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User`s manual and tutorial for the image rotating RISTRA OPO

1. Figure 19 a Reversal of walkoff in two crystal oscillators In this example the resonated e wave is displaced toward the outside of the cavity but displacement in the opposite direction works as well b Determining the direction of walkoff in a birefringent crystal Place the crystal so a straight reference line can be viewed in transmission Clean paper with a fine line is acceptable for this purpose but to minimize collecting particles on the crystal a clean microscope slide can be set on the paper If handled carefully the dielectric coatings on the crystal will not be damaged from contact with the microscope slide or with the paper Now rotate the film polarizer to selectively view the o and e waves as shown The e wave will be displaced from the reference line as viewed through the crystal dashed line Observing e wave displacement for crystals with small walkoff angles may require magnification Try a jeweler s loupe Important note You are viewing light scattered back through the crystal toward your eye from the surface below so the direction of walkoff for a wave propagating forward through the crystal away from your eye is opposite to the direction of displacement you view for the e wave In other words we saw the line move to the right so going into the page a ray would walk off to the left It is a good idea to use a hard pencil to identify the crystal and also to mark the direction of birefringent walkoff The intra cavity
2. Following initial alignment and before you increase the pump energy you might be able to carefully observe the position of the pump beam on the cavity mirrors For a one crystal RISTRA only two mirrors are involved but with two crystals the pump impinges on all four mirrors with two being high reflectors Weak scatter from high reflectors might be observable from the backside of the mirrors but some coatings notably ion beam sputtered coatings can have such low scatter that the beam spot is difficult to see This is especially true for IR pump beams If the pump enters through MI and exits trhough M4 you can also test alignment by comparing the positions of the reflected and transmitted spots on the RISTRA s base plate For an injected seeded pump laser an interference fringe pattern may be observable below M1 indicating the pump is reasonably well aligned Note however that the positions of the pump beam spots on mirrors or a fringe pattern below M1 on the baseplate indicate approximate but not final alignment After it s safe to increase pump energy final alignment is obtained by optimizing output energy near the oscillation threshold For two crystal cavities it s best to optimize energy after both crystals are rotated to have the same phase matching angle For injection AS Photonics LLC 32 RISTRA OPO user s guide seeded cavities the seed wavelength provides a reference for Ak 0 whereas free running oscillation places les
3. 13 9 13 cm ieca r Wavelengths 1 must be zero Red 1 Red 2 Blue 1588 1 e e 800 0 0 ja S 32 0 e alkoff mrad kl 61 93 0 00 63 03 hase velocities c 1 619 1 660 1 647 roup velocities c 1 644 1 684 1 691 rpDelDisp fsa2 mm 19 3 75 1 125 4 27 3 deg 1 62E0 pa v 7 21E7 Watt 0 76 mradecm 44 30 Kecm 1 58 0 78 mradeca 25 01 25 01 cam ieca 1588 1 o je 800 0 ejm S32 0 e alkoff mrad 0 00 72 13 73 72 4 Figure 6 Output from SNLO s QMIX for flux grown KTP and for BBO with a 532 nm pump and 800 nm signal Note that the temperature is 300 K The refractive index temperature derivatives are large for certain crystals so using the correct temperature is important See text for additional details r Wavelengths 1 must be zero Red 1 Red 2 Blue emperature range 112 38 Keem PO accpt ang 4 33 1 91 mradeca PO accpt bw 56 33 6 33 cm iecm 3393 4 e 1550 0 0 1064 0 0 alkoff mrad 14 38 0 00 0 00 hase velocities c 1 810 1 776 1 787 roup velocities c 1 903 1 3803 1 3819 rpDelDisp sA2 mm 513 7 56 7 139 6 t theta phi 79 6 0 0 deg ett 3 12E0 pa v So x La 1 04E8 Watt rystal ang tol 12 60 mradec a 28 05 Kecam 12 60 5 86 mradeca 5 52 9 92 cm 1ecm r Wavelengths 1 must be zero Red 1 Red 2 Blue ert 0 00E0 pm V 3393 4 0 1550 0 e 1064 0 0 alkoff mrad kd 0 00 45 00 0 00 hase velocities c 1 743 1 806 1 787 roup velocities c 1 818 1 836 1 819 rpDelDisp fs
4. so that s polarization at M3 corresponds to an o wave in For some crystals the effective nonlinearity deg is large enough that a crystal length of 15 mm may be too long for optimum performance This is discussed in section 3 AS Photonics LLC 5 RISTRA OPO user s guide Figure 4 RISTRA OPO mechanical assembly for a two crystal oscillator C2 and so on Inspection of Figure 5 should make this clear but realizing that nonplanar geometry can be confusing at first glance we ve included example one and two crystal cavity mirror specifications in Appendix C For a one crystal cavity C2 is omitted keeping CI placed in the lower leg and only one 4 2 retardation plate is required typically in the position of WP1 For a single crystal cavity the waveplate can also be located at the position of WP2 This could be advantageous for a cavity containing the crystal ZGP pumped at gt 2 um with a signal wavelength lt 3 7 um Most readily available birefringent materials used for waveplates might absorb the resulting idler wavelength gt 4 5 um and suffer thermal distortion The additional loss of idler following reflections from M3 and M4 can help eliminate such problems Otherwise one and two crystal cavities are similar with one important exception When a single waveplate is used the signal polarization between M3 and M4 will be elliptical but it will revert to linear following reflection from M4 as long as s and p reflective phase shift
5. depletion to 79 6 See text for additional details AS Photonics LLC 21 RISTRA OPO user s guide 4 Thermal effects Biaxial Crystals a Uniaxial Crystals a YCOB lu ZGP LBO OM mel al KTP li GAAS ra KTA DKDP 8 CSP ees CLBO ees me MT KNBO3 i GCOB BBO AGS BiBO AGSE 1 0 5 0 0 5 1 1 5 2 2 1 1 3 4 Thermal Expansion a 1 K x10 Thermal Expansion o 1 K x10 a b o Figure 15 Coefficients of thermal expansion along principal axes for a biaxial and b uniaxial crys tals 4 1 Introduction Optical parametric oscillators do not have intrinsic heating like optically pumped lasers or Raman oscillators However it is not unusual to have absorption of one or more of the waves in the nonlinear crystal Because the waves are not equally absorbed the heating is nonuniform along the length of the crystal Heating is also nonuniform in the transverse dimension because it mimics the transverse beam profiles Such nonuniform heating can cause disruption of phase matching along the longitudinal and transverse directions It also can cause thermal lensing which disrupts the cavity mode profiles The steady state temperature profile can be computed from the heat deposition profile and from the crystal thermal conductivity and thermal boundary conditions If we assume the crystal has a square d x d cross section a length L and is cooled on one side as is usually true for a RI
6. s acceptable to initially use a low x grid density for faster computation and increase the density as you refine your model parameters The two parameters for crystals length and deff can have dramatic effects which we ll demonstrate in the following sections In a one crystal model we use a single entry for the crystal length However in a two crystal version use two entries for length in the same text box Note that the two crystals can have different lengths but the nonlinearity deg has the same value for each OPO crystal For either one or two crystals the grid size should accommodate the beam with largest diameter which is usually the pump and can be automatically determined by the model This works well for lowest order Gaussians However for flat topped beams it may be better to manually set the size to maximize the filled in portion of the grid but make sure that none of the waves walk off the grid The phase mismatch Ak while usually zero can be set to other values as necessary and again is the same in each crystal Finally the text box for signal idler swap will be zero unless you re modeling a special case of oscillation at degeneracy where the signal and idler polarizations are rotated 90 after each successive cavity pass 3 3 Basic modeling concepts How to optimize performance of nanosecond OPOs Once the input text boxes contain values none can be empty you can run the model What you ll then do is optimize
7. 25 A 532 nm s polarization best effort R lt 0 5 acceptable R 70 2 A 800 nm p polarization R lt 4 A 1588 nm s polarization Side 2 outside R lt 0 25 A 532 nm s polarization R lt 0 25 A 800 nm p polarization R lt 4 A 1588 nm s polarization Mirror 3 Side 1 inside R lt 1 A 532 nm unpolarized R gt 99 A 800 nm unpolarized R lt 4 A 1588 nm unpolarized Side 2 outside R lt 0 25 A 532 nm s polarization R lt 0 25 A 800 nm p polarization R lt 4 A 1588 nm s polarization Mirror 4 Mirror 4 is identical to mirror 3 AS Photonics LLC 46 RISTRA OPO user s guide a Corner Cube b Beam dump c Piezo mirror adjustment Figure 27 RISTRA Accessories AS Photonics LLC 47 RISTRA OPO user s guide Figure 28 Bearing retainer spanner tool AS Photonics LLC 48 RISTRA OPO user s guide D Accessories for the RISTRA OPO Beam block assembly and turning mirror for pump beams that exit at M4 See Fig 27b For two crystal RISTRA OPOs that use M4 as the pump exit mirror we offer an assembly that attaches to the RISTRA cylinder that provides a beam block and an optional turning mirror to redirect the beams parallel to the optical table The redirection mirror can be wavelength selective to separate the pump beam and resonated wave This simplifies observation of cavity fringes as required for cavity length stabilization Can be used in conjunction with the
8. RIS TRA software can be downloaded from http www as photonics com RISTRA Modeling html AS Photonics LLC 8 RISTRA OPO user s guide 3 1 Selection of nonlinear crystals Designing an OPO begins by selecting the best crystal for your application Sometimes crystal selection is simple and sometimes it isn t but either way SNLO s QMIX provides most of the information you ll need to make the best choice Three important criteria a crystal must possess are adequate transmission at the signal idler and pump wavelengths the ability to phase match and sufficiently large nonlinearity detr Also important are the good operating parameters discussed in the latter part of subsection 1 1 which involve polarizations of the mixing waves and adequately large birefringent walkoff A final consideration not addressed by QMIX is the availability of well developed high quality crystals You ll find the list of crystals in QMIX is large but many are unavailable or unsuitable for use in OPOs Crystals that are commonly used in RISTRA OPOs pumped by the harmonics of Nd YAG and Nd YLF include KTP BBO KTA and perhaps BiBO For 2 um pumping with lasers such as Ho YLF the crystal ZGP is a very good choice Other well developed crystals such as bulk LiNbO not PPLN and LBO have undesirable polarizations for the mixing waves have small derr or smaller than desirable birefringent walkoff To provide an example of crystal selection that offers severa
9. an important issue for the RISTRA We explain elsewhere that small mirror tilts do not destroy the optical axis of the cavity but merely shift the angle and location of the axis slightly The same is true of small thermally induced tilts in the crystal The maximum allowed tilt is limited by the shift in beam position that can be tolerated It is typically 1 mrad which causes a shift in the axis location of approximately 100 microns However because the beam diameters are typically quite large in a RISTRA the beam quality of the resonated wave can be strongly affected by even weak thermal lensing AS Photonics has devel oped thermal models that can quickly compute the thermal profile inside the crystal based on the heat deposition profile and the crystal thermal conductivity We are also developing self consistent models of RISTRA OPOs that include thermal effects The RISTRA OPO model is iterated with the thermal model and the computed temperature profile is used in propagating the beams through the crystal on the next iteration of the OPO model This modeling allows us to predict beam quality for the OPO assuming the absorption and thermal properties of the crystal are known with sufficient accuracy Note that the method of cooling the crystal has little influence on thermal lensing Lensing depends on the transverse gradients of the temperature and these gradients are determined by the transverse profile of the heat deposition and the crystal condu
10. and undepleted pump signal and idler temporal profiles exiting the right mirror are shown in Figure 10 for two different crystal lengths The message to read from the temporal profiles in Figure 10 is that parametric back conversion where energy from the signal and idler sum frequency mix to generate new pump energy can reduce conversion efficiency And because the new pump is 180 out of phase with the original pump back conversion also affects beam quality In Fig 10a the recovery in amplitude of the depleted pump pulse following the onset AS Photonics LLC 12 RISTRA OPO user s guide of oscillation is the characteristic signature of back conversion Modeling can be used to understand this effect and what steps are taken to reduce it For example in Fig 10b the crystal was shortened to 12 mm there is less back conversion and the output energy increased slightly You should also investigate how changing the output coupling pump energy and the pump beam spatial profile can all affect conversion efficiency Finally changes in conversion efficiency can be quantified by calculating the percent pump deple tion from the model outputs This is done by dividing the difference of the undepleted and depleted pump energies by the undepleted pump energy at the mirror where the pump beam exits the cavity usually M2 right when using one crystal and M4 left when using two crystals Undepleted pump energy is obtained by running the model w
11. field fluence profile of the signal wave in Fig 2a is elongated in the direction perpendicular to birefringent walkoff denoted by the far field angle 6 and is compressed in the parallel direction denoted by 6 The improved beam quality in the direction is due to walkoff although for a three mirror ring OPO it is also enhanced by image inversion With the Dove prism inserted in the cavity the central portion of the far field fluence is round and symmetric and confined within a far field divergence angle of lt 1 mrad The weak shoulder with four fold symmetry in Fig 2b appears to be an artifact of scattering from the Dove prism and from the 2 retardation plates that were added to the cavity To better illustrate the small amplitude of this shoulder Figure 3 shows surface plots of all the contours in Figure 2 To demonstrate what image rotation can achieve for large F Fig 2c shows the same three mirror ring cavity OPO pumped by the large diameter lower quality beam in Fig 2f With F increased to 200 we observe poor beam quality that includes a far field divergence angle that is much larger in the perpendicular direction In contrast the far field fluence profile for the RISTRA OPO in Fig 2d with F gt 400 has a very tightly focussed symmetric central spot surrounded by a weak shoulder The surface plot of the far field fluence in Fig 3d again provides a better illustration of the relative heights of the peak and shoulder As the fluen
12. in a time efficient manner using SNLO s PW OPO BB which models a central ray without diffraction or walkoff Meaningful results will be obtained from PW OPO BB as long as memory requirements don t exceed 2 Gb the maximum that can be allotted to SNLO Oscillation near or exactly at degeneracy Crystal transmission versus wavelength can be viewed in the file CRYST_TR DAT located in the directory C SNLO Low idler transmission increases the threshold for oscillation and can cause unwanted heating of the crystal 7For oscillation on a four times around vortex mode use an injection seeded idler resonant cavity offset the idler seed beam about l 2 mm use a large diameter pump beam say 6 mm with super Gaussian coefficient of 4 and add 7 2 of R L phase to the resonated idler wave You might need to increase the x gird value to 64 to generate a beam with a deep symmetric hollow center AS Photonics LLC 10 RISTRA OPO user s guide can t be accommodated in Type I phase matching but it can be approximated Now that we ve described the limits of the RISTRA models let s begin by examining the inputs they require from the GUI shown in Figure 8 The crystal is 532 nm pumped KTP with data taken from the example QMIX output shown in Figure 6 You ll notice that we ve set all crystal reflections to zero and all mirror reflections to zero as well except for the signal Although we can guess these missing values at this early st
