oscilloscope If this fails to produce a visible pulse we will try to find the three beam spots that correspond to Beam Beam and the auto correlation pulse placing a card in front of the filter and looking for them as we rotate the crystal In this manner we can determine whether or not these signals are getting to the PMT 23 Asthe crystal 1s rotated a squarish pulse should appear on t
80. nsation Required to Properly Measure Ultrashort PUISCS wiv Reda ous oo a Doo s Qs OR eat lee qox dd A aos D 1 Calculating Pulse Broadening 0 0 ccc eee eee tenet enna D 4 Notes Report Form for Problems and Solutions viii Table of Contents List of Figures Figure 2 1 These standard safety warning labels would be appropriate for use as entry warning signs EN 60825 1 ANSI4 3 10 1 o oooocooocoercan eee eens 2 2 Figure 2 2 Folded Metal Beam Target 2 2 Figure 3 1 Interferometric Collinear Autocorrelation 3 2 Figure 3 2 Background free Non collinear Autocorrelation 3 2 Figure 3 3 Scanning Mechanism Model 409 3 3 Figure 3 4 Pulse Position and Overlap as the Block Rotates 3 4 Figure 3 5 Transform limited sech 3 5 Figure 3 6 Correction Factor of Calibration Etalons Relative to Delay at 800 nm 3 6 Figure 3 7 The Model 409 Autocorrelator Schematic 3 7 Figure 3 8 Outline Drawing a 3 8 Figure 4 1 Model 409 Components Controls Indicators and Connectors 4 2 Figure 4 2 The Model 409 Components and Optical Path 4 3 Fig
81. oe Geel Geese See taria 3 5 Hime Caloralo aiutera ee ETT 3 6 x aun dict bt eri eed direc ye 3 8 AA A 3 8 Chapter 4 Controls Indicators and Connectors 4 1 COMPONEN 4 1 eene cT PT Oe BES Maes PSN See 4 6 CONNeciors pisan RR 4 6 AGIUSIMNCRIS UTER a o ara ados 4 7 vil Chapter 5 Setup and Operation 5 1 SEID 5 1 SUSANA od 5 1 NA AAA ERRORES tes a da Cake oe he See ees He ee oe ee eru 5 1 Operation see ae eee Be ed 5 5 Op ration SUMMAN E SR ERRARE i 5 5 Sta cup RrOCCCUIG O a ea ae 5 6 Cannot Find an Autocorrelation 5 9 Testing for Autocorrelation ecc aaa lea a ass 5 12 Calibrating the Oscilloscope RR I rr 5 12 Determining the 8 Xr Eme 5 13 Interpreting the Signal 4 4 5 dr erri Up RIS CER Dog EE EE a A E V 5 14 Shut down E cs ei A e 5 15 Changing the Block Elalon sa ace Rn ame he hae eee A AIRE 5 15 Changing ine UV Flo aid wie soa he a a oe 5 16 Chapter 6 Maintenance
82. of the unit does not degrade when they get dirty anywhere near what happens when laser intracavity optics get dirty However dust on these optical surfaces can still cause damage to the optics when the dust 1s hit with a laser beam and burned into the surface Therefore cleanliness 1s still essential and you must apply the same laser optics maintenance tech niques to the autocorrelator optics with extreme care and with attention to detail Clean is a relative description nothing is ever perfectly clean and no cleaning operation can ever completely remove contaminants Cleaning is a process of reducing objectionable materials to acceptable levels e dry filtered nitrogen canned air or rubber squeeze bulb e hemostats e optical grade lens tissue e clean lint free finger cots or powderless latex gloves 6 1 Model 409 Autocorrelator Cleaning Solutions Required spectroscopic grade acetone and or methanol Methanol tends to clean better but may deposit a water based film on the surface being cleaned 1f not fresh If this occurs follow the methanol wipe with an acetone wipe to remove the film As always use fresh solvent from a bottle with little air in it Standard Cleaning Procedures Caution 6 2 Follow the principles below whenever you clean any optical surface Clean only one element at a time Work in a clean environment and whenever possible over a soft lint free cloth or pad if you have to remove
83. on or the distance between the two tips in one leg of a four prism sequence The calculated L 1s shorter than recommended above but since the material dispersion value of SF 10 prisms 15 so high sliding just a bit more glass in will add enough positive GVD to balance out the prism spacing D 5 Model 409 Autocorrelator D 6 Notes Notes 1 Model 409 Autocorrelator Notes 2 Notes Notes 3 Model 409 Autocorrelator Notes 4 Notes Notes 5 Model 409 Autocorrelator Notes 6 Report Form for Problems and Solutions We have provided this form to encourage you to tell us about any difficulties you have experienced in using your Spectra Physics instrument or its manual problems that did not require a formal call or letter to our service department but that you feel should be remedied We are always interested in improving our products and manuals and we appreciate all suggestions Thank you From Name Company or Institution Department Address Instrument Model Number Serial Number Problem Suggested Solution s Mail To FAX to Spectra Physics Inc Attention ISL Quality Manager ISL Quality Manager 650 961 7101 1330 Terra Bella Avenue M S 15 50 Post Office Box 7013 Mountain View CA 94039 7013 U S A E mail sales splasers com www spectra physics com
84. on the peak of the measured autocorrelation trace is the true peak of the actual pulse unlike that for a dye laser The full width half maximum FWHM point is measured at one half its full height as shown and the width of the actual pulse at that point 15 0 65 of the mea sured displayed autocorrelation pulse width FWHM At lo At 0 65At Actual Figure 5 8 Transform limited sech Pulse Note that the autocorrelation function must always be symmetric If the output signal is asymmetric the autocorrelator 1s either misaligned relative to the input beam or its internal optics are misaligned Refer to the begin ning of this chapter for information on aligning the autocorrelator to the input beam refer to Appendix C System Alignment for information on aligning the autocorrelator The pulse measured by the Model 409 at the experimental point is very likely not the same as that produced by the laser 1 e it 1s usually broad ened due to group velocity dispersion GVD Appendix D Prism pair Compensation covers compensation for GVD and shows how to ensure the shortest possible pulse arrives at the experiment and how to measure it f you are measuring the output of an older dye laser please refer to Appendix B for information specific to pulses generated by these systems Setup and Operation Shut down Procedure The Model 409 autocorrelator 15 simple to shut down 1 Block the input beam 2 Turn of
