44XX and 48XX revE.fm
Contents
1. 14 20 M6 52 14 2 Bo Mounting Hole 9Y TH SMA Input Connector 2 28 57 9 Frequency Adjust 1 55 6 S 08 2 0 X 04 1 0 39 4 Y Optical Aperture 1 00 Both Sides 25 4 i D Delrin Base 22 9 Sa Sal Lo no 53 13 5 Las 8 9 79 JL 79 20 1 20 1 2X 8 32 M 4 M ounting Hole 1 4 20 M 6 M ounting Hole Operation 7 Quick Start This section presents a brief introduction to using your high frequency phase modulator 1 Align 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 horizontally Be careful not to exceed the maximum recommended optical power or damage to the electro optic crystal could result See page 11 for a discussion of optical damage 2 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 observation of sidebands Generally an optical CAUTION spectrum analyzer with suitable finesse and free spectral range is used to observe the modulation sidebands To prevent damaging the electro optic crystal do not exceed the modulators maximum RF drive power 4 watts for Models 44xx 3 watts for Models 485x 3 Use the tuning slug2. across the electro optic crystal is so Bln where A is the free space wavelength A commonly used figure of merit for electro optic modulators is the half wave voltage Ma which is the voltage required to produce a phase shift Substituting into the preceding equation yields Ad Ce WER Vos T For these high frequency phase modulators the crystal is put into a resonant microwave cavity that enhances the voltage applied across the crystal This results in a voltage across the crystal that can be more than nine times the applied input drive voltage 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 _ 27 13 2PQI Es A Lis 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 um 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 radians volt Principles of Operation 17 Cavity Design Models 442 and 443X 0 25 4 6 GHz For the Models 44xx modulators the crystal is placed in a resonant microwave cavity to achieve a high Q_ gt 100 system see Quality Factor Q on page 24 The microwave c
3. 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 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 frequency to precisely match the RF drive frequency Alternately tune the RF drive frequency until it matches the mod ulator s resonant frequency Preventing Photorefractive Damage The electro optic crystals used in these modulators are susceptible to optical damage through the photorefrac tive effect This phenomenon is caused by the migra tion of photoexcited charge carriers from illuminated 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 Operation 11 12 Operation Photorefractive damage is a serious concern for visible wavelengths high optical power and tightly focused beams The photorefractive damage process can occur gradually over days or hours or for high optical pow ers and short wavelengths this effect can occur over seconds A damaged cryst
4. frequency to minimize propagation losses The optical alignment of the modulator can be disturbed by the RF cable so it is a good idea to use a strain relief on the cable The Models 44xx and 485x high frequency phase modulators are resonant devices with a 50 62 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 modulator and your application Feel free to contact us for assistance Phase Modulators The RF driver typically consists of an oscillator or synthesizer 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 sufficient 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 ther mal 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
5. the time the modulator is ordered RF Bandwidth The bandwidth of the modulator s resonant frequency otherwise known as the 3 dB frequency It is the range over which at least one half of the electrical drive power will be transferred to the modulator Material The modulators use MgO doped LiNbO and the IR modulators use LiNbO3 Max Optical Intensity This is the maximum optical intensity assuming a 1 mm diameter beam that can be passed through the crystal before photorefractive damage occurs Note that this optical damage threshold is strongly wave length dependent See page 11 for a discussion of photorefractive damage Phase Modulators 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 problem 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 wave
