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1. legend description reference FM401 3 FM413 Rmax45 800 nm C040003 FM420 FM424 Rmax45 532 nm C040008 BS12 BS7 uncoated 1 wedge plate C200003 BS8 Beam splitter 532 nm C160006 L1 Spherical lens 800 nm C280068 L2 Spherical lens 800 nm C260049 L3 Spherical lens 800 nm C280059 L4 Spherical lens 800 nm C260056 L5 Spherical lens 532 nm C260001 L6 Spherical lens 532 nm C260001 L7 Spherical lens 532 nm C260084 L8 Spherical lens 532 nm C260084 L9 Spherical lens 800 nm C260051 L10 Spherical lens 800 nm C280067 L11 Spherical lens 800 nm C260049 L12 Spherical lens 800 nm C280067 C Ti Sa Crystal C420009 P Photodiode E600002 CAM1 amp CAM2 High Resolution camera D600009 BP1 amp BP2 Beam pointing monitor D600004 EM1 amp EM2 Energy measurement device OF1 Optical fiber C400002 Table 6 22 References of optical components included in compressor module 250 TW Dulsar Second Multipass Amplifier 85 C Amplitude TECHNOLOG User s manual 6 8 3 ALIGNMENT A lot of mirrors are used in the second amplifier but only a few of them have to be adjusted for a power optimization In this paragraph we will assume that the mirrors of the multipass amplifier have not to be re adjusted and only the mirrors FM401 and FM402 will be used for the injection of the beam in the second amplifier At last the beam must pass through the both pinhole PH21 using the last mirror on the preamplifier breadboard
2. elevations in the compressor 4 Adjust the mirror FM603 to send the beam to the smallest grating of the compressor g at a beam height of 150 mm The beam between FM603 and g has to be perfectly horizontal Grating 2 Figure 6 26 Beam height values into the compressor 5 Adjust the grating g orientation so that the beam is reflected horizontally at the zero order reflection 6 Adjust the lines orientation of the grating g so that the beam is reflected horizontally on the first order of diffraction 7 Rotate the grating g in order to have an incident angle of the beam on the grating of about 30 degrees 8 Set the second grating G on the first order reflection at the distance of 600 mm 50 TW Dulsar Compressor 93 Q9 Amplitude I TECHNOLOGIE User s manual 9 Adjust the grating G orientation so that the beam is reflected horizontally at the zero order reflection 10 Adjust the lines orientation of the grating G so that the beam is reflected horizontally on the first order of diffraction 11 Rotate the grating G in order to have an incident angle of the beam on the grating of about the same value as the grating g The two gratings have to be as parallels as possible 12 Check with an infrared viewer that the beam which is dispersed is well centred on the gratings G 13 Set the reflector C on the beam diffracted by the grating G 14 Adjust the orientation of the reflector C to reflect
3. Button used to check a laser status as far as flash lamps Pockels cell and shutters are concerned 8 Validation of the switching on of a selected element 9 Validation of the switching off of a selected element 250 TW Dulsar 89 C Amplitude TECHNOLOG User s manual 5 START UP AND SHUT DOWN PROCEDURES Before starting up the laser it is absolutely essential to check that safety requirements are Satisfied e Make sure that all people within the laser area wear suitable adapted to radiation wavelength emitted protective glasses e All the interlocks systems must be active no shunt e The IR output beam must be blocked 5 1 DAILY START UP PROCEDURE Follow this step by step guide for the daily start up procedure of the laser system This procedure does not include the computer controlled mode it is a strictly manual mode Start up of the oscillator In this section the DPSS CW pump laser is supposed to be powered in the standby regime refer to the user s manual for more information about the complete starting up procedure e Switch on the cooling unit of the DPSS laser of the oscillator see picture 6 1 The temperature should stabilise after a couple of minutes Note that for Laser Quantum Finesse pump Femtolaser recommends to work with a high temperature of water coolant 25 C Thus to avoid thermal constrains in the oscillator behavior it is important to turn this chiller off as less often as
4. Pulsar 250 250 TW 710 HZ FEMTOSECOND SYSTEM User s Manual 7 aihe Narionale di Fisica Mucleare Amplitude _ TECHNOLO Tel 33 0 1 69 11 27 90 Fax 83 0 1 64 97 58 17 2 rue du Bois Chaland CE 2926 LISSES 91029 EVRY Cedex FRANCE Amplitude litude TTECHNOLOG User s manual TABLE OF CONTENTS 1 INTRODUCTION CHIRPED PULSE AMPLIFICATION esse esse eessse esse sesse esse ee se ee ei se ee 5 2 JIEASER SYSTEM MODULES iss beet see Else ese Ges en dee ene RS ee ee ge dee se PERS ERREUR Ro SER NE SEE eed 8 2 1 GENERAL SETUP OF THE FEMTOSECOND CHAIN ees es sesse se ee ee ee ee ee se ee ee ee ee ee ee ee ge ee ee ee ee ee ee ee 8 2 2 OSCILATOR BEE uc GE EE E M MdL ME 10 2 3 CONTRAST RATIO BOOSTER OPTIONAL ie ee ee ee ee ee ee ee ee ee ee ee 10 2 4 PULSE STRETCHER sesse ees ee es es se se eed ee ee ee ee ee ee ee ee ee ee ek be Re ee sees estate Re assesses sata seas se eese ta an 10 2 5 ACOUSTO OPTIC PROGRAMMABLE DISPERSIVE FILTER OPTIONAL 11 2 6 REGENERATIVE AMPLIFIER sees sesse ee se se se ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee nete Re nasse etse Re ee ee ee ee 11 2 7 ACOUSTO OPTIC PROGRAMMABLE GAIN CONTROL FILTER AOPGCF OR MATTER DEE 12 2 8 MULTEPASS AMPLIFIERS ES Ee eee tret eege ect EES se SU sos EEN se Ge be De ee e EP Eege 13 2 9 TYPICAL COMPRESSOR DESIGN 14 3 SAFETY WARNING Sisciscsscscccssicscesciesisteveccs cvaccevonseneccsceaveneccs
5. diffuse and fire resistant targets Whenever possible work with high ambient light in order to keep eye pupils contracted Make sure the laser beam is not accidentally pointed towards a reflecting surface People working on the laser must avoid wearing reflective objects wedding ring watch etc The reflections of the laser beam are generally in the plain of incidence of the laser and it is strongly recommended not to have the eyes in this plain The interaction between a laser beam and certain classes of materials flammable explosives or volatile solvents may be a source of fire Do not use the laser in the presence of such materials 3 2 ELECTRICAL SAFETY Some components used in the femtosecond amplifier system are supplied with high voltage These devices are protected with housings Never remove the protection covers of elements using high voltages Only an authorized and qualified person can manipulate these devices 250 TW Dulsar Safety 17 QD Arplitude litude TECHNOLOGIES User s manual Electrical safety label stickers are placed near sensitive components of the laser system 3 3 GENERAL SAFETY FEATURES Safety labels are affixed near sensitive components of the laser system Please make sure you have located those labels and rigorously follow the safety instructions e Label 1 Certification Label This label gives information on the model type and serial number of the laser sy
6. 40 5 2 DAILY SHUT DOWN PROCEDURE eder e pe p de bdo e ve dextre ar e EEN 46 6 CONTROL DIAGNOSTIC AND OPTICAL ALIGNMENT 4 ee eee eee see se Ge ge 49 6 1 EQUIPMENT NEEDED FOR ALIGNMENT cessere ee ee ee ee ee ee en ee ee ee ee rne sese tras enteras ette nn ee 49 6 2 CLEANING THE OPTICS i e ees sesse se ee ee ee ee ee se be ee ee eg ee SR be Re ee EEEE ER be ee Ad ee ER be Re ee de PR ee ee ee ER Ee 50 6 2 1 TOOLS 50 6 2 2 CLEANING PROCEDURE cccccccssssssssecececsensnnssccsceesessnausecececeessnnuacsecessessnsseesecsceesesseanees 50 6 8 TESI GONIROLPROGEDBURE END Reese DES Ee ee oe Ede We Ek ees See Gee Re Ee De ee ee AEE gees eed 51 6 4 SHORT PULSE OSCILLATOR esse sees esse se se ee ee ee ee ee ee ee ee ee ee Ee RR ee ee ee Re Re ee ee ee ee ee ee ee ee Ee 53 6 4 1 OUTPUT POWER MEASUREMENT se ee ee es ee se se se se ee ee es see se ENEE 53 6 4 2 PULSE TRAIN MONITORING esee ee ee ee ee ee ee enne nen ee enne tren en nenne entere sete nnns entere nis 54 6 4 3 SPECTRUM Ee e E 54 6 5 THE CONTRAST RATIO BOOSTER MODULE ese ese se ee ee ee se ee ee ee ee ee ee nnne entran enne tren en ee eit 57 6 5 1 OPTICAL SE TUP ie eege See 57 250 TW Dulsar Table of content 3 Amplitude litude TECHNOLOG Users manual 6 5 2 LIST OF COMPONENT EE 60 6 5 3 ALIGNMENT OF THE CONTRAST BOOSTER ee see ee ee ee es se se nennt inen tenete eres ee 60 6 6 THE STR
7. 680 730 780 830 880 Wavelength mn 4500 4000 3500 33000 2 82500 S o e EA 2 1000 500 0 630 680 730 780 830 880 Wavelength mn Figure 6 2 Typical spectra obtained at the output of the oscillator a Spectrum when there is only CW operation b Spectrum in mode locking operation with partial CW operation c Spectrum when the laser is perfectly mode locked without any CW The best setting is case c a broader spectrum without any CW operation obtained by adjustment of the cavity length Pump lasers warm up It is recommended warming up the pump lasers simultaneously with the oscillator 250 TW Daar Start up and shut down procedures 42 C Amplitude TECHNOLOG User s manual e Turn on the Genpulse electro optic controller The trigger necessary for the pump lasers flash lamps is immediately available e Reset the synchronization controller by pressing the highlighted green LED on the front panel e Propulse YAG lasers Turn on the main water Turn the keys located on each front panel of the lasers cabinet Switch on the cooling units and power supplies of each laser Press the Triggering button on the power supplies e CFR Ultra and CFR 200 Turn the key located on the front panel of the combo power supply and cooling unit Push one time the Run Stop button e With the remote control or via the computer turn on the flash lamps of all the lasers you might
8. Label 3 d fe e Label 4 Laser Hazard The laser is enclosed in protective housing that prevents to emission of visible and invisible radiation Avoid eye or skin exposure to direct or scattered radiation Do not open or disconnect the interlock system in order to avoid eye or skin exposure to visible and invisible laser radiation 250 TW Dulsar Safety 20 Amplitude litude TECHNOLOG Users manual DANGER VISIBLE AND INVISIBLE LASER RADIATION WHEN OPEN AND INTERLOCK DEFEATED AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION These labels are located on top of the protective housings Label 4 e HE im i i ol e Defeatable safety interlocks Defeatable safety interlocks shut the emission down of lasers laser pumps and oscillator laser when the protective housing is removed The interlocks are located on each laser housing in order to ensure immediate inhibition of light emission even if the protective housing is slightly lifted and not totally removed 250 TW Dulsar Safety 21 O Amplitude I TECHNOLOGIE User s manual A green LED is on when the interlock defeats are secured Defeatable safety interlock Remote electrical connector 250 TW Dulsar Safety 22 Guy Amplitude 4 CONNECTION AND SY
9. Optical cleaning paper Kodak Fisher e Acetone e Surgical tweezers e Cotton swab 6 2 2 CLEANING PROCEDURE First clean the optics with the dry neutral gas spray Do not put the spray into direct contact with the optics If the optics is still dirty proceed with the following procedure 1 Correctly clean your hands or wear clean protective gloves 2 Fold up the optical cleaning paper several times to obtain a little cushion of the diameter of the optical element Do not touch the cleaning surface of the optical paper 3 Humidify the cleaning paper with acetone 4 Gently drag the paper over the surface to be cleaned up 5 If it is necessary to repeat the operation take another cleaning tissue never use a cleaning paper twice WARNING FOR THE STRETCHER AND COMPRESSOR MODULES DO NOT TOUCH 250 TW Dulsar Oscillator 50 O Amplitude TECHNOLO User s manual THE SURFACE OF THE GRATING IT IS VERY FRAGILE AND EXPENSIVE AND CANNOT BE CLEANED 6 3 TEST CONTROL PROCEDURE Protective housing interlocks Remove the protective housing and check that the laser stops Replace the protective housing and make sure that the key has to be set on the STANDBY position in order to be able to restart the laser Defeat of safety interlocks When the action to defeat the safety interlocks is run make sure that the laser can operate and that the green indicator is on
10. The pulse build up time has to be as short as possible and the beam profile must be circular 10 It is also possible to optimize the Pockels cell PC2 orientation by checking the spectrum and the pulse build up time The spectrum must be without any modulation and the pulse train occurring into the cavity must be as stable as possible 250 TW Daar Regenerative Amplifier and PreAmplifier 79 QI Amplitude TECHNOLOG User s manual 11 The second Pockels cell PC3 might be optimized by watching the signal at the output of the regenerative cavity after the polarizer P3 and with a very long delay on the Pockels cell PC3 The right orientation is obtained when the output signal is as low as possible 12 Now proceed to the injection seeding following the procedure as described before in the figure 7 17 13 The other Pockels cells the pulse picker PC1 and the pulse cleaner PC4 must be aligned as described in the points 3 to 6 but with the direct beam centered on the scattering rings like in point 7 6 7 4 REGENERATIVE AMPLIFIER INJECTION SEEDING OPTIMIZATION In this paragraph the regenerative amplifier is assumed to be working properly If it is not the case refer to the previous paragraph The injection seeding in the regenerative amplifier may be optimized using two adjustable reflectors which control the direction of the beam coming from the stretcher In order to keep the system well aligned during a long period it is s
