#44xx & #48xx Man, Rev D7 TS (Page 2)
Contents
1. 1 55 39 4 Delrin Base NX Frequency 08 2 0 X 04 1 0 I Adjust Optical Aperture Both Sides 90 22 9 1 04 26 4 53 13 5 39 20 1 35 8 9 J9 L5 20 1 2X 8 32 M4 Mounting Hole 1 4 20 M6 Mounting Hole Principles of 10 Operation Vv Phase modulators are typically used to generate fre quency sidebands on a cw optical beam A sinusoidal electronic drive signal applied to the modulator pro duces optical sidebands which are separated from the cw optical carrier by the drive frequency These modu lation sidebands can be observed using an optical spectrum analyzer Given an induced peak optical phase shift of Ad in radians the fraction of power transferred to each of the first order sidebands is J Ao 2 where J 7 is the Bessel function of order one The fraction of power that remains in the carrier is Jo 9 7 where J is the Bessel function of order zero For example imposing a phase modulation with peak phase shift of 1 radian will transfer 19 of the optical carrier power to each of the first order side bands and leave 59 of the power in the carrier The maximum power that can be transferred to each of the first order sidebands is about 34 and this requires a peak phase shift of 1 8 radians For the Model 442x operating with 532 nm light a 1 8 radian phase shift requires a peak drive vol2. Vv Model Series 44xx and 485x User s Manual High Frequency Electro Optic Phase Modulators Patent No 5 414 552 Vv Contents I Quick Start 3 II Introduction 4 HI Principles of Operation 10 IV Operation 14 V Specifications 20 VI Warranty Service amp Support 25 VII Performance Data 26 LU 1s a registered trademark of 440018 Rev D Nei Focus Ine 2 l Quick Start Vv This section presents a brief introduction to using your high frequency phase modulator For detailed operating instructions and some background on how these devices work please refer to the sections in the manual that follow Start by aligning a collimated optical beam through the mechanical apertures of the modulator For Models 44xx the beam should be polarized vertically with respect to the modulator casing and for the Models 485x the beam should be polarized horizon tally Be careful not to exceed the maximum recommended optical power or damage to the electro optic crystal could result See page 17 for a discussion of optical damage Drive the modulator with a 50 Q RF driver tuned to the modulator s resonant frequency RF powers from 0 1 to 0 5 watts should be sufficient to allow observa tion of sidebands Do not exceed 4 watts of RF drive power Generally an optical spectrum analyzer with suitable finesse and free spectral range is used to observe the modulation sidebands Be careful not to damage your RF driver wit
3. caused by the migration of photoexcited charge carriers from illu minated regions to darker regions The localized refractive index variations resulting from the space charge field and the electro optic effect reduce the effectiveness of the modulators and cause distortion to the optical beam traveling through the modulator Photorefractive damage is a serious concern for visi ble wavelengths high optical power and tightly focused beams The photorefractive damage process can occur gradually over days or hours or for high optical powers and short wavelengths this effect can occur over seconds A damaged crystal will distort a beam usually by elongating it along one axis If operating close to the damage threshold it is a good idea to monitor the transmitted beam periodically for indications of optical damage If you input more optical intensity than recommend ed photorefractive damage will occur In reality this 17 18 Vv damage is not permanent Photorefractive damage can be at least partially reversed by carefully annealing the crystal and thus mobilizing the charge carriers Due to the sensitive parts contained inside the modulator housing however this process should only be done at New Focus Please contact us for more details Our modulators use two types of electro optic materi als LiNbO and magnesium oxide MgO doped LiNbO The LiNbO material has a lower damage threshold and so it is used in ou
4. crystal that can be more than nine times the applied input drive voltage 11 12 Vv leading to reduced half wave voltages and larger modulation depths For these modulators the peak phase shift obtained by applying a sinusoidal signal of average power P at the input SMA connector is Ag 5 T A 2 ewbd where Q is the quality factor of the resonant cavity is the drive frequency and is the crystal permittivity For the Model 442x high frequency phase modulators V is typically 45 volts at 1 06 pm corresponding to a modulation depth of 0 07 radians volt Note that these values scale with wavelength so at 532 nm V is 23 volts and the modulation depth is 0 14 radi ans volt Modulators operating from 0 5 to 3 GHz For the Models 44xx modulators the crystal is placed in a resonant microwave cavity to achieve a high Q gt 100 system The microwave cavity is designed to replicate a transmission line terminated by the crys tal Given the crystal s capacitance the transmission line length is chosen so that the line resonates at the desired frequency Typically the resonance has bandwidth of 0 5 1 of the resonant frequency allowing the device to be operated over this narrow frequency range In addi tion these modulators are equipped with a tuning slug that probes the interior of the microwave cavity and provides frequency tuning over a range of up to 200 MHz Vv Modulators operating from 3 to 13 GH