13. is more typical of Ho YLF lasers pumped at 1 9 um by cw fiber lasers Also the beam profiles are lowest order Gaus sian where the extended wings limit the maximum beam diameter accommodated by the RISTRA mechanical assembly to about 3 6 mm r Je or dewum 4 2 mm Beginning with the inputs in Figure 12 and iterating to optimize performance we find Roe l 0 65 and a crystal length of 12 mm provides 15 4 mJ at 3800 nm with M2 M 1 77 The M values could AS Photonics LLC 15 RISTRA OPO user s guide probably be lower but there is only a small amount of back conversion The resulting pulse temporal profiles at the output coupler M2 are shown in Fig 13a As usual we arrived at these specifications by adjusting crystal length and Recht to optimize energy but found that using a longer crystal reduced performance due to increased back conversion For example a crystal length of 15 mm results in 15 1 mJ with M Mi 1 96 a reduction in signal energv and beam qualitv The pulse temporal profiles for the 15 mm crystal are shown in Fig 13b Increasing the crystal length for this OPO is clearly couterproductive For a two crystal ZGP RISTRA we retain Recht l 0 65 and set ies 0 99 and after testing various combinations of crystal lengths find Cl 8 mm and C2 15 mm give the best performance with signal energy of 21 4 mJ pump depletion of 81 2 and M and M 1 85 The difference in crystal lengths is almost a factor of two but with
14. lt 10 ns Seed beam cw spatially filtered 2 mm 1 e diameter Output Maximum energy at 1550 nm 170 mJ Pump depletion Highest conversion efficiency 55 Beam quality Mj 3 8 Mi 24 2 denotes in direction of walkoff Reference SPIE 5337 71 80 2004 Comments Undesirable ooe mixing dictated by small deff for oeo mixing in KTA One KTA crystal had refractive index inhomogeneities resulting in reduced conversion efficiency and reduced beam quality Application Prototype 3 4 um source for laser ultrasonic testing Configuration One crystal 3400 nm resonant unseeded Mixing 2050 o 3400 e 5163 e Crystal Aperture NA length 10 mm ZGP 0 55 8 Output coupler R 0 5 at 3400 R not reported for 2050 and 5163 Waveplates One Multi order A 2 for 3400 Pump beam Broadband Ho YLF 1 lowest order Gaussian with 4 0 4 5 mm 1 e diam eter duration gt 14 ns Seed beam unseeded AS Photonics LLC 42 RISTRA OPO user s guide Output Maximum energy at 2400 nm gt 10 mJ Pump depletion 35 Beam quality M not measured but far field divergence indicates lt 1 8 x diffraction limited References SPIE 6875 687507 1 10 2008 Opt Express 15 14404 14413 2007 Comments This repetition rate for this system was as high as 500 Hz At 100 Hz following post amplification in another ZGP crystal the power output was 30 40 W Application Remote sensing source at 1627 nm Configurati
15. performance by adjusting crystal length output coupler reflectivity and perhaps beam diameter but in many cases pump parameters such as maximum energy pulse duration and the beam profile will be constrained by what the pump laser can actually deliver The resulting model outputs include energy power and normalized power fluence in the near and far field spectra and M as it evolves in time and also its averaged values Movies of the spatial profiles are also provided so you can observe their evolution in time The one crystal model output window corresponding to the inputs in Figure 8 is shown in Figure 9 Of all the outputs the two we ll find most useful are energy and power with power in the form of pulse temporal profiles The temporal profiles allow us to graphically observe conversion efficiency and how it s affected by back conversion Our goal is to achieve the highest efficiency for a given pump energy while minimizing back conversion We d also like to build an OPO that operates with fluence and peak power well below typical damage thresholds for crystals and mirror coatings Starting with the inputs in Figure 8 we ll illustrate two important concepts How to read the temporal profiles and how using two crystals especially crystals of unequal length can enhance conversion efficiency We ll initially change two of the inputs in Figure 8 by increasing pump energy to 200 mJ and decreasing Recht l to 0 60 The depleted
16. pulse doesn t appear truncated if you select power to plot the pulse temporal profiles The models offer spatial profiles with super Gaussian coefficients of 1 5 A coefficient of 1 pro duces the familiar lowest order Gaussian and a super Gaussian coefficient of 4 provides a good ap proximation to a globally flat topped beam profile Real beam profiles usually contain small scale fluence variations such as ring patterns or lumpiness for lack of a better description Fortunately small fluence variations can usually be ignored in simulations and are therefore not accommodated by the standard RISTRA models Note that beam diameters in all SNLO models use FWHM and not the more familiar 1 e relevant to lowest order Gaussian profiles Also the models accept irradiance pro files and not fields so a lowest order Gaussian has the form Iqe 27 a r the radial coordinate and a the 1 e radius with dewym 1 18a Continuing down the input list birefringent walkoff from QMIX is entered for the e waves and pump beam offset can be adjusted to optimize efficiency Be sure to monitor its effect on spatial fluence profiles especially on the near field depleted pump beam so that offset increases azimuthal symmetry but does not have the opposite effect This will be obvious when you run the models As mentioned previously mirror reflectivity is zero except at 800 nm but can be set to more realistic values as the model is refined Also
17. the beam during initial alignment The turning mirrors labeled HR 45 should be selected for s p polarization when M1 M2 is the input coupler For large beam diameters gt 7 mm flat top 5 mm 1 e Gaussian pump beam alignment can be critical for the RISTRA cavity because the beam must pass through the bores of the cylindrical body without clipping and must clear the apertures formed by mirror retaining rings and waveplate holders 4 2 half waveplate HR high reflector With any alignment procedure you should use two mirrors to facilitate walking in the beam as shown in Figure 21 And because the RISTRA s removable base is indexed you can pre align the pump beam to the center of the cavity bore without the cavity in place You might also consider compensating in advance for the vertical offset at M1 or the horizontal offset at M2 depending on where the pump enters the cavity This correction can be important for high index IR substrate materials such as ZnSe or ZnS when a small diameter pump beam must overlap an injection seeded cavity mode or if a large diameter beam fills most of the input coupler s clear aperture You should also pay attention to the walkoff displacement for an e polarized pump wave when its diameter is gt 6 mm for a flat topped spatial profile For Gaussian pump beam profiles be aware of the extent of the wings For a true lowest order Gaussian the maximum diameter is about 5 mm 1 e or lt 3 mm FWHM
18. the two missing mirrors M3 and M4 in Figure 5 would be accommodated from the product RjenR3 Ra for each wavelength and for a two crystal cavity we would set Richt 10 a high value such as 0 99 so the pump beam also passes through the second crystal C2 Below the R values the nine entries for phases that will generally be set to zero for the flat mirror where Jp is the peak irradiance 8Degeneracy can usually be accommodated with reasonable accuracy by increasing the difference between the group velocity indices for the signal and idler If PW OPO BB stops running because your machine runs out of memory increase the difference until the calculation will finish Also allocate 2Gb of memory to SNLO if your machine has sufficient resources See the instructions with SNLO on how to set memory allocation AS Photonics LLC 11 RISTRA OPO user s guide RISTRA cavity unless you want to model a vortex beam The z and x grid numbers are nominally 30 and 32 although you ll occasionally need to increase the spatial grid density to higher values at the price of increased computation time If your pulse temporal profiles aren t smooth or the spatial fluence has more structure than you expect the grid density may be too low or the pump energy too high Also monitor changes in output energy with grid density If energy changes appreciably by increasing the x grid number you may need to use a higher density grid for accurate results Note that it
19. v RISTRA OPO user s guide 1 Background material Nanosecond optical parametric oscillators OPOs are versatile sources of tunable coherent light that generate pulse energies ranging from less than 1 uJ to greater than 100 mJ When their output is mixed in subsequent sum or difference frequency generation stages they can produce wavelengths from the deep UV to the mid IR and beyond To generate wavelengths shorter than approximately 3 6 um nanosecond OPOs are typically pumped by the harmonics of Q switched flashlamp pumped Nd YAG lasers while for longer wavelengths diode pumped Ho YLF is a popular choice Although wavelength agility and solid state pumping make OPOs versatile and practical they have one major shortcoming OPO beam quality tends to be poor if output energies exceed more than a few mJ The reason for this is simple Higher output energy requires higher pump energy but the damage thresholds for nonlinear crystals and for dielectric coatings on cavity mirrors set limits for the peak pump irradiance W em and for its time integral fluence J cm To reduce the risk of optical damage requires increasing the pump beam diameter However to obtain high conversion efficiency OPO cavities are typically short relative to the length of the pump pulse Consequently for a fixed cavity length an increase in the beam diameter also increases the number of higher order transverse cavity modes that can oscillate For essentially all conventional
20. walkoff angles of 11 mrad this shouldn t complicate operation of the OPO Also the pump depletion is very high even for a two crystal RISTRA but does it represent a realistic value In an attempt to answer this question we approximate a real device by setting Rerystal 0 01 for all three wavelengths somewhat lousy AR coatings and also set RI Rea Rito 0 01 because real mirrors don t have R 0 and we also include crystal loss 0 008 mm at Apump 2050 nm obtained from Inrad s data sheet for ZGP http www inrad com pages crystals html These changes result in signal energy of 18 2 mJ pump depletion of 79 6 and M M 1 86 Of course we don t accurately know the values for the various surface reflections but barring other unforseen loss mechanisms our estimates suggest the high performance predicted by the model is reasonable Figure 14 shows pulse temporal profiles for the two crystal ZGP RISTRA OPO with and without the nonzero R values expected in real device AS Photonics LLC 16 RISTRA OPO user s guide r Wavelengths 1 must be zero 7 Red 1 Red 2 Blue 159898 1 e 800 0 e 532 0 0 t theta phi 46 6 0 0 deg eit 0 00E0 pav 1588 1 0 800 0 e 532 0 0 alkoff mradi 0 00 47 65 0 00 hase velocities c 1 736 1 817 1 790 roup velocities c 1 766 1 875 1 909 rpDelDispifsAa2 mm 29 0 243 2 405 2 58 2 0 0 deg 3 21E0 pav 2 34E7 Watt 0 92 mradecm 19 80 Keen 292 0 92 mradeca
21. waveplates are glued into their holders using UV curing glue It is very important to use as little glue as possible and to use a high viscosity flexible curing glue such as Norland 68 otherwise capillary action can carry the glue around the perimeter of the waveplate inside its holder If the glue is cured with complete contact the waveplate can be difficult to remove in case that becomes necessary The axes of the waveplates must have the correct orientation relative to the polarized light in the cavity The orientation is obtained by marks on the waveplate holder at 0 for two crystal cavities and at 22 5 for one crystal cavities see Fig 20 Aligning to the marks is inexact so the waveplate mount allows 10 of rotational adjustment Unmounted stock half waveplates usually have a flat ground on their side that is parallel to their slow axis This flat is used for orientation For some single crystal applications the RISTRA can use stock multiple order plates while two crystal designs require custom double A 2 plates for the pump and resonated wave For broader tunability zero order plates composed of two or more multiple order plates may be necessary and these also have flats or some other mark indicating their orientation Note that the custom waveplates required for most two crystal cavities may not have a flat ground on their side unless it is requested when they are ordered For a one crystal cavity you will need to decide in advance wh
22. 2 zm 410 3 67 3 139 6 41 6 0 0 deg 2 00E0 pa v 2 48E3 Watt 1 91 mradecm 112 38 Kecm 4 33 1 91 mradeca 56 33 56 33 cm iecm Figure 7 Output from SNLO s QMIX for xz cut KTA for 1064 o 1550 e 3393 0 at 6 41 6 and 1064 o 1550 o 3393 e at 0 79 6 See text for additional details AS Photonics LLC 17 RISTRA OPO user s guide po RO a JIT deff om V delta k 1 mm Ba Sig id swap 1 yes 0 no p Signal wave by default is perfectly aligned in cavity beams will not be if they walk off Right input W J 1 00E 3 1 00E 12 Left input W J J 1 00E 12 1 00E 12 1 00E 1 Left output energy J 54E 4 1 00E 12 121E 81 Right output energy J 237E 2 122E2 6 38E 2 Figure 9 The output GUI for the one crystal RISTRA model for KTP with 532 0 800 e 2588 0 corresponding to the inputs in Figure 8 See text for additional details AS Photonics LLC 18 RISTRA OPO user s guide Undepleted pump Depleted pump Pe Signal Idler Undepleted pump 7 k Depleted pump 7 A Signal 1 Idler Power Arb Power Arb 12 8 4 0 4 8 12 Time ns Time ns Figure 10 a OPO pulse temporal profiles for the RISTRA model inputs in Figure 8 but with pump energy increased to 200 mJ and R 2 reduced to 0 60 The KTP crystal length is 15 mm the signal en ergy is 45 9 mJ and the pump depletion is 35 Note the sudden decrease
23. 98 at 532 R lt 0 04 at 1576 4 Waveplates Two Custom multi order double 2 for 532 and 803 Pump beam Injection seeded Nd YAG 2 6 mm diameter flat topped spatial profile dura tion 10 ns Seed beam Pulsed 50 wJ 1 mJ 6 mm diameter flat topped spatial profile Output Maximum energy at 803 nm 14 mJ Pump depletion Measured with high accuracy at 90 Beam quality M not measured Approximately 60 of 803 nm energy within diffraction limited spot in far field Reference Opt Lett 31 380 382 2006 Comments Designed for low energy output but very high pump depletion Developed before we appreciated using unequal length crystals Higher pump depletion may be possible Flat topped 803 nm beam in near field not well characterized by M7 Application High energy UV generation at 320 nm for prototype ozone DIAL system using intra cavity sum frequency generation Configuration Two crystal 803 nm resonant with pulsed injection seeding self seeded Mixing for OPO 532 0 803 e 1576 4 o Crystal for OPO Aperture 10 x 10 mm length 15 mm xz cut KTP 0 58 4 0 Mixing for SFG 532 o 803 e 320 e Crystal for SFG Aperture 10 x 10 mm length 10 mm type II BBO 0 48 2 Output coupler R 0 85 at 803 R gt 0 98 at 532 R lt 0 04 at 1576 4 R lt 0 02 at 320 Waveplates Two Custom multi order double 2 for 532 and 803 Pump beam Injection seeded Nd YAG 20 6 7 mm diameter flat topped spati
24. Beam height Center of lower bore at 2 25 in Max beam diameter Flat topped 7 mm Gaussian 5 mm 1 e Base dimensions 3 75 x 2 75 in 1 4 20 hole pattern 3 0 x 2 0 in Assembly height One crystal 3 51 in Two crystal 4 31 inch Crystal apertures Max width 10 mm Height 10 mm AS Photonics LLC 39 RISTRA OPO user s guide Crystal length Nominal max 15 mm Absolute max 17 18 mm for 10 x 10 mm aperture Crystal rotation Range 10 for Resolution 20 4 mrad 1 17 10 x 10 x 15 mm crystal per actuator turn Cavity mirrors Dia 0 5 0 0 0 010 in Thick 0 125 0 010 in Angle of incidence All mirrors 32 765 Waveplates Dia 0 5 0 0 0 010 in Max thickness 3 mm PZT assembly Total displacement 15um Midpoint voltage 50V Note Be sure to order mirrors which do not have beveled edges on the outward facing side To mini mize strain on the mirrors which would affect the beam quality the mirrors are secured only by three short clips on the mirror retaining ring These clips might not securely fasten a mirror with beveled edges B Example configurations and performance specifications Previous uses of the RISTRA OPO include remote sensing platforms and prototype development for various applications Some published and un published examples are described below Wavelengths are in nm unless otherwise stated Table 2 provides a brief overview of specifications for a few applications Detailed descriptions fol
