Second harmonic generation crystal

Abstract

The optical system (10) generates a beam of ultraviolet laser radiation from a beam of visible laser radiation and spatially separates the ultraviolet laser beam from the visible laser beam. The optical system (10) includes two crystals (12, 14) made of the same optically nonlinear material contact-bonded along a planar interface (16). One crystal (12) has a major crystal axis oriented for Type I second-harmonic generation. The ultraviolet laser beam exits the optical system (10) through an uncoated surface (26) of the other crystal (14). The major crystal axes of the two crystals (12, 14) have different orientations and have reflection symmetry about the planar interface (16).

Description

[Technical Field]

[0001] Inventors: Wolf Seelert and Rüdiger von Elm (Priority) This application claims priority to U.S. Patent Application No. 16 / 801,857, filed February 26, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0002] (Technical field of the invention) The present invention relates generally to optical elements for generating second harmonic radiation, and more particularly to optical nonlinear crystals for generating ultraviolet wavelength radiation by harmonic conversion of visible wavelength radiation and separating the ultraviolet wavelength radiation from the remaining visible radiation.

[0003] (Background field discussion) In laser devices for providing ultraviolet wavelength radiation, it is common to generate ultraviolet (UV) radiation by harmonic conversion of visible radiation in an optical nonlinear crystal. Typically, visible radiation is generated by second harmonic conversion of near-infrared (NIR) wavelength radiation, which is generated by a solid-state laser such as an optically pumped semiconductor (OPS) laser.

[0004] As an example, in one common arrangement for generating continuous wave UV radiation, visible radiation is generated by intracavity second harmonic conversion of NIR radiation within a solid-state laser. The visible radiation is coupled into an impedance-matched resonant enhanced cavity for visible radiation. An optical nonlinear crystal within the resonant cavity then converts the visible radiation to UV radiation via Type I second harmonic conversion. It is possible to generate UV radiation at wavelengths of 266 nanometers (nm) or shorter using an OPS solid-state laser. The difficulty with Type I second harmonic conversion is that there is no inherent spatial separation between the generated UV radiation and the remaining visible radiation.

[0005] Typically, UV radiation is directed out of the resonant cavity by a dichroic mirror with a thin-film dielectric coating. This dichroic mirror can serve as one of the resonator mirrors, reflecting visible radiation and transmitting UV radiation. Alternatively, the dichroic mirror can be a separate intracavity element, transmitting visible radiation and reflecting UV radiation out of the resonant cavity.

[0006] A particular limitation of the laser configuration described above is damage to thin-film dielectric coatings caused by UV radiation. This limitation is more problematic for short-wavelength UV radiation. The intracavity elements can be translated, transitioning damaged regions out of the radiation and exposing unused regions; such transitions add significant complexity and cost to the laser device. Furthermore, even coatings that are not yet damaged typically have losses due to manufacturing variations in layer thickness or absorption by the coating material. Such losses reduce the efficiency of resonant cavities using intracavity harmonic generation and ultimately reduce the useful life of these resonant cavities.

[0007] An apparatus that overcomes these limitations is described in commonly assigned U.S. Patent No. 10,474,004, the complete disclosure of which is incorporated herein by reference. An uncoated birefringent prism receives visible and UV radiation generated by an optical nonlinear crystal. The visible and UV radiation have orthogonal linear polarizations. The birefringent crystal is oriented at Brewster's angle for the visible radiation and for total internal reflection of the UV radiation. The birefringent crystal is an additional element with optical surfaces that must be oriented with relatively high precision. Both the visible and UV radiation must pass through two surfaces of the birefringent crystal, resulting in unavoidable reflection losses.

