WO2000071342A1

WO2000071342A1 - Contacted crystal surface protector and method

Info

Publication number: WO2000071342A1
Application number: PCT/US2000/014520
Authority: WO
Inventors: Gregory J. Mizell
Original assignee: Ii-Vi Incorporated
Priority date: 1999-05-26
Filing date: 2000-05-26
Publication date: 2000-11-30
Also published as: AU5293200A

Abstract

Disclosed is a method of protecting optically polished surfaces of a hygroscopic nonlinear optical crystal (100) comprising the steps of optically bonding an inert material (120, 130) to each optically polished surface of the nonlinear crystal (100) and thereby protecting the optically polished surfaces for contact by water vapor. A harmonic generator made by this method is also disclosed.

Description

CONTACTED CRYSTAL SURFACE PROTECTOR AND METHOD BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to nonlinear optical materials and a technique for preventing the degradation of optically polished surfaces in the presence of atmospheric water vapor.

2. Description of the Prior Art

Nonlinear optical materials are commonly used for the frequency conversion of laser radiation. For example, commercial Nd:YAG laser products typically use one or more such crystals to provide outputs at wavelengths that are sub-multiples of the 1.064 micrometer laser wavelength. By passing an infrared laser output through temperature-controlled nonlinear crystals of the appropriate crystallographic orientation, a visible output at 0.532 micrometers can be generated by second harmonic generation while an ultraviolet output at 0J55 micrometers is produced by summing the 1.064 micrometer fundamental with the second harmonic. In some cases, it is also desirable to frequency double the second harmonic, thereby generating a fourth harmonic output at 0J66 micrometers; or to use a higher order sum process to generate the 0J13 micrometer fifth harmonic through a sum mixing process.

Nonlinear optical crystals can also be used to generate wavelengths that are greater than that of the input(s). For example, an optical parametric oscillator effectively divides the energy of a single input beam between two output frequencies with values that are determined by the phase-matching conditions and conservation of energy. Such a device is, in many respects, similar to a laser oscillator using an optical resonator to concentrate at least one of the output beams in the nonlinear crystal and exhibiting an input power threshold for the onset of oscillation. Difference frequency mixing, in which two beams are focused into a nonlinear crystal, can also be used to produce a long wavelength output beam with an energy equal to the difference between the two inputs.

Crystals that are useful for nonlinear frequency conversion must possess a combination of properties. These include a large value for the effective nonlinear coefficient, sufficient birefringence to phase-match the process of interest, low optical losses and immunity to optical damage by either the input or output beams. In addition, it must be possible to polish the crystal surface to high optical quality and reproducibly grow samples that are large enough for use as optical devices. Crystals satisfying these requirements are few in number and include potassium titanyl phosphate (KTP), beta barium borate (BBO), lithium triborate (LBO), cesium lithium borate (CLBO), potassium niobate and lithium niobate. Of particular interest since the mid-1980s are members of the borate family including BBO, LBO and CLBO. These materials combine a relatively high nonlinear coefficient, good resistance to optical damage (high damage threshold) and good transparency at ultraviolet wavelengths. In addition, they are highly birefringent and can be used to generate ultraviolet radiation from the 1.064 micrometer output of the NdNAG laser via sequential third, fourth and fifth harmonic processes. They have also been used for the manufacture of visible-emitting, widely-tunable optical parametric oscillators.

While the attractive features of these crystals are well known, their widespread use has been limited by surface degradation issues. Specifically, polished optical surfaces on CLBO, BBO and LBO have been found to degrade over a comparatively short period of time (weeks or months) due to the action of atmospheric water vapor. As a result, they are difficult to incorporate into products that must function for long periods of time under uncontrolled atmospheric conditions.