25. For others with less R amp D laboratory experience say those that want to incorporate a RISTRA OPO in a remote sensing platform where the alternative might be a commercial solid state laser deploying the RISTRA may pose a significant challenge Understanding this device from a more fundamental perspective should make that task less intimidating AS Photonics distributes numerical models to aid in designing mirror sets for the RISTRA cavity and for selecting the appropriate nonlinear crystal including its length This is the subject of section 3 AS Photonics can also provide assistance with modeling and cavity design including suggesting ven dors for mirror sets crystals and for custom intra cavity waveplates Feel free to contact us if you have any questions or feel you need assistance Unfortunately we cannot provide services for prototyping and laboratory validation We are also not currently equipped for delivering breadboard or brass board optical assemblies however we may be able to suggest others that can provide these services Export control restrictions The RISTRA can be exported to most EU and NATO member countries and a few others this includes Australia Japan South Korea New Zealand Sweden and Switzerland among others with a few restrictions For most other countries an export license must be obtained This takes time and is available only for certain unrestricted end use applications AS Photonics LLC
26. IR crystal ZGP pumped at 2050 nm by a laser with spatial and temporal characteristics typical of Q switched diode pumped Ho YLF As we noted in subsection 3 2 the single frequency RISTRA models can provide accurate predictions for pump lasers that oscillate on multiple longitudinal modes such as unseeded Nd YAG Experience suggests this remains true for Ho YLF even though its spectral bandwidth can be much greater than that of Nd YAG 9 10 While pump bandwidth is an important consideration of perhaps greater importance in our final example is the dependence of OPO performance on crystal length especially when derr is large as it is for ZGP We ve selected Asienal 3800 nm where the mixing is 2050 0 3800 e 4451 e with ps8 11 22 mrad pide 11 23 mrad and de 76 5 pm V The polarizations are not ideal but for these wavelengths it s the only mixing available in ZGP We selected these wavelengths because they are transmitted by sapphire which facilitates fabrication of custom waveplates and because the idler wave length may be useful for applications such as IR countermeasures IRCM We begin with a one crystal design with the initial model inputs shown in Figure 12 Note that the inputs in subsection 3 4 and subsection 3 5 were typical for OPOs pumped by com mercial research grade Q switched Nd YAG lasers with rep rates of 10 30 Hz The ZGP OPO we re modeling here uses lower pulse energy and a longer pump pulse of 30 ns which
27. OPO cavity designs increasing the ratio of beam diameter to cavity length reduces beam quality so we are confronted with a fundamental problem how do we accommodate large diameter pump beams without destroying beam quality A good solution is to use an advanced OPO cavity design such as the image rotating RISTRA 1 1 Fresnel numbers and the operating principles of the RISTRA OPO Good beam quality is obtained from any nanosecond OPO if the pump beam diameter is sufficiently small relative to the round trip length of the cavity This condition is conveniently expressed by the cavity Fresnel number D AL where D is the pump beam diameter the resonated wavelength and L the round trip length of the cavity Physically provides a measure of diffractive coupling across the transverse dimensions of the beam with F 1 indicating the cavity supports a single lowest order spatial mode resulting in a beam profile similar to the lowest order Gaussian from a high quality HeNe laser Unfortunately in nanosecond OPOs F 1 is almost impossible to realize unless the pulse energy is very low otherwise the peak optical power might exceed the damage thresholds for optics and crystals in the cavitv Therefore beam diameters are relatively large so that F gt 30 is common with the result that beam quality can be significantly diminished even for pulse energies of only a few mJ For output energies gt 100 mJ maintaining the pump fluence significa
28. P1 or WP2 is required and can be a single wavelength plate Because nonplanar cavities rotate polarization and support right and left circularly polarized resonances WP1 and WP2 are required to maintain linear polarizations parallel to the eigen polarizations of the crystals See text for additional details is again p polarized M2 would be the output coupler for the p polarized 800 nm signal e wave in C1 If instead we chose type II BBO with 532 e 800 o 1588 e then we should achieve better input coupling through M2 where the pump is p polarized e wave in C1 so that the pump beam exits through M3 with M1 the output coupler where the signal is p polarized o wave in C1 While the relationship between phase matching and the direction of propagation is an important one any cavity design should also consider that dielectric coatings are generally easier to make when they reflect a short s polarized wave and transmit a longer p polarized wave For phase matching available with some crystals this condition can t always be met but when possible it can make the task of the optical coater easier because it allows relaxed specifications for reflection and transmission at the idler wavelength The cavity mirror coating designs in Appendix C incorporate these ideas 2 1 Insensitivitv to tilt of cavity mirrors In addition to offering good beam quality when F is large the RISTRA OPO cavity also possesses the useful property that its cavity mir
29. PZT assembly described below when the PZT is attached at the location of M3 Polarizing beam splitter cube holder See Fig 27a A diagnostic tool for two crystal RISTRA cavi ties with the propagation direction MI M2 M3 M4A that simplifies adjusting the orientation of the intra cavity waveplates to achieve high linear polarization purity Attaches temporarily to the RISTRA cylindrical body near M4 PZT assembly for single frequency oscillation See Fig 27c If you need to stabilize the OPO output to a specific wavelength then you ll need to lock the OPO cavity to a seed laser This can be done using a PZT assembly that can be attached at the location of M3 or M4 as shown The PZT is a low voltage stack with midpoint voltage of 50V and total displacement of 15 um The cavity mirror must be glued to the PZT assembly with precision alignment AS Photonics offers this service with the purchase of the PZT assembly Cavity mirror installation jig See Fig 17 Cavity mirrors for the RISTRA OPO can be expensive so don t risk damaging them during installation This jig holds the cylindrical body so that the plane for the cavity mirror is horizontal Having both hands free facilitates placement of the cavity mirrors and retaining rings to reduce the risk of scratching a mirror during installation This jig is included in all RISTRA purchases at no extra cost Bearing retainer spanner tool See Fig 28 When assembling the rotation housing a smal
30. STRA the temperature rise AT at the center of a beam of radius r is roughly equal to apm 2 0 Kd where Q L is the time averaged heat absorbed per unit length and K is the thermal conductivity There will be a thermal lens created by the thermo optic effect which has a focal length of roughly 2 1 FL 2mr K Q k dn L L dT AS Photonics LLC 22 RISTRA OPO user s guide Because the cooling is asymmetric there will also be a thermally induced beam tilt at an angle Q L dn 2Kd dT Note that all three effects are inversely proportional to K For crystals K is a tensor with two principal values for uniaxial crvstals or three principal values for biaxial crvstals The values of K vary by a large amount from crystal to crystal but the multiple principal values are approximately equal for any particular crystal Figs 16a and 16b show the principal values for several popular nonlinear crystals Conductivities vary from 46 W m K for GaAs to less than 2 W m K for several crystals A high thermal conductivity does not ensure minimized thermal effects because thermal lensing and tilt are inversely proportional to the thermo optic coefficient dn dT This coefficient also varies by large amount among crystals and it depends on wavelength Its value can be computed using the SNLO function Ref Ind by computing the refractive index for the chosen wavelength at two different temperatures 4 2 RISTRA specific Thermal tilt is not usually
31. User s manual and tutorial for the image rotating RISTRA OPO AS Photonics LLC www as photonics com June 28 2011 Figure 1 Dual crystal RISTRA OPO Contents Laser Safety iv Warranty iv How to use this manual v Export control restrictions v 1 Background material 1 1 1 Fresnel numbers and the operating principles of the RISTRA OPO 1 2 Physical characteristics of the RISTRA cavity 5 2 1 Insensitivity to tilt of cavity mirrors 4 3 21 Ga eS eR Se eS 7 2 2 Modes of the RISTRA cavity A i e d 8 3 Modeling performance of the RISTRA OPO using SNLO 8 3 1 Selection of nonlinear crystals 14 ali i b tA Babu Sb tA 9 3 2 Getting started with the RISTRA models Capabilities limitations and a brief descrip tion of input parameters 2 3 Bea Gol ee tien eg Oe ee ee eh ee RS 10 3 3 Basic modeling concepts How to optimize performance of nanosecond OPOs 12 3 4 Model results for Apump 532 nm Asignal 800 nm for KTP BBO and BiBO 14 3 5 Model results for Apump 1064 nm Asionat 1550 nm for KTA 14 3 6 Model results for Apump 2050 nm Asionai 3800 nm forZGP 15 4 Thermal effects 22 Al Introduction 24 e ce eo s tis ee b 33 paea d Eee ES ES Eee Ee es 22 4 2 ORISTRA specifi 3 is in tai ka aate kal a ee a ae le Se lel oes 23 4 3 Higher order thermal effects 3 38 4a ed ace aa od ene Ge i Gc Gee eo Geen eG eo YG 24 5 Assembling the RISTRA OPO 24 5 1 Ins
32. age in our design we keep things simple to allow for quick comparisons to other configurations In addition we ve set all crystal loss to zero which is valid for these wavelengths in KTP However loss should be included when it s gt 10 per cm of crystal Use Beer s law I Ine 1 where L is the crystal length to obtain 1 mm for inclusion in the model You ll also notice there are only two mirrors This is because the model assumes 100 reflection on the other two so if we later refine our inputs we need to take R lt 100 into account In this example where the left mirror is a high reflector at 800 nm with Rjert 0 99 and the right mirror the output coupler with R ight 0 70 we would accommodate two additional R 0 99 reflectors at 800 nm by resetting Rie 0 99 9 or x 0 97 The RISTRA model has five inputs for pump signal and idler energies three left and two right In this example we set both idler energies and the left signal to tiny values of 10 while the right signal is set to 1 mW for injection seeding and the pump set to 100 mJ The pulse duration for the signal seed beam is 0 because it is cw although the model also accommodates pulsed injection seeding by setting the seed duration to a nonzero value and by using an appropriate pulse delay Note that the 10 ns duration of the non resonant idler exceeds the pump duration by 2 ns This extends the calculation time so the tail of the signal
33. al alignment of the seed beam to the RISTRA cavity may seem a bit daunting but you ll find it s relatively straight forward When the seed beam is initially injected dithering the seed laser frequency or equivalently the OPO cavity length will reveal a resonated wave that usually has little resemblance to a cavity mode The spatial profile for leakage from a high reflector may consist of an interference pattern containing tens to hundreds of spots At this point focusing the light onto a detector and looking for cavity fringes on an oscilloscope in xy mode is usually of little use The best way to proceed is to view the weak leakage using a beam profiler and walk in the seed beam until the number of spots diminishes and modes begin to appear A lens can then be used to focus the light onto the active area of a detector typically a Si or InGaAs PIN photodiode and look for fringes on an oscilloscope Adjustments will continue until the fringe pattern observed on the scope consists entirely of the one time around mode described in subsection 2 2 Initial alignment can be simplified using the seed beam propagation direction indicated in Figure 22 for a beam injected through the output coupler M2 that circulates in the direction MI M2 M3 MA and Figure 23 shows typical optical setups for seeding one and two crystal RISTRA OPOs Figure 24 shows typical evolution of the seed beam s spatial mode from initial to final alignment and Figure 25 show
34. al profile duration 10 ns AS Photonics LLC 41 RISTRA OPO user s guide Seed beam Pulsed 400 uJ 6 7 mm diameter flat topped spatial profile Output Maximum 803 nm energy not known maximum UV energy 2 140 mJ Pump depletion Not accurately measured Conversion efficiency 532 to 320 approximately 33 Beam quality Not known for 803 nm poor for UV References SPIE 5710 1 8 2005 IEEE J Sel Topics Quantum Electron 13 721 731 2007 Comments Pump passes through SFG crystal first Conversion efficiency and beam quality degraded by two photon absorptive heating in BBO SFG crystal Extra cavity SFG to gener ate 320 nm was also demonstrated using the OPO described in Opt Lett 31 380 382 2006 Using post amplification of the OPO s 803 nm beam to an energy of 100 mJ then SFG with additional 532 nm pump 320 nm energy reached 190 mJ Extra cavity SFG conversion efficiency also suffered from absorptive heating in BBO Application Demonstration of high energy eye safe source at 1550 nm Configuration Two crystal 1550 nm resonant with cw injection seeding Mixing 1064 o 1550 0 3993 4 e Crystals Aperture 10 x 10 mm length 17 mm xz cut KTA 0 79 6 0 Output coupler R 0 7 at 1550 R gt 0 98 at 1064 R 0 04 at 1576 4 Waveplates Two Custom multi order double 4 2 for 1064 and 1550 Pump beam Injection seeded Nd YAG 1 approximately 2nd order super Gaussian with 5 mm 1 e diameter duration
35. all mixing pan and sharp tipped tool for applying epoxy For two crystal cav ities a film polarizer lined paper and a microscope slide for determining the direction of bire fringent walkoff in a crystal Optional fiber tipped tweezers such as Techni Tool 758TW0304 powder free nitrile gloves reading glasses or jeweler s loupe e Materials required Crystal s Low outgassing optical epoxy such as opto packaging epoxies from Epoxy Technology If thermal conductivity is not a concern crystals are glued to rotation platters in the rotation assemblies using high quality low outgassing optical epoxy As shown in Figure 18 the surface of a rotation platter is cut with small trapezoidal shaped flutes to retain glue and form a strong bond Nonlinear crystals are almost always prepared with frosty sides and this is obviously a requirement for attaching them using epoxy Crystals should be glued to the platter near the cen ter and not on each end to reduce the chance of induc ing stress in the crystal For the 10 x 10 x 15 mm in Figure 18 glue would be applied to the two flutes in the center of the platter In principle high quality opti cal epoxies will shrink very little during curing but it is still a good idea to avoid inducing unwanted stress Al ways apply just the minimum amount of glue required to form a strong bond Excess glue serves no purpose Figure 18 Rotation assembly A very small screwdriver or other sharp tipped tool su