5. the modulator is matched to 50 Q when driven at its res onant frequency Resonant modulators are designed to have impedances close to 50 Q at resonance and a high return loss indicates a good impedance match between the driving source and the modulator With a high return loss power transfer to the modulator is optimized and reflected power which can harm RF drivers is minimized All New Focus resonant phase modulators are tested by measuring return loss versus frequency around the resonant frequency The results of this test are provid ed at the end of this manual For a power reflection coefficient K the return loss in dB is 10 logR A Return loss of 14 dB corresponds to 4 of the incident RF power reflected back to the driver VSWR the voltage standing wave ratio is another way to specify the quality of impedance matching between RF driver and resonant modulator VSWR is defined as the voltage ratio between the maximum and minimum of the standing wave that occurs because of impedance mismatch Given a return loss RL in dB the VSWR can be found from 1 10 R4 20 1 10 R2 20 _ A VSWR value of 1 indi cates a perfectly matched system A VSWR of 1 5 cor responds to 4 of the incident RF power reflected back to the driver Vv Q the quality factor or Q of a resonant cavity is a measure of the sharpness of the resonant cavity s fre quency response Generally a larger Q means a higher modulation depth For the high fre
6. Vv Engineers at New Focus are happy to provide assis tance with the operation of your phase modulator If you have any questions or comments please contact us at the address below Warranty New Focus Inc guarantees its phase modulators to be free of defects for one year from the date of shipment This is in lieu of all other guarantees expressed or implied and does not cover incidental or consequen tial loss Service In the event that the modulator malfunctions or becomes damaged please contact New Focus for a return authorization number before shipping the unit back for evaluation and repair You can reach us at NEW FOCUS Inc 2630 Walsh Avenue Santa Clara CA 95051 0905 USA Phone 408 980 8088 Fax 408 980 8883 Email Contact NewFocus com Internet www NewFocus com 25 26 Vil Performance Data Vv Model number Serial number Resonant frequency Wavelength Input RF test power Return loss VSWR Vv 27
7. ase contact New Focus 19 V Specifications 20 Vv RAM Residual amplitude modulation is a source of unwanted noise in a phase modulation system and a perfect phase modulator will exhibit no RAM However etalons in the modulator and misalignment of the optical beam will lead to some amplitude mod ulation With careful adjustment of an optical beam s alignment and polarization our modulators will exhibit less than 60 dB of RAM for a 1 radian peak phase shift Wavelength two standard broadband AR coatings are available 0 5 0 9 hm and 1 0 1 6 pm Each coat ing has a 1 maximum reflectivity per surface The optical losses in the modulators are determined by the absorption and scatter of light in the electro optic crystal and by the quality of the anti reflection coat ings on the end faces The crystals typically have loss es of 0 3 cm at 1 0 pm So for a 2 cm long crystal the total insertion loss will be about 2 6 at 1 0 pm Operating Frequency the range of resonant fre quencies over which these modulators can be designed to operate The particular resonant frequen cy of a given modulator is specified at the time the modulator is ordered RF Bandwidth the bandwidth of the modulator s resonant frequency otherwise known as the 3 dB fre quency It is the frequency range over which at least one half of the electrical drive power will be trans ferred to the modulator Material the visible modulators use Mg0