85. orrectly 2 Verify the two pins on the main beam splitter are facing downward and that the oval window is oriented horizontally If this 1s not the case a Block the incoming beam b Unscrew the knurled ring and remove the beam splitter The beam splitter 15 very thin and easy to break Be very careful as you handle it c Turn the beam splitter over so the pins face downward Hold onto the two small pins with your fingers and turn the optic so the oval window 1s horizontally oriented e While holding the pins to keep the oval window properly posi tioned screw the ring back in place and tighten it f Unblock the incoming beam Aligning the Beam Paths 3 With the autocorrelator properly aligned to the input beam the input beam spot should be centered horizontally and vertically on the main beam splitter and on mirror M If this 15 true skip to Step 4 If it 1s not true the alignment beam splitter is not properly realigned It will be realigned later At this time a Unclamp the rear of the bottom plate and move the back of the autocorrelator so the input beam spot 1s horizontally centered on the main beam splitter Then reclamp the bottom plate b If necessary adjust the rear height of the autocorrelator to center the input beam spot vertically on the main beam splitter System Alignment 4 c Verify the input beam 15 still centered on the alignment iris If it 1s not centered unclamp the front and
86. ose using the autocorrelator only occasionally As you continue to use your unit you will find 1ts setup becoming second nature and the summary list will serve as a quick reference Note The Model 409 is designed to measure vertically polarized light For horizontally polarized light measurements use a polar rotator external to the unit to rotate the light vertically do not rotate the nonlinear crys tal to compensate The label on the crystal should always be upright and the long axis of the crystal should be horizontal Setup summary e Place the autocorrelator on the table near the laser beam pick off point e Connect the autocorrelator to an oscilloscope and set the oscilloscope for 200 mV div and 5 ms div sweep and external trigger e Turn on the autocorrelator and oscilloscope and verify the connections are correct and the unit is working properly by adjusting the GAIN and DELAY controls e If required set up a beam splitter to pick off less than 50 mW of the sampled beam and direct it parallel to the table toward the autocorrela tor e Align the autocorrelator to the laser beam using the alignment iris and alignment beam and target and clamp the autocorrelator to the table Setup Procedure Set up the autocorrelator so it 1s convenient to use NOT drop the unit or jar it while placing it upon a surface When mov Caution The autocorrelator can be misaligned by sharp shocks or impacts DO ing the Model 409 support
87. ou are changing pulse width you might need to change the block and etalon Refer to Changing the Block and Etalon at the end of this chapter If you are changing wavelengths refer to Changing the UV Filter also at the end of this chapter Finding a Pulse di While watching the oscilloscope adjust the nonlinear crystal to get an autocorrelation signal If an autocorrelator signal 15 observed skip to Testing for Autocorrelation later in this chapter and continue from that point The autocorrelation phase matching angle of the nonlinear crystal also changes with a change in laser wavelength Therefore the crystal angle should be adjusted every time you change laser wavelengths If you cannot find the signal after following the rest of this step and Steps 4 through 8 if you have a Tsunami laser continue with the next section Cannot Find an Autocorrelation Trace Rotate the crystal and look for any gain response on the oscilloscope Any increase in gain means pulses are present If no gain increase 1s observed and you are positive pulses are present refer to the trouble shooting guide in Chapter 6 to determine the cause Two large squarish pulses should appear on the oscilloscope one for each of the frequency doubled beams and midway between the sech autocorrelation pulse Figure 5 3 The autocorrelation pulse 15 often very small and you might have to change the gain of the autocorrelator in order to see it I
88. oving the single tension mounting screw and grasping the crystal mount lift the whole assem bly off the pivot pin 5 Loosen the lens clamping screw with a in driver just enough so that the lens can be moved up and down by hand but does not fall out 6 Hold a white card directly over the crystal assembly pivot pin and move the lens up and down until beams 1 and 2 merge into a single spot on the card A small tilt correction of HRR might be necessary to place one beam on top of the other If you cannot focus the two spots on the card a realignment of the autocorrelator might be required Refer to Appendix C System Alignment Internal Optical Align ment 7 Once the two beams are focused on the card remove the card and replace the crystal assembly Do not tighten the crystal assembly tension mounting screw too much it applies friction to the wheel keep the crystal from moving on its own yet allows the wheel to be moved manually for angle adjustment Setup and Operation 8 Repeat Step 3 under Start up Procedure to obtain an autocorrelation trace a If you still cannot find a pulse place a white card in front of the UV filter Figure 5 5 and adjust the crystal angle until one of the two frequency doubled bright blue beam spots appears on the card Note the position of the crystal angle adjustment wheel Then continue to rotate the crystal until the second bright blue spot appears and again note the pos