[0008] A need exists for a laser device that generates UV radiation and separates it from visible radiation without exposing any coatings to the UV radiation. Preferably, the generation and separation is achieved while adding minimal optics, optical surfaces, complexity, and cost to the laser device. Summary of the Invention [Means for solving the problem]

[0009] In one aspect, an optical system for converting radiation having a fundamental wavelength to radiation having a second harmonic wavelength according to the present invention includes a first crystal made of an optically nonlinear material. The first crystal has a major crystal axis oriented to convert the fundamental radiation to second harmonic radiation by second harmonic generation. A second crystal made of the same optically nonlinear material as the first crystal is provided. The first and second crystals are bonded together along a first planar interface. The first planar interface is tilted with respect to the fundamental radiation. The major crystal axes of the first and second crystals have a mutual angular separation and reflection symmetry with respect to the first planar interface. The thermal expansion coefficients of the first and second crystals are thereby matched along the first planar interface.

[0010] In another aspect, a laser apparatus for generating a beam of laser radiation at a second harmonic wavelength according to the present invention includes a laser delivering a beam of laser radiation at a fundamental wavelength that is twice the second harmonic wavelength. An impedance-matched resonant enhanced cavity is provided to receive the fundamental laser beam. The resonant enhanced cavity is defined by a plurality of cavity mirrors, the plurality of cavity mirrors having a high degree of reflectivity at the fundamental wavelength and arranged to direct the fundamental laser beam along a closed path within the resonant enhanced cavity. An optical system is provided and located in the closed beam path of the resonant enhanced cavity. The optical system includes a first crystal and a second crystal made of the same optical nonlinear material. The optical nonlinear material has a major crystal axis. The major crystal axis of the first crystal is oriented to convert the fundamental radiation to second harmonic radiation by second harmonic generation. The first and second crystals are bonded together along a planar interface that is tilted relative to the fundamental laser beam. The major crystal axes of the first and second crystals are separated from each other by an angle of at least 2°. The major crystal axes of the first and second crystals have reflection symmetry about the planar interface, and the thermal expansion coefficients of the first and second crystals are thereby matched along the planar interface.