36. avity can produce even better beam quality We ll now test performance of the xz cut at 0 41 6 where the mixing is 1064 0 1550 e 3393 o ps8 45 00 mrad and deff 2 00 pm V Changing the inputs for walkoff derf refractive indices and adding 0 35 mm of pump beam offset we find the signal energy drops to 42 2 mJ How ever M and Mi improve to 1 56 and 1 55 respectively We increase the length of C1 to 17 mm to compensate for the smaller deg and also increase Re l to 0 75 and obtain a signal energy of 79 4 mJ We note that R 0 75 is relatively low output coupling for a nanosecond OPO and although we observe no back conversion we won t further increase Recht l We also locate the 1 mJ oscillation threshold and find it occurs with pump energy slightly less than 160 mJ For the higher signal energy M2 and Mi increase slightiv to 1 74 and 1 72 respectivelv While the beam qualitv is impressive the low conver sion efficiencv and high threshold are clearlv undesirable so for most applications we can tolerate a reduction in beam quality as suggested in subsection 3 1 Note however that a real pump beam with wavefront aberrations and a lumpy spatial irradiance profile will most likely result in lower beam quality than the model suggested for the xz cut of KTA at 9 79 6 3 6 Model results for Apump 2050 nm Asional 3800 nm for ZGP As a final example we ll model one and two crystal RISTRA OPOs using the
37. can be locked to the cavity and the PZT s are not needed In that case the servo amplifier sends a correction signal to the laser controller instead of to the cavity mirror PZT In either case we prefer to generate an error signal by modulating the laser frequency rather than slowly dithering the cavity length See Figure 26 for information on electrical components required for cavity stabilization and see text for additional details RF radio frequency LO local oscillator 2 are half wave retardation plates AS Photonics LLC 35 RISTRA OPO user s guide Figure 24 Weak leakage of the resonated wave through a high reflecting mirror of the RISTRA cavity during seed beam alignment In these examples the laser frequency or the cavity length is being swept over at least one free spectral range so that a and b represent snapshots of patterns that otherwise evolve in time a During initial alignment image rotating cavities can produce very complicated patterns b As the seed beam is walked in patterns suggesting actual cavity modes begin to appear At this point the light can collected with a lens and focussed on a detector to observe fringes on an oscilloscope c Well aligned seed beam exiting the cavity through a high reflector The upper right corner in a c contains a weak secondary reflection c 2 w i o L Four times Four times Four times Two times Figure 25 Tvpical fringes obs
38. ce profiles in Figure 2 and Figure 3 suggest image rotation is very effective for im proving beam quality in nanosecond OPOs especially when is large and the pump beam quality is less than ideal However the effectiveness of image rotation is influenced by several parameters and is therefore not the same under all operating conditions The requirements for image rotation to work effectively are that the ratio of walkoff displacement to beam diameter where walkoff displacement crystal length x walkoff angle and the ratio of the pump pulse duration to cavity round trip time together provide enough time for the diffractive clean up process to effectively fill in the entire beam profile during the pump pulse Beam quality also depends on the polarizations of the mixing waves where ideally the polarization of the resonated wave is orthogonal to the polarization of the pump and unresonated wave For the RISTRA OPO with cavity length of 109 mm typical good operat ing parameters for a 6 mm diameter pump beam might be a crystal length of 15 mm with birefringent AS Photonics LLC 2 RISTRA OPO user s guide 2 O 1 2 2 1 O 1 2 X mm X mm Figure 2 a Far field fluence profile for a three mirror ring cavity OPO with Asig 800 nm and F 33 Oj and 0 are far field angles where is the direction of walkoff b Same as a with intra cavitv Dove prism for 90 image rotation c Same as a with F 200 d Far field flue
39. ce the hazard posed by a high power pump beam that exits through M4 AS Photonics offers a beam dump assembly that attaches to the RISTRA body that blocks this beam This assembly also incorporates a mirror that redirects beams exiting through mirror 4 so that they propagate parallel to the optical table One use for this mirror is to separate the resonated wave from the pump beam to observe cavity fringes for a lock signal when a PZT mirror assembly is attached at the location of M3 See Appendix D for this assembly and other accessories 6 2 Pump beam delivery and alignment Aligning the pump beam to the RISTRA cavity is relatively simple but unlike a typical open cavity the pump must propagate within the round bores of the RISTRA s cylindrical body Given it s a solid chunk of metal with apertures formed by mirror retaining rings and waveplate holders it s important to begin alignment using very low pulse energy A misaligned high energy beam that strikes metal and forms a plasma can damage expensive optics and crystals Some high energy Q switched lasers suitable for AS Photonics LLC 31 RISTRA OPO user s guide pumping the RISTRA offer pulse energy control but if this is not available we suggest the optical setup shown in Figure 21 HR 45 Thin film polarizer Input coupler Pump laser Figure 21 Alignment of the pump beam If your pump laser lacks energy control use a half waveplate and thin film polarizer to attenuate
40. ch as a scribe works well for applying small drops of epoxy Note that the nominal aperture size for crystals used in the RISTRA is 10 x 10 mm so they can usually be handled without tweezers However it is a good idea to wear powder free nitrile gloves For two crystal cavities it is usually a good idea to orient the crystals so the direction of birefringent walkoff is reversed in C1 and C2 This will improve efficiency by increasing spatial overlap of the pump and resonated waves and if the resonated wave is e polarized obtain propagation closer to the geometric central ray of the cavity Figure 19 shows one of two orientations for reversing walkoff and includes a diagram and instructions for determining the direction of birefringent walkoff in a crystal 5 3 Installation of intra cavity waveplates e Tools required A needle for applying glue fiber tipped tweezers such as Techni Tool 758TW 0304 UV source for curing UV glue blower duster preferably N2 CO2 or rubber bulb air blower Optional powder free nitrile gloves reading glasses or jeweler s loupe e Materials required Waveplate s UV curing glue lBecause the cavity is singly resonant and there are two mirror reflections between crystals we can ignore the orientation of the d tensor in the second crystal as discussed in Ref 15 AS Photonics LLC 27 RISTRA OPO user s guide b A Film polarizer Lines viewed through Birefringent crystal crystal
41. ctivity The gradient depends weakly on the net temperature rise at the beam center The cooling conditions can however strongly affect the disruption of phase matching along the length of the crystal The thermal conductivity of the aluminum crystal mounting plate is 250 W m K or about 100 times the conductivity of most nonlinear crystals Assuming good thermal contact with the mounting plate and good conductivity of heat out of the mounting plate the temperature rise at the beam center is determined primarily by the crystal conductivity and absorptivity as expressed above in the introduction As a rough estimate with good thermal contact between the plate and crystal and between the rotation shaft and its housing the temperature rise of the crystal where it contacts the mounting plate is 0 7 K per watt absorbed Thermal contact between the plate and crystal can be improved by using a thermal paste or a thermal pad between crystal and plate Pads and pastes typically have conductivities in the range 1 10 W m K and because they are only 100 microns or so thick their thermal resistance is negligible compared with the crystal resistance AS Photonics offers thermal pads AS Photonics LLC 23 RISTRA OPO user s guide and thermal pastes suitable for crystal mounting Heat removal from the RISTRA crystal mounting plate is by conduction along the rotating shaft holding the plate This shaft is designed with a small clearance to the body of the rota
42. ctrum that works well for phase sensitive detection although it will contain a small amount of residual AM RAM Following demodulation the DC offset that occurs from RAM is easily eliminated by adjusting the DC input offset on most servo amplifiers Regardless of how the seed laser remains resonant with the OPO cavity adjust the seed frequency or adjust the cavity length we recommend using phase sensitive detection for stabilization This technique is robust simple to implement and electrical components such as balanced mixers voltage controlled oscillators VCOs and low pass filters are available in convenient coaxial packages that are small and inexpensive An expensive lock in amplifier is not necessary Figure 26 shows a block diagram using discrete components for cavity length stabilization using phase sensitive detection As discussed in a previous footnote this is sometimes referred to as PDH stabilization but in our examples we re assuming the low amplitude first order modulation sidebands modulation index lt 1 lie within the OPO s cavity resonance For true PDH stabilization the modulation frequency exceeds the cavity resonance width but will be less than the cavity s free spectral range Applying this technique to the RISTRA is unnecessary and might unintentionally excite the RISTRA s vortex modes as described in subsection 2 2 After the OPO can be locked and remain locked barring major perturbations such as pou
43. e KTP KTA and LiNbO but we reject LiNbO because the cut with deg 4 01 pm V at 90 47 0 phase matches with 1064 e 1550 o 3393 0 while the cut for 9 58 6 phase matches with 1064 e 1550 0 3393 e but has deff 0 447 pm V which is impractically small Unfortunately KTP is also unacceptable because transmission at 3393 nm is approximately 57 for a length of 10 mm or in terms of Beer s law the absorption coefficient 0 057 mm That leaves KTA but as shown from the QMIX output in Figure 7 xz cut KTA offers two relatively poor choices which we men tioned previously in subsection 1 1 For the desirable oeo mixing with 1064 0 1550 e 3393 0 at 0 41 6 derr is only 2 0 pm V which is smaller than we d like for pumping at 1064 nm For mixing with 1064 0 1550 o 3393 e at 0 79 67 deff 3 12 pm V which is adequate for a 1064 nm pump wavelength but the pump and signal share o polarization so the OPO s signal beam quality will be more dependent on the beam quality of the pump This is particularly true given p is only 14 88 mrad Nonetheless the lower oscillation threshold and higher conversion efficiency force us to choose ove mixing even though we are also choosing lower beam quality We ll validate this choice from model results in subsection 3 5 3 2 Getting started with the RISTRA models Capabilities limitations and a brief description of input parameters Before we test our crystal
44. e polarization component as the light exits along the path M1 gt M2 Rotate the waveplate as necessary to extinguish or at least minimize the unwanted s or p polarization component For a two crystal cavity Install M3 alone and the wavelplate between the locations of M2 and M3 Install the polarizing beam splitter cube holder described in Appendix D with a PBS cube in place to extinguish one polarization component Inject linearly polarized laser light at the resonant wavelength through the opening for M2 along the path M2 M3 It can be s or p polarized but will preferably have the polarization of the resonated wave as it leaves Cl in lower leg Rotate the waveplate between M2 and M3 as necessary to extinguish or at least minimize the unwanted polarization component for light exiting through the hole for M4 Now install M4 and MI and the waveplate between M4 and MI After aligning the beam to the cavity use a polarizer aligned to transmit one polarization component as the light exits along the path M1 gt M2 Rotate the waveplate between M4 and M1 as necessary to extinguish or at least minimize the unwanted polarization component e Lock the adjuster s for orientation angle and remove the waveplate s If you were able to adequately adjust the orientation of the wavelplate s then add two more very small drops of UV glue and cure the glue If you didn t have a test laser avai
45. e specifications 2 2 0 2 0 0 000022 eee 40 AS Photonics LLC lii RISTRA OPO user s guide Laser Safety Exposure to the laser radiation emitted by the RISTRA optical parametric oscillator OPO and by the class 3B and class 4 lasers used to pump the RISTRA OPO poses a significant ocular hazard Owners of these laser systems and of OPOs like the RISTRA bear the responsibility of providing appropriate safeguards for the users of high power laser systems within their facility They are also responsible for implementing safety programs to adequately control the hazards associated with laser use In the United States the accepted governing standard for laser safety is ANSI Z136 1 The Safe Use of Lasers Outside of the United States international standards such as IEC 60825 14 Safety of Laser Products are commonly used It is strongly recommended that laser owners follow the governing standard in their country to ensure regulatory compliance and to provide safety programs to protect their employees their facilities and the public Additional resources for laser safety are available from the Laser Institute of America e http webstore ansi org search for Z136 1 e http www laserinstitute org Warranty AS Photonics LLC warrants to the end user customer that the RISTRA products will be free from defects in materials and workmanship under normal use and service for a period of one 1 year from the date of original purchase The
46. end user assumes responsibility for the burning of eyeballs crystals mirrors or waveplates AS Photonics LLC iv RISTRA OPO user s guide How to use this manual This document is a combination of a manual and a tutorial Like any manual it contains instructions on how to assemble and use the RISTRA OPO and it also includes appendices with physical dimen sions example performance specifications and example cavity mirror coating specifications It also describes some useful accessories for assembling and working with the RISTRA cavity If you are al ready well acquainted with the operation of nonplanar image rotating OPOs such as the RISTRA then you can devote your attention to section 5 amp section 6 and Appendices A D that refer to assembly and accessories If you are like most users and aren t well acquainted with the RISTRA OPO then you should consider reading Sections 1 3 before working with this device These sections describe the theory of operation of the RISTRA OPO provide guidance on when use of the RISTRA is advantageous and introduce the power and utility of numerical modeling We ve included these subjects because unlike many laser products the RISTRA is delivered in a form that may require the user to specify and order its optical components and nonlinear crystals and carry out assembly installation and initial alignment For experienced workers in the field of crystal nonlinear optics these tasks may pose no challenge
47. ere to install the waveplate because the two wavelplate holders have different lengths For most applications the waveplate will be located AS Photonics LLC 28 RISTRA OPO user s guide Figure 20 Intra cavity waveplate holder Note the markings at 0 and 22 57 between M2 and M3 WPI in Figure 5 However sometimes it makes sense to put it between M4 and M1 at the location of WP2 For example in a 2 4m pumped ZGP RISTRA with a resonated wavelength 3 7 um and idler wavelength 4 5 um with propagation direction MI gt M2 M3 M4 you could eliminate idler absorption in a waveplate made from sapphire by placing it between M4 and M1 In choosing this location we re assuming the cavity is singly resonant and therefore any remaining idler is weak following three mirror reflections a reliable assumption We re also assuming identical s and p reflective phase shifts on M3 and MA as described in section 2 If for some reason M3 and M4 are not identical mirrors it s OK to use two waveplates oriented at 0 in a one crystal cavity The following procedure is suggested for installing the waveplates Note that steps for optimizing linear polarization purity can be omitted if a suitable test laser is unavailable Although less rigor ous optimization of waveplate orientation is possible by monitoring output energy near the oscillation threshold e Place a waveplate holder on clean flat surface and gently drop in a waveplate using fiber