Prior art solutions to the surface degradation problems with these and other hygroscopic crystals include continuously maintaining the polished nonlinear- optical crystal at an elevated temperature, using optical coatings to seal the surface or optically coupling the surface to an environmentally-inert material using an index- matching fluid. While effectively minimizing the interaction of the water vapor with the crystal surface, both of these approaches have significant practical drawbacks. Specifically, the heating approach requires the crystal to be placed in an oven that is continuously supplied by a source of electrical power. In at least one commercially available frequency converter, this condition is met by incorporating a battery backup system into the device. This arrangement increases both the cost and the complexity of the final assembly in a way that is disadvantageous for the end user. Dielectric coatings, often used to minimize reflectivity from the crystal surfaces, also provide some degree of isolation from atmospheric water vapor. Particularly effective in this regard are the high-density, low porosity coatings that are applied using ion-assisted deposition techniques. Unfortunately, coatings of this type do not provide a perfect seal and often produce mechanical stresses at the crystal surface when the crystal/coating combination is heated. This stress is caused by dissimilar thermal expansion coefficients for the crystal and coating and can change the crystal properties and/or cause the coating to de-bond from the surface. It can also interfere with the coating process, increasing the scattering losses to an unacceptably high level.

Coupling to an inert material using an index-matching fluid has been proposed as a method for protecting the surface without introducing coating stresses. Mounts that are sealed with quartz windows and filled with an index matching fluid have been used for many years to prevent the degradation of water-soluble crystals like ADP and KDP. More recently, workers from the University of St. Andrews described an LBO optical parametric oscillator based on this approach (see T.R. Stevenson, F.G. Colville,

M.F. Dunn, and J J. Padgett, "Doubly-resonant optical parametric oscillator formed by index-matching cavity mirrors directly onto an uncoated LBO crystal", Digest of the 1995

Conference on Electro-Optics and Lasers, Optical Society of America, Washington,

1995, page 402). In this device, the plano-convex cavity end-mirrors were coupled to the ends of the LBO nonlinear crystal using a thin layer of index matching fluid. Stated advantages of this approach included the elimination of coating-induced mechanical stresses as the crystal was heated and cooled in addition to the protection from water vapor-induced surface damage. While suitable for initial laboratory demonstrations, index-matching fluids are found to degrade over time when exposed to the intense optical fields typically used for nonlinear frequency conversion. This situation is worsened for systems that have ultraviolet input and/or output beams as is commonly the case for borate-based frequency converters.

It would be an advantage to provide a method for protecting the surfaces of hygroscopic nonlinear crystals that does not require a continuous supply of electrical power, degrade in optical transmission with time or produce unwanted mechanical stresses at the crystal surface. Crystals protected by such a method could then be readily incorporated in a wide range of laser and optical frequency conversion devices. It would be a further advantage if a method of protection could be developed that was compatible with curved- as well as flat-surfaced crystals and was compatible with standard coating techniques that could be used to adjust the surface reflectivity. Such a technique would enable the design of multi-functional optical structures in which the nonlinear crystal surfaces were used as lenses and/or reflectors.

Finally, it would be advantageous if the protective technique also provided a method for affixing other optical components to the crystal surface in a manner that was mechanically robust and stable with respect to time, temperature and vibration. Such assemblies could be used as lasers, optical parametric oscillators, walkoff-compensated nonlinear converters and other multi-component devices.

SUMMARY OF THE INVENTION I have developed a method for protecting the optically polished surfaces of a hygroscopic nonlinear optical crystal. According to the invention, an inert material is bonded to the surface of the hygroscopic crystal via intermolecular, van der Waals forces making it impossible for water vapor to reach the crystal surface. Due to the strength of the van der Waals bond, the contacted assembly is mechanically stable with respect to time, temperature and vibration.

In one embodiment of this invention, an optical contacting technique is used to protect the surfaces of a single, hygroscopic nonlinear crystal as be might used to frequency double the output of a pulsed, solid-state laser. In this device, fused silica windows are bonded to the crystal ends to create a monolithic frequency doubling structure. Optionally, antireflection coatings designed to minimize the reflection of the input and/or output beams are applied to the outer surfaces of the fused silica windows. This embodiment directly addresses one of the principal problems associated with the use of hygroscopic nonlinear materials like lithium triborate and beta-barium borate in commercial laser devices. Appropriately-coated assemblies can be treated like the non- hygroscopic crystals (lithium niobate and potassium titanyl phosphate, for example). Other transparent optical materials including optical glass, undoped Nd:YAG, etc., may be substituted for fused silica in this embodiment. One surface of the protective window may also be curved, thereby giving the contacted assembly an optical power that can be used to focus or collimate the input and/or output beams. When compared to prior art techniques for protecting the surfaces of a single, hygroscopic nonlinear crystal, this embodiment has the advantages of completely sealing the crystal surface against water vapor and a negligible increase in optical damage threshold with respect to the unbonded crystal. In an alternative embodiment of this invention, two or more nonlinear crystals (at least one of which is hygroscopic) are bonded together to create a multistage, nonlinear generator. The outer face(s) of the hygroscopic crystal(s) is(are) protected by bonding a window of fused silica or other transparent material to it. Such an assembly is possible if the polarization of the nonlinear output of the first crystal can be oriented with respect to the crystallographic axes of the second nonlinear crystal and/or the input beam polarization in such a way that efficient nonlinear conversion is possible in the second crystal. In cases where this is not possible, an appropriately-oriented birefringent plate may be placed between the two crystals and optically contacted to their surfaces.