[0011] In yet another aspect, an optical system for converting radiation having a first fundamental wavelength and radiation having a second fundamental wavelength into radiation having a sum-frequency wavelength according to the present invention includes a first crystal made of an optically nonlinear material. The first crystal has a major crystal axis oriented to convert the first fundamental radiation and the second fundamental radiation into sum-frequency radiation by sum-frequency generation. A second crystal made of the same optically nonlinear material as the first crystal is provided. The first and second crystals are bonded together along a planar interface. The planar interface is tilted relative to the first and second fundamental radiations. The major crystal axes of the first and second crystals have a mutual angular separation and reflection symmetry about the planar interface. The thermal expansion coefficients of the first and second crystals are thereby matched along the planar interface. The present invention provides, for example, the following. (Item 1) 1. An optical system for converting radiation having a fundamental wavelength into radiation having a second harmonic wavelength, comprising: a first crystal made of an optically nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert fundamental radiation into second harmonic radiation by second harmonic generation; a second crystal made of the same optically nonlinear material as the first crystal, the first crystal and the second crystal being bonded together along a first planar interface, the first planar interface being inclined with respect to the fundamental radiation; and Equipped with An optical system, wherein the major crystal axes of the first and second crystals have a mutual angular separation and have reflection symmetry about the first planar interface, and wherein the thermal expansion coefficients of the first and second crystals are thereby matched along the first planar interface. (Item 2) Item 1. The optical system of item 1, wherein the major crystal axes of the first and second crystals are oriented such that the fundamental radiation is not refracted and the second harmonic radiation is refracted at the first planar interface. (Item 3) 3. The optical system of claim 1, wherein the fundamental radiation enters the optical system through an uncoated input surface at a Brewster's angle relative to the fundamental radiation. (Item 4) 4. The optical system according to any of items 1 to 3, wherein the fundamental radiation exits the optical system through an uncoated output surface at Brewster's angle relative to the fundamental radiation. (Item 5) 5. The optical system of claim 4, wherein an inclination angle of the first planar interface relative to the fundamental radiation and a distance between the first planar interface and the output surface are selected to separate the fundamental radiation from the second harmonic radiation on the output surface. (Item 6) 5. The optical system of claim 4, wherein an inclination angle of the first planar interface relative to the fundamental radiation and a distance between the first planar interface and the output surface are selected to overlap the fundamental radiation and the second harmonic radiation on the output surface. (Item 7) 7. The optical system according to any of items 4 to 6, wherein the second harmonic radiation exits the optical system through another uncoated output surface at Brewster's angle relative to the second harmonic radiation. (Item 8) 8. The optical system according to any of items 1 to 7, wherein the fundamental radiation enters and exits the optical system through a parallel uncoated surface at Brewster's angle relative to the fundamental radiation. (Item 9) 9. The optical system according to any one of items 1 to 8, wherein the first and second crystals are bonded together by optical contact bonding. (Item 10) 10. The optical system according to any one of items 1 to 9, further comprising a third crystal made of the same optically nonlinear material as the first and second crystals, the third crystal being bonded to the first crystal along a second planar interface, the second planar interface being parallel to the first planar interface, the major crystal axes of the first and third crystals having a mutual angular separation and reflection symmetry about the second planar interface, and the thermal expansion coefficients of the first and third crystals being thereby matched along the second planar interface. (Item 11) Item 11. The optical system of item 10, wherein the major crystal axes of the second and third crystals have the same orientation. (Item 12) Item 11. The optical system of item 10, wherein the first and third crystals are bonded together by an optical contact bond. (Item 13) 13. The optical system according to any one of items 1 to 12, wherein the major crystal axes of the first and second crystals are separated from each other by at least 5°. (Item 14) Item 14. The optical system of item 13, wherein the major crystal axes of the first and second crystals are separated from each other by at least 12°. (Item 15) 15. The optical system according to any one of items 1 to 14, wherein the second harmonic generation is type I second harmonic generation. (Item 16) 16. The optical system according to any one of items 1 to 15, wherein the optical nonlinear material is selected from the group consisting of beta-barium borate (BBO), lithium triborate (LBO), and cesium lithium borate (CLBO). (Item 17) 17. The optical system according to any one of items 1 to 16, wherein the fundamental radiation has a wavelength of about 426 nanometers and the second harmonic radiation has a wavelength of about 213 nanometers. (Item 18) 1. A laser apparatus for producing a beam of laser radiation at a second harmonic wavelength, comprising: a laser delivering a beam of laser radiation at a fundamental wavelength that is twice the second harmonic wavelength; an impedance-matched resonant-enhanced cavity for receiving a fundamental laser beam, the resonant-enhanced cavity being defined by a plurality of cavity mirrors, the plurality of cavity mirrors being highly reflective at the fundamental wavelength and arranged to direct the fundamental laser beam along a closed path within the resonant-enhanced cavity; an optical system located in a closed beam path of the resonant enhanced cavity, the optical system including a first crystal and a second crystal made of the same optical nonlinear material, the optical nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert laser radiation at the fundamental wavelength into laser radiation at the second harmonic wavelength by second harmonic generation, the first and second crystals being bonded together along a planar interface that is tilted with respect to the fundamental laser beam; Equipped with The major crystal axes of the first and second crystals are separated from each other by an angle of at least 2°, the major crystal axes of the first and second crystals have reflection symmetry about the planar interface, and the thermal expansion coefficients of the first and second crystals are thereby matched along the planar interface. (Item 19) Item 19. The optical system of item 18, wherein the fundamental laser beam enters the optical system through an uncoated input surface and exits the optical system through a parallel uncoated output surface, the input surface and the output surface being at Brewster's angle relative to the fundamental laser beam. (Item 20) 20. The optical system of claim 19, wherein a tilt angle of the planar interface relative to the fundamental laser beam and a distance between the planar interface and the output surface are selected to separate the fundamental laser beam from the second harmonic laser beam on the output surface. (Item 21) 21. The optical system according to any one of items 18 to 20, wherein the first and second crystals are bonded together by optical contact bonding. (Item 22) 22. The optical system according to any one of items 18 to 21, wherein the second harmonic generation is type I second harmonic generation. (Item 23) 1. An optical system for converting radiation having a first fundamental wavelength and radiation having a second fundamental wavelength into radiation having a sum frequency wavelength, comprising: a first crystal made of an optically nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert first fundamental radiation and second fundamental radiation into sum frequency radiation by sum frequency generation; a second crystal made of the same optically nonlinear material as the first crystal, the first and second crystals being bonded together along a planar interface, the planar interface being inclined with respect to the first and second fundamental radiations; and Equipped with An optical system, wherein the primary crystals of the first and second crystals have a mutual angular separation and have reflection symmetry about the planar interface, and the thermal expansion coefficients of the first and second crystals are thereby matched along the planar interface. (Item 24) Item 24. The optical system according to item 23, wherein the first and second fundamental radiations exit the optical system through an uncoated output surface at Brewster's angles relative to the first and second fundamental radiations. (Item 25) 25. The optical system of claim 24, wherein an inclination angle of the planar interface relative to the first and second fundamental radiations and a distance between the planar interface and the output surface are selected to separate the first and second fundamental radiations from the sum frequency radiation on the output surface. (Item 26) 26. The optical system according to any one of items 23 to 25, wherein the first and second crystals are bonded together by optical contact bonding. [Brief explanation of the drawings]