11. and PH22 using the mirror FM401 for the input beam Look at PH23 moving the mirror FM402 and PH24 using FM411 for the output During the optimization of the infrared beam adjustment it is absolutely necessary to stop the pump beam First adjust slightly the mirrors FMO3 to send the beam through the amplifier in order to be center on the wedge plate BS1 while checking that the beam is not hitting the edges of the crystal in its holder As the input beam has a quite large diameter compared to the optics aperture the beam must not clip on the lens L1 or on the wedge plate BS1 And with the mirror FM04 adjustments make the beam going out of the amplifier properly 250 TW Dulsar Second Multipass Amplifier 86 Guy Amplitude TECHNOLOG User s manual 6 9 THE MAIN CRYO COOLER MULTIPASS AMPLIFIER MODULE 6 9 1 OPTICAL SETUP AND ALIGNMENT The optical set up is shown in figure 7 23 for the infrared optics 750 nm 850 nm optics and the set up for pump optics 532 nm is shown on figure 7 24 Figure 6 23 below Configuration of the main amplifier 250 TW Dulsar Main Cryo Cooler Multipass Amplifier 87 O Amplitude TECHNO User s manual To the vacuum room From Amplifier 2 Probe beam J Amplitude TECHNOLOG User s manual The beam coming from the second amplifier is reflected by mirrors FM501 FM502 FM503 FM504 FM505 FM506 FM507 FM508 to make four passes in the Ti Sa crystal C inside the cryogenic cooler CR
12. beam coming from the stretcher is completely misaligned compared to the regenerative amplifier direction In this case it is recommended to observe the beam coming out of the regenerative amplifier in front of the mirror FM301 with a small screen no reflecting business card for example and to superimpose the beam coming from the stretcher with help of the mirror FM210 Note that the beam is not visible because of the low repetition rate imposed by the frequency of the Pulse Picker Then place the screen in front of the mirror FM210 and superpose the two beams with help of the mirror FM301 Repeat the procedure two or three times the injection seeding should occur It is then possible to follow the fine adjustment described below with the help of an oscilloscope Then as the axis of the seeded beam is well defined without Dazzler the mirror FM109 can be flipped down and the beam can pass through the Dazzler again The seeding must be optimized again without touching any of the previous mirrors witch are already aligned FM105 FM110 and FM 301 Only with mirrors FM107 and FM108 the injection seeding must be optimized in the same way that what have been explained before 250 TW Daar Regenerative Amplifier and PreAmplifier 81 C Amplitude TECHNOLOG User s manual 6 7 5 PREAMPLIFIER ALIGNMENT A lot of mirrors are used in the power amplifier but only a few of them have to be adjusted for a power optimization In this paragraph we wi
13. cell Be sure that only one pulse is dumped out see figure 7 18 a Intensity a u a wm o 90 e d o 2 4 D E wo om Delay nsec Delay nsec Figure 7 18 Optimization of the fine delay of the regen out Pockels cell for pulse extraction a No pulse after the maximum the pulse is correctly dumped out b There is a little pulse after the maximum the delay of the Hegen out Pockels is wrong FINE ALIGNMENT First switch off the power supply of the Pockels cells in order to avoid any electrical risk The alignment of the regenerative amplifier uses the seed beam coming from the stretcher without pumping the cristal 1 Adjust the mirrors FM301 to send the beam successively to the polarizer P1 P2 the mirror CM1 and through the crystal C It is possible to use a diffusing device 250 TW Paar Regenerative Amplifier and PreAmplifier 77 Amplitude litude TECHNOLOG User s manual cleaning tissue for example in front of the crystal and to observe the transmitted light in order to centre the beam on the Ti Sa rod 2 Adjust the mirror CM2 to centre the beam on the aperture of the Pockels cell PC2 3 Proceed to the orientation adjustment of the Pockels cell Place a polarizer and a white screen after the Pockels cell and diffuse the input beam as described in figure 7 19 Screen Diffusing element Polariser Pockels cell Fig
14. component 250 TW Dulsar Stretcher 63 gt Amplitude TECHNOLO User s manual The beam coming from the Booster hits folding mirror FM201 passes through WP201 hits the mirror FM202 and goes into the stretcher through the periscope PR The stretcher itself is made of grating G concave mirror CVM convex mirror CXM corner cube CC and a prism P The optical alignment of the stretcher will be described in the Optical Alignment part The beam comes out of the stretcher at FM203 The beam then passes through the telescope L1 L2 to reduce the beam size and hits folding mirror FM204 FM205 and FM206 Folding mirror FM207 can be flipped up and down If one wants to bypass the Dazzler it can be done by simply flipping up FM207 Then the beam is directly seeded inside the regenerative cavity 6 6 2 LIST OF COMPONENTS Table 7 8 summarizes the different optical components of the stretcher module with Amplitude Technologies references legend description reference FM201 3 FM210 Rmax45 800 nm C040003 L1 Spherical lens C260049 L2 Spherical lens C280057 WP201 gt WP203 Half wave plate 800 nm C220001 PR Periscope C040003 AOPDF Acousto optic modulator E600033 CC Corner cube C560014 CVM Concave Mirror C560017 CXM Convex Mirror C560018 P Prism C340004 G Grating C500009 250 TW Dulsar Stretcher 64 gt Amplitude TECHNOLOL User s manual Table 7 8 References of optical c
15. ee ee ee ee eb dee PUE 90 6 10 THE COMPRESSOR MODULE esene therein e esee einer nnns ee ee ee ee 91 6 10 1 GENERAL PRESENTATION ee ee ee se entes essei aiite ness ee ee eate neas e s eae ee ee 9 6 10 2 COMPLETE ALIGNMENT OF THE COMPRESSOR eher ee ee se se ee 92 6 10 3 PARALLELISM ADJUSTMENT BETWEEN THE GRATINGS ee ee ee ees se se ee ee ee ee 95 6 10 4 OPTIMIZATION OF PULSE DURATION ee ee se se se se se se ee ee ee ee nnne nnne Re Re Re Re annua 95 7 TECHNICAL SPECIFICATIONS wisssssessscscsessonsssnsesssscnsccessscesessesecsscsnascssssnccssesessossexsccsssesssces 98 ros n PERFORMANCE EE 98 7 2 DIMENSIONS sc HE HM E 99 250 TW Dulsar Table of content 4 J Anplitude TECHNOLOG User s manual 1 INTRODUCTION CHIRPED PULSE AMPLIFICATION The 250 TW Pulsar laser system is a compact femtosecond laser source providing more than 7 5 J pulse energy at 10 Hz repetition rate The pulse length is about 22 fs and leads to a peak power higher than 250 TW with an average power of rougly 75 W This new and compact high performance laser source was designed by Amplitude Technologies The system is a Titanium Sapphire laser based on the so called Chirped Pulse Amplification CPA scheme It consists of a full integrated Ti Sa oscillator with its DPSS Diode Pumped Solid State pump laser a stretcher a regenerative amplifier two multi pass amplifiers pumped by three Nd YAG lasers and a vacuum compressor The CPA technique invo
16. essential for researchers working with this system to be able to handle it well and solve problems which may arise This chapter describes how to control and realign the different parts of the system 6 1 EQUIPMENT NEEDED FOR ALIGNMENT The entire alignment of the system requires the following equipment Laser safety goggles to protect against the second Harmonic of the Nd YAG laser 532 nm and 700 900 nm Ti Sapphire laser emission gt Powermeters able to measure 10 mW to 100 W of the second harmonic Nd YAG wavelength as well as the Ti Sapphire emission wavelength 700 900 nm An infrared viewer for the visualization of the IR beams Fast photodiodes with rise time better than 1 ns if possible An oscilloscope with a bandwidth of 300 MHz or higher Metric allen wrenches A spectrometer 1 nm min resolution in the range of 700 900 nm VV VV ON WV Polaroid polarizers 250 TW Dulsar Oscillator 49 Q9 Amplitude TECHNOLOL User s manual 6 2 CLEANING THE OPTICS In the laser the optical elements are submitted to a significant energy density Pollutants are susceptible to contaminate the optics and create hot spots These hot spots degrade the quality of the optical surfaces and coating which may result in reduced laser efficiency Cleaning the optics rarely needs to be done This operation should only be performed if power loss or mode deterioration is observed 6 2 1 TOOLS e Dry neutral gas spray e
17. is needed to produce gain This explains why ultra short pulses must be stretched prior to amplification Safe operation of amplifiers requires effective intensities below 5 GW cm Obviously the longer the stretched pulse is the higher the amplified pulse energy which can be reached without damage is Grating 1 Grating 2 Mirror Figure 1 3 Principle of a pulse compressor After amplification the pulse must be compressed back to its initial duration A compressor device based on a wavelength dispersion system very similar to the stretcher see figure 1 3 is commonly used This compressor is theoretically able to compensate for any stretching introduced into the pulse but the gratings are required to be perfectly aligned In particular the incident angle onto the compressor has to be finely adjusted in order to compensate for the stretcher and the dispersion effects through the amplifier Another important issue is the beam quality Regenerative amplifiers use a TEMoo laser resonator and deliver diffraction limited beams High power multipass amplifiers use only flat mirrors and do not affect the beam quality 250 TW Dulsar Introduction 7 J Amplitude TECHNOLOG User s manual 2 LASER SYSTEM MODULES 2 1 GENERAL SETUP OF THE FEMTOSECOND CHAIN The Pulsar 250 TW is divided into three parts the front end the main amplifier and the vacuum compressor The general set up of the two first parts of the laser
18. is shown on the figure 2 1 250 TW Daar Laser system and modules 8 O Amplitude TECHNOLOGE User s manual OUTPUT FIRST TABLE OUTPUT SECONDTABLE Figure2 1 General setup of the front end and the main amplifier The femtosecond front end and the main amplifiers system are installed on two optical table of 4 50 x 1 50 m Different modules such as booster stretcher 10 Hz amplifier and amplifier 2 are set on bread boards A single cover prevents the optics of the amplifier from the dust and air flow perturbation 250 TW Paar Laser system and modules 9 O Amplitude TECHNOLOGIES User s manual 2 2 OSCILLATOR The Oscillator is a commercial Synergy manufactured by Femtolasers Refer to the manual for details It is delivered with its own DPSS CW pump laser a Laser Quantum Finesse as well as its closed loop chiller figure 2 2 Figure 2 2 Synergy Oscillator 2 3 CONTRAST RATIO BOOSTER OPTIONAL In order to improve contrast ratio an optional module is available This module consists in a compact multipass amplifier to amplify the oscillator output up to the microjoule level The pulse is then cleaned by a saturable absorber that removes residual ASE background of the oscillator pulses before seeding 2 4 PULSE STRETCHER The stretcher design is based on an all reflective triplet combination Offner see figure2 3 The triplet combination is composed of two spherical concentric mirrors The fi
19. need For instance the sequence to turn on the CFR 200 flash lamps is CFR Flash On Use a similar sequence to turn on the Pockels cells or to check the status of a laser Allow 10 minutes of warm up time Synchronisation box Once the oscillator is into pulsed mode the RF clock necessary for the Genpulse and the Synchronisation box delay generator is available Contrast Booster Switch on the High Voltage CH1 CH4 Switch on the Pulse Picker CH1 Pockels cell Allow the CRF Ultra Q Switch with the remote control The CFH Ultra is the common pump laser for the regenerative amplifier and the contrast booster multipass amplifier Regenerative Amplifier Start up 250 TW Daar Start up and shut down procedures 43 C Amplitude TECHNOLOG User s manual Switch on the power supply of the acousto optic modulator controller device Dazzler and Mazzler Activate the desired acoustic waves with the laptop computer controlling to the acousto optic modulators Carry on with the start up of the electro optic module Activate the High Voltage CH2 CH3 Activate the electro optics switch CH2 located on the front panel to make the cavity lasing Open the regen shutter Open the seed shutter If the manual shutter of the pump beam was closed open it Extract the beam by activating the CH3 switch If necessary the fine delays for the four Pockels cell can be adjusted using the correspo