In a disclosed fifth harmonic generator, three nonlinear crystals are optically contacted together in the correct crystallographic orientations to generate a 212.8 nm output when pumped by a 1064 nm input beam. An optional oven assembly is used to stabilize the output power for optimal power conversion.

Other disclosed embodiments of the optical contacting method include frequency-converted laser devices incorporating hygroscopic nonlinear materials. For example, a 355 nm output can be produced by bonding a coated neodymium yttrium vanadate gain crystal to a composite nonlinear frequency converter consisting of a crystal of potassium titanyl phosphate, a waveplate and a crystal of beta barium borate. The output end of the nonlinear assembly is optically contacted to a curved, fused silica end piece with a dielectric coating on its outer (curved) surface. A stable laser cavity is formed by the curved output face and the flat, coated face of the yttrium vanadate crystal and, when pumped by a laser diode whose output wavelength is matched to a strong absorption of the vanadate crystal, laser oscillation takes place. Outputs at the second and third harmonic wavelengths are produced by second harmonic and sum frequency mixing in the intracavity nonlinear crystals. This, and similar embodiments make it possible to use hygroscopic borate crystals, with their desirable phase matching properties in microchip laser designs. Such assemblies are particularly advantageous for the generation of blue and ultraviolet beams from a compact, diode-pumped assembly. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a first embodiment of a harmonic generator in accordance with the present invention;

Fig. 2 is a schematic view of a second embodiment of a harmonic generator in accordance with the present invention;

Fig. 3 is a schematic view of a third embodiment of a harmonic generator in accordance with the present invention; Fig. 4 is a graph showing the orientation of the axes of the BBO crystals shown in Fig. 3;

Fig. 5 is a schematic view of a fourth embodiment of a harmonic generator in accordance with the present invention; and

Fig. 6 is a schematic view of a fifth embodiment of a harmonic generator in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is described in a preferred and in several alternate embodiments with respect to Figs. 1-6. While these embodiments demonstrate the best modes for achieving the objectives of this invention, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the invention.

As used in this description, the terms "optical contact" or "optical contacting" refer to a joining of two materials through the steps of: (1) polishing the two surfaces so that they are essentially free of scratches and digs (scratch/dig figure of 60/40 or less) and have curvatures that are matched to better than 1 fringe over the area of contact; (2) cleaning the surfaces to remove all lint, dust and other contaminants from the surface; (3) placing the two surfaces in physical contact with one another; and (4) applying a light pressure to the two components to force out the air that is trapped between the two parts. When this process is carried out correctly, the surfaces will be drawn together by van der Waals forces. In the case of identical glass pieces, the force holding the surfaces together is in the range of 29 to 43 psi and a shear force of approximately 114 psi acting parallel to the contacted surface is required to separate them. In the optical shop, glass pieces are usually separated with a sharp blow from a wooden mallet or by applying local heat to one edge of the joint with a Bunsen burner or other heat source. In the latter case, differential expansion across the contacted surface acts to break the bond. Spatially-uniform temperature variations, however, have little effect on the contacted bond and assemblies incorporating hygroscopic BBO crystals are stable with respect to uniform heating over a range exceeding 50° C.