[0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0013] [Figure 1]Figure 1A is a perspective view, Figure 1B is a plan view, and Figure 1C is a side view schematically illustrating a preferred embodiment of an optical system according to the present invention, the optical system being for converting a beam of fundamental radiation into a beam of second harmonic radiation and separating the two beams, the optical system comprising a first crystal bonded to a second crystal along a planar interface, the separated beams exiting the optical system through an uncoated output surface. [Figure 2] FIG. 2A is a cross-sectional plan view, and FIG. 2B is a cross-sectional side view that schematically shows further details of the optical system of FIGS. 1A to 1C. [Figure 3] FIG. 3 is an enlarged cross-sectional side view showing further details of the optical system of FIGS. 1A-1C. [Figure 4] FIG. 4A is a cross-sectional plan view and FIG. 4B is a cross-sectional side view schematically illustrating another preferred embodiment of an optical system according to the present invention, similar to the optical system of FIGS. 1A-1C, but including a third crystal bonded to the first crystal along another planar interface. [Figure 5] FIG. 5 is a side view that schematically illustrates a method for manufacturing the optical system of FIGS. 4A and 4B according to the present invention. [Figure 6A] FIG. 6A is an example of the optical system of FIGS. 1A-1C having a planar interface and major crystal axes tilted at an angle selected to maximize separation between the beams of fundamental and second harmonic radiation on the output surface. [Figure 6B] FIG. 6B is an example of the optical component of FIGS. 1A-1C having a planar interface and major crystal axes tilted at an angle selected to maximize overlap of the beams of fundamental radiation and second harmonic radiation on the output surface. [Figure 7] Figure 7A is a perspective view, Figure 7B is a plan view, and Figure 7C is a side view schematically illustrating yet another preferred embodiment of an optical system according to the present invention, which is similar to the optical system of Figures 1A to 1C, but in which the fundamental radiation beam exits the optical system through one uncoated output surface and the second harmonic beam exits through another uncoated output surface. [Figure 8]FIG. 8 shows a schematic diagram of one embodiment of a laser system according to the present invention, including a laser, a resonant enhanced cavity, and the optical system of FIGS. 1A-1C. DETAILED DESCRIPTION OF THE INVENTION

[0014] Detailed Description of the Invention Referring now to the drawings, in which like components are designated by like numerals, FIGS. 1A-1C schematically illustrate a preferred embodiment of an optical system 10 in accordance with the present invention. FIG. 1A is a perspective view, FIG. 1B is a plan view, and FIG. 1C is a side view of optical system 10. The outer edges of optical system 10 are highlighted in bold in the drawings. Optical system 10 includes a first crystal 12 and a second crystal 14 made of the same transparent optically nonlinear material. First crystal 12 and second crystal 14 are bonded together along a planar internal interface 16.