20. possible 250 TW Daar Start up and shut down procedures 40 QD Amp litude TECHNOLOGE User s manual Picture 6 1 Front panel of cooling unit e Switch on the power supply of the DPSS oscillator pump laser see figure 6 2 Turn the key to the ON position Push the Shutter button to open it Push the Push to start button Key Power level 1 Shutter open e Wait about 10 minutes corresponding to the warm up time of the pump beam in the oscillator e Drive the oscillator into pulsed mode Mode lock by pushing the button located at the back of the oscillator module The oscillator should start mode locking It is recommended to check the spectrum with a spectrometer The spectrum should be broad without any narrow lines corresponding to CW component see figure 6 2 The adjustment of the mode locking operation can be optimised using the 250 TW Daar Start up and shut down procedures 41 O Amplitude ATT User s manual screw corresponding to the adjustment of the cavity length see the oscillator manual for more information 4500 4000 3500 ed 5 3000 D z 2500 n 2000 E 600 E 1000 500 0 630 680 730 780 830 880 Wavelength mn 4500 4000 3500 gt 3 3000 SS 2500 gt 2000 e o 1500 2 1000 500 0 630
21. see in figure 1 2 the blue path is longer than the red one Therefore blue wavelengths take more time to travel through the system than red ones Due to Fourier transform properties a femtosecond pulse exhibits a broad spectrum typically 26 nm for a 30 fs pulse Since the bluer part of the spectrum is delayed compared to the redder part when travelling through the stretcher the output pulse is stretched and looks like a temporal rainbow red in the leading edge and blue in the trailing edge The stretching factor depends on the spectral width of the input pulse and on the intrinsic characteristics of the stretcher grooves density of the gratings distance between the gratings number of roundtrips in the stretcher incidence angle etc For a given stretcher configuration the wider the input spectrum is the longer the stretched pulse is 250 TW Dulsar Introduction 6 O Amplitude TECHNOLOG User s manual Once stretched the pulse is amplified in several amplifier stages a regenerative amplifier followed by a 5 pass amplifier then a 4 pass amplifier and the main amplifier which is a 3 pass At the output of the amplifying system the energy does not depend on the input pulse duration delivered by the oscillator The only constraint comes from possible damage that could be caused to the amplifying material A 10 mJ 30 fs pulse has a peak power higher than 300 GW Very few solid state materials can withstand such a high density that
22. shutter SH1 allows or not the beam to seed the regenerative amplifier RGA The RGA is made up with two curved mirrors CM1 and CM2 two flat mirrors FM304 and FM305 two Pockels cells PC2 and PC3 and two polarizers P2 and P3 The AOPGCF is located between the two polarizers The beam is seed at P2 and extracted at P3 The seed Pockels cell is PC2 and the extraction Pockels cell is PC3 An automatic shutter SH2 allows or not cavity lasing A brewster cut Ti Sa crystal C1 is located between the two curved mirrors For safety reasons the Mazzler driver must be linked to the cavity shutter When the acoustic wave is changed the cavity must not lase to avoid damages caused by a too narrow spectrum during the spectral transitions thanks to Mazzler action So a specific photodiode MPD allows a control of the regenerative cavity when the Mazzler acoustic wave is changed This photodiode monitors the brewster reflection leak of the crystal thanks to mirrors FM335 FM336 and FM337 The RGA is pumped by a CFR Ultra The pump beam is split into two parts using the combination of a wave plate WP2 and a polarizer P8 After P8 two mechanical shutters MSH1 and MSH2 allows the user to block or not the pump beams For the RGA the pump beam successively hits FM322 FM323 and FM324 passes through WP4 before being focused by L1 The other part of the beam passes through the same kind of devise with WP3 and P9 which allows a fine adjustment of the Booster pump be
23. the beam at this step on the grating on the figure 7 13 O9 OO PH3 PH4 Figure 7 13 Position of the second pass on the grating 10 Proceed to the adjustment of the reflector C to reflect back the beam horizontally at a height of 145 mm The height of the reflected ER BS beam can be adjusted by the vertical position of the upper mirror of the e e reflector The horizontal adjustment of the beam can be controlled by Ss Ss the orientation of the reflector It is possible to check the horizontality of the beam when the beam hit the convex mirror again at the same PH3 PH4 position Rotate the reflector so that the spots on the convex mirror are well superimposed OO OO OO OO 11 The beam should be reflected by the concave mirror for the fourth Bei nA time at a height of 105 mm It is then propagating to the prism reflector oo oe P 250 TW Dulsar Stretcher 69 C Amplitude TECHNOLOG User s manual 12 Adjust the prism position and orientation so that the beam exits from OO OO oe oe the stretcher without being clipped It exits at the same height than the e A input beam but with a lateral shift OO OO OO OO PH3 PH4 13 Itis now necessary to check that there is no spatial chirp in the output beam of the stretcher If the convex and concave mirrors are not in a perfect afocal position the different wavelengths have different angles of propagation at the output of the stretcher This can be che
24. 0 TW Dulsar Main Cryo Cooler Multipass Amplifier 91 VIN Amplitude TECHNOLOGIE User s manual Since two conditions have to be fulfilled at once two free parameters are needed The angle of incidence on the grating and the compressor length are the two adjustable parameters of this system Note that it is necessary to keep all the information about the pulses and especially the entire spectrum to be able to re compress correctly to short pulses This means that it is very important to transmit all the wavelength of the spectrum in each part of the system Input gt FM601 Output FM602 FM604 FM603 Figure 6 25 Setup of the compressor It is suggested to check the amplified pulse spectrum before any alignment of the compressor Remember that the minimum spectral width to get 30 fs is around 40 nm 6 10 2 COMPLETE ALIGNMENT OF THE COMPRESSOR The alignment procedure of the compressor can be achieved as follow 1 First reduce the output energy by increasing the delay of the extraction Pockels cell 2 Adjust the last mirror FM510 of the multi pass amplifier to send the beam to the centre of FM601 Note that the beam must be simultaneously centred on the afocal system L1 L2 and on the mirror FM601 50 TW Dulsar Compressor 92 O Amplitude User s manual 3 Adjust FM601 to direct the beam to the beam elevator FM602 FM603 in order to have a beam height of 150 mm at the output of FM603 see figure 7 26 for the beam
25. 02001 1 FM3073 FM321 Rmax45 800 nm C040003 FM335 Silver mirror C140011 FM3362 FM337 Rmax45 800 nm C040003 CM1 Curved mirror 800 nm C020013 CM2 Curved mirror 800 nm C020013 FM322 2 FM334 Rmax45 532 nm C040008 P1 P2 P3 P6 P7 Polarizer 800 nm C320003 P4 P5 Polarizer 800 nm C320007 P8 P9 Polarizer 532 nm C320004 WP1 Wave plate 800 nm C220001 WP2 WP3 WP4 Wave plate 532 nm C220006 L1 Spherical lens 532nm C260074 L2 Spherical lens 800 nm C260076 L3 Spherical lens 532 nm C260063 L4 Spherical lens 532 nm C260063 C1 Brewster cut Ti Sa Crystal C420003 C2 Flat Flat Ti Sa Crystal C420004 AOPGCF Mazzler E600002 BS1 Beam splitter 532 nm C160006 BS2 Wedge plate C200003 BS3 Beam splitter 800 nm C160008 SH1 SH2 SH3 Mechanical shutter B350081 MSH1 amp MSH2 Manual shutter F510127 PR Periscope C040003 PD1 amp PD2 Photodiode E600002 PM1 amp PM2 Power measurement monitor BP1 Beam pointing monitor D600004 EM1 Energy measurement monitor MPD1 Mazzler Photodiode CR Ceramic C580011 Table 7 16 References of optical components included in the power amplifier module 250 TW Dulsar Regenerative Amplifier and PreAmplifier 75 J Anplitude TECHNOLOG User s manual 6 7 3 REGENERATIVE AMPLIFIER CAVITY OPTIMIZATION The important thing in the regenerative amplifier is to decorrelated the behaviour of the cavity itself from the beam seeded inside it So when you check the cavity itself first block t
26. 10 Metallic mirror C140001 FM111 Rmax60 800 nm C060001 FM112 gt FM114 Rmax45 800 nm C040003 FM115 2 FM119 Rmax45 532 nm C040008 P Prism C340004 C Ti Sa crystal C420006 CM1 amp CM2 Curved mirror 800 nm C020029 LO Spherical lens 800 nm C260079 L1 Spherical lens 800 nm C260052 L2 Spherical lens 800 nm C260055 L3 Spherical lens 800 nm C260055 L4 Spherical lens 532 nm C260068 L5 Spherical lens 532 nm C260068 P1 amp P2 Polarizer 800nm C320008 WP1 Half wave plate C220001 PC1 Pockels cell C460005 PD Photodiode E600002 BS101 Beam Splitter 532 nm C160003 BS102 amp BS103 Wedge plate C200003 SA Saturable absorber C380009 Table 7 6 References of optical components included in the contrast booster module 6 5 3 ALIGNMENT OF THE CONTRAST BOOSTER The pointing of the oscillator may change from day to day due to a re alignment or to a temperature variation of the room In this case the beam direction 250 TW Dulsar Contrast Ratio Booster 60 QI Amplitude TECHNOLOG User s manual has to be re adjusted in order to go through the Booster properly Use the following procedure to readjust the input beam in the stretcher 1 With adjustments of mirrors FM100 and FM101 make the beam passing well trough pinholes PH10 PH11 and PH12 and check with a scattering paper that the beam passes well centered through the Pockels cell 2 Before entering the ring cavity with mirror FM105 c
27. 200mA SMA connectors used to supply the photodiodes set on the system Interlock this connector is used for mechanical shutters Int Ext Selector for switching between computers controlled mode Ext or Manual mode Int Control SubD 15 pin connector Connector for external control Channel Function Voltage 1 Pulse Picker 6000 V 2 Regenerative seed 2995 V 3 Regenerative Extraction 2995 V 4 Pulse Cleaner 6000 V Table 4 6 Electrical requirements for all the modules provided with the system supplied by the Genpulse 250 TW Dulsar 28 Amplitude litude TECHNOLOG User s manual Mazzler photodiode signal in Q Switch 10 Flash lamps Hz trigger Mazzler photodiode signal 10 Hz trigger signal control Oscillator sianal Regen shutter RF signal control Main 10 HZ sync switch signal Mazzler f rns trigger E 5 j 8 Mains Photodiodes power Pockels cell Pockels cell Synchronization High Voltage Mechanical shutters signal Local remote Pockels cell Computer control control switch electronic suplly Figure 4 7 Rear panel of the Genpulse module and functional description of the connectors 4 2 3 2 FRONT PANEL OF THE GENPULSE The front panel of the Genpulse is shown in figure 4 8 e The seed and regen switches correspond to the mechanical shutters set between the stretcher and the regenerative amplifier and into the regenerative amplifier respectively The re
28. D1 Analog Pockels 2 Sync out Q delay 2 PD2 Coarse PERE Pockels 3 Analog Pockels 4 delay 2 x 10 Hz Flash Lamps Figure 4 10 Genpulse synchronization principle The signals switching the Pulse Picker and the Regen Input are activated according to independent analogical fine delays for each Pockels cell A coarse delay adjustable with the thumbwheel of the front panel is applied before switching the Regen Output and the Pulse Cleaner Pockels cells Two analogical fine delays allow one to adjust precisely the switching time of the Pockels cells The Flash lamps signal used to trigger all the YAG lasers flash lamps is not re synchronized and is taken just after the divider The nearly jitter free re synchronized signal for all Q switch triggering is generated after AD1 Table 4 11 summarises the typical output signals delivered by the Genpulse 250 TW Dulsar 32 O Amplitude _ User s manual Input Connector Signal characteristics RF in BNC from 0 1V to 1 5V Output Connector Signal characteristics Synchro out 10 Hz BNC TTL 6 usec Out 3 10Hz BNC TTL 6 usec 10 Hz Flashes BNC TTL 6 usec 10 Hz Pockels BNC TTL 6 usec Table 4 11 Electrical characteristics of the output signals of the Genpulse 250 TW Dulsar 83 O Amplitude PTECHNOLG User s manual Oscillator pulse train ri E Internal clock Clock after divider Re synch
29. ETCHER MODULE nnn ee ee ee ee ke ee ee ee ee ee tenens assa eae ke ee ee 63 6 6 1 OPTICAL EE 63 6 6 2 LIST OF COMPONENTS oe Se ee eg eeuse See ese see ue ede Ge oe Ge Ee ee ee dd ee reg EY ee do ee Rer ed 64 6 6 3 ALIGNMENT OF THE STRETCHER ee ees ee ese ees es es es es ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee 65 gt COARSE ALIGNMENT IN THE STRETCHER ese ee ee ee ese se se ee ee ee ee ee se ede ee ee ee ee ee ee ee ee ee 65 P FINE ALIGNMENT IN THE STEICHEN 66 6 6 4 ALIGNMENT OF THE DASZ 71 6 7 THE REGENERATIVE AMPLIFIER AND THE PREAMPLIFIER MODULE see esse se ee ee ee 72 6 7 1 ele 72 6 7 2 LIST OF een Ee 75 6 7 3 REGENERATIVE AMPLIFIER CAVITY OPTIMIZATION eee se se ee ee ee es se tenentes 76 GOARSE ALIGNMENT heri e EE Ee sue rte 76 gt FINE ALIENMENT ee EES 77 6 7 4 REGENERATIVE AMPLIFIER INJECTION SEEDING OPTIMIZATION 80 6 7 5 PREAMPLIFIER ALIGNMENT ees esse see ese ees es es se ee ee ee es ese ese ee ee ee be ee ee ee ee ee ee ee ee ee ses ER 6 8 THE SECOND MULTIPASS AMPLIFIER MODULE eren ee inerte nenne 83 6 8 1 OPTICAL E 83 6 8 2 LIST OF COMPONENT Sots esse tee ete rt eeepc ette eee ee estos 85 6 8 3 ALIGNMENT EE EE EE the 86 6 9 THE MAIN CRYO COOLER MULTIPASS AMPLIFIER MODULE Jie esse se se ee ee ee ee ee ee se ee ee ee 87 6 9 1 OPTICAL SETUP AND ALIGNMENT ee ee ee se se se se se se ee ee ee ee ee nnne nennen Re ee 87 6 9 2 LIST OF COMPONENTS x cc tte tese tec GE Gee tee e esee eU eg