6. um Aperture 1mmx2mm 1mmx2 mm Connector SMA SMA Impedance 500 500 Max RF Power 3W 3W Modulation Depth 0 05 rad V 0 05 rad V at 1 06 um gt 0 04 gt 0 04 Max Vy at 1 06 um 79V 79V VSWR lt 1 5 lt 1 5 Return Loss gt 14dB gt 14 dB Definitions of Specifications Phase Modulators RAM Residual amplitude modulation RAM is a source of unwanted noise in a phase modulation system An ideal phase modulator will exhibit no RAM However etalons in the crystal and misalignment of the optical beam will lead to some amplitude modulation 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 Characteristics 21 22 Characteristics Wavelength Two standard broadband AR coatings are available 0 5 0 9 um and 1 0 1 6 um Each coating has a 6 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 coatings on the end faces The crystals typically have losses of 0 3 cm at 1 0 um 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 frequencies over which these modulators can be designed to operate The particular resonant frequency of a given modulator is specified at
7. 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 sidebands 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 voltage 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 rj The induced refractive index change caused by an external electric field has the form An 1 2n3r33E where An is the change in the index of refraction ng is the unperturbed index of refraction r33 is the appropriate element in the electro optic tensor and E is the applied electric field The New Focus phase modulators consist of an electro optic crystal of length width b and thickness d The electric field is applied along the crystal s z axis 16 Principles of Operation Phase Modulators and transverse to the direction of optical propagation Modulation is induced onto the laser beam by aligning 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 voltage V
8. USER S GUIDE High Frequency Electro Optic Phase Modulators Models 442x 443x amp 485x U S Patent 4 5 414 552 p New Focus A Newport Corporation Brand 3635 Peterson Way Santa Clara CA 95054 USA phone 408 980 5903 fax 408 987 3178 e mail techsupport newfocus com gt www newfocus com MASTER Warranty Newport Coporation guarantees its products 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 consequential loss Information in this document is subject to change without notice Copyright 2012 2001 1998 Newport Corporation All rights reserved The New Focus logo and symbol are registered trademarks of Newport Corporation Document Number 440018 Rev E Contents Operation 5 oda wc win es ARE 5 Quick Marti 8 Using the Modulator 00200222 9 Principles of Operation 15 The Electro Optic Effect dg dao 15 Creating E e 16 Cavity Des A eet egy eee ae ee 18 Characteristics 19 Model 442x Specifications 0 0 0 0 ee ee eee 19 Model 443x Specifications e 20 Model 485x Specifications nouenn naunan anean 21 Definitions of Specifications 05 21 Customer Service 25 TEGHIMCALS UP POL EE 25 o Hite Modu d i Raa ee 25 Performance Data 27 Phase Modulators Contents 3 Ae Contents Operation Introduction Phase Modulators The New Focus Models 44xx
9. al 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 recommended photorefractive damage will occur In reality this damage is not permanent Photorefractive damage can be at least partially reversed by carefully anneal ing 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 materials LiINbO3 and magnesium oxide MgO doped LiNbO3 The LiNbO material has a lower damage threshold and so it is used in our IR modulators which operate from 1 0 to 1 6 um In this wavelength range photorefractive damage is generally not a serious problem we recommend a maximum optical intensity of 1 W mm2 at 1 3 um The visible modulators which operate from 500 900 nm come standard with MgO doped LiNbO crystals The MgO doping increases the resistance to photorefractive damage enabling this material to be used in the visible wavelength range For MgO doped LiNbO3 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
10. and 485x high frequency electro optic phase modulators provide an efficient means of single frequency optical phase modulation in the 0 25 to 13 GHz frequency range These modula tors are useful components in a variety of experimental techniques including FM spectroscopy laser fre quency stabilization atom cooling laser linewidth broadening and laser guide star systems These modulators feature low drive voltages large modulation depths a wide range of operating frequen cies from 0 25 to 13 GHz a broad range of wave lengths from 0 5 to 1 6 um low optical insertion loss and high optical power handling capability Their 1 to 2 mm apertures make them compatible with most laser sources Finally the electro optic materials used in these devices are nonhygroscopic so they can be left on an optical table for indefinite periods without requiring a sealed enclosure These high frequency phase modulators are classified into three resonant frequency ranges Model Frequency Range 4421 4423 0 25 to 2 0 GHz see Figure 1 4431 4433 2 0 to 4 6 GHz see Figure 2 4851 4853 6 8 or 9 2 GHz see Figure 3 Operation 5 Figure 1 Mechanical views of the Model 442x 6 Operation The modulator is shipped to you with the resonant frequency set to the frequency specified when your order was placed with New Focus The operating wavelengths are determined by the broadband anti reflection coating appl