[0015] The first crystal 12 is oriented and arranged to convert a fundamental radiation beam 18 having a fundamental wavelength into a second-harmonic radiation beam 20 having a second-harmonic wavelength by Type I second-harmonic generation. Two photons of the fundamental beam 18 are converted into each photon of the second-harmonic beam 20. The fundamental wavelength is twice the second-harmonic wavelength for energy conservation. In Type I second-harmonic generation, as is known in the art, the fundamental radiation has an ordinary linear polarization and the second-harmonic radiation has an extraordinary linear polarization.

[0016] Here, "orientation" refers to orienting the major crystal axis relative to the polarization plane and propagation direction of the fundamental beam 18 to achieve momentum conservation or "phase matching." Phase matching takes advantage of the birefringence of optical nonlinear materials and is necessary for efficient second-harmonic generation. The major crystal axis of the second crystal 14 has a different orientation relative to the fundamental beam 18. Therefore, the crystal 14 is not phase-matched and does not generate significant second-harmonic radiation.

[0017] Here, the fundamental beam 18 enters the optical system 10 through an uncoated input surface 22, which is oriented at the internal Brewster angle β of the fundamental radiation to minimize reflection losses. F 1B , and the second harmonic beam 24 is oriented at a normal incidence angle γ in the plane of FIG. 1C . The optical nonlinear material has different refractive indices for the orthogonally polarized beams. The ordinary polarized fundamental beam has the same refractive index in each crystal, while the extraordinary polarized second harmonic beam has a different refractive index, resulting in the second harmonic beam being refracted away from the remaining fundamental beam at the internal interface 16.

[0018] Both the remaining fundamental beam 24 and the second harmonic beam 20 exit the optical component 10 through an uncoated output surface 26, which is at an internal Brewster angle β with respect to the remaining fundamental beam to minimize reflection losses at the fundamental wavelength. F 1B. The input surface 22 and output surface 26 are parallel so that the entire optical system 10 does not cause angular displacement of the fundamental radiation; i.e., fundamental beam 18 and remaining fundamental beam 24 propagate in the same direction outside of optical system 10. The different refractive indices of the orthogonally polarized second harmonic and remaining fundamental beams cause these beams to be refracted at different angles by output surface 26, creating an additional internally reflected beam 28 at the second harmonic wavelength depicted in FIG. 1B. The refracted remaining fundamental beam 24 and second harmonic beam 20 become divergent and spatially separated as they propagate away from output surface 26 of optical system 10.

[0019] 2A and 2B are cross-sectional schematic diagrams showing further details of optical system 10. FIG. 2A is a plan view and FIG. 2B is a side view. In these figures, the internal beams have been omitted for clarity of illustration. FIG. 2A shows the external Brewster angle α of fundamental beam 18 emerging from output surface 26. F and the external angle α of the second harmonic beam 20 SH These beams diverge at an angle Δα in the plane of FIG. 2A and at an angle δ in the plane of FIG. 2B.

[0020] 2A and 2B, major crystal axes x, y, and z are oriented in first crystal 12 and second crystal 14. The major crystal axes are oriented symmetrically with respect to internal interface 16. Specifically, the major crystal axes have reflection symmetry with respect to internal interface 16. One particular advantage of this symmetric arrangement is that the thermal expansion coefficients of the first and second crystals match in all directions at the internal interface.

[0021] When bonding crystals together, it is important to match their thermal expansion coefficients. Crystals can be bonded using commercially available polymer adhesives. For example, one light-curing optical adhesive is sold by Norland Products Inc. of Cranbury, New Jersey. These adhesives are nominally transparent over a relatively wide wavelength range, can be cured at room temperature, and the adhesive itself provides some compatibility when the bond is stressed by heating. However, such adhesive layers still absorb weakly and are degraded by high-power ultraviolet radiation. Also, sufficient heating by optical radiation or other means can cause bond failure if the crystals have different thermal expansion coefficients.