30. Electro optic devices On the front panel of Genpulse switch the High voltages labelled CH CHAT and CH2 CH3 off On the front panel switch off the sync signals labelled CH1 CH 2 CH 3 and CHAT e Mazzler AOPGCF With the dedicated laptop computer exit the Mazzler program f desired turn of the RF Generator controller Main switch on the rear panel 250 TW Paar Start up and shut down procedures 47 Amplitude TECHNOLOG User s manual gt Turn off the Dazzler Acousto optic modulator optional e With the dedicated laptop computer exit the Dazzler program e Turn off the RF Controller Button on the rear panel if desired gt Stop the oscillator and its CW pump laser e Push the Shutter open e Turn the key to the standby position Shut down the electro optic controller Genpulse e Switch off the main power switch of the Genpulse controller e Switch off the main power switch of the synchronization module Other Power supplies e Turn off if desired all power supplies attached to optional devices such as cameras cameras controller main computer oscilloscope 250 TW Daar Start up and shut down procedures 48 J Amplitude TECHNOLOG User s manual 6 CONTROL DIAGNOSTIC AND OPTICAL ALIGNMENT The 250 TW femtosecond system will be first installed and aligned by a Amplitude Technologies service staff Nevertheless a femtosecond system is quite complex and it is
31. Mazzler creates losses in the laser cavity unexpected high energy peak might occur and damage the optical components To prevent from such situations before loading a new waveform a shutter closes the cavity and a photodiode PD checks that there is no laser beam Once we are sure there is no laser into the cavity the waveform is loaded and the photodiode checks that the laser radiation is back again 250 TW Dulsar 25 O Amplitude TECHNOLOGIE User s manual Regen shutter 240 V 1 5A Computer Figure 4 4 Mazzler connections and security loop connection 4 2 3 ELECTRO OPTIC DELAY GENERATOR GENPULSE The Genpulse is an electronic module which controls all the Pockels cells except Q switch Pockels cells of YAG lasers involved in the system It manages the high voltage supplying the fast switching devices and the synchronisation of the different Pockels cells The Genpulse is also used to power several photodiodes around the system as well as the Mazzler security loop The Pockels cells assemblies and the photodiodes should be connected as described in figure 4 5 The complete scheme of synchronisation of all devices and events will be developed in the next paragraph see figure 4 12 for a synchronisation overview 250 TW Pasar 26 User s manual 1000 V 2A EN EN E E ES z E Mazzler II controll Pockels cells Photodiodes Shutters B Mazzler Photodiode Figure 4 5 Connection sketch for the Genpulse de
32. NCHRONISATION 4 1 WATER CONNECTIONS Only a few modules need to be cooled with water e The base plate of the oscillator e Thecrystal mount of the second multipass amplifier e The Helium compressor used for the cryo cooler e The 11 Propulse Nd YAG laser User s manual The oscillator is delivered with its own closed loop chiller and so is the crystal mount in the second multipass amplifier Note that the power supply of the DPSS CW pump laser has to be powered permanently in order to prevent any damage to the laser diodes 4 2 ELECTRICAL SUPPLY CONNECTIONS The table 4 1 summarizes the electrical requirements and consumptions for each module delivered with the system Maximum Voltage Module Absorbed phase nb Power DPSS OW laser Power supply 110 240V 1 2400W DPSS CW laser chiller 110 240V 1 2000 W CFR Ultra Nd YAG laser 240V 3 3500 W 250 TW Pasar 28 O Amplitude TECHNOLO CFR 200 Nd YAG laser 240V 3 800 W Propulse YAG 1 11 200 V 5000 W Cryostar 200 V 3400 W Electro optic modules controller 200 V 500 W Compressor controller 200 V 2500 W User s manual Table 4 1 Electrical requirements for all the modules provided with the system 4 2 1 CRYOSTAT DEVICE The cryostat used for the main amplifier crystal cooling is composed of several elements the vacuum chamber the primary pump the controller the helium compressor and its tran
33. RT PULSE OSCILLATOR The Femtosource Synergy Femtolasers oscillator is set on a breadboard including the CW DPSS laser Laser Quantum Finesse the main part of the oscillator cavity and some holders with optical elements see figure 7 1 For more details about the oscillator and its pump laser please refer on the respective manual of these modules Figure 7 1 Femtoseconde oscillator cavity 6 4 1 OUTPUT POWER MEASUREMENT Use a suitable Power meter 0 1W for the measurement of the output of the oscillator When a power meter is used the pulse energy can be obtained by dividing the average power by the pulse repetition rate The pulse duration has no influence on the measurement of its energy on most volume absorber power or energy meters The specific performances of the short pulse oscillator are described in the Femtosource user s manual p 17 Note that the energy level of the short pulse oscillator should be in the range of 6 nJ about 500 mW for 80 MHz repetition rate oscillator 250 TW Dulsar Oscillator 53 Amplitude litude TECHNOLOG Users manual 6 4 2 PULSE TRAIN MONITORING It is possible to monitor the pulse train of the oscillator with a fast photodiode and a high bandwidth oscilloscope This control might be very useful during the mode locking optimization of the oscillator A typical pulse train record is shown on the figure 7 2 EE Figure 7 2 Typical pulse train out of the oscillator monitore
34. Remote interlock Remove the connector and make sure that the laser stops Replace the connector and make sure that the key has to be set on the STANDBY position in order to be able to restart the laser With a voltmeter record the voltage between the pins of the remote interlock Key actuator Make sure that the key can not be removed in the ON position Emission indicator Make sure that the blue emission indicator is lightning after the Key actuator switch on and lasers emission Beam attenuator Make sure that the attenuator slides properly Make sure that the position is consistent with the labeling Check with a sensitive screen that no light is emitted while the attenuator is closed Manual reset Make sure that the laser cannot be restarted after a safety fault or an interruption of mains without setting the key on the STANDBY position first Labels 250 TW Dulsar Oscillator 51 O Amplitude TECHNOLOG User s manual Affix the four types of labels Certification Identification Protective housing Aperture label and Warning logotype on their specified locations Output energy Measure with a calibrated power meter the average power of the output beam for the nominal operating point and divide this value by the repetition rate to get the output energy Pulse duration Measure the pulse duration with a SPIDER module 250 TW Dulsar Oscillator 52 O Amplitude TECHNOLOGIE User s manual 6 4 SHO
35. TATION The main adjustment of the compressor is the distance between each grating This can be adjusted in order to minimize the pulse duration at the output of the system It is also possible to adjust the injection of the beam at the entrance of the compressor The optical setup of the compressor is shown on figure 7 25 First adjust the mirror FM510 located at the output of the main amplifier to center the beam of the pinhole located in front of the mirror FM601 Then adjust the mirror FM601 in order to center the compressed beam on the pinhole located at the output of the compressor The compressor is used to shorten the pulses after the amplification in the regenerative amplifier As explained in the introduction the compressor will compensate for the group delay dispersion delay versus wavelength introduced by the stretcher and the amplifiers This group delay can be expended in a Taylor series t 1o A o oo B o wo Note that the group delay is obtained by differentiation of the phase law do 1 0 0 2 o 0 2 3 0 3 6 and hence that A24 corresponds to the second order dispersion and B 93 2 to the third order dispersion In order to compensate for the group delay of the stretcher and amplifier both the second and third order dispersion terms must be adjusted These terms depend on the grating groove density compressor length and the angle of incidence on the grating 25
36. W Dulsar 30 O Amplitude User s manual Regenerative Amplifier Figure 4 9 Functional sketch of the different modules driven by the Genpulse The internal synchronisation principle is shown in figure 4 10 The Genpulse module includes an internal clock which is divided in order to obtain the repetition rate of the pump lasers All the synchronisation outputs for the pump lasers are delayed with individual delay lines AD that allow accurate synchronisation of the pump pulses as much as possible with the amplified pulse Two extra output signals Sync out and Out 3 from the divider are also available for the synchronisation of external devices Sync out is used for the synchronization of the Mazzler if necessary The RF signal coming from the oscillator monitor is re synchronised with the output of the divider providing a low repetition rate signal synchronised with the oscillator pulse train Note that this re synchronisation produces a small natural jitter corresponding to one oscillator period between the re synchronised signal and the pump signal synchronisation 250 TW Dulsar 81 QD Amplitude litude TECHNOLOGIES User s manual A long delay corresponding to the pulse build up time of the pump lasers is applied to the re synchronised signal This long delay is driven by the internal clock of the Genpulse and does not produce any significant jitter 10 Hz Q Switch RF in Mes ied E Pockels 1 delay PP P
37. YO The amplified beam is extracted out of the amplifier on folding mirror FM509 FM510 sends the beam through the attenuator A first telescope made up with large aperture lenses L1 and L2 expand the beam then the beam passes through the motorized waveplate WP and hits the two polarizers P1 and P2 After a second expander made with lenses L3 and L4 the beam is carried towards the vacuum compressor Several infrared diagnostics are available in the main amplifier The fluorescence of the Ti Sa crystal is monitored thanks to a photodiode P and alignment of the pump beams can be monitored with the high resolution camera CAMS The leak through FM509 is imaged with L6 L7 telescope Two beam splitters BS1 and BS2 take a fraction of the beam to send into an energy measurement detector EM and into a beam pointing monitoring device BP And an other camera CAMA monitors the IR output beam profile For alignment of the IR beam two diaphragms are set on the table to materialize the input line The beam must pass through PH25 and PH26 using mirrors outside the second amplifier The other mirrors inside the amplifier must not be touched Before sending the pump at full power check the cryostat cooling compressor is ON the temperature of crystal must absolutely not exceed 30 C and the vacuum pressure in cryostat is correct i e less than 10 mbar refer to the Cryostar manuel You can then send the pump power one by one into the crystal an
38. am energy This part of the pump beam is then directed by FM325 towards the contrast booster in order to pump the ring multipass amplifier The unused part of the pump beam is sent in a ceramic CR The leak through FM323 is used to monitor the CFR Ultra power by sending it to the power measurement device PM1 FM326 and FM327 are used to send the beam to the detector 250 TW Daar Regenerative Amplifier and PreAmplifier 73 J Anplitude TECHNOLOG User s manual The beam coming from the regenerative amplifier is reflected successively by the polarizers P4 and P5 used in reflection There are two reflections on each polarizer The polarizer P6 reflects the beam into the pulse cleaner made up with Pockels cell PC3 and folding mirror FM306 Once the polarization is flipped the beam is transmitted through P6 and P7 and reflected by FM307 The leak through FM307 is used to measure the RGA energy using EM1 An uncoated wedge plate splits this leak and sends the beam into photodiode PD2 to monitor the sliced pulse A piece of glass sends part of the leak into the detector BP1 while the transmitted part goes into EM1 Another photodiode PD1 is used to monitor the RGA pulse train behind the folding mirror FM305 After FM307 the beam is then seed into the 5 pass multipass amplifier by FM309 and FM310 Its polarization is flipped to match the pump polarization using WP21 The multipass amplifier is of the butterfly type the different passes overlap