Optical contacting techniques have been used for the fabrication of optical components for many years, although it is common practice to use identical materials. Optical contacting of dissimilar, non-hygroscopic crystals was first developed by this inventor in an effort to develop compact, frequency-doubled microlaser devices. Certain embodiments of this invention are disclosed in U.S. Patent No. 5,651,023, which describes several monolithic, optically contacted, microlasers incorporating non- hygroscopic crystals like Nd:YAG, Nd-doped yttrium vanadate and potassium titanyl phosphate. The current invention is based on the discovery that the optical contacting technique described above can be used to affix an environmentally inert material, such as fused silica, to a hygroscopic nonlinear material including lithium triborate (LBO), beta barium borate (BBO) and cesium lithium triborate (CLBO). thereby producing an environmentally rugged, long-lived assembly.

The embodiment shown in Fig. 1 is a harmonic generator consisting of a hygroscopic nonlinear crystal 100 with optically contacted fused silica window surface protectants, including an input window 120 and an output window 130. In a preferred configuration, the nonlinear crystal 100 is a lithium triborate crystal (LBO) that is oriented for the sum frequency mixing of 1064 nm and 532 nm radiation to generate a 355 nm output. The crystallographic c-axis is oriented at a 45 degree angle with respect to the propagation direction of input beams 105. The lithium triborate crystal is typically 10 to 20 mm in length and has a square cross section with 3 mm long sides. The fused silica windows 120, 130 are circular in cross section with a diameter of approximately 7 mm. Dissimilar component radii have been found to facilitate the optical contacting process and yield a more robust assembly. The opposed ends of the nonlinear crystal 100 are polished flat and parallel and optically contacted to inner surfaces 122, 131 of the fused silica windows 120, 130, respectively, according to the procedure above. Dielectric coatings designed to minimize the reflection of the 1064 nm and 532 nm input beams

(dual antireflection coatings) may be applied to outer surfaces 121, 132 of the fused silica windows 120, 130, respectively. In operation, the input beams 105 are spatially overlapped using a dichroic mirror or other beam combiner before passing through the nonlinear generator. Optionally, a temperature stabilized oven assembly 140 surrounds the assembly and can be used to adjust the crystal temperature for optical output power.

Phase matched sum frequency mixing occurs in the LBO nonlinear crystal producing a 355 nm sum frequency output 110 that exits the nonlinear crystal assembly with the power-depleted inputs.

In an alternative embodiment shown in Fig. 2, the output window 130 of

Fig. 1 is replaced with a plano-convex lens or output window 230 that focuses

(collimates) an output beam 210. In this configuration, a planar side 231 of the output window 230 is optically contacted to the polished end of an LBO nonlinear crystal 200 and a dual band antireflection coating (AR 532 nm and 1064 nm) applied to the other side. A fused silica planar window 220 with an inner surface 222 and an antireflection coated outer surface 221 is optically contacted to the other surface of the LBO crystal 200 and receives an input beam 205. Although fused silica is a preferred window material in the embodiments of Figs. 1 and 2, it is possible to substitute a wide range of alternative environmentally-inert window materials that can be polished to that flatness and surface finish required for optical contacting. Such materials include but are not limited to glasses like BK-7 that are commonly used for the fabrication of lenses, mirrors and other optical components, undoped gain materials like yttrium-aluminum-garnet and yttrium orthovanadate, and crystal quartz, etc.

In those cases where the index of refraction of the window material is not matched to that of the hygroscopic nonlinear crystal, it is advantageous to apply a dielectric index matching layer to the inside surface of the window material in order to reduce reflections at the optically contacted interface. In the embodiment of Fig. 1, this coating is applied to the surfaces 122 and 131 prior to contacting. In Fig. 2, the coating would be applied to surfaces 222 and 231. A wide range of coating materials and designs is known in the art and could be used to minimize reflections at the nonlinear crystal/window interface.