48. erved during alignment of the seed laser to the RISTRA cavitv When the spatial mode pattern in Figure 24 b is initiallv focused onto a detector the corresponding fringe pattern not shown might initially resemble a sine wave but following small adjustments will begin to display recognizable fringes a The cavity is poorly aligned with the two and four times modes described in subsection 2 2 easily observed b Cavity alignment is better but the amplitude of the red and blue shifted four times modes is still too large c The cavity is well aligned so that only one time modes are observed Note that there may always be a small amount of coupling of the seed laser to the off axis modes but this will not result in any significant admixture to the one time mode once the cavity it locked because they are non degenerate in frequency with shifts 0 25 free spectral range AS Photonics LLC 36 RISTRA OPO user s guide assembly that attaches to the RISTRA s cylindrical body using the threaded holes for a mirror retaining ring Figure 26 illustrates locking the OPO to the laser where a servo amplifier sends a correction sig nal to a PZT To lock the laser to the OPO the servo amplifier would instead supply a correction signal to the laser controller For either locking method we prefer modulating the laser light rather than slowly dithering a cavity mirror For example modulating the current in semiconductor lasers can produce a phase modulated seed spe
49. final crystal angle adjustments can be made by maximizing OPO pulse energy with the pump set just above the threshold for seeded oscillation Of course with a seed wavelength longer than about 2 um video cameras are scarce and expensive so the initial search for Ak 0 may be restricted to observing OPO pulse energy and may therefore be more tedious A few final comments are in order Even if you have experience with injection seeding of nanosec ond OPOs or with alignment of stable build up cavities the geometry of the RISTRA can present a few 47f your OPO contains two crystals you will likely observe three wavelengths The seed laser and one wavelength for each crystal How may wavelengths you observe and how easily they can be observed will depend on how hard the OPO is pumped AS Photonics LLC 37 RISTRA OPO user s guide Seed beam Optical Seed laser DFB or DBR isolator amplifier Lo pass filter xy oscilloscope Figure 26 Block diagram showing discrete inexpensive electronics for cavity length stabilization using phase sensitive detection This example uses a distributed feedback DFB or distributed Bragg reflector DBR semiconductor laser as the seed source When available these lasers are convenient because their frequency stability and linewidth are adequate for seeding low finesse OPO cavities and their packaging sometimes includes SMPM fiber pigtails with FC APC connectors Optical phase modulation i
50. for error correction then this is true PDH The high bandwidth is usually unnecessary for stabilizing ns OPOs and sidebands outside the one time around resonance of the RISTRA may cause problems as they could couple to the vortex modes described in subsection 2 2 AS Photonics LLC 34 RISTRA OPO user s guide Cavity mode Figure 22 Alignment of the injection seeding beam through cavity mirror M2 for propagation in the direction M1 M2 M3 gt M4 Begin by aligning the seed beam parallel to the flat surface of the RISTRA assembly s base plate Then use a protractor to set the incident angle on M2 as shown This is acommon configuration for one and two crystal cavities If the direction of propgation is reversed M4 M3 M2 M1 with MI the output coupler and and M2 the input coupler it s easiest to inject the seed beam through M3 Approximate initial alignment helps reduce the complexity of the interference pattern shown in Figure 24 a Pump a beam out Figure 23 Two typical optical setups for injection seeding the RISTRA OPO cavity a One crystal cavity b Two crystal cavity The nonplanar RISTRA cavity is projected onto the page as a rectangle with mirrors M1 M4 indicated In these examples we re assuming a spectroscopic application where the OPO cavity must be stabilized to a specific seed wavelength and therefore requires piezo electric transducers PZTs If an exact wavelength is not required the seed laser
51. g resonator Appl Opt 42 3550 3554 2003 7 A V Smith and D J Armstrong Generation of vortex beams by an image rotating optical parametric oscillator Opt Express 11 868 873 2003 8 G Anstett M Nittman and R Wallenstein Experimental investigation and numerical simula tion of the spatio temporal dynamics of the light pulses in nanosecond optical parametric oscilla tors Appl Phys B 79 305 313 2004 9 A Dergachev D Armstrong A Smith T Drake and M Dubois 3 4 um ZGP RISTRA nanosecond optical parametric oscillator pumped by a 2 05 um Ho YLF MOPA system Opt Express 15 14404 14413 2007 10 A Dergachev D Armstrong A Smith T Drake M Dubois High power high energy ZGP OPA pumped by a 2 05 um Ho YLF MOPA system Proceedings of SPIE 6875 6 2008 11 R W P Drever J L Hall F V Kowalski J Hough G M Ford A J Munley and H Ward Laser phase and frequency stabilization using an optical resonator Appl Phys B 31 97 105 1983 12 E D Black An introduction to Pound Drever Hall laser frequency stabilization Am J Phys 69 79 87 2001 13 D J Armstrong and A V Smith 90 pump depletion and good beam quality in a pulse injection seeded nanosecond optical parametric oscillator Opt Lett 31 380 382 2006 14 D J Armstrong and A V Smith All Solid State High Efficiency Tunable UV Source for Air borne or Sa
52. gh accuracy but use somewhat idealized reflectivity values for the pump and un resonated wave Typically in nanosecond OPOs the middle wavelength is resonated so actual specifications for the idler can be relaxed because it has the longest wavelength and will generally achieve high transmission through thin coatings de signed for the two blue waves What we want is a cavity that is singly resonant and with four mirror reflections that can be achieved rather easily for the un resonated wave Consequently mirror specifi cations should give the thin film coater some room to adjust the design to achieve the best performance while retaining adequately high optical damage thresholds Both of the example specifications shown below for a 532 nm pumped KTP RISTRA cavity are similar to those used in Ref 5 and they reflect this concept where Rigier lt 4 although some of the values for Rpump and Rsignal May be optimistic In AS Photonics LLC 43 RISTRA OPO user s guide contrast the model inputs in section 3 assume Rigier 0 but even if we had set it to 1 this is lower than required in a four mirror cavity and for some coating designs may be difficult to achieve Note that for the one crystal specifications M3 is identical to M4 with side 1 unpolarized and to reduce the number of coating runs we use the AR coating from side 2 of M2 on side 2 of M3 and M4 We begin with specifications for the mirror substrates followed by required damage thresho
53. guide and the power of modeling at our disposal we choose the length of Cl to be shorter than C2 After a few trial runs we achieve pump depletion of 72 with signal energy of 98 7 mJ using crystal lengths of 7 mm followed by 14 mm The pulse temporal profiles exiting the right and left mirrors are shown in Figure 11 You may be wondering if it s wise to use two crystals with such different lengths It depends on polarization beam diameter and the size of the walkoff angle p When circulating an e wave like we re doing here the unequally compensated walkoff displacement might cause problems especially if a large diameter beam clips one of several apertures in the RISTRA assembly It could also be bothersome for injection seeding that is if you re expecting a high degree of symmetry when using two crystals because the unequal walkoff displacement in the two long legs of the cavity will result in a cavity mode that s displaced from the geometric central ray of the cavity For small diameter beams or when circulating an o wave the unequal walkoff displacement might be ignored However a large diameter e polarized pump beam could also potentially clip one of the apertures in the RISTRA assembly Of course any one crystal design that circulates an e wave suffers similar problems regardless but it s worth keeping this issue in mind when you design an OPO AS Photonics LLC 13 RISTRA OPO user s guide 3 4 Model results for Apump 532
54. hich can t be cleaned with common solvents such as methanol or acetone will ruin your day The RISTRA is delivered fully assembled minus the optical components Final assembly requires attaching mirrors and gluing crystals and waveplates in place Procedures for carrying out these steps are described below Note that the order of assembly given here is optional and should be changed to accommodate your particular application It may help to read through subsection 5 1 subsection 5 3 before you begin assembly to determine what will work best The optical components must adhere to the specifications outlined in Appendix A An example of a mirror set is listed in Appendix C 5 1 Installation of cavity mirrors e Tools required Small screwdriver for 0 80 shoulder bolts fiber tipped tweezers such as Techni Tool 758TW0304 cavity mirror installation jig see Figure 17 blower duster preferably N2 CO or rubber bulb air blower Optional powder free nitrile gloves reading glasses or jew eler s loupe e Materials required Cavity mirrors The cavity mirrors are attached to flat faces on the RISTRA s cylindrical body by spring loaded retaining rings where three points on the rings align with three points on each face to define a plane Without a ring to hold it in place a mirror will slide off the face unless the face is horizontal When assembled and mounted on its baseplate none of the RISTRA s mirrors are horizontal so the eas
55. iest AS Photonics LLC 24 RISTRA OPO user s guide Biaxial Crystals Uniaxial Crystals zr CBO IT LNB p KDP jaq CAAS A CSP GOO KNBO3 R Leo AGS BiBO m AGSE q 0 0 5 1 1 5 2 2 5 3 3 5 4 0 10 20 30 40 Thermal Conductivity W m k Thermal Conductivity W m K a b Figure 16 Thermal conductivities along principal axes for a biaxial and b uniaxial crystals Figure 17 Cavity mirror installation jig 50 AS Photonics LLC 25 RISTRA OPO user s guide way to attach the mirrors is to first remove the crystal rotation assemblies from the cylindrical body and mount it on the cavity mirror installation jig described in Appendix D To attach the mirrors follow these steps e Remove the 0 80 shoulder bolts that hold the retaining rings to the cylinder Be careful handling the springs as they are easily lost e Decide which faces will hold M1 M4 and mark the cylinder with a pencil if necessary You should also use a hard pencil to mark the sides of all of your mirrors with M1 M4 for future reference e Mount the cylinder to the jig To do so insert the dowel pin into one of the holes on the jig s plate and secure it with 2 56 screw in the other hole Because the mirror retaining rings overhang the flat face on the side with the shorter waveplate holder assembly and prevent the cylinder from mounting flush on the mirror jig the mirrors should be mounted to the longer side first e Use tweezers to
56. ignal for the seed laser might also indicate maximum talon transmission It will probably be easiest to optimize transmission while scanning the seed laser frequency or cavity length with the servo loop open 6 Use of the RISTRA OPO In subsection 6 1 subsection 6 3 we describe how to safely use the RISTRA OPO and how to carry out initial optical alignment We also describe in detail how to injection seed the RISTRA for applications requiring single frequency oscillation 6 1 Eye safety with nonplanar geometry Exposure to the high power laser radiation emitted by nanosecond OPOs and by the Class IV Q switched lasers that pump them poses a significant ocular hazard Unfortunately these hazards are increased by the nonplanar geometry of the RISTRA cavity in particular for one that contains two crystals If the pump wave is o polarized M4 will likely be the pump beam exit mirror for two crystal designs with M1 the input coupler For this configuration the pump beam leaves the cavity at an angle of approximately 65 with respect to horizontal and therefore posses a significant hazard Because peak intra cavity irradiance can easily exceed 100 MW cm the beam for the non resonant wave and leakage of the cavity mode through high reflecting mirrors also pose significant hazards When using the RISTRA OPO always use the utmost care to locate all stray beams exiting the cavity and use beam dumps or some form of enclosure to capture them To redu
57. in amplitude for the depleted pump in a followed by a sudden recovery This is the signature of parametric back conversion b The crystal length was decreased to 12 mm to reduce back conversion so the signal energy increased to 51 4 mJ The pump depletion is now 39 From the parameters in Figure 8 the model tells us this one crystal RISTRA OPO is unlikely obtain high conversion efficiency where pump depletion would exceed 50 The temporal profile for the undepleted pump is generated by setting deg 0 in the model as indicated in Equation 1 The plots of pulse profiles shown here are not generated by the RISTRA model but were plotted using data in the model generated file RIPWR R DAT See text for additional details Undepleted pump 173 Depleted pump Signal Undepleted pump 7 ir Depleted pump Signal Idler Power Arb Power Arb Time ns Time ns Figure 11 a OPO pulse temporal profiles at right mirror output coupler for the two crvstal RISTRA model using inputs in Figure 8 but with pump energy increased to 200 mJ and crystal lengths of 7 mm and 14 mm The signal and idler are normalized relative to the undepleted pump at the left mirror pump exit for proper power scaling b Pulse temporal profiles at left mirror The signal and idler pulse heights are exaggerated so they don t overlap the profile for the depleted pump In a note the absence of back conversion due
58. istortion due to its smaller walkoff angle Finally we know we can obtain more energy using two crystals However subsection 3 3 already discussed two crystal designs so we won t test them here 3 5 Model results for Apump 1064 nm Asional 1550 nm for KTA In subsection 3 1 we gave an example of crystal selection for a 1064 nm pumped 1550 nm eyesafe OPO that amounted to choosing the lesser of two evils The chosen crystal was KTA and the anticipated performance for its two available xz cuts forced us to compromise beam quality for higher conversion efficiency We ll now justify that choice using the RISTRA model Unfortunately conversion efficiency decreases as the pump wavelength increases and deff for 1064 nm pumped KTA is actually smaller than deff for the 532 nm pumped KTP OPO in subsection 3 4 Consequently the factor of two increase in Apump suggests we consider a two crystal design to begin with otherwise the pump fluence may be impractically high We ll begin by modeling the xz cut of KTA at 0 79 67 with 1064 0 1550 o 3393 e and deff 3 12 pm V For consistency we ll retain the 6 mm diameter 4th order super Gaussian pump beam but increase the pump energy to 300 mJ and we ll start with two 17 mm long crystals Otherwise pump pulse du ration and the injection seed power remain the same as shown in Figure 8 In our previous calculations for KTP BBO and BiBO we optimized efficiency by setting pump beam offset to a
59. ith deff 0 to accurately account for loss of pump energy due to any absorption in the crystal and due to loss from mirror and crystal coatings For example when the pump beam exits through M4 percent pump depletion would be calculated from fred 0 Ef a 0 1 Eri d T 0 Percent depletion 100 x Although a small amount of pump energy leaks through M2 in a two crystal design or M1 for reversed propagation where Roe gt 0 99 and the leakage when deff O differs from when deft 4 0 due to pump depletion in the first crystal that small difference in leakage can be safely ignored when calculating percent pump depletion Note that if the direction of propagation is reversed you must monitor EAN or Ea as appropriate Now let s test a two crystal design by running the RISTRA model Starting again with the inputs in Figure 8 we set Recht 0 99 so the pump beam continues through the second crystal and then we try various combinations of crystal lengths until we achieve the highest signal energy with lowest back conversion A simple way to think about the operation of this two crystal oscillator is we deplete just enough pump energy in the first crystal to achieve oscillation After two mirror reflections we ve rejected essentially 100 of the unresonated idler then we deplete the remaining pump energy in the second crystal to achieve maximum amplification of the signal wave with little back conversion With a little intuition as our