89. p section of Chapter 5 A low power HeNe laser can also be used to perform the beam alignment portion of the alignment Use a high speed photodiode such as the ET2000 to verify the laser 15 emitting pulses Verify the correct block etalon and UV filter are installed Optimize both pump and pulse laser outputs If measuring fs pulses compensate for GVD pulse broadening by using prism pair compensa tion see Appendix D Then perform the Test for Autocorrelation found in Chapter 4 If it is a true autocorrelation trace the unit does not require realignment Do not clean the optics of the Model 409 unless they appear truly dirty If you find you need to clean the optics refer to Chapter 6 Mainte nance for cleaning procedures Heed the warnings to prevent acci dental damage to your unit During the alignment procedure loosen the mounting screws for the three mirrors and beam splitter only if you are absolutely sure align ment of these optics is needed and then loosen only one mount at a time loosening more than one at a time often results in a poorly aligned system when the mounting screws are tightened again Verifying the Correct Components are Installed Danger Laser Radiation C 2 Turn off power to the autocorrelator Remove the autocorrelator cover Note remove the cover only after the autocorrelator has been aligned to the incoming beam and the base plate has been clamped to the table as outlined in the S
90. pectra Physics manufactures many different lasers and laser accesso px ms ries The maximum radiant input powers of these devices vary from a few microwatts to tens of watts Units utilizing higher output powers normally have the potential for a greater safety hazard especially those products that utilize pulsed or invisible output The following general laser safety precautions are especially important for users of high power laser products e Use protective eyewear at all times selection depends on the wave length and intensity of the radiation the conditions of use and the visual function required Protective eyewear 1s available from vendors listed in the Laser Focus World Lasers and Optronics and Photonics Spectra buyer s guides Consult the ANSI and ACGIH standards listed at the end of this section for guidance e Maintain a high ambient light level in the laser operation area This keeps the eye s pupil constricted thus reducing the possibility of eye damage e the protective covers on the laser and its accessories at all times 2 1 Model 409 Autocorrelator 2 2 SEE MANUAL Avoid looking at the laser beam even diffuse reflections are hazard OUS Avoid wearing jewelry or other objects that may reflect or scatter the beam while using the laser Use an infrared detector or energy detector to verify that the laser beam 1s off before working in front of the laser Operate the laser at the lowest beam
91. r out Replace the filter with the appropriate one from the optics kit refer to Table 4 3 and set its holder flush with the input side of the holder then tighten the release setscrew to hold it in place 12 Vdc should be found at pin E measure with respect to chassis ground or pins E Es or E45 of the printed circuit board With the autocorrelator off remove the uv filter assembly Turn the unit on and observe the response to room light If there is still no signal proceed With the autocorrelator off open the PMT case 1 screw and remove the tube Clean and inspect the pins and socket contacts Replace the tube in its socket Turn the unit on and check for a signal If there is still no signal pro ceed With the unit off remove the PMT Turn the unit back on and using a DVM carefully measure the high voltage from contact 11 of the socket to chassis ground see Figure 6 1 It should be about 200 to 1200 Vdc it varies with the setting of the GAIN control on the panel 2 to 9 Vdc should be found at pin E when measured with respect to chassis ground or pin Eg Verify oscilloscope trigger is working properly Use clip leads to display the signal across the PMT load resistor located on the PMT base mount on the oscilloscope If there is no PMT response at a full GAIN setting the PMT may be bad 7 2 Service and Repair Symptom Signal evident but faulty Possible Causes Corrective Action Signal e
92. relator signal is found midway between the two square shaped beam signals e Use the calibration etalon to verify the signal is a true autocorrelation pulse the signal shifts once the etalon 15 in out e Readjust the GAIN control on the autocorrelator and the vertical and horizontal controls on the oscilloscope to display a good pulse The time base should be set between 0 1 and 5 ms e If you are measuring pulses less than 100 fs use a prism pair to com pensate for group velocity dispersion GVD Refer to Appendix D 5 5 Model 409 Autocorrelator Start up Procedure Note d 5 6 Once the autocorrelator is set up each time it is used during that setup simply confirm its alignment to the input beam by verifying the input beam is centered on the input iris and the alignment beam is centered on the alignment target If either 1s not the case refer to the setup section again to realign the autocorrelator to the input beam Once aligned proceed as fol lows for day to day operation 1 Verify the laser 15 pulsing and mode locked An ultrafast photodiode such as the ET2000 can be used or if you have a fs system a grating can be used instead to spread the 1 order spectrum If no pulses are present no spreading will result If you are using a dye laser verify the input beam is collimated and that its mode is TEM p If you are not changing pulse width or laser wavelength from that used last time skip to Step 3 If y