8. doped LiNbOz and the IR modulators use LiNbO Vv Max Optical Intensity this is the maximum optical intensity assuming a 1 mm diameter beam that can be passed through the crystal before photore fractive damage occurs Note that this optical damage threshold is strongly wavelength dependent See page 17 for a discussion of photorefractive damage Aperture the size of the mechanical aperture at the input and output of the modulator The aperture aids optical alignment and ensures that the beam passes through the center of the crystal Connector all modulators have female SMA input connectors Impedance resonant phase modulators are matched to 50 Q and this is the input impedance seen by the RF driver Max RF Power the maximum recommended RF drive power Above this power thermal effects in the crystal such as thermal lensing will become a prob lem and the modulator s resonant frequency will drift significantly Modulation Depth the resulting optical phase shift when a 1 volt signal is applied to the modulator The modulation depth is specified at 1 06 pm The modulation depth varies inversely with wavelength So for example the modulation depth at 532 nm is twice that at 1 06 pm 21 22 Vv Max YC this is the voltage required to achieve a 180 degree phase shift at 1 06 pm Ya varies linearly with wavelength and so Vy at 532 nm is half that at 1 06 pm Return Loss Return loss describes how well
9. e of thumb is that the beam diameter should be about one third the aperture size to mini mize clipping For a 2 mm aperture a good beam size is 0 5 1 mm and for a 1 mm aperture a good beam size is 250 500 im Larger beams can be focused slightly and then colli mated after the modulator using a pair of lenses If you do this keep in mind the intensity of the beam inside the modulator crystal and make sure the intensity does not exceed the damage threshold see the discussion of optical damage on page 17 Driving the modulator Connect the SMA jack on the modulator to an RF dri ver using an RF cable with operating bandwidth greater than the modulation frequency to minimize propagation losses The optical alignment of the modulator can be disturbed by the RF cable and so it is a good idea to use a strain relief on the cable 15 16 Vv The Models 44xx and 485x high frequency phase modulators are resonant devices with a 50 Q imped ance when driven at their resonant frequency These modulators require an RF driver matched to 50 Q and tuned to the resonant frequency of the modulator New Focus does not sell RF synthesizers oscillators or amplifiers but suitable sources are available from other companies New Focus engineers can provide help in finding the source that s right for your modu lator and your application Feel free to contact us for assistance The RF driver typically consists of an oscillator or syn thesiz
10. er followed by an RF amplifier The RF driver should be capable of generating output powers in the 1 to 4 watt range For many applications 1 watt is suf ficient to generate a suitable phase shift Note that if the modulator is driven with RF powers greater than about 3 watts the modulator casing can heat up noticeably This heating can cause some shifting of the modulator s resonant frequency and it can lead to thermal lensing in the crystal Finally note that if the modulator is not driven at or close to its resonant frequency most of the RF drive power will be reflected back to the driver Excessive RF power reflected back from the modulator to the RF driver will not harm the modulator but can damage the driver So when driving the modulator be sure that the RF source is matched to the modulator s resonant fre quency Ensuring that the drive frequency is matched Vv to the modulator can be done either by observing the optical sidebands on an optical spectrum analyzer or by measuring and minimizing the amount of RF power that is reflected from the modulator Use the tuning slug to fine tune the modulator s resonant fre quency to precisely match the RF drive frequency Alternately tune the RF drive frequency until it matches the modulator s resonant frequency Optical damage The electro optic crystals used in these modulators are susceptible to optical damage through the pho torefractive effect This phenomenon is
11. h reflected power If the modulator is not driven at or close to its resonant frequency most of the RF drive power will be reflected back to the driver Use the tuning slug to fine tune the modulator s resonant frequency and precise ly match it to the RF drive frequency Introduction Vv The New Focus Models 44xx and 485x high frequency electro optic phase modulators provide an efficient means for single frequency optical phase modulation in the 0 5 to 13 GHz frequency range These modula tors are useful components in a variety of experimen tal techniques including FM spectroscopy laser fre quency stabilization atom cooling laser linewidth broadening and laser guide star systems for astrono my The features of these modulators include low drive voltages large modulation depths a wide range of operating frequencies from 0 5 to 13 GHz a broad range of wavelengths from 0 5 to 1 6 pm low opti cal insertion loss and high optical power handling capability Their 1 to 2 mm apertures make them compatible with most laser sources Finally the elec tro optic materials used in these devices are nonhy groscopic and so the modulators can be left on an optical table for indefinite periods without requiring a sealed enclosure Operation of the New Focus electro optic phase mod ulators is based on the linear electro optic or Pockels effect whereby an applied electric field induces a change in the refractive index of