39. ansdvectccsenccucceseecsedecssutassaccsseseveossevees 16 Su LASER LIGHT SAFETY ees ss sesse sees ee es ee se se se ee ee ee oe ee ee ee ae ee eo GR be Re ed ee GE Re Re ed ee GR Re ee ee ER Re 16 3 2 ELEGTRIGALSAFETY tussen See ee Ges ee ee gee ee Bee eo e FEE Oe OE e See ee ee bee ee E De ep Gie gek 17 3 3 GENERAL SAFETY FEATURES iese ees esse se be ee ee ee ee ee ee ee ee ee ee ee ee ee be ee ee ee ee ee ee ee eed ee ee ee ee Re ee ee ee ee 18 4 CONNECTION AND SYNCHRONISATION e ee esse ee see esse soto Gee Se ee Ge ee seta Ge ge 23 4 1 KE ee Eng Le 23 4 2 ELECTRICAL SUPPLY CONNECTIONS cccccccssssecessseeecesssececssnsecseaeeeceesueceessnsececseeeeesnaeees 23 4 2 1 CRYOSTAT DEVIGE T 24 4 2 2 AOPDF AND AOPGCF OPTIONAL ee ee ee ee ee ee se ee ee ee ee ee ee ee ee ee ee 25 4 2 3 ELECTRO OPTIC DELAY GENERATOR GENPULSE eeuse ee ees se se ee ee ee ee se se se ee nnne 26 4 3 SYNCHRONISATION os e ee Ed 30 4 3 1 INTERNAL SYNCHRONISATION PRINCIPLE OF THE GENPULSE cete 30 4 3 2 PULSAR CONTROLLER gege eege teste Eie teda tego vs 35 gt Front panel of the Zeen controller Au 36 gt Rearpanel Pulsar of the Paar controller sss 37 gt Remote Control Description eerte 38 5 START UP AND SHUT DOWN PROCEDURES sees se esse esse esse es ee ee es Ge se Ge be aetas se 40 5 1 DAILY STARTUPTPROGEDURE 5 weed des veuve ise GE ee ee orb GES Ep oe eek
40. cells by successively selecting Y AG Pockels Off Close shutters for YAG 2 to 11 by successively selecting YAG Shutter Off Turn off YAG 2 to 11 flash lamps by successively selecting YAG flash Off e Cryogenic Cooler Turn off the compressor unit by pressing the green button on the front panel of the controller e Stop the water flow if desired Amplifier 2 e YAG 1 pump laser 250 TW Daar Start up and shut down procedures 46 J Anplitude TECHNOLOG User s manual Switch off YAG 1 Pockels cells by successively selecting YAG Pockels Off Close shutters for YAG 1 by successively selecting YAG shutter Off Turn off YAG 1 flash lamps by successively selecting YAG flash Off e Stop the water flow if desired gt Amplifier 1 e CFR 200 Big Sky Laser With the remote controller Turn off Pockels cell and flash lamps by using the sequences Pockels CFR2 Off and Flash CFR2 Off Stop the Q switch by pressing O Switch on the front panel of the power supply Turn the key on the Big sky power supply chiller combo RGA Amplifier and Contrast Booster e CFR Ultra Pump laser With the remote controller switch off the Pockels and flash lamp trigger by using the sequences Pockels CFR1 Off and Flash CFR1 Off Turn the key on the laser power supply chiller combo e
41. cked by inserting a grazing incidence screen in the output beam and checking the spectrum at different location of the beam see figure 7 14 If the spectrum changes when you move the fiber into the beam it means that there is some spatial chirp TEST KC EC TK TTT Figure 7 14 Check for spatial chirp from the stretcher 14 Itis then necessary to change the distance between the concave and the convex mirrors To do this translate a little the convex mirror and re adjust the stretcher alignment touch only the concave mirror CVM should be enough Otherwise follow the procedure as described before Then iterate if the spectrum is still changing 15 A more precise method consists of observing the behavior of the far field output beam At the output of the stretcher place a long focusing lens f 1meter for example and place a CCD camera at the focus point Then place a stripe of paper about 2 cm wide in front of the concave mirror Move this stripe along the mirror and 250 TW Dulsar Stretcher 70 J Amplitude TECHNOLOG User s manual if somewhere you can see two focusing spots at the camera screen there is some spatial chirp inside the stretcher 16 Then follow point 14 6 6 4 ALIGNMENT OF THE DAZZLER It is then important to be sure that the beam goes properly through the Dazzler AO Two pinholes PH5 and PH6 allow a quick check to see the alignment of the beam that goes out from the stretcher To enter proper
42. cooler compressor unit e Open the water flow for Cryostar compressor e Turn on the compressor unit by pushing the green button on the front panel of cryostat controller gt Start up YAG 2 2 to 11 Propulse e Open the water flow for YAG cabinets e Turn on all the units power supplies and cooling units if they were off e Press the triggering yellow button on power supplies e On the remote control select the sequence YAG Flash On YAG lasers should start flashing e Allow 15 mn of warm up time e Open the shutters for YAG lasers by selecting YAG Shutter On Warning Before switching the Pockels cells on wait until the Cryostar temperature has stabilized at about 180 deg Celsius e Switch the Pockels cells on by successively selecting YAG 7 Pockels On 250 TW Daar Start up and shut down procedures 45 Q9 Amplitude TECHNOLOGIES User s manual 5 2 DAILY SHUT DOWN PROCEDURE Follow the procedure below to shut down the femtosecond system for a short period overnight in the case of daily operation for example Refer to the respective manual of each module if you want to shut down the system for a longer period gt Block all infrared laser beams e Close the shutter seed on the front panel of the Genpulse e Close the shutter regen on the front panel of the Genpulse Amplifier 3 e YAG pump lasers Switch off YAG 2 to 11 Pockels
43. d observe on the CCD camera CAMS if the pump lasers are centered on the crystal Check output energy and IR amplified beam profile at each additional pump for 250 TW Dulsar Main Cryo Cooler Multipass Amplifier 89 Q9 Amplitude I TECHNOLO User s manual safety With half of the pumping lasers on the crystal you can check alignment of the amplifier looking at the fluorescence depletion on CAMS If the depletion is not symmetrical touch very sofly FM502 to adjust it Figure 6 24 Non symmetrical depletion on the left and on the right symmetrical depletion after alignment 6 9 2 LIST OF COMPONENTS legend description reference FM5012 FM5012 Rmax45 800 nm C040022 FM513 gt FM524 Rmax45 800 nm C040029 BS1 uncoated 1 wedge plate C200003 BS2 uncoated 1 wedge plate C200003 L1 Spherical lens 800 nm C260083 L2 Spherical lens 800 nm C260012 L3 Spherical lens 800 nm C280085 L4 Spherical lens 800 nm C260015 L6 Spherical lens 800 nm C2800XX L7 Spherical lens 800 nm C2600XX C Ti Sa Crystal C420016 WP Waveplate 800 nm C220007 P1 amp P2 Polarizor 800 nm C320010 P Photodiode E600002 CAM3 amp CAM4 High Resolution camera D600009 BP Beam pointing monitor D600004 EM Energy measurement device 250 TW Dulsar Main Cryo Cooler Multipass Amplifier 90 J Anplitude TECHNOLOG User s manual 6 10 THE COMPRESSOR MODULE 6 10 1 GENERAL PRESEN
44. d by the photodiode 6 4 3 SPECTRUM CONTROL The temporal duration of the pulse is directly related to the width of the spectrum at the output of the oscillator It is easy to check the Full Width at Half Maximum of the spectrum at the output of the oscillator with a spectrometer The spectrum should be centered on 800 nm with a FWHM of approximately 80 100 nm The Femtosource oscillator is not a tunable system so no attempt should be made to change the central wavelength or FWHM using the crystal position screw at the back of the oscillator module 250 TW Dulsar Oscillator 54 O Amplitude TECHNOLOG User s manual Typical spectrum records are shown in figure 7 3 The presence of CW operation in the laser is easily visible when a narrow spike appears in the spectrum It is possible to optimize the mode locking operation using the cavity length fine adjustment of the oscillator located on the back of the blue box The mode locking effect has to be started by pushing the button 630 680 730 780 830 880 Wavelength mn 630 680 730 780 830 880 Wavelength mn D 630 680 730 780 830 880 Wavelength mn Figure 7 3 Typical spectra obtained at the output of the oscillator a Spectrum when there is only cw operation b Spectrum in mode locking operation wit
45. ge plate BS102 is used to monitor the pulse train in order to synchronize the Pockels cells via the electro optics driver Genpulse When the oscillator is realigned it may be necessary to re adjust this synchronization device e Plug the output of the photodiode on the oscilloscope and observe the optical pulse train coming from the oscillator e Optimize the output level by adjusting the mirror FM200 which send the beam into the photodiode P1 a signal between 100 mV and 500 mV is required e Plug the output of the photodiode back into the RF input of the Genpulse 250 TW Dulsar Contrast Ratio Booster 62 O Amplitude TECHNOLO User s manual 6 6 THE STRETCHER MODULE 6 6 1 OPTICAL SETUP The stretcher module includes the Offner stretcher and the Dazzler The Dazzler is a dispersive programmable acousto optic filter that enables an accurate control of the phase and the amplitude of the light wave This control is able to pre compensate the group velocity dispersion involved in the whole laser chain Thus shorter pulses could be generated Refer to the Dazzler user s manual for further information Figure 7 7 shows the stretcher setup and all the devices installed on the same breadboard All the optical elements are labeled in order to locate easily each component for the alignment procedure To the Regenerative amplifier cp o 3 gt D UJ o o o D Figure 7 7 Setup of the stretcher module and index for each
46. gen shutter is also controlled by the Mazzler software in case of modification of the acoustic waveform e The other switches ON OFF activate independently the different Pockels cell of the system according to table 4 6 e The Coarse delay thumbwheels is used to adjust the delay between the input and the output of the beam in the regenerative amplifier e The four toggle buttons correspond to the fine delay adjustment of the Pockels cells CH1 to CH4 250 TW Dulsar 29 C Amplitude litude TECHNOLOG User s manual e The Fine delay screen displays the relative delay value of the Pockels cell selected with the Ch button HV On Off activates the high voltages for the pair of Pockels cell CH1 together with CH4 and CH2 with CH3 Analog Delay display Swicth for Pockels cell activation Regen and seeding Shutters control 8 kV high voltage switch Coarse PC delay adjustment Fine Pockels cell 4 kV high delay adjustment voltage switch Figure 4 8 Front panel of the Genpulse module and functional description 4 3 SYNCHRONISATION 4 3 1 INTERNAL SYNCHRONISATION PRINCIPLE OF THE GENPULSE The Genpulse module allows one to synchronise the triggering of the different Pockels cells of the system with the pulse train of the oscillator see figure 4 9 It can also manage external device synchronisation such as an acousto optic module or Pockels cell for Q switched Nd YAG lasers 250 T
47. gh power delivered by the system is ensured by three multi pass amplifiers The first multi pass amplifier is pumped by 120 mJ 9 532 nm issue from a single YAG laser CFR 200 by Quantel The second multi pass amplifier is pump by 250 TW Daar Laser system and modules 13 O Amplitude TECHNOLOG User s manual 2J delivered by one Propulse Nd YAG laser and the third amplifier is pumped by 10J delivered by 10 Propulse lasers manufactured by Amplitude Technologies Due to the high average power of the pump beams a cryogenically cooled Ti Sa mount is used in the third multipass amplifier see figure2 6 This innovative component significantly increases the thermal conductivity of the crystal and consequently decreases the thermal lens effect in the amplifier medium The spatial beam quality in high energy and high average power femtosecond amplifiers is then significantly improved Figure2 6 Cryogenically cooled Ti Sa device Cryostar 2 9 TYPICAL COMPRESSOR DESIGN The amplified pulses are re compressed to short duration using a classical compressor design see figure2 7 Two gratings with an optimized number of lines transmit the very broad spectrum bandwidth with excellent efficiency The geometry of the stretcher compressor is designed to obtain the flattest phase dispersion in the overall system 250 TW Dulsar Laser system and modules 14 VIN Amplitude TECHNOLOGIES User s manual Figure2 7 Typical compressor des
48. gy after compression gt D 5 2 calculated gt 50 mJ Pulse duration lt 20 fs 23 fs 30 fs 23 fs Synchronisation with Linac 1 ps 0 15 ps 1 ps 0 15 ps Spectral width gt 50nm 80 nm 50nm 80 nm Beam size 100 mm 120 lt 15 mm Beam quality M 1 5 M 1 5 Energy stability RMS 1 5 96 0 8 96 2 96 Contrast ns 10 3 107 10 3 107 Contrast ps 1 ps 10 210 10 2 10 Contrast ps 9 5 ps 10 5 105 10 5 10 Contrast ps 10 20 ps lt 5 10 1 107 5 10 1 107 Contrast ps ASE 10 4 10 lt 10 4 100 Beam pointing Stability 2 urad 15 urad 1 FWHM full width half maximum 50 TW Dulsar Technical Specifications 98 O Amplitude TECHNOLOG User s manual 7 2 DIMENSIONS dep e e 2 de Il EE e 2 sa I EI i II li 50 TW Dulsar Technical Specifications 99