60. ized resonances These include nondegenerate hollow modes that are radially polarized or have hybrid radial tangential polarization and also linearly polarized filled modes with the specific mode depending on the polarization of the light injected into the cavity and the on the length of the cavity 6 When configured for use as an OPO the RISTRA supports an on axis mode that closes after one round trip of the cavity This is the dominant high gain mode with good beam quality We note that the RISTRA also supports two linearly polarized nondegenerate hollow vortex modes with charge m 1 7 Broadband oscillation can support an admixture of vortex modes for certain mixing conditions especially when F is large while injection seeded operation can be configured to obtain essentially pure m 1 vortex modes if desired These modes can be excited by laterally displacing the seed beam and tuning the seed frequency 1 4 free spectral range FSR away from the one time around mode Adding a small tilt to the seed beam will further help excite these modes and a pump beam with donut shaped spatial profile will also enhance excitation of the vortex modes Unless you are explicitly trying to excite vortex modes donut shaped pump beams are not a good choice for pumping a RISTRA OPO See subsection 6 3 for further details on injection seeding the RISTRA OPO and see Figure 25 for how to recognize cavity fringe patterns indicating the presence of the four times ar
61. l good choices let s consider a 532 nm pump to generate an 800 nm signal After examining various crystals we narrow the list to the xz cut of KTP xz cut of KTA type II BBO and the xz cut of BiBO From the QMIX output in Figure 6 we see KTP at 0 58 27 and 0 phase matches with 532 0 800 e 1588 0 and has deff 3 21 pm V and p 47 65 mrad so it appears to be a good choice KTA at 0 61 77 and 0 offers the same phase matching as KTP but has smaller p and smaller deff and it s more expensive so we won t consider it further in this example Type II BBO at 0 27 37 phase matches with 532 e 800 0 1588 e so it meets the criteria for good polarizations but deff 1 62 pm V which is small However the walk off angles p gt 2 63 03 mrad and rho gt 88 61 93 mrad are larger than for KTP so BBO may be of interest for large diameter beams and pump pulse lengths lt 8 ns Finally BiBO at 9 45 9 and 0 phase matches with 532 0 800 e 1588 0 the same polarizations as KTP has deff 2 12 pm V and p 88 94 mrad so with it s large walkoff it s also attractive for large beam diameters For lowest oscillation threshold and perhaps the highest conversion efficiency KTP may be the best choice but BBO and BiBO are definitely in the running So which crystal do we choose We ll consider a few more of their characteristics and if we still can t decide we ll compare the relative performance
62. l nut used to secure the rotation shaft in place The nut which threads into the rotation body is tightened by a spanner tool The two pins on the spanner tool fit into holes on the nut This tool is included in all RISTRA purchases at no extra cost References 1 G Hansson H Karlson and F Laurell Unstable resonator optical parametric oscillator based on quasi phasematched RbTiOAsO4 Appl Opt 40 5446 5451 2001 2 J N Farmer M S Bowers and W S Scharpf Jr High brightness eye safe optical parametric oscillator using confocal unstable resonators in Advanced Solid State Lasers M M Feyer H Injeyan and U Keller eds Vol 26 of OSA Trends in Optics and Photonics Series Optical Society of America Washington D C 1999 pp 567 571 3 A V Smith and M S Bowers Image rotating cavity designs for improved beam quality in nanosecond optical parametric oscillators J Opt Soc Am B 18 706 713 2001 4 D J Armstrong and A V Smith Demonstration of improved beam quality in an image rotating optical parametric oscillator Opt Lett 27 40 42 2002 AS Photonics LLC 49 RISTRA OPO user s guide 5 A V Smith and D J Armstrong Nanosecond optical parametric oscillator with 90 image rotation Design and performance J Opt Soc Am B 19 1801 1814 2002 6 D J Armstrong M C Phillips and A V Smith Generation of radially polarized beams with an image rotatin
63. lable for the previous steps waveplate orientation can optimized by maximizing output energy near the oscillation threshold The signature of optimized polarization will likely be difficult to observe for higher pump fluence so oscillation near threshold is strongly suggested 5 3 1 Adjusting the angle of incidence for the waveplates The waveplate holders also allow adjustment of tilt about an angle of incidence near 0 This ad justment was included because waveplates often behave like talons even though we don t want them to Because the resonated light passes through the intra cavity waveplates many times during a pump AS Photonics LLC 30 RISTRA OPO user s guide pulse a small loss associated with talon transmission can reduce the efficiency This effect can be ob served in broadly tunable cavities resulting in periodic oscillation of output energy as the wavelength is changed and it can also affect performance of single frequency oscillation For broad tuning little can be done except to get waveplates with very good anti reflection coatings However for single frequency oscillation at a fixed wavelength output energy can be optimized by adjusting the angle of incidence of the waveplates After oscillation in a single frequency RISTRA OPO has been optimized pump beam well aligned and Ak 0 as described in subsection 6 2 and subsection 6 3 try tilting the angle of incidence to see if it influences output energy The fringe s
64. lds for the optical coatings Mirror specifications are then given for two crystal and one crystal cavities Note Be sure to order mirrors which do not have beveled edges on the outward facing side SUBSTRATE SPECIFICATIONS Diameter 0 5 0 0 0 010 in 12 7 0 0 0 254 mm Thickness 0 125 0 010 in 3 175 0 254 mm Wedge lt 1 arc minute Transmitted wavefront A 10 at 633 nm over 2 80 of clear aperture Surface quality Super polish if applicable for selected substrate material Material Fused silica PUMP LASER SPECIFICATIONS AND DAMAGE THRESHOLDS Laser Nd YAG 20 A 532 nm Pulse length 10 15 ns Repetition rate 10 Hz Spa tial profile Approximately flat topped with diameter 6 mm Peak power lt 200 MW cm peak fluence lt 2 0 J cm Optical damage thresholds gt 4 J cm for A 532 nm 2 2 J cm for A 800 nm 2 2 J cm for A 1588 nm MIRROR SPECIFICATIONS FOR TWO CRYSTAL CAVITY All angles of incidence are 9 32 8 Nonlinear mixing 532 0 800 e 1588 0 Mirror 1 Input coupler Side 1 inside R lt 0 25 A 532 nm p polarization best effort R lt 0 5 acceptable R gt 99 A 800 nm s polarization R lt 4 A 1588 nm p polarization Side 2 outside R lt 0 25 A 532 nm p polarization R lt 0 25 A 800 nm s polarization R lt 4 A 1588 nm p polarization Mirror 2 Output coupler Side 1 inside R gt 99 A 532 nm s polarization AS Photonic
65. low below in the same order Table 2 Example performance specifications Configuration Mixing amp Crystal s Pump beam Output mJ two crystal 532 0 803 e 1576 4 0 6 mm dia flat top 14 at 803 nm 803 nm resonant xz cut KTP duration 10 ns injection seeded 2 10 x 10 x 15 mm intra cavitv SFG 532 0 803 e 1576 4 0 6 7 mm dia flat top 140 at 320 nm 803 nm resonant xz cut KTP 10 x 10 x 15 mm duration 10 ns injection seeded 532 0 803 e 320 e type II BBO 10 x 10 x 10 mm two crystal 1064 o 1550 0 3993 4 e 5 mm dia 2nd order 1550 nm resonant xz cut KTA super Gaussian injection seeded 2 10x 10x 17 mm duration lt 10 ns 170 at 1550 nm AS Photonics LLC 40 RISTRA OPO user s guide one crystal 2050 o 3400 e 5163 e 4 0 4 5 mm dia 1 e 10 at 3400 nm 3400 nm resonant ZGP Gaussian unseeded length 10 mm duration gt 14 ns two crystal 1064 o 1627 0 3074 8 e 6 mm dia semi 100 at 1627 nm 1627 nm resonant xz cut KTP donut profile unseeded 10x 10 x 14 mm duration lt 7 ns 10 x 10x 17 mm Application Demonstration of very high conversion efficiency and very high beam quality using pulsed injection seeding Configuration Two crystal 803 nm resonant with pulsed injection seeding self seeded Mixing 532 0 803 e 4 1576 4 o Crystals Aperture 10 x 10 mm length 15 mm xz cut KTP 0 58 4 0 Output coupler R 0 7 at 803 R gt 0
66. n a combination of o and e tilt in the upper crystal C2 6 3 Injection seeding for single frequency oscillation When a singly resonant nanosecond OPO is injection seeded by a cw laser and pumped by a single fre quency laser it can generate temporally transform limited pulses at the seed laser wavelength Despite some claims to the contrary using a broadband pump laser and an injection seeded OPO will usually not obtain a true transform limited bandwidth This is the case for a pump pulse with strong amplitude modulation a characteristic common to homogeneously broadened solid state gain media such as Nd YAG With an amplitude modulated pump the resonated spectrum will consist of a strong carrier at the seed laser frequency accompanied by weak AM sidebands For many applications the spectral nar rowing obtained for broadband pumped injection seeded oscillation will be sufficient but it s unlikely to be temporally transform limited Under these conditions the spectrum of the unresonated wave can also be narrowed slightly but it largely retains the broadband character of the pump Injection seeding a ring cavity even a nonplanar cavity like the RISTRA is relatively easy A ring geometry often allows injecting a seed beam through a partial reflector like the output coupler without inserting a beam splitter in high power beams as they leave or enter the cavity OPO cavities also circu late light at all times because they don t contain active or
67. nce for RISTRA OPO with Asig 800 nm e Small diameter Gaussian beam used to pump OPO in a and b f Low quality large diameter beam used to pump OPOs in c and d All OPOs pumped 3 4 x threshold See text for additional details AS Photonics LLC 3 RISTRA OPO user s guide a ar 2 2 bA s v v V V c c D D 3 2 L L g 2X Z e E k Fluence Arb Figure 3 Surface plots of fluence profiles in Figure 2 Note the small amplitude of the shoulders on the profiles in b and d relative to their respective peak heights See text for additional details AS Photonics LLC 4 RISTRA OPO user s guide walkoff angle gt 50 mrad and a pump pulse duration of at least 10 ns Using xz cut KTP as an example desirable phase matching conditions would include 532 0 800 e 1588 0 where the resonated wave at A 800 nm has extraordinary polarization and undergoes birefringent walkoff and the pump and unresonated wave have ordinary polarization The walkoff angle of 47 65 mrad is sufficient but is probably smaller than ideal for the large diameter of the low quality pump beam shown in Figs 2f amp 3f Another example of good phase matching for these wavelengths could be obtained from a type II BBO crystal with 532 e 800 0 1588 e where the walkoff angles for the e polarized pump and idler are 63 03 mrad and 61 93 mrad respectively Although the pump a
68. nd resonated waves in the two previous examples of good phase matching pa rameters have orthogonal polarizations this is not an absolute requirement for using the RISTRA OPO When these polarizations are parallel the beam quality of the resonated wave will be more strongly in fluenced by the beam quality of the pump but image rotation will retain its clean up capabilities For example a 1064 nm pumped RISTRA using the xz cut of the crystal KTA that resonates a 1550 nm sig nal wave will likely use 1064 0 1550 0 3393 e at 79 67 with der 3 12 pm V rather than 1064 o 1550 e 3393 0 at 0 41 6 deff 2 00 pm V even though the pump and resonated signal share the same polarization And why make this choice It s a matter of choosing increased conversion efficiency as opposed to optimum beam quality This difference in deff is not large it s big enough to substantially increase the threshold for oscillation and decrease the overall conversion efficiency This compromise is often encountered for generation of eye safe wavelengths using a pump wavelength of 1064 nm 2 Physical characteristics of the RISTRA cavity As its name implies the RISTRA geometry is derived from a rectangle that is twisted to form a non planar cavity that produces exactly 90 of image rotation The configuration for a RISTRA cav ity that contains two nonlinear crystals and two 4 2 retardation plates is shown in Figure 5 along with name conventions for
69. nding on the optical table or changing the crystal rotation angle a significant amount you must rotate the crystal s to locate Ak 0 for the seed laser wavelength This can be done by pumping just above the threshold for unseeded oscillation and observing OPO pulses using a scope and detector that each have bandwidths 200 MHz While the crystal is rotated to change the phase matching angle the OPO pulse energy will increase significantly when Ak 0 This technique works well if you know that the crystal is rotated close to the correct angle to begin with and requires only minor adjustments If instead the error in the phase matching angle is larger than a few degrees you may need to simultaneously compare the seed laser wavelength and the wavelength s for unseeded oscillation You can use a pulsed wave meter to observe the separate wavelengths but if your seed wavelength is already accurately known then a secondary absolute measurement using an expensive instrument is unnecessary A good alternative is an inexpensive grazing incidence grating followed by a lens with focal length 1 m to observe the spectrum in the far field using a video camera Each crystal angle is then rotated until its wavelength and the wavelength for the seed laser coincide Note that rotating the crystals through large angles may force the cavity out of lock so large angle tuning may be easier with an open servo loop Independent of the initial technique to find Ak 0
70. ng concepts Pulse temporal profiles for the RISTRA model 19 11 Modeling concepts Pulse temporal profiles for the two crystal RISTRA model 19 12 RISTRA model inputs for single crystal 2050 nm pumped ZGP 20 13 Pulse temporal profiles for one crystal ZGP RISTRA 0 0 20 14 Pulse temporal profiles for two crystal ZGP RISTRA 0 0 21 15 Coefficients of thermal expansion for various crystals 22 16 Thermal conductivities for various crystals 2 0 2 0 2 00002000004 25 17 Cavity mirror installation jig dei il ba jw Ge eee Ke ee Bee eS 25 18 Rotation assembly Se Sal we Slate se a eee WORE EER OR SRE 27 19 Reversal of walkoff with two crystals 00000 28 20 Ihntra cavity waveplate holder oaaae 29 21 Optical setup for aligning the pump beam to the RISTRA cavity 32 22 Aligning a seed beam to the RISTRA cavity LL 35 23 Optical setups for injection seeding the RISTRA cavity LL ooa 35 24 Spatial modes during RISTRA seed alignment L 000000 36 25 Cavity fringes during RISTRA seed alignment 0 36 26 Electronics for cavity length stabilization oaoa 38 21 TRISTRATACOESSOTIEN li ea i ana ar aus a a dae YEE dab ie Be ageing 47 28 Bearing retainer spanner tool 0 hee eek oe ee ee Abe he A 48 List of Tables 1 Dimensions and specifications for the RISTRA OPO 39 2 Example performanc
71. nm Asignal 800 nm for KTP BBO and BiBO Following our brief introduction to modeling of nanosecond OPOs we can pick up where we left off at the end of subsection 3 1 and make a final choice between KTP BBO and BiBO We ll start with our result from subsection 3 3 for the KTP RISTRA where 200 mJ of 532 nm pump energy combined with a crystal length of 12 mm produced a maximum energy of 51 4 mJ at 800 nm If we now update all input parameters relevant to type II BBO cut at 27 3 and apply our modeling methodology we find a maximum energy of 43 3 mJ for a crystal length of 17 mm with Recht l 0 70 9 Repeating this process for type II xz cut BiBO at 0 45 9 we find a maximum energy of 49 9 mJ for a crystal length of 17 mm with Ra l 0 62 Because psisnal 88 84 mrad for BiBO is large we also offset the pump beam by 0 7 mm From this exercise we find KTP generates slightly more signal energy than BiBO but BBO produces less by about 15 For some applications we would be done at this point but there are two other things to consider If the pump beam quality is less than optimum and it almost always is we might achieve better signal beam quality from BiBO than from KTP especially when the beam diameter is large or the pump pulse duration is shorter than 7 8 ns On the other hand if the beam diameter is small and in particular if the pump profile is lowest order Gaussian then KTP should offer higher efficiency and less beam profile d