[0022] Optical contact bonding is preferred when the joint will be exposed to ultraviolet radiation (especially for high-power applications). Contact bonding forms a direct chemical bond between the crystals, eliminating an intermediate adhesive layer and therefore being extremely reliable and durable. Contact bonding is achieved by pressing extremely flat, polished surfaces together at relatively high temperatures. For example, when bonding beta-barium borate (BBO), temperatures exceed 800°C. Therefore, even small differences in thermal expansion coefficients can cause significant stresses during cooling, causing the crystals to separate or crack. BBO has a nine-fold difference between the thermal expansion coefficients along orthogonal crystal axes. The optical system of the present invention has crystals made of the same material oriented to match the thermal expansion coefficients at the internal interface 16, allowing it to be contact bonded at high temperatures without such failure due to thermal stress.

[0023] Another advantage of optical system 10 is that there is no change in refractive index when a beam having ordinary polarization propagates therethrough, so there is minimal reflection of fundamental beam 18 at internal interfaces 16. Minimizing power loss of the fundamental beam is particularly important when optical system 10 is incorporated into a laser resonator or resonant enhancement cavity. Loss of the circulating fundamental beam significantly reduces the efficiency of such a resonator, and the effects of such loss are exacerbated by the nonlinearity of second harmonic generation.

[0024] FIG. 3 is an enlarged view of the internal interface 16 as depicted in the cross-sectional side view of FIG. 2B. The major crystal axes of the first crystal 12 and the second crystal 14 are centered on the internal interface in the drawing to emphasize their symmetry with respect to the internal interface. The z-axis of the first crystal 12 is tilted at an angle φ with respect to the internal interface, and the z-axis of the second crystal 14 is tilted at an angle φ' in the opposite direction with respect to the internal interface. These tilt angles φ and φ' are the same to match the thermal expansion coefficients. The z-axis of the first crystal is at an angle θ with respect to the fundamental beam 18, which is selected to achieve phase matching. The z-axis of the second crystal is at a different angle θ' with respect to the fundamental beam 18. The second harmonic beam 20 is, in the plane of the drawing, in the paraxial approximation

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[0025] 4A and 4B schematically illustrate another preferred embodiment of an optical system 30 according to the present invention. FIG. 4A is a cross-sectional plan view of the optical system 30, and FIG. 4B is a cross-sectional side view. The optical system 30 is similar to the optical system 10, but includes an additional third crystal 32 made of the same optically nonlinear transparent material as the first crystal 12 and the second crystal 14. The first crystal 12 and the third crystal 32 are bonded together along a planar internal interface 34 that is preferably parallel to the internal interface 16. The major crystal axes of the third crystal preferably have the same orientation as the respective major crystal axes of the second crystal. An uncoated input surface 36 is located on the third crystal of the optical system 30, and the internal Brewster angle β of the fundamental beam 18 is 1 / 2 . F The input surface 36 and the output surface 26 are preferably parallel.

[0026] An advantage of optical system 30 over optical system 10 is that the fundamental beam 18 propagates through a constant path length L within first crystal 12, regardless of where it is incident on input surface 36. This constant path length allows for translation of optical system 30 without changing the overall efficiency of second-harmonic generation. Parallel input and output surfaces 36 and 26 allow for translation of optical system 30 without displacing any of the external beams. Such translation can be used to extend the useful life of optical system 30, since areas or volumes degraded or otherwise damaged by UV radiation can be shifted relative to the beam. Optical system 30 can be translated in two dimensions, as indicated by the double-headed arrow T in the drawing, making a two-dimensional array of incidence locations available on the input surface. The maximum number of incidence locations depends on the diameter of the fundamental beam relative to the dimensions of the optical system.