11. avity is designed to replicate a transmission line terminated by the crystal 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 oper ated over this narrow frequency range In addition these modulators are equipped with a tuning slug that probes the interior of the microwave cavity and pro vides frequency tuning over a range of up to 200 MHz Model 485 6 8 or 9 2 GHz 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 to match the microwave velocity through the resonant cavity with the optical velocity through the crystal 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 to achieve significant modulation depth 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 1x 2 mm aperture and the optical beam must be horizontally polarized with respect to the modulator housing 18 Principles of Operation Characteris
12. diameter and the particular batch of crystal material being used The damage thresholds are conservatively Phase Modulators 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 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 please contact New Focus Operation 13 14 Operation Principles of Operation The Electro Phase Modulators Optic Effect Operation of the New Focus electro optic phase modulators is based on the linear electro optic or Pockels effect whereby an applied electric field induces a change in the refractive index of the crystal 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 T
13. he material used in these modulators are lithium niobate LiNbO3 and magnesium oxide doped lithium niobate MgO LiNbO3 These materials are well suited for use in these types of modulators because they have wide optical transparency windows large electro optic coefficients and low RF losses Having low RF loss 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 voltages 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 voltage while still allowing a relatively large optical aperture Principles of Operation 15 Creating Sidebands Phase modulators are typically used to generate frequency sidebands on a cw optical beam A sinusoidal electronic drive signal applied to the modulator produces optical sidebands which are separated from the cw optical carrier by the drive frequency These modulation 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 Ad where J is the Bessel function of order one The fraction of power that remains in the carrier is Jo Ad where Jo is the Bessel function of order
14. ied to the surfaces of the electro optic crystals Two standard wavelength ranges are offered 0 5 0 9 um and 1 0 1 6 um For applications requiring even better anti reflection coatings contact New Focus to obtain a customized narrow band V coating The physical characteristics and performance specifi cations for these modulators are listed beginning on page 19 Mechanical drawings of the three types of modulators are shown in Figures 1 to 3 SMA Input 7 Connector 442X Vis IR Phase Modulator E new Focus Frequency Adjust Se 2 19 55 6 a 11 4 fee a 16 3 7 t m2 CY mS I 7 08 2 0 X 08 2 0 Optical Aperture 7 Cees 1 82 46 2 bk Delrin Base 1 4 20 M6 Mounting Hole A 2 08 52 8 Height D is frequency dependent 91 23 1 O Figure 2 Mechanical views of the Model 443x Figure 3 Mechanical views of the Model 485x Phase Modulators SMA Input 2 Connector tee dl Frequency Adjust eo 32 40 1 14 8 A pie SR aoa 1 i Sl H H 04 1 0 X 04 1 0 Optical Aperture Both Sides Delrin Base Height D is frequency dependent 1 03 D
15. length So for example the modulation depth at 532 nm is twice that at 1 06 um Max The voltage required to achieve a 180 degree phase shift at 1 06 um V varies linearly with wavelength and so V y at 532 nm is half that at 1 06 um Return Loss Describes how well the modulator is matched to 50 2 when driven at its resonant frequency 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 Characteristics 3 24 Characteristics modulator is optimized and reflected power which can harm the driving source 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 provided at the end of this manual For a power reflection coeffi cient R 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 RL 20 110 D0 VSWR A VSWR value of 1 indicates a perfectly matched system A VSWR of 1 5 corresponds to 4 of the i