72. nonzero value but for xz KTA at 0 79 6 where p is only 14 88 mrad we ignore this correction For Recht l 0 65 we obtain signal energy of 126 mJ but see evidence of back conversion We next try reducing the length of CI and find 14 mm results in a small increase in signal energy to 131 mJ The left and right depleted pump temporal profiles are now similar to those in Figure 11 We also run the model several times with lower pump energy to find the oscillation threshold arbitrarily defined as lt 1 mJ of signal energy occurs at slightly less than 90 mJ of pump energy Finally we check the average M values at full JNote that for a crystal with a 10 x 10 mm aperture the maximum length accommodated by the RISTRA assembly is approximately 17 mm A crystal this large may result in a reduced tuning range due to the restricted crystal bays in the RISTRA body IOMeasurements that demonstrate beam distortion due to walkoff for Gaussian beams are shown in Ref 8 AS Photonics LLC 14 RISTRA OPO user s guide energy Although the model s pump beam profile has smooth irradiance and no wavefront aberration common polarization of the pump and resonated wave will usually increase M and that is the case here For the signal we find M 2 97 and Mi 2 96 where subscripts w and p denote the directions parallel and perpendicular to walkoff respectively These are good M values for a high OPO but with different mixing the RISTRA c
73. ntly below typical damage thresholds say a fluence of l 2 J cm requires even larger beam diameters so that safe operating conditions can easily result in F gt 200 For a conventional nanosecond OPO cavity with two flat mirrors little can be done to reduce F except lengthening the cavity which reduces conversion efficiency so we must consider other cavity designs to obtain good beam quality One well developed class of high cavities that do improve beam quality are the diffractively coupled unstable resonators used in nanosecond solid state lasers In these unstable cavities where magnification is gt 1 magnification improves beam quality by smoothing variations in phase and amplitude across the beam diameter Unstable resonators can similarly improve Here is four times larger than the more commonly used definition of F r AL where r is the pump beam radius We note that F 1 is possible especially for stable resonators but pulse energies are generally restricted to lt 1 mJ AS Photonics LLC 1 RISTRA OPO user s guide beam quality in nanosecond OPOs but their use is usually limited to phase matching with periodically poled materials 1 where is small due to the 0 5 3 mm crystal apertures or with non critical phase matching in bulk crystals 2 otherwise the birefringent walkoff that accompanies angle critical phase matching disrupts the azimuthal symmetry about the cavity axis For a flat mirror high F cavit
74. of OPOs containing these crystals using the RISTRA models Environmental conditions such as elevated humidity could affect our choice and we find BBO is mildly hygroscopic while KTP and BiBO are inert with respect to moisture We note that BiBO is attractive because its deff is larger than that of BBO However BBO and KTP are very well developed can be cut with large dimensions and high quality crystals are available from numerous vendors BiBO on the other hand is relatively new and some of its physical characteristics may be less well known So lacking an obvious choice at this point we ll see what the models tell us in subsection 3 4 Before we do that let s consider another example of crystal selection that s not as simple and as we ll see amounts to a choice between the lesser of two evils A common application for nanosecond OPOs is generation of eye safe wavelengths using the fun damental of an Nd YAG or Nd YLF laser for the pump A signal wavelength of 1550 nm is con venient because it s widely used in the telecom C band making stabilized lasers for injection seed ing readily available so let s select a crystal for Apump 1064 nm QMIX suggests three choices It s well known that type II xz cut KTA at 90 and 0 phase matches non critically with 1064 0 gt 1533 5 0 3475 3 e at 300 K This is an obviously bad choice for the RISTRA OPO because p 0 mrad AS Photonics LLC 9 RISTRA OPO user s guid
75. on Two crystal 1627 nm resonant unseeded Mixing 1064 o 1627 0 3074 8 e Crystals Aperture 10 x 10 mm length 17 mm xz cut KTP 9 72 9 0 Output coupler R 0 55 at 1627 R gt 98 at 1064 R lt 4 at 3074 8 Waveplates Two Custom multi order double 2 for 1627 and 1064 Pump beam Broadband Nd YAG 10 semi donut profile not dark in center 6 mm diame ter duration lt 7 ns Seed beam Unseeded Output Maximum energy at 1627 nm 100 mJ for 450 mJ pump energy Pump depletion Not measured Beam quality M not measured but far field divergence indicates substantial admixture of vortex modes At least 4x diffraction limited probably worse References Not published Comments This is a good example of why the eye safe region is a challenge resulting in low conversion efficiency and poor beam quality An idler near 3000 nm allows use of KTP rather than the more expensive KTA but we are still forced to choose ooe mixing with larger deff and very small walkoff rather than oeo mixing with larger walkoff and lower deff The pump laser was a Continuum Inlite which was not designed for pumping OPOs but is attractive to lidar developers owing to its small ruggedized package It has a donut like profile which for unseeded oscillation in the RISTRA enhcances excitation of vortex modes C Example cavity mirror specifications The example model results in section 3 determine output coupler reflectivity with hi
76. ound vortex modes 3 Modeling performance of the RISTRA OPO using SNLO In addition to distributing the standard SNLO nonlinear optics modeling software AS Photonics also distributes free software designed specifically for modeling the RISTRA OPO Given the high cost of nonlinear crystals and custom cavity mirror coatings selecting crystal lengths and determining the optimum output coupling for any OPO especially for the RISTRA is no place for guesswork The SNLO based RISTRA models are easy to run and provide very good guidance for selecting crystal lengths and output coupling to optimize performance Because the RISTRA is a four mirror ring that accepts one crystal in each of its two longer cavity legs and because there are two mirror reflections between each crystal it can be configured for various modes of operation These include a conventional OPO using one or two crystals OPO intra cavity sum frequency generation SFG with sFG Mpump Osignal OPO intra cavitv difference frequency generation DFG with prG signal Midler and also OPO intra cavitv 2 generation Although models exist that accommodate these additional mixing processes we ll consider only the most commonly used those for one and two crystal OPOs AS Photonics can provide assistance with modeling the other cavity configurations on special request 4The standard SNLO distribution can be downloaded at http www as photonics com SNLO htm1 and the
77. passive Q switches so robust cavity locking techniques that generate a continuous error signal such as first derivative dither lock i e traditional AS Photonics LLC 33 RISTRA OPO user s guide phase sensitive detection or Pound Drever Hall PDH 11 12 are simple to implement Although discrete sampling techniques such as cavity build up time or feed forward approaches such as ramp and fire can be used with nanosecond OPOs continuous error correction usually provides a tighter lock and better frequency stabilization The laser that is used to injection seed the OPO must oscillate on a single longitudinal mode and the injected beam must have little wavefront aberration and be well collimated Although a lowest order Gaussian spatial profile works well for seeding it is not mandatory Other profiles are useful for special applications such as pulsed injection seeding where the seed beam spatial profile can be selected to strongly influence the spatial profile of the OPO s resonated wave 13 For most seeding applications the beam that emerges from a single mode polarization maintaining fiber following collimation pro vides an excellent spatial profile The optical power required for seeding can be as low as a few uW or even lower but should be sufficient to generate an error signal that is well above the baseline electronic noise in the detector and in the electronics comprising the servo system Owing to its nonplanar design initi
78. pick up the correct mirror with its high reflecting side down away from the tweezers and check for any dust that may be on the coating Very carefully remove the dust by blowing gently with a blower duster Do not use any pressurized duster whose contents can condense and stick to the mirror coating Dry N3 is best if available Second best is probably a rubber bulb air blower e Carefully set the mirror on the horizontal face on the cylinder e Pick up a mirror retaining rig with the tweezers Note that the bolt holes are not uniformly spaced so line the ring up to the hole pattern on the cylinder in advance Set the ring over the mirror Make sure the three tabs where the bolts pass through the ring are all in contact with the backside of the mirror e Pick up a bolt with a spring on its shoulder and drop it through the ring and thread it into the cylinder The bolt can be dropped it place using the tweezers but you can also use you fingers if you re comfortable doing so but try not to touch the mirror even if you re wearing gloves Install all of the bolts Tighten the bolts until the bolt shoulders are tight against the RISTRA body then back them out one turn to avoid binding of the retaining ring e Remove the cylinder from the jig and remount for installation of the next mirror Repeat above steps until M1 M4 are in place AS Photonics LLC 26 RISTRA OPO user s guide 5 2 Installation of nonlinear crystals e Tools required Sm
79. rors require no adjustments in fact in our design they can t be adjusted This characteristic of the RISTRA results from a universal property of nonplanar resonators namely low sensitivity to small tilts of their cavity mirrors These cavities have a unique axis even when they are formed by flat mirrors Unlike familiar two mirror cavities when a mirror is tilted slightly the cavity still has an optical axis although the axis is displaced a small amount relative to its previous position For this reason the RISTRA was designed without cavity mirror adjustments As shown in Figure 4 the RISTRA mechanical assembly consists of a solid cylinder Machined faces position the mirrors with three points on spring loaded retaining rings aligned with three points on the machined faces to define planes The assembly is quasi monolithic and very stable with low sensitivity AS Photonics LLC 7 RISTRA OPO user s guide to vibration Note that insensitivity to tilt does not apply to beams that are injected into the cavity For injection seeded operation the seed beam must be interferometrically aligned to the RISTRA s axis 2 2 Modes of the RISTRA cavity Owing to its nonplanar image rotating geometry the RISTRA cavity supports modes of oscillation that may be unfamiliar to some users When empty no birefringent crystals and no A 2 retardation plates the RISTRA cavity supports modes consisting of superpositions of right and left circularly polar
80. s LLC 44 RISTRA OPO user s guide R 70 2 A 800 nm p polarization R lt 4 A 1588 nm s polarization Side 2 outside R lt 0 25 A 532 nm s polarization R lt 0 25 A 800 nm p polarization R lt 4 A 1588 nm s polarization Mirror 3 Side 1 inside R gt 99 A 532 nm s polarization R gt 99 800 nm p polarization R lt 4 A 1588 nm unpolarized Side 2 outside R lt 0 25 A 532 nm s polarization R lt 0 25 A 800 nm p polarization R lt 4 A 1588 nm unpolarized Mirror 4 Pump exit Side 1 inside R lt 0 25 A 532 nm p polarization best effort R lt 0 5 acceptable R gt 99 A 800 nm s polarization R lt 4 A 1588 nm p polarization Side 2 outside R lt 0 25 A 532 nm p polarization R lt 0 25 A 800 nm s polarization R lt 4 A 1588 nm p polarization MIRROR SPECIFICATIONS FOR ONE CRYSTAL CAVITY Note All angles of incidence are 0 32 8 Nonlinear mixing 532 0 800 e 1588 0 Mirror 1 Input coupler Side 1 inside R lt 0 25 A 532 nm p polarization best effort R lt 0 5 acceptable R gt 99 A 800 nm s polarization R lt 4 A 1588 nm p polarization Side 2 outside R lt 0 25 A 532 nm p polarization R lt 0 25 A 800 nm s polarization R lt 4 A 1588 nm p polarization Mirror 2 Output coupler and pump exit AS Photonics LLC 45 RISTRA OPO user s guide Side 1 inside R lt 0
81. s achieved through the laser driver s RF input In this example the OPO cavity is locked to the seed laser using a low voltage PZT stack for cavity length stabilization To lock the laser to the cavity the PZT assembly is removed and the dashed line marked Servo in supplies a correction signal to the laser driver s servo input Note that the phase shifter may be optional as small changes in the VCO frequency may obtain the correct phase for demodulation AS Photonics can suggest vendors for many of the components shown here Definitions IF intermediate frequency LO local oscillator RF radio frequency PZT piezo electric transducer SMPM single mode polarization maintaining FC APC FC angle polished connector TV tuning voltage VCO voltage controlled oscillator unexpected pitfalls so here are a few things to keep in mind Don t saturate the fringe detector and start out by scanning slowly through at most about three free spectral ranges If the one time around fringe saturates the detector the relative height of the four times around fringes the ones you re trying to eliminate will be exaggerated and it might appear that you can t achieve good seed beam alignment Most PIN photodiodes can accommodate about V into 1 MQ before they begin to saturate although it s generally a good to keep their output voltage much lower Also be sure to maintain complete spatial overlap of the cavity leakage with the fringe de
82. s are identical on M3 and M4 in other words M3 and M4 are identical mirrors The correct orientation for s and p polarizations can be obtained at CI with the A 2 plate placed in the location of either WP1 or WP2 Note that with two crystals the polarization between M3 and M4 must be linear and this condition along with matching cavity polarizations to the eigenpolarizations of the crystals can only be achieved using two waveplates Because s and p polarizations are exchanged between M4 and M1 the direction of propagation in the RISTRA cavity can be selected to achieve the best pump beam input coupling as determined by phase matching in a particular crystal For example a 532 nm pumped two crystal cavity that resonates an 800 nm wave using xz cut KTP with 532 0 800 e 1588 0 would efficiently couple a p polarized pump o wave in C1 through M1 and use M4 as the pump exit mirror where the pump wave AS Photonics LLC 6 RISTRA OPO user s guide WPI 3 Figure 5 Two crystal RISTRA cavity denotes Rotated Image Singly Resonant Twisted RectAngle Ml MA are cavity mirrors WP1 and WP2 are A 2 retardation plates C1 and C2 are nonlinear crystals For the two crystal configuration shown here with propagation through Cl from left to right WP1 must be a double wavelength A 2 plate for the pump and resonated wave while WP2 can be a single wavelength A 2 plate for the resonated wave alone For a single crystal configuration only W
83. s constraint on the phase matching angles Additional details for injection seeded operation are given in subsection 6 3 Finally for two crystal cavities with an e polarized pump beam pure o tilt vertical or pure e tilt horizontal in the lower crystal CI results in a combination of o and e tilt in the upper crystal C2 This consequence of nonplanar geometry may be confusing at first but you ll get used to it Just make adjustments to pump beam tilt in small increments and re optimize crystal rotation angles as necessary Here are a few things to remember when aligning the pump beam e Begin alignment with very low pulse energy e Don t rush and damage expensive optics or crystals e Install a thin film polarizer and half waveplate to control pulse energy if your pump laser lacks energy control e If possible set beam height and propagation direction before installing the RISTRA assembly in the beam path This step will result in a pump beam that is fairly well aligned to begin with The center of the lower bore of the RISTRA is 2 25 above table and it is laterally centered on the base plate e Use two mirrors to walk in the pump beam as shown in Figure 21 e Exercise caution when observing beam spots on the back sides of high reflecting cavity mirrors or the positions of beams exiting the cavity e And finally for two crystal cavities with an e polarized pump beam be aware that pure o or e tilt in the lower crystal C1 results i
