Abstract:
In an optical parametric frequency conversion arrangement first and second optically nonlinear crystals ( 18, 20 ) mounted on respectively first and second drive-shafts ( 26, 30 ). The drive shafts ( 26, 30 ) are counter-rotatably driven by a single stepper-motor ( 50 ) via a gear-train. The first drive-shaft ( 26 ) includes a lead screw. When the first drive-shaft ( 26 ) is rotated, the first crystal ( 18 ) is rotated and simultaneously translated, while the second crystal ( 20 ) is simultaneously counter-rotated but is not translated.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of United Kingdom Patent Application No. GB1420741.9, filed on Nov. 21, 2014, the content of which is incorporated by reference herein in its entirety for all purposes. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to laser apparatus including frequency-conversion by one or more optically nonlinear crystals. The invention relates in particular to an optical parametric oscillator (OPO) tuned by varying the incidence angle of a beam on a pair of optically nonlinear crystals arranged for walk-off compensation. 
     In such an arrangement, it is usual to a have second optically nonlinear crystal (compensating crystal) counter-rotatable toward or away from the OPO crystal, for providing walk-off compensation and canceling changes in beam pointing resulting from rotating the OPO crystal. The counter rotation is preferably done simultaneously by a common stepper motor rotating both crystals using suitable gearing. The OPO and compensation crystals can be referred to as a crystal-pair. 
     In such an OPO, the pump-radiation is focused to a beam-waist in the OPO crystal for maximizing electric field intensity and corresponding increasing parametric conversion efficiency. Because of this, depending on the pump-wavelength and power, it is not unusual for optical damage to gradually occur on the OPO crystal. Damage does not usually occur in the compensation crystal as the beam-diameter has expanded from the beam-waist and the electromagnetic field intensity is consequently reduced below a damage threshold. 
     In any frequency conversion arrangement an unavoidable problem of optical damage to an optically nonlinear crystal can be mitigated by periodically moving (shifting) the crystal with respect to an incident beam. This is usually termed “crystal-shifting” by practitioners of the art. The period can be selected, for example, by monitoring output power and shifting the crystal when power has fallen by some predetermined percentage. 
     When only a single crystal is involved, with a fixed (non-tunable) conversion, the crystal can be mounted on a simple, small translation-stage and translated in one or two axes. In the case of the above described counter-rotatable crystal-pair, the shifting arrangement including motor, gearing, and crystals has to be shifted. This requires a much bigger, and consequently more expensive and space-consuming, translation stage. 
     Separate stepper-motors and associated control systems would be needed for crystal rotation and crystal shifting. There is a need for a simpler arrangement for the crystal-shifting which does not require such a translation stage, and does not require separate motors and control systems. 
     SUMMARY OF THE INVENTION 
     In one aspect, opto-mechanical apparatus in accordance with the present invention comprises first and second optically nonlinear crystals mounted on respectively first and second drive-shafts, having respectively first and second rotation-axes. The drive-shafts are counter-rotatably driven by a stepper-motor via a gear-train. 
     Only the first drive-shaft includes a lead-screw arranged such that when the first drive-shaft is rotated, the first crystal is rotated about the first rotation-axis and simultaneously translated in the direction of the first rotation-axis, while the second crystal is simultaneously counter-rotated about the second rotation-axis, but is not translated in the direction of the second rotation-axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
         FIG. 1  is a three-dimensional view schematically illustrating a preferred embodiment of apparatus in accordance with the present invention including a housing including mechanisms (not shown) for rotating first and second drive-shafts on which are mounted respectively first and second crystal holders supporting respectively first and second optically nonlinear crystals with the drive-shafts counter-rotatable with respect to each other and with the first drive-shaft simultaneously translatable about a drive-shaft axis thereof. 
         FIG. 1A  is a three-dimensional view schematically illustrating details of the crystal holders and crystals of the apparatus of  FIG. 1 . 
         FIG. 2  is an exploded three-dimensional view schematically illustrating details of the rotating and translating mechanisms not shown in  FIG. 1 . 
         FIG. 2A  is an expanded three-dimensional sub-assembly view schematically illustrating further details of the mechanisms of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, wherein like features are designated with like reference numerals.  FIG. 1  and  FIG. 1A  schematically illustrate a preferred embodiment 10 of crystal rotating and shifting apparatus in accordance with the present invention. The embodiment is described with to use thereof in an above-discussed OPO. 
     Apparatus  10  includes a housing  12  having an extension portion  16 . The housing includes a base flange  14  for mounting the apparatus in an OPO apparatus. Within the housing, not shown in either  FIGS. 1 and 1A , is a single stepper-motor and gearing arranged to simultaneously counter-rotate drive-shafts  26  and  30  about drive-shaft axes  28  and  32  respectively (see  FIG. 1A ), as indicated by arrows R 1  and R 2  respectively. The mechanism also simultaneously translates drive-shaft  26  in the drive-shaft axis direction as indicated by arrows T. Details of the mechanism are described in detail further herein below with reference to  FIG. 2  and  FIG. 2A . 
     Optically nonlinear crystals  18  and  20  are supported on crystal holders  22  and  24  respectively. In terms of the background art discussed above, crystal  18  is an OPO crystal in which a beam being frequency-divided (parametrically converted) is focused. 
     Crystal  20  is a compensating crystal. The direction of incidence of a laser beam being frequency-divided is indicated as such. The propagation axis of the beam is indicated as axis  42 . It should be noted that OPO crystal  18  is longer than crystal  20  in the drive-shaft axis direction to accommodate translation of the crystal as drive-shaft  26  is translated. 