93. rrelator operates in a back eround free configuration according to the principles of noncollinear auto correlation described above Double Pass E t gt ad E The Scanning Mechanism 4 4 4 27 s 7 s 2 ace 27 x Pu s 7 bos gt N s s s I 2 DA Rotat otaun M Block Figure 3 3 Scanning Mechanism Model 409 The scanning mechanism of the Model 409 15 shown in Figure 3 3 The length of the two beam paths are changed by passing both of them through a rotating block of fused silica The two beams enter the block at comple mentary angles with respect to the normal of the block surface and as the block rotates the angle of incidence that each arm makes with the surface of the block 1s varied and a change in optical path lengths results By passing both beams through the quartz block the variation in path length of one beam relative to the other becomes nearly linear over an angle of rotation of approximately 72 degrees Figure 3 4 shows the sequence of pulse positions and overlap for each beam path as the block rotates At the beginning of the scan path A is at a minimum and path B is at a maximum As the block rotates the pulses move together at a constant relative rate At the point where the angle of incidence for path and 15 the same the pulses overlap As the block continues to rotate the pulses mo
94. see the whole pulse if necessary Optimizing the Pulse 40 Once you have obtained the autocorrelation pulse for the proper block SIZE readjust the GTI or prism compensation control to generate a nor mal sech pulse 41 Then iterate between adjusting the tilt of HRR adjusting the lens up and down slightly and adjusting the rotation and tilt of M to maximize the output signal If you have a ps system proceed to Adjusting the Alignment Beam Split ter 42 Fs users must center the pulse in the scan one more time Figure C 12 shows typical fs pulses in a single scan The good pulses are at 0 and 180 and they appear a bit wider than those at 90 and 270 The latter are created when the block is at an oblique angle to the beams and the beams become distorted Do not use these pulses for measure ment a Center the 0 and 180 pulses on the screen as shown in the graph in Figure C 12 The 90 270 pulse may or may not be centered Model 409 Autocorrelator hae ci 0 90 2 70 180 Figure C 12 0 907 270 and 180 fs pulses as shown on the oscillo scope b Iterate adjustments of the HRR position and tilt controls to move the 90 270 pulse with respect to the 0 and 180 pulses until the 90 270 pulse 15 centered between the 0 and 180 pulses and all pulses are centered on the screen Use the DELAY control to recenter the pulses on the gr
95. serted into one or both beams of the M HRR beam path When inserted into both beams the delay 15 doubled For a quick delay estimate you can use delay times from Table A 2 below However because the actual delay 15 affected by the wavelength being measured use the calibra tion correction factor listed in Figure A 4 to determine the exact delay for calibrating the oscilloscope Note insertion of the etalon into the optical path of the autocorrelator also allows you to examine an additional 40 ps or 600 fs into the wings of the pulse Additional information about alignment and use of the calibration etalon is provided in Chapter 3 The Scanning Mechanism Table A 2 Etalon Delay Time Etalon Delay Time Single Double Pass Large 20 40 ps Medium 1 5 3 ps Thin 300 600 fs 1 0200 1 0150 1 0100 1 0050 1 0000 0 9950 0 9900 0 9850 l l 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 Wavelength nm Figure A 4 Calibration Etalon Correction Factor Relative to Delay at 800 nm For more information on autocorrelation refer to A J De Maria et al Pico second Laser Pulses IEEE Vol 57 No 1 p 2 Jan 1969 A 5 Model 409 Autocorrelator A 6 Appendix B Signal Interpretation Dye Lasers Accurate interpretations of autocorrelation measurements 1 e actual pulse width determinations are complicated by two factors e Th
96. sily bumped and the etalon moved into one or both beam paths by accident This can impact your calibration or pulse measurement Always reposition the etalon lever toward the outside of the unit when you are done removing or installing the cover The three etalons are made of fused silica By their thin nature the medium and thin etalons are very fragile and care must be taken when handling or cleaning them or when moving about the cavity at any time Rotating block spins in the paths of the split beams to create an optome chanical change in path length for each beam with respect to one another as one path becomes longer the other becomes shorter Thus pulses in one beam are swept past the pulses 1n the other When these pulses overlap in the nonlinear crystal a time related autocorrelation signal 1s created The autocorrelation signal is sent to the vertical channel of the oscillo scope while a sync signal provided by the motor driver is sent to the oscilloscope EXT time base input Together they produce a real time view able measurable output pulse One of three different sized blocks 15 installed to optimize pulse width res olution for pulses from 30 fs to 65 ps see Table 4 2 A single setscrew holds the block to the motor shaft and the shoulder stop on the block assembly places the block at the correct position on the shaft relative to the beams when it is pushed all the way onto the shaft All three blocks are made of
97. sured Table C 2 UV Filters Filter Color For Input Wavelength Regions Black 550 to 680 nm Opt Green 680 to 1080 nm Std Light Blue green 1080 to 1600 nm Opt a Slide the light shield off the filter snout and set it aside for now b Note the color of the filter If the wrong UV filter 1s installed loosen the filter release setscrew on the top corner of the holder Figure C 1 and pull the filter out c Replace the filter with the correct one from the optics kit refer to Table C 2 Push the filter all the way in then tighten the release setscrew Place the filter just removed into the optics kit Verifying Beam Alignment 1 Verify autocorrelator 15 still aligned to the incoming beam the beam 15 centered on the alignment iris and the alignment beam 15 cen tered on the alignment target Make sure you are viewing the target from directly overhead Rotate the block by hand so the normal of the block faces the main beam splitter Figure C 2 C 3 Model 409 Autocorrelator HRR Rotating Fused Calibration Etalon Lever HRR Tilt Adjust Silica Block Tension Setscrew Lockdown Slide Setscrew HRRo Slide Position Adjust HRRo Tilt Adjustment Tension Setscrew Photomultiplier Tube PMT Covered n Tension Setscrew ad Height Adjust BEEN EH A P 6 8 Etalon gt M1 Ma Main Beam Splitter Alignment Beam Splitter Under cover amp Adjustment Lever