12. hen your order was placed with New Focus The operating wavelengths are determined by the broadband anti reflection coating applied to the sur faces of the electro optic crystals Two standard wave length ranges are offered 0 5 0 9 hm and 1 0 1 6 pm 5 Vv The table on page 24 is the product matrix for the high frequency phase modulators and lists the physi cal characteristics and performance specifications for these modulators Mechanical drawings of the three types of modulators are shown in figures 1 to 3 Fig 1 Vv Mechanical views of the model 442x phase modulator SMA Input S Connector Frequency Adjust 45 r 208 528 11 4 p Height D is frequency dependent 64 e 2 19 55 6 16 3 LA A 08 2 0 X 08 2 0 Optical Aperture la Ae 1820462 Delrin Base 1 4 20 M6 Mounting Hole Fig 2 Vv Mechanical views of the model 443x phase modulator SMA pt Connector L Frequency Adjust 1 14 2 1 4 es 0 F 290 7 82 POR 10 1 i i bk pe 04 1 0 X 04 1 0 Optical Aperture Both Sides o Delrin Base Height D is frequency dependent 1 03 al 52 14 2 O Q 1 4 20 M6 0 Mounting Hole 26 2 Fig 3 LU Mechanical views of the model 485x phase modulator SMA Input Connector
13. ose a polarization rotation as well as a phase modulation which can lead to unwanted amplitude modulation if the modulator is followed by any polarizing optics It is important to carefully align the polarization since the crystals used by New Focus are cut so that the beam propagates along the y axis of the crystal This orientation minimizes the effects of acoustic resonances but makes it critical that the optical beam be linearly polarized along the crystal s z axis Aligning an optical beam through the mod ulator Use the 1 4 20 M6 for metric versions tapped hole located on the base of the modulator to mount it on a positioning device for alignment We recommend the New Focus Model 9071 9071M for metric versions tilt aligner because of its convenient tilt and transla tion capabilities Orient the optical beam so it is linearly polarized along the z axis of the electro optic crystal With the Model 44xx modulators the polarization should be Vv oriented vertically with respect to the modulator cas ing and with the 485x modulators the polarization should be horizontal Aligning an optical beam through the modulator is straightforward Simply position and align the mod ule so that the beam passes through the mechanical apertures clearing them without clipping The beam should be collimated with a waist size less than the aperture size and such that the Rayleigh range is at least the length of the crystal A good rul
14. quency phase modulators Q is defined as Af where fis the modu lator s resonant frequency and Af is the full width of the modulator s resonance measured at the 3 dB points where the modulator absorbs one half of the incident RF drive power For the high frequency phase modulators Q is typical ly between 100 and 200 The measured Q for each modulator is written in the modulator performance data section at the end of this manual 23 Model Series 44xx amp 485x Specifications Vv Wavelength 0 5 0 9 pm 1 0 1 6 pm 0 5 0 9 um 1 0 1 6 um 0 5 0 9 um 1 0 1 6 um Operating Freq 0 5 1 5 GHz 0 5 1 5 GHz 1 5 3 0 GHz 1 5 3 0 GHz 9 2 GHz 9 2 GHz RF Bandwidth 0 5 freq 0 5 freq 0 5 freq 0 5 freq 0 5 freq 0 5 freq Material MgO LiNb0s LiNbO3 MgO LiNb0s Mg0 LiNbO3 Mg0 LiNbO LiNbO Max Optical Power 2 2 2 2 2 2 in a 1 mm beam 5 W mm 647 nm 1 W mm 1 3 um 5 W mm 647 nm 5 W mm 1 3 um 5 W mm 647 nm 1 W mm 1 3 um Aperture 2 mm 2 mm 1 mm 1 mm 1x2 mm 1x2 mm Connector SMA SMA SMA SMA SMA SMA Impedance 500 50 Q 500 500 500 500 Max RF Power 4W 4W 4W 4W 3W 3W Modulation Depth at 1 06 um 0 07 rad V 0 07 rad V 0 05 rad V 0 05 rad V 0 05 rad V 0 05 rad V Max V at 1 06 um 45V 45V 63V 63V 63V 63V VSWR 15 15 15 15 15 5 Return Loss 14 dB 14 dB gt 14 dB 14 dB 14 dB 14 dB 24 VI Warranty Service amp Support
15. r IR modulators which operate from 1 0 to 1 6 pm In this wavelength range photorefractive damage is generally not a seri ous problem we recommend a maximum optical intensity of 1 W mm at 1 3 pm The visible modulators which operate from 500 900 nm come standard with MgO doped LiNbO crystals The MgO doping increases the resistance to photore fractive damage enabling this material to be used in the visible wavelength range For Mg0 doped LiNbO the recommended maximum optical intensity is 5 W mm at 647 nm for a 1 mm diameter beam Keep in mind that the optical damage threshold depends on many factors including wavelength beam diameter and the particular batch of crystal material being used The damage thresholds are con servatively stated to avoid this problem However it is difficult to guarantee damage free performance at a specific wavelength and power Typically the damage issue is most problematic for wavelengths shorter than 600 nm where the photorefractive damage Vv process becomes more efficient and the maximum optical power drops off sharply as the wavelength gets shorter Also note that the damage specifications given here assume a 1 mm diameter beam The damage process is more of a problem for tightly focused beams and so for smaller diameter beams the damage threshold intensities are lower than the values given here If you have a concern about photorefractive damage in your particular application ple