49. h a partial cw operation c Spectrum when the laser is perfectly mode locked without any cw operation The best setting is case c a broader spectrum without any cw operation obtained by adjustment of the cavity length The beam coming from the oscillator is going to the contrast Booster module by the periscope PRO and the mirror FM100 on the optical table 250 TW Dulsar Oscillator 55 VIN Amplitude TECHNOLOGI User s manual The mirror FM100 enables adjustments before entering the contrast Booster module The residual transmission of the mirror FM100 is used for checking the measurements of the oscillator This leak is split into two parts by beam splitter BS103 The reflected part of it goes into the power measurer BPOSC while the transmitted part of it is reflected by the mirror FM101 and then goes into the beam pointing monitor PMOSC The optical setup is shown in figure 7 4 Figure 7 4 Setup of the measurement detectors of the oscillator 250 TW Dulsar Oscillator 56 gt Amplitude TECHNOLOG User s manual 6 5 THE CONTRAST RATIO BOOSTER MODULE 6 5 1 OPTICAL SETUP The contrast booster is a pulse cleaning apparatus It consists in an n pass 6 lt n lt 20 multipass amplifier for direct amplification of the oscillator pulse and a saturable absorber that cleans it This module also contains an electro optical devise the pulse picker which creates a 10 Hz pulse train from the 60 MHz oscillator pulse train The optica
50. he beam that is reflected by BS1 is sent into a high resolution camera CAM1 an energy measurement device EM1 and a beam pointing monitor BP1 To do so we use three beam splitters BS2 BS3 and BS4 A focusing lens L9 and a diverging lens L10 help adapting the beam size to the aperture of the detectors An optical fibre F located after BS4 checks the pre amplifier spectrum As for the input beam the output amplified beam is analyzed in terms of beam profile energy and beam pointing In this purpose the leak through FM411 is used Folding mirrors FM415 FM16 and FM417 sends the beam into the different detectors by means of three beam splitters BS5 BS6 and BS7 A focusing lens L11 and a 250 TW Dulsar Second Multipass Amplifier 84 O Amplitude TECHNOLO User s manual diverging lens L12 are also added to adapt the beam size to the detectors aperture BS7 sends the beam into another high resolution camera CAM2 BS5 into the energy measurement device and BS6 into the beam pointing monitor In addition to theses measurements folding mirror FM413 sends the crystal fluorescence into a photodiode P This feature is very helpful for optimizing the YAG laser energy and the timing between the infrared and the pump beam 6 8 2 LIST OF COMPONENTS Table 6 22 summarizes the different optical components which compose the amplifier module with Amplitude Technologies references
51. he injection seeding of the regenerative amplifier Thus you will also prevent any damage which can occur during the alignment optimization COARSE ALIGNMENT The main criteria for the optimization of the regenerative cavity alignment are the pulse build up time i e the delay between trigger on the second Pockels cell CH2 of the Genpulse and the maximum of the pulse train and the output beam profile which must be as round as possible If those two criteria are 1 Set the switching time of the Regen Out Pockels cell to a very large value in order to observe on the oscilloscope all the nanosecond pulse train propagating in the cavity Or you can switch the CH3 button off 2 Reduce the pulse build up time as much as possible adjusting slightly the rear mirror FM304 and the pump beam mirror FM324 Observe the beam profile at the output in front of the polarizer P3 3 Switch on the second Pockels cell and adjust the delay in order to maximize the output power after the polarizer P3 4 Proceed to the injection seeding It is possible that the new setting has changed the direction of the regenerative cavity it is then necessary to proceed to the injection seeding optimization see the next paragraph 250 TW Paar Regenerative Amplifier and PreAmplifier 76 QS Arplitude litude TECHNOLOGIES User s manual 5 Optimize the output power of the seeded regenerative amplifier with the adjustment of the delay of the Regen out Pockels
52. he operation several times WARNING FOR THE COARSE ADJUSTMENT DO NOT TOUCH OTHER 250 TW Dulsar Stretcher 65 QD Amplitude litude TECHNOLOGIES User s manual Figure 7 9 Representation of the different beams hitting the stretcher grating when the oscillator is in CW operation left and in ML operation right gt FINE ALIGNMENT IN THE STRETCHER It may be necessary to re align the stretcher completely The setup of the stretcher and the different heights of the beam are summarized on following figures e e e e e o Convexe mirror e 9 e 9 e e e e e e D D o 2 0 oe Figure 7 10 Picture and ray tracing of the stretcher 250 TW Dulsar Stretcher 66 C Amplitude TECHNOLOGIE User s manual Figure 7 11 Beam height values into the stretcher and alignment tool used The alignment procedure is performed as follows 1 Use the oscillator in CW mode take the alignment tools provided with the system see figure 7 11 and place them in front of the grating G and in front of the corner cube C in their corresponding clamps PH3 PH4 2 The direction of the beam between elements P and G must follow a straight line materialized by the both pinholes holes at height 135 mm on the alignment tool the red one on the tool sketch You can make some walking tuning with mirrors FM101 and FM103 in order to help you 250 TW Pasar OO OO e
53. heck PH13 with mirror FM110 check PH14 The beam must go out of the ring cavity properly without touching anything else inside it And on the concave mirrors CM1 and CND the next figure must be seen that corresponds to the fourteen passes in the cristal Second round trip in the cavity after the vertical i shift 3 If the spots are not aligned as shown in the previous figure usually a vertical shift can be seen in the first passes To optimize the alignment the prism P must be blocked with a paper Then to avoid the vertical shift the figure observed on CM2 must be optimized with the vertical adjustment of the concave mirror s mount CM1 using the pinhole PH15 The figure observed on CM1 must be optimized with the vertical adjustment of the flat mirror s mount FM111 And then release the beam that hits the prism P and adjust its mount in lateral to align the second line of the spots just above the first one and correctly above the mirror FM107 4 The beam then goes out and is sent properly in the stretcher when it passes through PH16 and PH17 thanks to mirrors FM113 and FM114 250 TW Dulsar Contrast Ratio Booster 61 O Amplitude TECHNOLOG User s manual 5 Note that when the infrared beam has been realigned the amplification might be not optimized So observe the signal from the photodiode PD2 at the output of the ring amplifier and optimize the level of the signal with the pump mirrors adjustments FM119 and FM117 The wed
54. ign For very high output power the compressor is usually placed into a vacuum chamber to prevent from non linear effects in air 250 TW Daar Laser system and modules 15 QD Amplitude TECHNOLOGIES User s manual 3 SAFETY WARNINGS 3 1 LASER LIGHT SAFETY Several laser beams are involved in a femtosecond amplifier system Because of its high intensity the laser beam can cause serious injuries if safety precautions are not followed The laser source is a potential hazard to the eyes not only from direct or secular reflection but also from a diffuse reflection Damage to skin and fire hazards may also be caused by this kind of source The following is a partial list of precautions to follow when using high power class IV pulsed lasers Only authorized and trained personnel should be allowed to operate the laser system When operating the laser system all people within the laser room must wear protective eye wear adapted to the emitted radiation wavelength 250 TW Dulsar Safety 16 J Anplitude TECHNOLOG User s manual Never look directly into the laser beam Even after secular or diffuse reflections a laser beam can cause serious injuries Set up experiments so that the laser beam is either well above or well below eye level Set up controlled access area for laser operation Post clearly visible warning signs near the laser operation area Block unused laser beams with absorbing
55. indicates that the input signal frequency 10 Hz from Genpulse is correct 250 TW Pasar 86 Q9 Amplitude TECHNOLOGIE User s manual gt Rear panel Pulsar of the Pulsar controller Figure 5 2 Pulsar Controller rear panel 1 Input flash lamps synchronization from Genpulse 2 Input Q switch Pockels cell synchronization from the Genpulse 3 Output Flash lamps synchronization signals for each YAG lasers CFR Ultra CFR 200 and Propulse lasers from 1 to11 4 Output Q switch Pockels Cell synchronization signals for each YAG lasers CFR Ultra CFR 200 and Propulse lasers from 1 to11 5 Shutters connectors for Propulse YAG lasers only 6 Main Switch 7 Main Power plug 250 TW Dulsar 37 O Amplitude TECHNOLOG User s manual 8 Chiller Interlock 9 Cryostat Interlock 10 SubD 15 pin connector for external control Remote Control Description Figure 5 3 Remote control Emergency Stop stops every shutters flashes and Pockels of the system Energy mode Alignment for low energy and Normal for Q switch mode Buttons used to select a laser which the user wants to change its state Button used to turn on or turn off the flash lamps of a laser Button used to turn on or turn off the Q switch Pockels cell of a laser oa P Wa N Button used to open or close a laser shutter does not operate for CFR 200 and CFR Ultra 250 TW Dulsar 88 O Amplitude TECHNOLOG User s manual
56. ing G and rotate the reflector C in order to re find the initial position of the output beam 3 Monitor the spatial chirp effect and repeat the procedure as described above until the chirp effect has disappeared As for the stretcher optimization you can also check this effect of spatial chirp with a CCD camera Place the camera at the focal point and observe the far field beam while a part of the beam is clipped inside the compressor Move the compressor as it is described previously until the clipping as no effect 6 10 4 OPTIMIZATION OF PULSE DURATION The pulse duration optimisation can be achieved using an autocorrelator or a phase measurement 50 TW Dulsar Compressor 95 O Amplitude TECHNOLOGIES User s manual 1 First adjust the distance between the two gratings with help of the translation stage fixed on the grating g in order to minimize the pulse duration 2 Locate the position of the beam at the output of the compressor 3 Rotate slightly the first grating g and rotate the second grating G in order to re find the initial position of the output beam 4 Re adjust the distance between the two gratings in order to minimize the pulse duration again 5 Repeat the procedure 2 3 4 to get the output pulse duration as short as possible Note that the pulse duration is completely optimized when the spectrum has been broadened with the Mazzler and when the phase has been corrected with the Dazzler Refer those device
57. inside the Titanium Sapphire crystal C2 The beam path is the follow the beam successively hits FM311 FM312 FM313 FM314 FM315 FM316 FM317 FM318 FM319 and FM320 which is the exit mirror Then FM321 sends the beam into a telescope L5 and L1 in the next amplifier to enlarge it An automatic shutter SH3 allows to block or not the beam coming from the RGA The pumping configuration is explained next The amplifier is pumped by a CFR 200 The beam is first levelled down by a periscope PR It is then splits into two parts with BS1 The beam portion that is transmitted through BS1 is reflected by FM328 and focused on the left side of the crystal C2 by L3 The reflected beam portion is sent onto the right side of the crystal using the folding mirrors FM329 and FM330 The beam is focused using the lens L4 The CFR 200 power is monitored by sending the leak through FM328 to a power measurement device PM2 FM332 FM333 and FM334 sends the beam into the detector 250 TW Daar Regenerative Amplifier and PreAmplifier 74 O Amplitude Tre CHNOLO 6 7 2 LIST OF COMPONENTS User s manual Table 7 16 summarizes the different optical components which order the regenerative amplifier and the pre amplifier module with Amplitude Technologies references Legend description reference FM301 3 FM303 Rmax45 800 nm C040003 FM304 2 FM306 Rmax0 800 nm C
58. is allows decreasing the pulse duration down to 20 fs 2 6 REGENERATIVE AMPLIFIER The first amplification stage consists of a regenerative amplifier producing around 1 mJ stretched pulses at 10 Hz The regenerative cavity includes two Pockels cells in order to optimise the contrast ratio out of the amplifier One is used to seed 250 TW Dulsar Laser system and modules 11 VIN Amplitude TECHNOLOGI User s manual the stretched pulse into the regenerative cavity and the other dumps out the pulse at the maximum energy level The regenerative amplifier technique provides an excellent beam profile according to the TEMoo transverse mode of the resonator An advanced electronic module see figure2 4 allows synchronizing and switching over the different Pockels cells in the system For ultra short pulse duration an optional Acousto Optic Programmable Gain Control Filter AOPGCF or Mazzler can be added into the cavity The use of this device is explained in the next section Figure2 4 Synchronization and switch over electronic module Genpulse 2 7 ACOUSTO OPTIC PROGRAMMABLE GAIN CONTROL FILTER AOPGCF OR MAZZLER The main limitation for laser amplifiers is the gain narrowing To circumvent this problem people used to use intracavity dispersive filters thin etalon birefrigent filters In this case one has to deal with many drawbacks such as pulse replicas cavity instability Recently the advent of the Dazz