16. ncident RF power reflected back to the driver Quality Factor Q The quality factor or Q of a resonant cavity is a measure of the sharpness of its frequency response Generally a larger Q means a higher modulation depth For high frequency phase modulators Q is defined as f Af where f is the modulator 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 typically between 100 and 200 The measured Q for your modulator is written in the performance data section at the end of this manual Customer Service Technical Support Information and advice about the operation of any New Focus product is available from our applications engineers For quickest response ask for Technical Support and know the model and serial number for your product Hours 8 00 5 00 PST Monday through Friday excluding holidays Toll Free 1 866 NUFOCUS 1 866 683 6287 from the USA amp Canada only Phone 408 980 5903 Support is also available by fax and email Fax 408 987 3178 Email techsupport newfocus com We typically respond to faxes and email within one business day Service Phase Modulators In the event that your New Focus product malfunctions or becomes damaged please contact New Focus for a return authorization number and instructions on shi
17. pping the unit back for evaluation and repair Customer Service 25 26 Customer Service Performance Data Model Number Serial Number Resonant Frequency Wavelength Input RF Power Return Loss VSWR Phase Modulators Performance Data 27 28 Performance Data
18. t it on an adjustment positioning device for align ment We recommend the New Focus Model 9071 or 9071M tilt aligner because of its tilt and translation capabilities 2 Turn on the optical beam and orient the beam so it is linearly polarized along the z axis of the electro optic crystal With the Model 44xx modulators the polarization should be oriented vertically with respect to the modulator casing and with the 485x modulators the polarization should be horizontal Operation 9 10 Operation 3 Position and align the module so that the beam passes through the mechanical apertures clearing them without clipping The beam should be collimated with a waste size less than the aperture size and such that the Rayleigh range is at least the length of the crystal A good rule of thumb is that the beam diameter should be about one third the aperture size to minimize 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 pm Larger beams can be focused slightly and then collimated 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 11 Driving the Modulator Connect the SMA jack on the modulator to an RF driver using an RF cable with operating bandwidth greater than the modulation
19. tics Model 442x Specifications Model 4421 4423 Wavelength 0 4 0 9 um 1 0 1 6 um Operating Frequency 0 25 2 0 GHz 0 25 2 0 GHz RF Bandwidth 0 5 freq 0 5 freq Material MgO LiNbO3 LiNbO Max Optical Power 2 W mm 4W mm2 in a 1 mm beam 532 nm 1 06 um Aperture 2mm 2mm Connector SMA SMA Impedance 500 500 Max RF Power 4W 4W Modulation Depth gt 0 05 rad V gt 0 05 rad V at 1 06 um Max Vy at 1 06 um 31 63 V 31 63 V VSWR gt 5 gt 5 Return Loss gt 14dB gt 14 dB Phase Modulators Characteristics 19 Model 443x Specifications Model 4431 4433 Wavelength 0 4 0 9 um 1 0 1 6 um Operating Fre 2 0 4 6 GHz 2 0 4 6 GHz quency RF Bandwidth 0 5 freq 0 5 freq Material MgO LiNbO3 MgO LiNbO3 Max Optical Power 2 W mm2 4W mm in a 1 mm beam 532 nm 1 06 um Aperture 1mm 1mm Connector SMA SMA Impedance 500 500 Max RF Power 4W 4W Modulation Depth gt 0 04 rad V gt 0 04 rad V at 1 06 um Max Vy at 1 06 um 45 79V 45 79 V VSWR lt 1 5 lt 1 5 Return Loss gt 14 dB gt 14 dB 20 Characteristics Model 485x Specifications Model 4851 4853 Wavelength 0 4 0 9 um 1 0 1 6 um Operating Fre 9 2 GHz 9 2 GHz quency or 6 8 GHz or 6 8 GHz RF Bandwidth 0 5 freq 0 5 freq Material MgO LiNbO3 LiINbO3 Max Optical Power 2 W mm A W mm in a 1 mm beam 532 nm 1 06
20. to fine tune the modulator s resonant frequency and precisely match it to the RF drive frequency If the modulator is not driven at or close to its resonant frequency most of the RF drive power will be CAUTION reflected which could damage to the driver 8 Operation Using the Modulator Phase Modulators 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 modulator will impose a polarization rotation as well as a phase modulation which can lead to unwanted amplitude modulation if the modulator is followed by any polar izing optics It is important to carefully align the polar ization since the crystals used by New Focus are cut so that the beam propagates along the y axis of the crys tal 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 Modulator To align the module to the optical beam 1 Use the 1 4 20 M6 for metric versions tapped hole located on the base of the module to moun
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