84. s seed beam cavity fringes at various stages of alignment The electronics and additional optics required to frequency stabilize an OPO are not too expensive and consist of standard optical and RF components Depending on the application the seed laser might be locked to the OPO or the OPO locked to the seed laser If your application requires single frequency oscillation but the exact wavelength is not critical then the laser can be locked to a standard RISTRA cavity An example application might be high efficiency sum frequency generation where the OPO s output pulse is mixed with the pulse from an injection seeded pump laser 14 If instead your appli cation requires that a specific wavelength falls within the spectral bandwidth of the OPO s pulse then the OPO must be locked to the laser This configuration requires that a cavity mirror be mounted on a piezoelectric transducer PZT to control cavity length For this configuration AS Photonics sells a PZT BThere may be some confusion about what constitutes PDH locking If the low amplitude first order modulation side bands modulation index lt 1 lie within a cavity resonance this is traditional phase sensitive detection even if the modula tion frequency is several hundred MHz Following demodulation and low pass filtering the error signal will be the familiar first derivative If the first order sidebands lie outside the resonance and following demodulation you retain a high band width
85. scillators produce donut like beam profiles that can excite the vortex modes at the expense of the one time around mode An admixture of these modes reduces beam quality because they remain hollow at all propagation distances they have helical wavefronts that combine with flat wavefronts to produce highly structured interference patterns and they have comparatively large far field divergence angles Fortunately these modes can be eliminated almost entirely by injection seeding the higher gain one time around mode If you have no choice but to use a donut like pump beam the overall conversion efficiency may be lower than anticipated but seeding will dramatically improve the beam quality for the resonated wave and even for the unresonated wave in most cases If your application requires the best possible beam quality especially for higher output energies you might consider the extra effort and cost of injection seeding the cavity even if you have a near perfect pump beam profile Appendices A Dimensions and specifications for the RISTRA OPO Table 1 contains the dimensions for the RISTRA OPO mechanical assembly and for its optical compo nents Not included are specifications for small hardware and other minutiae Table 1 Dimensions and specifications for the RISTRA OPO Cylindrical body Length 50 017 mm OD 37 64 mm Cavity Physical length 109 mm Cavity Bore ID 10 mm Cavity Legs Long 2 31 925 mm Short Long V 2 22 575 mm
86. selections from subsection 3 1 we need to discuss a few basic concepts for modeling nanosecond OPOs Modeling is a simple task because the RISTRA models run fast on a desktop PC and also because a little intuition further speeds the iteration process used to find a good design If you ve modeled cw OPOs or cw buildup cavities for 2 generation where mode stability and mode matching are critical vou ll find modeling of nanosecond OPOs with flat mirrors comparatively simple The RISTRA models are based on SNLO s 2D cav LP module They include two dimensional spatial profiles walkoff diffraction and image rotation but cannot accommodate broadband oscilla tion They are only capable of treating the case of single frequency oscillation using a single frequency pump but they still provide good predictions of output energy for most broadband systems Single fre quency operation is not a choice but a necessity imposed by limitations in available computer memory and processing speed Although a broadband equivalent to 2D cav LP does exist it s too numerically intensive for everyday use and therefore an equivalent broadband model for the RISTRA cavity has not been developed This means single frequency oscillation occurs on the one time around mode as described in subsection 2 2 unless we force oscillation on one of the RISTRA s four times around modes If you re developing a broadband system and need to estimate its line width this can be done
87. tallation of cavity mirrors 2 2 ee es 24 5 2 Installation of nonlinear crystals 2244 2 dae 2406 dee Ree Se KE Oe SES 27 5 3 Installation of intra cavity waveplates 2 2 0 2 2 00200 000004 27 5 3 1 Adjusting the angle of incidence for the waveplates 30 6 Use of the RISTRA OPO 31 6 1 Eye safety with nonplanar geometry 0 200000000004 31 62 Pump beam delivery and alignment 4 244 a04 9 424 24 Odea i L See 31 6 3 Injection seeding for single frequency oscillation o oo 33 6 3 1 Injection seeding improves beam quality LL 39 Appendices 39 A Dimensions and specifications for the RISTRA OPO 39 AS Photonics LLC ii RISTRA OPO user s guide B Example configurations and performance specifications 40 C Example cavity mirror specifications 43 D Accessories for the RISTRA OPO 49 References 50 List of Figures 1 Dual erystal RISTRA OPO 2 2 6 koe ba Me ia Sele oe Oe ee A i 2 Contour plots of OPO spatial fluence profiles 0 0 3 3 Surface plots of OPO spatial fluence profiles 004 4 4 RISTRA OPO mechanical assembly 2 2 0200 02 eee 6 5 RISTRA OPO cavity configuration and name conventions 7 6 QMIX output for KTP and BBO i 9 38 ab a PE ate he an ean ee BY 17 T QMIX output for xz cut KTA aoaaa ee ee ee 17 8 RISTRA model inputs for 532 nm pumped KTP aoaaa a 18 9 RISTRA model output for 532 nm pumped KTP LL 0000 18 10 Modeli
88. tector s active area As shown in Figure 24 b spatial separation between lobes for the various modes of this cavity can result in missing information unless the entire leakage beam impinges on the fringe detector If you use a fiber coupled detector it goes without saying that you shouldn t try to use a single mode fiber Finally the RISTRA mechanical assembly may be rock stable due to its quasi monolithic design but most mirror mounts are compar atively flimsy When making fine adjustments to the turning mirrors used for seed beam alignment lightly touch and then release the knobs on mirror mounts When you touch a mirror mount during alignment you may find the fringe pattern changes abruptly upon contact AS Photonics LLC 38 RISTRA OPO user s guide 6 3 1 Injection seeding improves beam quality Although nanosecond OPOs are usually injection seeded for applications requiring single frequency oscillation seeding can also improve beam quality For a nonplanar cavity like the RISTRA broadband oscillation can allow simultaneous oscillation of off axis modes such as the four times around vortex modes described in subsection 2 2 How many of these modes might oscillate and with what fraction of the total pulse energy depends on the cavity Fresnel number F the mixing parameters for a given crystal and the pump beam spatial profile with the latter sometimes enhancing excitation of these modes For example some diffractively coupled Nd YAG laser o
89. tellite Based Ozone DIAL Systems IEEE J Sel Topics Quantum Electron 13 721 731 2007 15 D J Armstrong W J Alford T D Raymond A V Smith and M S Bowers Parametric amplification and oscillation with walkoff compensating crystals J Opt Soc Am B 14 490 474 1996 AS Photonics LLC 50 RISTRA OPO user s guide
90. th Recht l 0 65 and crystal length increased to 12 mm The signal energy is 15 4 mJ and the pump depletion is 58 The signal and idler are normalized relative to the undepleted pump for proper power scaling b Same as a but with crystal length increased to 15 mm The longer crystal reduces the 3800 nm signal energy slightly from 15 4 mJ to 15 1 mJ and increases M2 and Mi from 1 77 to 1 96 The pump depletion is now lt 57 See text for additional details AS Photonics LLC 20 RISTRA OPO user s guide Undepleted pump ba eni Depleted pump Signal Undepleted pump z e Depleted pump Signal Idler i Power Arb Power Arb 48 36 24 12 0 12 24 36 48 48 36 24 12 0 12 24 36 48 Time ns Time ns Figure 14 a OPO pump pulse temporal profiles at left mirror M4 pump exit mirror and signal and idler pulses at mirror M2 output coupler for a two crystal ZGP RISTRA using the model inputs for the one crystal version in Figure 12 but with C1 and C2 lengths of 8 mm and 15 mm respectively The signal energy is 21 4 mJ and the pump depletion is 81 2 The signal and idler are normalized relative to the undepleted pump for proper power scaling b Same as a but with Rerysta 0 01 for all three wavelengths Roe Ree Ru 0 01 and crystal loss 0 008 mm at Apump 2050 nm Attempting to mimic the losses in a real OPO reduces signal energy to 18 2 mJ and reduces pump
91. the cavity mirrors nonlinear crystals and A 2 plates The rectangle s ra tio of length width v2 and after twisting the angle of incidence on all four of its mirrors M1 M4 is 32 765 or 32 8 for the purpose of specifying dielectric coatings The cavity is twisted such that the planes containing the paths M4 gt M1 M2 and M1 gt M2 M3 are perpendicular with s p polarizations at M1 becoming p s polarization at M2 after passing through crystal Cl The length width ratio and angles of incidence were determined from a rigorous but general method for designing image rotating cavities presented in Ref 5 For the purpose of building a RISTRA OPO you can neglect the rigor but if you need to order cavity mirrors you will need to understand a few of its characteristics and they are explained below As described in subsection 1 1 the RISTRA cavity achieves high beam quality through the use of angle critical birefringent phase matching which requires rotation of the crystals Because the cavity polarizations must match the eigenpolarizations of the crystals and must also correspond to s and p polarizations at the cavity mirrors the crystals are oriented in the cavity so they rotate in the following manner p polarization at mirror M1 corresponds to an o wave in crystal Cl and s polarization at M2 s polarization at MI corresponds to an e wave in Cl and p polarization at M2 The convention for M3 C2 and M4 is the same but rotated by 90
92. tion stage so good heat removal can be achieved by using thermal paste in this gap Ordinarily this is omitted but it can be added at the customers request or the customer can add the paste in which case we recommend contacting AS Photonics for advice on how to do so 4 3 Higher order thermal effects The only thermal effect discussed above was the thermo optic effect which is usually the dominant effect However the thermal profile causes optical distortions via other effects as well For example nonuniform thermal expansion caused by nonuniform temperature profiles can cause slight bulges in the end faces of the crystal centered on the beams This adds or subtracts from the thermal lensing due to the thermo optic effect Other higher order effects include refractive index changes due to thermal expansion via the strain optic effect via the electro optic effect due to electric fields induced by thermal expansion and the inverse piezo electric effect or fields induced by the pyro electric effect Changes caused by these higher order effects are usually of order 10 times smaller than the thermo optic changes but in some circumstances they must be considered as well 5 Assembling the RISTRA OPO The RISTRA OPO is simple to assemble However the cavity mirrors waveplates and crystals it contains must be treated with care by individuals acquainted with the proper handling of delicate optical components One fingerprint on a crystal like BBO w
93. tipped tweezers e Visually align the flat on the plate so that the appropriate alignment mark is in the middle of the flat Rotate the plate by gently pushing on the exposed edge of the waveplate e Apply a very small amount of UV glue to the edge of the plate at just one point Use a needle for applying the glue Cure the glue with the UV light source e Install the plate in the RISTRA s cylinder and install the appropriate mirrors for testing orientation of the polarization e Repeat all steps but the preceding one if you have a two crystal cavity The following steps can be omitted if you don t have a suitable test laser Unfortunately there aren t many birefringent materials suitable for making waveplates with sufficiently high damage thresholds for A gt 4 5 um Sapphire is a good choice if it can be used AS Photonics LLC 29 RISTRA OPO user s guide For a one crystal cavity The RISTRA should be completely assembled so that it can be mounted on the optical table or a breadboard Remove M2 The wavleplate can be installed between M2 and M3 or between M4 and MI as discussed previously Inject linearly polarized laser light at the resonant wavelength through the opening for M2 along the path M2 M3 It can be s or p polarized but will preferably have the polarization of the resonated wave as it leaves Cl in lower leg After aligning the beam to the cavity use a polarizer aligned to transmit on
94. to the short crystal and in b note the near 100 depletion of the pump in the second half of the pulse These are signatures of a very efficient nanosecond OPO where the signal energy is now 98 7 mJ and the pump depketion is 72 The temporal profile for the undepleted pump is generated by setting des O in the model as indicated in Equation 1 The plots of pulse profiles shown here are not generated by the RISTRA model but were plotted using data in the model generated files RIPWR R DAT and R2PWR_L DAT See text for additional details AS Photonics LLC 19 RISTRA OPO user s guide Indexes of refraction Crystal left reflectivity Crystal right reflectivity Crystal loss per mm Enrgy Puwr left J W Enrgy Pwr right J W Pulse duration ns Pulse delay ns Beam diameter FWHM mm deff pm V detta k 1 mm S9 1d swap 1 yes 0 no Pump must be bluest wave Signal and idler designations are interchangeable Signal wave by default is perfectly aligned in cavity Other beams will not be if they walk off Undepleted pump TA Undepleted pump TA Depleted pump Depleted pump AS Signal Be Signal a pe idler idler u u z z o o a a 0 0 7 48 36 24 12 0 12 24 36 48 48 36 24 12 0 12 24 36 48 Time ns Time ns Figure 13 a OPO pulse temporal profiles at right mirror M2 output coupler for a one crystal ZGP RISTRA using inputs in Figure 12 but wi
95. y there is little diffractive coupling however birefringent walkoff if present behaves much like magnification in one transverse dimension giving essentially the same beam clean up as an unstable resonator While walkoff is one dimensional its clean up effects can easily be extended to both transverse directions using image rotation 3 4 This is the idea behind the RISTRA OPO where its nonplanar geometry was designed to produce a convenient image rotation angle of 90 5 To illustrate how the Fresnel number F affects beam quality and also demonstrate beam clean up from birefringent walkoff with and without image rotation Figure 2 a d contains contour plots of spatial fluence profiles from two different OPOs for various operating conditions It also includes profiles for small and large diameter pump beams that were used for varying F from 33 to gt 400 The OPOs contained one or two xv cut KTP crystals with 0 58 0 and p 47 65 mrad where p is walk off They were pumped by the 532 nm second haromonic of a Q switched injection seeded Nd YAG laser The OPO cavities were singly resonant and injection seeded at the signal wavelength of 800 nm In Fig 2a the OPO cavity was a three mirror ring pumped by the near perfect Gaussian beam shown in Fig 2e with F 33 In Fig 2b a Dove prism was inserted into the cavity with it s base oriented at 45 to the plane of the cavity to induce 90 image rotation 4 Note how the far

User`s manual and tutorial for the image rotating RISTRA OPO

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