While the preferred embodiment of Fig. 1 is designed to generate the third harmonic of the NdNAG laser by summing the fundamental (1064 nm) with the first harmonic (532 nm), primary features can be applied to a wide range of nonlinear frequency conversion devices. Specifically, appropriately oriented crystals of BBO, LBO, CLBO and other hygroscopic crystals may be used for second, third, fourth and fifth harmonic generation, sum and difference frequency generation and optical parametric oscillation. Composite nonlinear crystal assemblies in which multiple nonlinear crystals are optically contacted to one another are an alternative embodiment of this invention that can minimize the number of input beams required to generate higher order harmonics (third, fourth or fifth). For example, the assembly of Fig. 3 is designed to generate as an output beam 315 the 212.8 nm fifth harmonic of a single 1064 nm input beam 300. In this device, the input beam 300 first passes through a nonhygroscopic KTP crystal 310 to generate a 532 nm second harmonic via Type II, critically-phase-matched second harmonic generation. The two beams travel together through a first or leftmost BBO crystal 320 that is oriented to phase-match the frequency doubling of the 532 nm beam to 266 nm. The 1064 nm beam travels through the first BBO crystal 320 without interacting, entering a second or rightmost BBO crystal 330 with the unconverted 532 nm beam and the 266 nm fourth harmonic beam. Sum frequency mixing between the 1064 nm input beam and the 266 nm fourth harmonic beam produces a fifth harmonic output at 212.8 nm. All four waves exit the assembly through a UV-grade sapphire protective window 340 and are available as the output beam 315. Optionally, a birefringent quartz waveplate 350 with a fullwave of retardation at 266 nm and a halfwave of retardation at 1064 nm or a halfwave of retardation at 266 nm and a fullwave of retardation at 1064 nm can be inserted between the two BBO crystals 320, 330 to adjust the relative polarization of the 266 nm and 1064 nm beams for optimal power conversion efficiency.

In the assembly shown in Fig. 3, all surfaces are optically contacted, i.e., the right surface of the KTP crystal 310 is optically contacted to the left surface of the first BBO crystal 320, such that the two BBO crystals 320 and 330 are joined by optical contacting at the interface between them, and the right surface of the second BBO crystal 330 is optically contacted to the inner surface of the sapphire end window 340. Optionally, an outer surface 311 of the KTP crystal 310 is antireflection coated at the 1064 nm input wavelength and an output face 341 of the sapphire protective window 340 has a broadband UV antireflection coating. In a typical assembly, all components would have square cross sections with 5 mm long sides. The KTP crystal 310 would be 3 mm in length while the two BBO crystals 320, 330 would be 5 mm long. Non-critical phase- matching of the desired frequency conversion processes is guaranteed by cutting the KTP crystal 310 at 0=24, and the first BBO crystal 320 at θ=47.6 and the second BBO crystal 330 at 0=51 J . The axes of the BBO crystals 320, 330 are oriented so the beams are polarized as shown in Fig. 4. In order for the 1064 nm beam to remain linearly polarized in the preferred direction, the entire assembly can be housed in a thermal enclosure 360 for purposes of tuning the combined birefringence of the KTP crystal 310 and first BBO crystal 320 to a fullwave of retardation. The temperature tuning range required to accomplish this task is estimated to be less than 10° C. Further control of the 1064 nm polarization may be accomplished by placing a waveplate that has a fullwave of retardation at 532 nm and a halfwave at 1064 nm between the two BBO crystals 320 and 330. This waveplate would be contacted to the two crystals using the procedures outlined above and oriented to rotate the 1064 nm polarization to a direction that is orthogonal to the 532 nm second harmonic beam. So oriented, the full 1064 nm power would be available to the sum frequency generation process, thereby increasing the fifth harmonic conversion efficiency.

Obvious extensions of this embodiment include assemblies for the generation of third and fourth harmonic radiation. The assembly identical to that of Fig. 3, but with the second BBO crystal 330 removed, would generate an output at the fourth harmonic. Similarly, substitution of a crystal phase matched for the third harmonic generation in place of the first BBO crystal 320 combined with appropriate polarization control would produce an output at 355 nm.

While the output of an Nd:YAG laser at 1064 nm has been used as an input beam in the above examples, it should be realized that the principles embodied in these devices are independent of input wavelength. As long as the hygroscopic crystals incorporated in the assembly are oriented to phase match the process of interest, a wide range of input wavelengths can be used. In addition, the demonstrated principles for affixing protective windows to frequency conversion assemblies consisting of one or more hygroscopic nonlinear crystals are applicable to difference frequency generation and optical parametric amplification as well as the second harmonic and sum frequency generation.

In another embodiment of this invention, composite crystal assemblies with protective end windows are fabricated from pieces of the same nonlinear crystal.