     Referring now principally to  FIG. 1A , each crystal holder is attached to an upper clamp-member  34  via a screw (not shown) through an arcuate slot  40  in the crystal holder. The upper clamp-member is clamped to a lower clamp-member by a screw (also not shown). This serves to attach the crystal holders to the drive-shafts. The crystals are edge-bonded to the crystal holders on a shelf-portion  38  thereof. In the drawing of  FIG. 1A , the crystal holders and clamp-members are identical. Reference numerals are provided for only one, for simplicity of illustration. Attaching crystal holders to the clamp via arcuate slots  40  allows the crystals to be initially aligned with each other during manufacture, as indicated by arrows A 1  and A 2  and an axis  41  perpendicular to the drive shaft-axis. This is a one-time manual alignment, which is made during assembly of the inventive apparatus, then fixed. For this reason, the adjustment arcs A 1  and A 2  are indicated by a dashed line. 
       FIG. 2  is an exploded three-dimensional view schematically illustrating details of the rotating and translating mechanisms not shown in  FIG. 1 . Further detail of the mechanisms is depicted as sub-assembly  10 A in  FIG. 2A . 
     In  FIG. 2 , housing  12  is disassembled into a front cover  13  including extended section  16  discussed above, a machined surround section  15  including base  14 , and a rear cover  17 . Holes  66  and  68  through cover  13  are provided to engage bearings (bushings)  62  and  64  on drive-shafts  30  and  26  respectively. 
     A stepper motor  50  is depicted withdrawn from extended section  16  of front cover  13 . Stepper motor  50  turns a gear  52 , which engages a larger-diameter gear  54  connected to drive-shaft  30 . Gear  54  is an anti-backlash gear having two components gears  54 A and  54 B, one thereof fixed and one floating under spring pre-load as is known in the art. In the drawing, gear  54 B is the spring loaded gear. Rotation of drive shaft  30  rotates crystal holder  24 . 
     Also attached to drive-shaft  30  is an anti-backlash gear  56  having component gears  56 A and  56 B. In the drawing, gear  56 B is the spring loaded gear. Gear  56  engages a gear  58  having the same diameter as gear  56 , but a greater thickness. Gear  58  is connected to drive shaft  26  via a lead-screw assembly  60 . This provides that when gear  58  is rotated by gear  56  drive, shaft  26  and crystal holder  22  thereon are rotated, in a direction opposite to crystal holder  24  and through exactly the same angle. Further, because of the lead screw assembly, rotation of the draft shaft  26  causes the crystal holder to be translated as indicated by arrows T. The anti-backlash gear arrangement of gears  54  and  56  is critical in ensuring that the rotation angles are indeed exactly the same, and exactly repeatable. 
     A preferred thread-pitch for lead screw  60 , in this embodiment of the present invention, is 125 μm, corresponding to 200 turns-per-inch (TPI). About 5 mm of travel is required. Lead-screws can be custom-made by most precision machine-shops. “Off-the-shelf” lead-screw assemblies are also commercially available. By way of example, a Model AJS254-0.5H-NL lead screw, available from Newport Corporation of Irvine, Calif. has a thread pitch of 254 TPI. It should be noted here, that in the arrangement described, gear  58  translates with lead screw  60  and drive shaft  26 , as indicated in  FIG. 2A  by arrow T. The additional thickness of gear  58  over gear  56  is made sufficient that gears  56  and  58  stay meshed through the contemplated translation range of drive-shaft  26 . In an example of operation of the inventive crystal shifter it is useful to consider an OPO pumped at a wavelength of 520 nanometers (nm) with signal radiation tunable through a range between about 680 nm and about 1300 nm. For optically nonlinear crystals  18  and  20  of β-barium borate (BBO), this requires rotating each crystal (in opposite directions) through a total angle of about 24 degrees. With the preferred lead-screw pitch of 200 TPI, this would translate crystal  20  a distance of only about 8 micrometers (μm). A typical 1/e2 beam-waist diameter in OPO crystal  18  would be about 60 μm, so there would always be a substantial degree of overlap of extreme-tuned positions of the beam-waist. 
     When a need for shifting the beam waist to a completely fresh spot on the crystal is indicated, for example, by a detected unacceptable power-drop as discussed above, the crystals are rotated through one or more revolutions of about 360°, bringing the crystal angles back into the tuning range. Considering again the preferred 200 TPI lead screw, one full 360°-revolution of drive-shaft  26  would translate crystal  18  a distance of about 125 μm. This is sufficient to shift the 60 μm beam-waist to a completely fresh spot on OPO crystal  18 . 
     In the embodiment of the present invention described above, crystal rotation for tuning purposes and crystal translation of crystal shifting are achieved with a single stepper motor which can be driven by a single control system. This provides for a considerable reduction in size and complexity compared with the above described prior-art approach, even with translation in only one axis. 
     The embodiment of the invention is illustrated by detailed engineering drawings prepared for building a prototype of the invention. Only those details necessary for understanding principles of the present invention are described and identified by reference numerals. From the quality of the drawings, the function of other details not described will be evident to a person of ordinary skill in the mechanical engineering art. The details are not intended to be limiting, and a person of such ordinary skill in the mechanical engineering art may make changes to the details, whether described or not described, without departing from the spirit and scope of the present invention. 
     In summary, the present invention is described above in terms of a preferred embodiment. The invention is not limited, however, by the embodiment described and depicted. Rather the embodiment is limited only by the claims appended hereto.