59. l setup is shown in figure 7 5 250 TW Dulsar Contrast Ratio Booster 57 gt Amplitude TECHNOLO User s manual Bee EE To the Stretcher mm mm a Figure 7 5 Setup of the contrast booster module The beam coming from the oscillator is reflected on folding mirror FM100 on the optical table and passes through a cube polarizer P1 an uncoated wedge plate BS102 and hits the mirror FM103 Thanks to the mirror FM102 one of the both reflections on BS102 is focused on a monitoring photodiode PD1 using lens LO while the other reflection hits an optical fibre F linked to the spectrometer The signal 250 TW Dulsar Contrast Ratio Booster 58 D gt Amplitude I TECHNOLO User s manual provided by the photodiode is used as a RF master clock signal for the all laser system and the fibre monitors the oscillator spectrum After FM103 the beam enters the 10 Hz Pulse Picker which is made of a half wave plate WP1 and a double Pockels cell PC1 used in transmission Once the high voltage applied to the Pockels cell is switched on the beam polarization is switched from horizontal to vertical and then reflected on polarizer P2 and on mirrors FM104 and FM105 After the pulse picker the out coming beam passes through the collimating lens L1 and then the beam goes into the contrast booster by hitting FM106 FM107 FM108 FM109 and the edge of FM110 Then the beam enters the ring multipass amplifier This multipass c
60. lay generator 4 2 3 1 REAR PANEL OF THE GENPULSE All the connectors of the Genpulse are set on the rear panel as shown in figure 4 7 e The main switch controls the supplying of power to the module e RF in BNC connector corresponding to the photodiode output signal that monitors the oscillator pulse train e Channel 1 to 4 HV High Voltage BNC Synchronization signal and SUBD15 connectors power supply Used to control the Pockels cells of the system Table 4 6 describes the corresponding Pockels cells function versus the channel number e Sync OUT corresponds to an output 10 Hz synchronisation signal available to trigger the Mazzler for example e Out3 is an extra 10 Hz signal that can be used as a trigger e Flash and Pockels Those 10 Hz signals are used to synchronise the flash lamps and Pockels cells for all YAG lasers CFR and Propulse via the synchronization module The synchronization scheme is developed in the next sections 250 TW Dulsar 27 QI Amplitude TECHNOLOG User s manual Mazzler security loop control It is compound of three connectors Photodiode in b pin connector receives the monitoring laser signal of the regenerative amplifier Photodiode out BNC connector connected to the Mazzler controller it transmits the laser status emission or not to the Mazzler controller Shutter Control BNC connector enables the Mazzler controller to remotely close or open the regen shutter 12V
61. ler has considerably improved the spectrum enlargement by flattening the global amplifier gain But the spectrum width is still limited to around 50 nm against 30 nm without the Dazzler To pick up even more amplifier bandwidth Amplitude Technologies use a very innovative technique that enables to handle spectra as wide as 80 nm 250 TW Dulsar Laser system and modules 12 gt Amplitude TECHNOLOG User s manual For ultra short pulse duration below 25 fs an optional Acousto Optic Programmable Gain Control Filter AOPGCF or Mazzler by Fastlite is added into the cavity The purpose of this device is to flatten the global amplifier gain in order to enlarge spectrum As mentionned before due to Fourier Transform properties the larger is the output spectrum the shorter the compressed pulse duration can be Spectral filter gt Pockels cell diffracted beam acoustic wave EI Figure 2 5 Principle of the Mazzler for a simple laser resonator The Mazzler is device similar to the Dazzler but with slight differences An acoustic wave diffracts the beam The unwanted spectral components belong to the diffracted beam and are sent away from the laser cavity This way one creates a hole in the non diffracted beam cavity beam for spectral components located where the gain is the highest figure 2 5 This results in gain flattening Very broad spectra can be achieved using this technique 2 8 MULTFPASS AMPLIFIERS The hi
62. ll assume that the mirrors of the multipass amplifier have not to be re adjusted and only the mirrors FM307 and FM309 will be used for the injection of the beam in the first amplifier During the optimization of the infrared beam adjustment it is strongly recommended to reduce or stop the pump beam First adjust slightly the mirror FM307 to send the beam through the pinhole PH19 set in front of the mirror FM309 and adjust the mirror FM309 to send the beam through the pinhole PH20 Then adjust the mirror FM310 in order to have the beam going through the pinhole PH20 while checking that the beam is not hitting the edges of the crystal in its holder At last make sure that the beam goes out correctly center with a maximum of energy Sometimes the pump beam must be adjusted to optimize the output energy with mirrors FM328 and FM330 250 TW Daar Regenerative Amplifier and PreAmplifier 82 Amplitude litude TECHNOLOG Users manual 6 8 THE SECOND MULTIPASS AMPLIFIER MODULE 6 8 1 OPTICAL SETUP This module includes all the components of the second multipass amplifier including the water cooler The optical setup is shown in figure 7 21 From the regen L9 FM413 Bem vr m m S 5 3 a Z EN LL TTA dam L13 EE MES S L3 da SEP ohh Wen as EM Tothe wy EM1 BP1 FM414 FM418 BP2 EM2 FM416 L11 amplifier Figure 6 21 Optical setup of the compressor module 250 TW Dulsar Second Multipass Am
63. lves temporal stretching of the ultra short pulse delivered by an oscillator by a factor 1000 to 10000 in order to safely amplify the pulses in solid states materials Stretching produces a chirped pulse After amplification the laser pulse is compressed back to a value as close as possible to its initial value After the compression stage one should obtain in principle a high intensity ultra short pulse free of chirp see figure 1 1 l 1 Ir Wi Wa w Compressor Il A il o iil AANA ha Oscillator adi Stretcher 4 A ii Amplification 1 if V l M tl I 1 IU IL I Figure 1 1 Principle of chirped pulse amplification 250 TW Dulsar Introduction 5 Amplitude litude TECHNOLOG Users manual Stretching and compression are usually achieved by means of dispersive systems such as gratings or prisms The principle is to create different optical paths for each wavelength of the spectrum Figure 1 2 shows a typical stretcher design using two gratings and a telescope system Note that this is not the Amplitude Technologies stretcher design but its use is convenient for tutorial purposes Grating 1 Imaging Grating 1 device em PF T LL Gz 1 Grating 2 Mirror Figure 1 2 Stretcher principle The stretching factor depends on the distance D between the two gratings As one can
64. ly the Dazzler adjustments would be done with mirrors FM106 and FM105 in order to make the beam passing well through the Dazzler check PH7 and trough PH8 for the output beam The mirror FM106 enables an adjustment of the beam height to overlay the beam axis with the Dazzler crystal s one With a scattering paper before and a screen behind the Dazzler the position of the beam inside the Dazzler s crystal can be quickly C checked The folding mirrors FM107 and FM108 enable an accurate adjustment of the injection seeding before entering the pulse picker 250 TW Dulsar Stretcher 71 Amplitude TECHNOLOG User s manual 6 7 THE REGENERATIVE AMPLIFIER AND THE PREAMPLIFIER MODULE 6 7 1 OPTICAL SETUP This module includes the regenerative amplifier and the first multipass amplifier The optical setup is shown in figure 7 15 From the To the PMI Stretcher Booster 4 dd MEME p m EDEN FM322 NS e PC4 M326 AN Oo CR NI Wy To the 17 amplifier 2 pm DE E NT RRE DE mu FM330 FEES II 4 FM318 aE 7 Ze adn FM314 LS PH20 FM332Y 33 us PH20 WP1 L2 Puig FM309 M312 Figure 7 15 Optical setup of the regenerative amplifier 250 TW Daar Regenerative Amplifier and PreAmplifier 72 QI Amplitude TECHNOLOG User s manual The seed beam coming from the stretcher is reflected by FM301 and in seeded into the regenerative amplifier with polarizers P1 and P2 An automatic
65. nding knobs and simultaneously checking the monitoring photodiodes for pulse picker and regenerative amplifier Pulse Cleaner and Multipass amplifier 1 MP1 Activate the pulse cleaner by switching on CH4 If necessary optimize the fine delay for channel 4 using the corresponding knob and simultaneously checking the pulse cleaner monitoring photodiode Turn on the Multipass amplifier 1 pump laser the CFR200 by switching on the Pockels trigger on the remote controller AND with the Q Switch button located on the front panel of the power supply Multipass amplifier 2 MP2 The Propulse YAG laser 1 is supposed to be warmed up According to the apertures and the camera monitoring this amplifier is also supposed to be aligned Start up the chiller Once the set temperature is reached allow the Q switch Pockels cells trigger for YAG 1 and activate the corresponding Pockels button on the remote controller 250 TW Paar Start up and shut down procedures 44 J Anplitude TECHNOLOL User s manual Open the shutters After few minutes for the laser power to stabilize seed the infrared radiation coming from MP1 Warning Before switching the Pockels cells on be sure that the chiller is on gt Cryogenically cooled Amplifier 3 MP3 e The Propulse YAG lasers 2 to 11 are supposed to be warmed up According to the apertures and camera monitoring this amplifier is also supposed to be aligned e Start the cryogenic
66. o eo OO OO OO OO PH3 PH4 Stretcher 67 O Amplitude TECHNOLOGII User s manual 3 Use the mirror FM101 to centre the beam on the pinhole PH3 And adjust the mirror FM103 to centre the beam on the pinhole PH4 The beam should be located on the grating G as shown on figure 7 12 Figure 7 12 Position of the incoming beam on the grating 4 Repeat the steps 3 and 4 as long as the beam hitting the grating is not perfectly horizontal at 135 mm eo 5 Adjust the vertical tilt of the grating G so that the reflected beam at the zero oo order is horizontal at 135 mm The same tool may be used for the beam height control 6 Use the adjustment of the grating G mount to rotate the grating grooves Thus OO optimize this adjustment to send to diffracted beam horizontally reflected on 9O the concave mirror CVM OO OO 7 Repeat the steps 5 and 6 as long as the both emergent beams from the grating are not perfectly at 135 mm 8 Adjust the orientation of the concave mirror CVM to send the beam to the Ss centre of the convex mirror CVX The beam height on the mirror CVX should OO be set at 125 mm OO 250 TW Dulsar Stretcher 68 O Amplitude TECHNOLOGI User s manual 9 Adjust the orientation of the convex mirror CVX so that the beam OO loo reflects vertically back to the concave mirror CVM at a height of 115 OO OO mm and passes through both pinholes PH3 and PH4 properly Note De 2 the position of
67. omponents included in the stretcher module 6 6 3 ALIGNMENT OF THE STRETCHER gt COARSE ALIGNMENT IN THE STRETCHER The pointing of the input beam may change from day to day due to a re alignment or to a temperature variation of the room In this case the beam direction has to be re adjusted in order to go through the stretcher properly Use the following procedure to readjust the input beam in the stretcher Use the pinholes provided with the system and put them in front the mirror FM1 and in front output of the stretcher breadboard Adjust the mirror in the booster module FM215 in order to send the output beam of the booster through the first pinhole PH1 Adjust the mirror FM101 to send the beam properly through the stretcher and in the pinholes PH2 PH3 and DHA note that PH3 and PHA are pinholes especially designed for the alignment of the stretcher as shown on figure 7 11 It is possible to check the spots hitting the grating of the stretcher Those should be as shown in figure 7 9 and the right setting is obtained when the beam comes out from the stretcher and goes through the output pinhole PH5 using mirror FM103 As the beam can exit from the stretcher only if it s well aligned so tune mirrors FM101 and FM103 until the beam exits only if you are sure that no optical element of the stretcher has moved For the best optimization of the oscillator beam alignment through the stretcher it might be necessary to repeat t