98. t Figure 3 1 supplies friction to hold this adjustment in place loosen it if the tilt adjust is too tight to turn Changing the tension on the tilt adjust can affect the autocorrelator alignment slightly HRR position adjust used to set the path length of beam 2 equal to that of beam 1 thus enabling the two separate pulses to temporally overlap in the nonlinear crystal for proper autocorrelation The position adjustment moves HRR on a dovetail slide and it 1s secured via a lockdown setscrew to prevent accidental movement Figure 4 1 The adjustment and set screw are accessible only when the cover is removed Adjust this control only when specifically told to do so in the alignment procedure in Appendix C The position adjust 1s set at the factory and should not be readjusted Mirror and main beam splitter mount clamping screws 4 used to secure the three mirrors and the main beam splitter to the center plate once they have been aligned at the factory Figure 4 4 Do not loosen these nuts unless advised to do so elsewhere in this manual Mo My Main Beam Splitter PC Board Rear Figure 4 4 The Four Clamping Screws for the Mirrors and Main Beam Splitter Chapter 5 Setup and Operation Setup The Model 409 autocorrelator 15 easy to set up The summary below 15 a check list for setting up the autocorrelator quickly Following the list 1s a detailed step by step setup procedure for first time users and for th
99. t is also possible you might have to reduce the gain if the signal appears saturated 1 e the top 1s flattened and the pulse appears to have wings Figure 5 1 If you can see squarish pulses but cannot find the autocorrelation pulse there might be too much light noise in the room Reduce the ambient light as much as possible Note that too much input beam power the GAIN set too high the wrong UV filter installed or a poorly focused lens will also cause this problem Setup and Operation Figure 5 3 Autocorrelation pulse bounded by the squarish frequency doubled beam pulses The pulses shown are typical but sizes and shape can vary widely The following Steps 4 through 7 applies to fs and ps Tsunami systems The precursor indication shown in Figure 5 4 a e however does not exist for fs systems 4 Increase the gain on the autocorrelator so a trace 15 seen on the oscillo scope noise on the baseline Figure 5 4a 5 Adjust the fine PHASE control on the Tsunami Model 3955 to maxi mize the amplitude of the pulse Figure 5 4 b d Small adjustments to the bi fi ps or prism pair fs may also be neces sary Turn the appropriate micrometer control turn at a time 6 Adjust the prism dispersion compensation control fs until a pulse occurs Figure 5 4f or the GTI POSITION control ps until a pulse begins to appear Figure 5 4 e f 7 Slightly adjust the angular adjustment knobs on the AOM mount to maximi
100. t replace the cover when told to do so Instead return here Remember to always change the etalon when you change the block as outlined Table C 3 Block Sizes Pulse Width Block Size 1 lt x lt 65 ps Large 0 2 ps lt x lt 5 ps Medium 80 fs lt x lt 500 ps Thin 30 fs lt x lt 80 ps Thin with prism compensation Note If you are going to install the thin block replace the large block with the medium block first before moving to the thin block If you omit the extra steps you may have difficulty finding the fs autocorrelation pulse using the thin block 38 Install the medium block then repeat the steps under Finding the Autocorrelation Pulse Using the Oscilloscope above to find the auto correlation pulse then refer to Centering the Pulse in the Scan above to adjust the position of HRR to recenter the pulse in the scan range Note that the pulses appear wider System Alignment Center of Pulse Center of Pulse _ bp WT 1 H Pt ite Vi Edge of Edge of Scan Scan Figure C 11 Typical pulses using the medium block shown off center If the thin block is not to be used proceed to Step 40 otherwise con tinue 39 Install the thin block and repeat the procedures in Step 38 to find an autocorrelation pulse and to recenter the pulse in the scan range Note that the pulses widen again Adjust the time base on the oscilloscope to
101. t to the proper angle for the laser wavelength used 11 the pulses in the two beams overlap and 111 the overlapped pulses are properly focused within the crystal A thumb wheel protruding from the optics side of the autocorrelator near the base plate sets the phase matching angle of the crystal The tension mounting screw and spring washer provide enough friction to maintain the crystal at the angle set by the operator yet allows the operator to move it at will This screw 15 removed in order to remove the crystal assembly from the unit Do not overtighten this screw The crystal is polarization sensitive and the SP label on the front of it should appear upright for sampling vertically polarized light The crystal 1s mounted in a sealed housing filled with a phase matching liquid never open the sealed unit When properly installed in its mount the crystal 1s flush with the input side of the mount A single setscrew on the top exposed corner of the mount holds the crystal in place Figure 4 1 UV Filter absorbs all but the frequency doubled UV light from the non linear crystal This UV light then passes on to the photomultiplier tube PMT for detection and amplification A single setscrew on the top exposed corner of the mount holds the filter in place A light shield shroud slides onto the input snout of the filter and minimizes the amount of ambient light that reaches the filter Three filters are used to cover the entire waveleng