16. tage of about 13 volts 1 7 W average power The effect of an applied electric field on a crystal s refractive index is described by a third rank tensor fij The induced refractive index change caused by an external electric field has the form 1 3 AN 5n hE Vv where Av is the change in the index of refraction 7 is the unperturbed index of refraction 733 is the appropriate element in the electro optic tensor and E is the applied electric field The New Focus phase modulators consist of an elec tro optic crystal of length width b and thickness d The electric field is applied along the crystal s z axis and transverse to the direction of optical propagation Modulation is induced onto the laser beam by align ing the polarization of the input beam with the z axis of the crystal An electronic signal is then directly modulated onto the laser beam through the electro optic effect The optical phase shift obtained by applying a volt age V across the electro optic crystal is where A is the free space wavelength A commonly used figure of merit for electro optic modulators is the half wave voltage Va which is the voltage required to produce a TT phase shift Substituting into the preceding equation yields Ad S n rl nz For these high frequency phase modulators the crys tal is put into a resonant microwave cavity that enhances the voltage applied across the crystal This results in a voltage across the
17. the crys tal With electro optic devices phase modulation is achieved by aligning the polarization of the optical beam along the z axis of the electro optic crystal By applying an electronic drive signal to the crystal the phase of the optical beam is then modulated through the electro optic effect Vv The materials used in these modulators are lithium niobate LiNb0 and magnesium oxide doped lithi um niobate MgO LiNbO These materials are extremely attractive for use in these types of modula tors because they have wide optical transparency win dows large electro optic coefficients and low RF loss es Having low RF losses is the key to making efficient high Q devices that operate at frequencies up to 13 GHz The large electro optic coefficient of lithium niobate means that these modulators require low drive volt ages and have large modulation depths In addition by putting the crystal in a resonant microwave cavity the resonant enhancement of the voltage across the crystal further reduces the required input drive volt age while still allowing a relatively large optical aper ture New Focus offers these high frequency phase modula tors classified into three resonant frequency ranges e Models 4421 4423 0 5 to 1 5 GHz see Fig 1 e Models 4431 4433 1 5 to 3 GHz see Fig 2 e Models 4851 4853 3 to 13 GHz see Fig 3 The modulator is shipped to you with the resonant frequency set to the frequency specified w
18. z For frequencies above 3 GHz the crystal length required to maintain phase matching becomes too short to obtain reasonable modulation depth and a different design is required The Model 485x employs a patented design in which the microwave velocity through the resonant cavity and the optical velocity through the crystal are matched This is accomplished with a microwave waveguide where the velocity of the microwave radiation is geometry dependent By adjusting the geometry so the optical and microwave velocities are equal the crystal length can be made long enough so that sig nificant modulation depth is obtained This ensures a good modulation depth and low drive voltage requirements The cavity is equipped with a tuning slug that allows manual adjustment of the resonant frequency over a range of up to 100 MHz The Model 485x has a 1x2 mm aperture and the optical beam must be horizon tally polarized with respect to the modulator housing 13 14 Operation Vv When used properly the New Focus electro optic phase modulators can provide efficient optical phase modulation with extremely low unwanted amplitude modulation and insertion loss The key to obtaining this pure phase modulation is good alignment of the optical beam with the crystal s propagation axis and accurate orientation of the polarization of the beam along the crystal s electro optic axis If the beam is not properly aligned a phase modula tor will imp
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