In one application, the assembly is designed to minimize the deleterious effects of the Poynting vector walkoff that is characteristic of non-critically phase-matched interactions. According to this well-known effect, light propagating at an angle with respect to the principal optical axes of a nonlinear crystal will generate a nonlinear output beam in a non-collinear direction. As this beam travels down the crystal, it spatially "walks off of the input beam significantly reducing the efficiency of the nonlinear frequency conversion process. In order to minimize the effects of Poynting vector walkoff, it is possible to change the direction of the nonlinear crystal axes (and hence the walkoff direction at periodic intervals). In the embodiment of Fig. 5, a single crystal of BBO, oriented for second harmonic generation of a 532 nm input (θ=47.6), is cut into ten segments (400 - 409) of identical length (2 mm). These segments are subsequently reassembled into a single structure by optically contacting the adjacent surfaces of the individual segments. Minimization of Poynting vector walkoff is achieved by rotating the crystallographic axes of adjacent crystals by 90 degrees in the plane perpendicular to the propagation direction before contacting them. Undoped NdNAG windows 410, 420 are optically contacted to the two ends of the assembly as described previously to protect the outermost surfaces of the walkoff-compensated frequency doubler from atmospheric moisture. Optional dielectric coatings may be applied to the window surfaces to minimize reflection of an input beam 411 and/or an output beam 421.

Like-crystal composite assemblies made of like-oriented crystals may also be used to increase the available nonlinear interaction distance in cases where the maximum crystal length is shorter than desired. In this case, the like-oriented crystals are contacted in an end-to-end relationship to produce an assembly of increased length with the nonlinear properties of a single crystal. Protective windows are applied to the ends of this assembly as shown in Fig. 5.

Optically contacted laser devices, formed from non-hygroscopic materials, have been reported by a number of workers and are described in U.S. Patent Nos. 5,651,023, 5,796,766 and 5,838,713. In the '023 patent, three platelets of materials were optically contacted to form an intracavity-doubled solid state laser for laser diode pumping. The '766 patent describes the use of optically contacted, non-hygroscopic materials for purposes of heat removal. The '713 patent describes an optically contacted tunable source that, in one embodiment, incorporates an LBO crystal that is coated on its output face. Advantages of these assemblies include small size, low-cost manufacture and improved heat removal when compared to prior art devices. These patents do not teach or suggest that it was possible to use hygroscopic nonlinear crystals in their devices ('766 and '023) or that inert windows contacted to both surfaces of a hygroscopic crystal provided optimal environmental protection. Accordingly, the embodiment of Fig. 6 is an Nd: YVO₄ laser in which two, nonlinear optical crystals are used to generate second and third harmonic outputs from the intracavity field. In Fig. 6, a 0.4 mm thick crystal of 3% Nd:YVO₄ 500 is optically contacted to a 3 mm long KTP crystal 510 oriented with 0=24 to phase match second harmonic generation of the 1064 nm intracavity field. The high-gain axis of the Nd:YVO₄ crystal 500 is oriented at 45 degrees relative to the axes of the KTP crystal 510 to maximize the production of second harmonic light at 531 J nm. A waveplate 520 with a halfwave of birefringence at 1064 nm and a fullwave at 532 nm or a fullwave at 1064 nm and halfwave at 532 nm is used to adjust relative polarizations of the two fields to directions that optimize the production of third harmonic radiation in a BBO crystal 530 that is oriented with 0=31 J. This BBO crystal 530 is optically contacted to the waveplate 520 on the left side and to a plano-convex, fused silica protective window 540 on the right side. Optical coatings that are highly reflective at the laser wavelength are applied to the outer surface of the Nd:YVO₄ crystal 500 and a curved surface 541 of the protective window 540. In addition, the Nd: YVO₄ crystal coating is highly transmissive (>95%) at the 808 nm pump wavelength. The coating on the curved output surface 541 is optimized for high transmission at the third, and possibly, the second harmonics. The curvature of the output window surface 541 is greater than the diffractive path length between the pump face of the Nd:YNO₄ crystal 500 and the output surface. Optimal performance in the device of Fig. 6 is expected with curvatures between 100 mm and 300 mm. In operation, the device is energized by a pump beam 550 that is focused onto the coated face of the Νd:YNO₄ crystal 500 and generates an output beam 555. This pump beam 550 is most advantageously supplied by a broad area diode laser or diode laser array.