102. th range and each is identified by a different color refer to Table 4 3 The green filter 15 pro vided as standard for use with the Tsunami laser The other two are optional 4 5 Model 409 Autocorrelator Controls Connectors Table 4 3 UV Filters Filter Color For Input Wavelength Regions Black 550 to 680 nm Opt Green 680 to 1080 nm Std Light Blue 1080 to 1600 nm Opt Photomultiplier case encloses the photomultiplier tube PMT which has a high spectral sensitivity in the UV range from 550 nm to 1 6 mm A small aperture in the case allows only the filtered UV light to enter the PMT Do not open the case Power supply located on the side opposite the optics provides dc power to the Model 409 from standard line voltages It also contains the GAIN amplifier the variable DELAY control circuit and provides a sync sig nal from the ac synchronous motor drive circuit The following controls are located on the rear panel of the autocorrelator Figure 4 3 GAIN control used to control the gain of the photomultiplier tube PMT Saturation of the PMT output may occur if input beam power 15 50 mW or the GAIN control setting 15 too high Normal GAIN setting 15 between 0 and 2 The values silkscreened around the control knob are for uncalibrated reference only DELAY control determines the time interval between the trigger pulse output and the autocorrelation signal Use it to center the au
103. the two table clamps provided in the accessory kit clamp one of the input side corners of the bottom plate to the table This provides a swivel point and prevents the front of the unit from moving laterally yet allows lateral movement of the rear during the next few steps For the following steps use a white business card transparent tape an IR viewer or fluorescent card to help you see the alignment beam spot 13 Adjust the autocorrelator yaw alignment While viewing the alignment target Figure 5 2 from directly over head if possible move the back of the autocorrelator from side to side to place the alignment beam spot 1n line with the center of the tar get Note the beam exits below and behind the alignment iris When done the input beam should still be centered on the input iris 14 Adjust the autocorrelator pitch alignment 5 4 Setup and Operation Operation Operation Summary Again view the alignment target from above and adjust the height of the back leg to center the alignment beam on the target If the wheel 1s too tight to turn loosen the tension setscrew on the side of the mount Figure 4 2 just enough so the wheel can turn yet you can still feel a drag on it 15 Clamp the rear of the bottom plate to the table using the second table clamp Verify the input beam is still centered on the input iris If it has moved loosen the front table clamp and repeat Steps 9 through 14 This completes the au
104. to the surface of the optic making it even harder to clean or ruin it d Wipe the surface in a single motion Be careful that the hemostat does not touch the optic surface or the coating may be scratched 4 Repeat the cleaning with a fresh swab if necessary never reuse a tis sue When you need to clean the crystal remove the single shoulder screw and lift the assembly off the pivot pin Do not disassemble the unit only clean the windows This completes the cleaning procedure Chapter 7 Service and Repair This chapter contains a general troubleshooting guide for use by you the user It is provided to assist you in isolating some of the problems that might while using the autocorrelator complete repair procedure 15 beyond the scope of this manual For information concerning repair by Spectra Physics see Chapter 8 Customer Service At the end of this chapter 1s a replacements parts table listing components that can be replaced by you Troubleshooting Guide Danger Laser Radiation Use this guide 1f the performance of the Model 409 drops unexpectedly If you try the following suggestions and are unable to bring your autocorrela tor up to specification call your Spectra Physics service representative for help Many of these procedures require you to adjust or replace optics with the cover off and the laser at high power For safety block the input beam every time you change an optic or interfere
105. tocorrelation trace displayed on the oscilloscope It is a 10 turn knob for fine position control Power switch turns power on and off to the autocorrelator It 1s lit when placed in the ON position and ac line power 15 available thus indicating the autocorrelator is operational The following connectors are located on the rear panel of the autocorrelator Figure 4 3 SIGnal OUT connector BNC provides attachment for BNC cable between the autocorrelator and the high impedance vertical amplifier of an oscilloscope It sends the output signal from the photomultiplier tube to the oscilloscope TRIGger OUT connector BNC provides attachment for a BNC cable between the autocorrelator and the external trigger of an oscilloscope It sends the DELAY trigger pulse signal to the oscilloscope Power connector provides attachment for the power cord and contains the fuse as well as the voltage select pc board that 1s used for setting the system for 100 120 220 or 240 Vac Adjustments Caution Controls Indicators and Connections Figure 4 3 Model 409 Rear Control Panel With the exception of the crystal angle adjust do not realign the internal components of the autocorrelator unless you are certain the system has been tampered with or dropped All day to day alignment adjustments including changing the rotating block etalon and UV filter are covered in Chapter 5 Setup and Operation Crystal angle adjust