68. onsists in two curved mirrors CM1 and CM2 a flat mirror FM111 a prism P and a Ti Sa crystal C After the needed number of passes here n 14 the beam is extracted in the same way as it entered the multipass cavity on mirrors FM110 FM109 FM108 but with a different height around 5 mm higher Thus the beam goes out passing just a little above the mirror FM107 and is focused with the L2 lens in the saturable absorber SA The lens L3 recollimates it The part of the beam transmitted by the mirror FM112 goes into a photodiode PD2 that monitors the level of absorption of SA After mirrors FM113 and FM114 the beam goes into the stretcher with an adapted size beam splitter BS101 the reflected part goes to FM118 FM119 and is focused on the right side of the crystal C by the lens L4 For the second part the beam is focused 250 TW Dulsar Contrast Ratio Booster 59 gt Amplitude TECHNOLO User s manual down on the left side of the crystal C by L5 and follows the way FM115 FM116 FM117 6 5 2 LIST OF COMPONENTS Table 7 6 summarizes the different optical components which compose the contrast booster with Amplitude Technologies references legend description reference FM100 FM101 Rmax45 800 nm C040003 FM102 Metallic mirror C140001 FM103 Rmax45 800 nm C040003 FM104 Metallic mirror C140001 FM105 2 FM107 Rmax45 800 nm C040003 FM108 2 FM1
69. plifier 83 J Amplitude TECHNOLOG User s manual The beam coming from the pre amplifier is sent in the second amplifier thanks to the mirror FM401 Then the beam hits the beam splitter BS1 a small fraction of it is used for the different measurements The transmitted part passes through the telescope L1 L2 to increase the beam size and then it seeds the 4 pass multipass amplifier by the mirror FM402 It successively hits the folding mirrors FM403 FM404 FM405 FM406 FM407 FM408 FM409 and is extracted by FM409 and FM410 The different passes cross at the Ti Sa crystal C A telescope made up with lenses L3 and L4 enlarges the beam before seeding the main amplifier with mirror FM412 The pumping configuration is the follow the amplifier uses a Drogalse YAG laser The pump beam is split in two with BS8 The reflected part passes through a convergent telescope made of L5 and L6 and a vacuum tube VT Then the beam is collimated on the left side of the crystal C with lens L7 and with mirrors FM418 FM419 go into the crystal C On the other side the transmitted beam hits mirrors FM420 FM421 and FM422 and passes through the collimated lens L8 before passing through the crystal C A large number of measurements are available in this module their purposes are listed below The beam coming from the pre amplifier is analyzed in terms of beam profile energy beam pointing and spectrum After being reflected by FM401 the small fraction of t
70. ronised Pump synchro output Regen pump pulse Ampli pump pulse Pump synchro output DelayPP PD1 A HV Pockels 1 PD2 HV Pockels 2 Lem Regen pulse train Amplified pulse _ _ Figure 4 12 General synchronisation diagram of the system 250 TW Dulsar 34 Amplitude litude TECHNOLOGIE User s manual 4 3 2 PULSAR CONTROLLER The pulsar controller is an electronic synchronisation module controlling independently 13 Nd YAG lasers with a single input synchronisation signal from the Genpulse The controller is connected to a single remote controller to drive easily all the pump lasers The synchronisation module interconnection is represented on the figure 4 13 The pulsar controller sends to each Pyopulse pump laser the flash lamp and Pockels cell synchronisation signal and drives the shutter Each YAG laser can be driven thanks to the remote control More over the controller integrates a safety interlock with the two cryostats 60 MHz oscillator signal Pockels cells Flash Pockels Le Shutter Figure 4 13 General synchronisation diagram of YAG lasers 250 TW Pasar 85 Amplitude TECHNOLOGIES gt Front panel of the Pulsar controller User s manual Figure 5 1 Front panel of Pulsar controller Sub D 15 connector to the remote control or to the computer 2 Line LED green 3 Reset push button for 10 Hz input check The green LED light
71. rst mirror is concave and the second convex This combination presents interesting properties for use in a pulse stretcher It is characterised by a complete symmetry so only the symmetrical aberration can appear spherical aberration and astigmatism This combination has no on axes coma and exhibits no chromatic aberration because all the optical elements are mirrors for more details see ref 250 TW Dulsar Laser system and modules 10 Amplitude litude T TECHNOLOGIE User s manual Aberration free stretcher design for ultra short pulse amplification G Cheriaux F Salin and al OPTICS LETTERS March 15 1996 2 gt 3 2 2 LJ 2 e e e o o 6 ea Figure 2 3 Aberration free stretcher design 2 5 ACOUSTO OPTIC PROGRAMMABLE DISPERSIVE FILTER OPTIONAL For Short pulse 30 fs requirements an optional AOPDF or Dazzler by Fastlite can be added usually right after the pulse stretcher Refer to the manual for more details For standard laser systems the Dazzler is used as a phase modulator to pre compensate for dispersion and phase distortions introduced through out the laser system but also as an amplitude modulator to optimize the laser output spectrum In the Pulsar 250 the problems of phase compensation and amplitude modulation are totally de correlated The Dazzler would only be used for phase control while another device the Mazzler see section 2 7 would be used to optimize the spectrum Th
72. s manual to have more information Figure 7 28 shows different computed pulse durations and spectral phase for different settings of the angle of incidence in the compressor 50 TW Dulsar Compressor 96 D erla 0 100 User s manual 0 080 0 060 e e N eo Group delay ps S 100 50 0 50 100 Pulse duration fs 0 060 0 080 0 100 755 763 T tee TN 795 804 t 13 nu 831 aveleng 841 850 860 0 100 0 080 0 060 0 040 0 020 0 000 0 020 0 040 100 50 0 50 100 Pulse duration fs Group delay ps 0 060 0 080 0 100 i 755 763 771 779 787 795 804 813 822 831 Wavelength nm 841 850 860 Figure 6 28 Different pulse shape and group delay dispersion for several angles of incidence 50 TW Dulsar Compressor 97 O Amplitude User s manual 7 TECHNICAL SPECIFICATIONS 7 1 PERFORMANCES Main Beam Parameters Probe Beam parameters Specified Measured Specified Measured Repetition rate 10 Hz 10 Hz 10 Hz 10 Hz Central wavelength 785 815nm 800 nm 785 815nm 800 nm Pulse energy before compression 8J 7 2J gt 80 mJ Pulse ener
73. sformer These modules are connected to each other as described in figure 4 2 For more details about this device see the user s manual of the Cryostar product provided with the system He compressor Transformer Water connection Electrical connection Controller el Tap water 3l min 240 V 10 A 3ph Primary pum Es gem 240 V 10 A 1ph Vacuum Chamber Figure 4 2 Connection sketch for the cryostat module 250 TW Dulsar 24 gt Amplitude TECHNOLOG User s manual It is highly recommended to connect the controller and the primary pump permanently to an electrical supply in order to maintain a constant vacuum in the chamber 4 2 2 AOPDF AND AOPGCF OPTIONAL Dazzler and Mazzler are made with an acousto optic modulator a RF programmable generator and a computer to control the shape of the filter The different parts of the device have to be connected as shown in figure 4 3 and 4 4 Refer to the user s manual of these devices for more details Dazzler 240V 1 5A HF Programmable Generator 240V 1 5A AO crystal Computer Figure 4 3 Connection sketch of the AO filter module The Mazzler system includes a security loop that ensures the regenerative amplifier does not operate when a waveform modification is processing This security loop is described below Between two waveform modifications of the Mazzler the RF signal might go to zero As the
74. stem AMPLITUDE TECHNOLOGIES C 91029 EVRY France T l 33 0 169112790 Fax 33 0 164975817 Type PULSAR 100 Date 07 2007 S N PUL 110 07 125 250 TW Dulsar Safety 18 QI Amplitude TECHNOLOG User s manual This label is located on the rear panel of the laser bench Label 1 e Label2 Warning logotype The laser output declaration label describes this laser is a high power Class IV pulsed laser The information present in this label is the stated energy output the pulse duration and the wavelength This label ensures that adequate eye protection and beam handling precautions are observed VISIBLE AND INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION 582 nm Wavelenght Max Average Power Ge w CLASS IV LASER PRODUCT IEC 825 1 2000 This label is located rear panel of the laser bench Label 2 250 TW Dulsar Safety 19 QI Amplitude TECHNOLOG User s manual e Label 3 Aperture Label This laser aperture warning label is posted near all laser outputs Avoid eye or skin exposure to radiation emitted from these apertures AVOID EXPOSURE VISIBLE AND INVISIBLE LASER RADIATION IS EMITTED FROM THIS APERTURE These labels are located on front panel of the laser bench
75. the beam back at an elevation of 100 mm Both mirrors of C have to be moved to reflect the beam on the grating and only the settings of the higher reflector allows having a horizontal reflected beam 15 The beams on the gratings should appear as shown in figure 7 27 Grating 1 g Grating 2 G Figure 6 27 Representation of the different beams hitting the compressor gratings 50 TW Dulsar Compressor 94 LZ Amplitude TECHNOLOG User s manual 16 Insert the mirror FM604 between the grating G and the beam elevator FM602 FM603 in order to reflect the beam out of the compressor without any clipping effect The compressor is now aligned but not optimized Several parameters have to be adjusted for the optimization of the compressor the grating parallelism the angle of incidence on the gratings and the distance between the two gratings 6 10 3 PARALLELISM ADJUSTMENT BETWEEN THE GRATINGS If the two gratings are not parallel it will produce spatial chirp effect at the output of the compressor The spatial chirp can be observed at the focal plan of a long focal length lens If the beam appears as a line and if the effect of a clipping in front of the reflector C is visible at the focal point it means that there is some chirp at the output of the compressor The angle between the two gratings has to be adjusted as follows 1 First locate the position of the beam at the output of the compressor 2 Move slightly the second grat
76. trongly recommended to use the same settings for each adjustment It is recommended to first optimize the seeding without passing through the Dazzler unit so flip up the mirror FM109 to bypass the Dazzler and start the alignment The mirrors FM105 and FM110 placed in the stretcher module are usually used to control the injection seeding The main criterion for the optimization is the pulse build up time in the regenerative amplifier cavity Typical records of the pulse evolution in the regenerative amplifier are shown in figure 7 17 The best adjustment is obtained when the pulse build up time is as short as possible and when the 250 TW Daar Regenerative Amplifier and PreAmplifier 80 Amplitude litude TECHNOLOG Users manual contrast ratio between the pulse seeded and the self running laser effect in the cavity is the highest a a a 3 b 4 4 i Intensity a u Ki Intensity a u Intensity a u Ki o N o N o Dy o 50 100 150 20 0 50 100 150 200 0 50 100 150 20 Delay nsec Delay nsec Delay nsec Figure 7 17 Optimization of the injection seeding in the regenerative amplifier a ns regime the cavity is not seeded no spike appears b the cavity is seeded but the contrast ratio is poor c the cavity is seeded the contrast ratio is high It is possible that no injection seeding occurs when adjusting the reflectors because the
77. ure 7 19 Setup for the Pockels cell orientation adjustment 4 An image similar to the one shown in figure 7 20 should appear Adjust the Pockels cell to centre the direct beam in one quarter of the diffused rings Scattered rings Direct beam Figure 7 20 Image of the transmitted light through the Pockels cell The correct setting is obtained when the direct beam is in one quarter of the diffused rings 250 TW Paar Regenerative Amplifier and PreAmplifier 78 O Amplitude TECHNOLOG User s manual 5 Remove the diffusing device screen cleaning tissue and polarizer and adjust the mirror FM304 to reflect the beam on itself One part of the beam should be transmitted by the polarizer P2 and P3 6 Adjust the mirror FM305 so that the beam is back reflected on itself 7 Adjust the orientation of the Pockels cell PC3 in order to minimize the reflection due to the polarizer P3 This Pockels cell can also be oriented with the same method than which is explained in point 4 But this time the direct be is well centered on the diffused rings 8 Block the beam coming from the stretcher and switch on the Pockels cells of the regenerative amplifier with a large delay between the two Pockels cells triggers Send the pump beam into the crystal at the same location as the infrared beam The laser effect should begin in the regenerative amplifier 9 Optimize the cavity alignment with the rear mirror FM304

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