While the surfaces in the embodiment of Fig. 6 are directly contacted, it may desirable to coat one or more of them with a reflectivity -minimizing dielectric coating. Such a coating would be designed to reduce the Fresnel reflections that are normally present at an interface between two surfaces of dissimilar index of refraction.

The use of a coating for this purpose is a further advantage of the present invention and does not degrade the environmentally-robustness of an optically contacted assembly.

Other embodiments and modifications of this invention may occur to those of ordinary skill in the art in view of these teachings. For example, the Νd: YNO₄ crystal of Fig. 6 may be physically separated from the KTP crystal and the two resulting surfaces antireflection coated at 1064 nm. Similarly, a single-crystal harmonic generator, similar in design to the third harmonic generator of Fig. 1 , may be inserted in the cavity of a flashlamp-pumped laser to generate a second harmonic output through intracavity second harmonic generation.

Although the present invention has been described in detail in connection with the discussed embodiments, various modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be determined by the attached claims.

Claims

I CLAIM:

1. A method of protecting optically polished surfaces of a hygroscopic nonlinear crystal comprising the steps of optically contacting a first inert crystal to a first optically polished outer surface of the nonlinear crystal and optically contacting a second inert crystal to a second optically polished outer surface of the nonlinear crystal, with the first and second optically polished surfaces opposed to each other, thereby protecting the optically polished surfaces of the nonlinear crystal from contact by water vapor.

2. The method of claim 1 further including the step of applying an anti-reflection coating to outer surfaces of each inert crystal opposite inner surfaces thereof which are optically contacted to the nonlinear crystal.

3. The method of claim 2 wherein the anti-reflection coating is a dielectric coating.

4. The method of claim 1 wherein one of the inert crystals is a collimating output lens having a curved outer surface.

5. The method of claim 1 wherein the nonlinear crystal is selected from the group consisting of potassium titanyl phosphate, beta barium borate, lithium triborate, cesium lithium borate, potassium niobate and lithium niobate.

6. The method of claim 1 wherein the inert crystals are selected from the group consisting of fused silica, optical glass and undoped neodymium yttrium- aluminum-garnet.

7. The method of claim 2 further including the step of coating the inner surfaces of the inert crystals with an index matching layer in order to reduce reflections at the optically contacted interfaces thereof.

8. The method of claim 1 wherein the nonlinear crystal is formed of multiple nonlinear crystals optically contacted in series to adjacent crystals to form a composite crystal.

9. A harmonic generator comprising a nonlinear crystal having a first inert crystal optically contacted to a first optically polished outer surface of the nonlinear crystal and a second inert crystal optically contacted to a second optically polished outer surface of the nonlinear crystal, with the first and second optically polished surfaces opposed to each other, thereby protecting the optically polished surfaces of the nonlinear crystal from water vapor.

10. The harmonic generator of claim 9 further including an anti- reflective coating on outer surfaces of each inert crystal opposite inner surfaces thereof which are optically contacted to the nonlinear crystal.

11. The harmonic generator of claim 10 wherein the anti-reflection coating is a dielectric material.

12. The harmonic generator of claim 9 wherein the nonlinear crystal is selected from the group consisting of potassium titanyl phosphate, beta barium borate, lithium triborate, cesium lithium borate, potassium niobate and lithium niobate.

13. The harmonic generator of claim 9 wherein the inert crystals are selected from the group consisting of fused silica, optical glass and undoped neodymium yttrium-aluminum-garnet.

14. The harmonic generator of claim 10 further including an index matching layer between the inner surfaces of the inert crystals and the outer surfaces of the nonlinear material, thereby reducing reflections at the optically contacted interfaces thereof.

15. The harmonic generator of claim 9 wherein the nonlinear crystal is formed of multiple nonlinear crystals optically contacted in series to adjacent crystals to form a composite crystal.

16. The harmonic generator of claim 15 wherein the each nonlinear crystal has its crystallographic axis rotated by 90 degrees with respect to an adjacent crystal.