106. tocorrelator setup Continue with Operation for instructions on using your unit The summary below is a check list to allow you to use the autocorrelator quickly on a day to day basis Following the list are step by step start up and shut down procedures for first time users and for those using the auto correlator only occasionally As you continue to use your unit you will find many of these procedures becoming second nature and the summary list will serve as a quick reference When the pulse width to be measured 1s changed considerably the rotating block and etalon might need to be changed in order to maintain optimum measurable resolution When there are large changes in laser input wave length the UV filter might need to be changed to accommodate the new wavelength and when and if the UV filter 1s changed the lens might need readjustment to refocus the beams in the nonlinear crystal The sections Changing the Block and Etalon and Changing the UV Filter at the end of this chapter explain how to make these changes and readjustments e Verify the autocorrelator is setup and aligned to the picked off input beam according to the setup instructions above e Verify the laser 15 pulsing and mode locked and if you have a dye laser that the output beam 15 collimated e Change the block etalon and UV filter 1f necessary for the pulse width being measured and laser wavelength used e Adjust the nonlinear crystal until the autocor
107. tro reflectors offset each beam and return them to the beam splitter Here they are placed in a single plane in line with the incoming beam and directed down to the lens The beam splitter is very thin to minimize the pulse broadening effects of group velocity dispersion GVD However because it 1s thin it 1s also very fragile and care must be taken when handling or cleaning it It is designed for broadband operation from 550 to 1600 nm 4 2 Controls Indicators and Connections M Mirror HRR Hollow Retro reflector Xtal Non linear Crystal PMT Photomultiplier Tube Calibration Etalon _ TAS _ Rotating Block Thin Block Shown 1 2 N Alignment Beam Splitter M fo Input My ASA Main Alignment Beam Iris Splitter Y IS Lens BS Xtal TS i M3 Alignment Target Figure 4 2 The Model 409 Components and Optical Path M M and routing mirrors route beams along the beam path M and M route beams and 2 respectively from the main beam splitter to HRR and HRR respectively and the retro reflected beams back to the beam splitter M4 routes the two beams from the lens to the nonlinear crystal Each mirror has a mount that can be rotated in a plane parallel to the center plate when the mounting bolt on the opposite side of the plate 1s loosened Figure 4 4 Each also has a tilt control that permits a
108. und Pulsbreite sind abhangig von Pumpquelle und Laserkonfigu ration Laserklasse 4 Peligro al abrir y retiar el dis positivo de seguridad exist radiacion laser visible e invisi ble evite que los ohos o la piel queden expuestos tanto a la radiacion directa como a la dis persa Potencia Longitud de onda y anchura de pulso dependen de las opciones de bombeo y de la configuracion del laser Producto laser clase 4 Al abrir y retiar el dispositivo de seguridad exist radiacion laser visible e invisible evite que los ohos o la piel queden expuestos tanto a la radiacion directa como a la dispersa Potencia Longitud de onda y anchura de pulso dependen de las opciones de bombeo y de la configuracion del laser Pro ducto laser clase 4 Gevarr zichtbare en neit zicht bare laserstraling wanneer geopend en bij uitgeschakelde interlock Vermijd blootstelling van oog of huid aan directe straling of weerkaatsingen daarvan Vermogen golfleugten en pulsduur afhankelijk van pomp optics en laser configuratie Klasse 4 Laser Produkt Zichtbare en neit zichtbare laserstraling wanneer geopend en bij uitgeschakelde inter lock Vermijd blootstelling van oog of huid aan directe straling of weerkaatsingen daarvan Vermogen golfleugten en puls duur afhankelijk van pomp optics en laser configuratie Klasse 4 Laser Produkt Caution exposure The use of controls or adjustments or the performance of procedures other than t
109. ure 4 3 Model 409 Rear Control Panel 4 7 Figure 4 4 The Four Clamping Screws for the Mirrors and Main Beam Splitter 4 8 Figure 5 1 Normal pulse vs a PMT saturated Pulse with Wings 5 3 Figure 5 2 Model 409 Controls Indicators and Connectors 5 4 Figure 5 3 Autocorrelation pulse bounded by the squarish frequency doubled beam pulses The pulses shown are typical but sizes and shape can vary widely 5 7 Figure 5 4 Precursor to mode locking a pulse ps as seen through an autocorrelator einM ipwerellezeoe M rrr 5 8 Figure 5 5 The Model 409 Components and Optical Path 5 11 Figure 5 6 The frequency doubled retro reflected beams and autocorrelation spots on a white card 5 12 Figure 5 7 Calibration Etalon Correction Factor Relative to Delay at 800 5 13 Figure 5 8 Transform limited sech Pulse 5 14 Figure 6 1 Lens Tissue Folded for Cleaning 6 3 Figure 7 1 Position of Pin 11 on PMT Socket 7 3 Figure 7 2 The frequency doubled retro reflected beams and autocorrelation spots on a white card 7 4 Figure A 1 Scanning Mechanism Model 409 A 1 Figure A 2 Variation in Path L
110. ve apart completing the scan When the pulses from the two beams overlap and are focused into the non linear crystal which 15 set to the correct angle of incidence autocorrela tion occurs The resulting emission from the crystal 15 filtered by the uv filter and then directed into the photomultiplier PMT tube It is the output from this PMT circuit together with a sync pulse generated by the rotating block motor that creates the display on the oscilloscope 3 3 Model 409 Autocorrelator Signal Interpretation n A B P L gt B a BA lt MN gt B P L gt B A Yo B A E P L gt B Figure 3 4 Pulse Position and Overlap as the Block Rotates Accurate interpretations of autocorrelation measurements 1 e actual pulse width determinations are complicated by two factors e The ratio of the actual pulse width to the width of the autocorrelation trace 15 a function of the pulse shape e The pulse shape can vary between two extremes dependent upon the operating parameters of the laser system Fortunately we can dismiss the second factor for unlike a dye laser the Tsunami laser outputs an easily measured sech pulse as does OPAL system Figure 3 5 illustrates the relationship of pulse width to the auto correlated pulse shape for a Tsunami or OPAL generated sech pulse It is transform limited and generally exhibits the shortest possible pulse width The peak of the measured autocorrelation tra
111. vident but no correla tion trace No sweep trace on the oscillo scope Less than full scan range can be seen Weak signal Signal is too strong Signal to noise ratio is too low Possible optical misalignment Refer to appendix C System Alignment Gain is set too low or too high Check for stray light getting into the PMT especially fluorescent light check for tight connections between the oscilloscope trigger input and the autocorrelator TRIG OUT connector Possible optical misalignment Refer to appendix C System Alignment Check for possible damage to the rotating block Possible optical misalignment of tilt adjust Refer to appendix C Sys temAlignment Possible optical misalignment of the lens Refer to Steps 4 through 7 under Adjusting the Focus of the Lens in Chapter 5 Insufficient laser input power Increase laser output power Dirty optics Clean the optics Refer to Chapter 6 Maintenance GAIN control is set too high reduce gain Laser input power is set too high close down the input iris to attenuate the beam Possible optical misalignment Refer to Appendix C System Alignment Not enough power verify beam is collimated beam width 3 mm mode is TEMOO and ambient light entering the Mode 409 is minimized especially fluorescent light Symptom Pulses are too wide fs systems only Possible Causes Corrective Action Dispersion of the input
112. y one pass through the prism 15 used the output is spa tially chirped While the spacing of the prisms provides negative disper sion the prism material actually adds more positive dispersion to the system This can be used to our advantage in the optimization of a prism pre compensator For an initial setup based on a Tsunami laser and the Model 409 autocorre lator set the prisms approximately 30 cm apart and at Brewster s angle to the beam with the high reflector a few cm from the second prism With this spacing the prism pair should start with excess negative GVD By moving the prism tips into the beam increasing the amount of optical material in the beam we can balance the GVD for minimum pulse width To do this place the first prism on a translation stage so the stage moves the prism in the direction of the bisector of the apex This way more glass can be pushed into the beam path without displacing the beam or changing its angular direction By moving the prism into the beam path and monitoring the output from a Model 409 the pulse should get narrower as the disper sion 15 balanced If a minimum cannot be found adjust the prism spacing and search for the minimum again Model 409 Autocorrelator Calculating Pulse Broadening Broadening tout tin Below are some simple formulae for calculating the effects of GVD and compensation B broadening 15 defined as the ratio of the output pulse width to the input pulse width where
113. y short lt 80 fs the pulse broadening in optical materials from group velocity dispersion GVD makes measurement of its true pulse width difficult Also because the GVD of glass causes the pulse width to broaden the pulse that reaches an experimental sample after traveling through beam splitters lenses etc may not be the same pulse measured by the autocorrelator It is thus important to ensure that the measurement technique and experimental setup incorporate the same amount of glass and some GVD compensation if the shortest pulses possible are to be measured and delivered to a sample Even before the pulse leaves the laser it travels through extra glass For example if we assume the pulse in a Tsunami laser 15 at its shortest as it passes through the coating of the output coupler it then travels through the output coupler substrate the photodiode beam splitter and the output win dow For the Tsunami laser the total thickness of these optics 1s about 1 9 cm Thus using the formulas found in Calculating Pulse Broadening later in this chapter a pulse that 15 60 fs at the output coupler coating becomes 66 fs by the time it exits the laser Include the glass of the autocor relator and that in any experimental setup and the pulse can be broadened substantially Since the Model 409 uses two beam splitters a lens and a spinning block the pulse from a Tsunami laser is broadened before it is measured This means the pulse out of th
114. ze the amplitude of the autocorrelator trace Repeat Steps 5 6 and 7 until the pulse locks The amplitude will drasti cally increase and the pulse will be well defined at this point If the auto correlator gain is too high and the signal is saturated Figure 5 45 lower the gain on the autocorrelator until a clean pulse is present and increase the oscilloscope sweep speed to broaden the pulse for viewing Figure 5 4h 8 Optimize Tsunami output power then skip to Testing for a True Autocorrelation 5 7 Model 409 Autocorrelator _ PA Pp CI RI LA dl Ll LI _ LI RENE SAUNE JA Figure 5 4 Precursor to mode locking a pulse ps as seen through an autocorrelator on an oscilloscope 5 8 Setup and Operation Cannot Find an Autocorrelation Trace If the autocorrelation trace cannot be found do not try to align the autocor relator optics yet If the unit worked the last time it was used 1t should work this time even if you changed the block and etalon and or UV filter It is more likely that the laser 1s either not emitting pulses or the autocorre lator is not set up properly for the pulse width or wavelength being mea sured Verifying the Laser is Emitting Pulses 1 Verify the laser 15 emitting pulses a Connect an oscilloscope to the MONITOR and SYNC outputs on the Tsunami electronics module or if you are using a different laser use a fast photodiode such as the ET2000

Model 409 Autocorrelator - Spectra

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