[0027] FIG. 5 is a side view diagram illustrating a method 40 according to the present invention for efficiently and precisely fabricating an optical system 30. A first preform 42 and a second preform 44 are bonded together along a planar interface 46. A first preform 42 and a third preform 48 are bonded together along a parallel planar interface 50. The major crystal axis of the first preform 42 is oriented relative to the parallel planar interface to provide phase matching for second harmonic generation. The second preform 44 and the third preform 48 have major crystal axes oriented symmetrically relative to the major crystal axis of the first preform 42 and oriented to match the thermal expansion coefficients in all directions along each parallel planar interface. The length L is selected to optimize second harmonic generation efficiency. The angle γ is a nominal free parameter that can be selected according to the considerations described below.

[0028] Multiple optical components 30 (two are depicted) can be fabricated from the bonded preforms of FIG. 5 by cutting the bonded preforms along cut lines 52. The material in the first, second, and third preforms becomes the first, second, and third crystals of each optical system 30, respectively. Planar interface 46 becomes internal interface 16, and planar interface 50 becomes internal interface 34. The input and output surfaces of the cut optical systems are then polished to the desired optical quality. Method 40 minimizes the amount of waste material removed in the fabrication of optical systems 30 and minimizes the number of discrete operations required to fabricate a batch of optical systems. A similar method can be used to fabricate multiple optical components 10 by cutting two oriented and bonded preforms.

[0029] A precise method for preparing the preforms is to cut them from one large block of optically nonlinear material. A first preform is cut at an angle γ along the intended planar interface 46. The cut surface is polished, and the offcut becomes the second preform. The second preform is rotated 180°, and the planar interface 46 is formed by contact-bonding the first preform to the second preform. Similarly, a third preform is cut from the first preform along the intended planar interface 50, the cut surface is polished, the third preform is rotated 180°, and the third preform is then contact-bonded to the first preform to form the planar interface 50.

[0030] FIG. 6A is a cross-sectional side view illustrating an example of an optical system 10 with an angle γ selected to maximize the separation between the remaining fundamental beam 24 and the second-harmonic beam 20 on the output surface 26. It is known that some optical nonlinear materials have lower damage thresholds and / or higher degradation rates when the surface is exposed to fundamental and UV radiation simultaneously, i.e., lower and higher, respectively, than when the surface is exposed to each radiation individually. Therefore, when using these optical nonlinear materials (especially in high-power applications), it is desirable to separate these beams. Another advantage is that the fundamental radiation does not propagate through areas on the output surface that are degraded by UV radiation, which extends the useful life of the optical system in intracavity applications.

[0031] FIG. 6A shows an example with an optical system 10 made of BBO for operation at a temperature of approximately 200°C. The fundamental wavelength is 426 nm, and the second-harmonic wavelength is 213 nm. The remaining fundamental beam 24 and second-harmonic beam 20 are depicted by their boundary rays. The fundamental beam 18 and the remaining fundamental beam 24 have diameters of approximately 330 micrometers (μm) near the internal interface 16. The angle γ is selected to be 29.25°. To achieve both phase matching in the crystal 12 and symmetry of the major crystal axes, the angle θ is 73.3°, and the angle θ' is 48.2°. This example takes advantage of the spatial walk-off of the second-harmonic beam from the fundamental beam within the first crystal 12, which is due to birefringence. The boundary rays on one side of the second-harmonic beam diverge from the fundamental beam at an angle of approximately 2.9°. After refraction at the internal interface 16, the second harmonic beam diverges from the remaining fundamental beam at an angle of approximately 6.4°. A distance d of approximately 3 millimeters (mm) between the internal interface 16 and the output surface 26 is sufficient to separate the beams on the output surface.

[0032] FIG. 6B is a cross-sectional side view showing another example of optical system 10, where angle γ is selected to maximize the overlap of the remaining fundamental beam 24 and second-harmonic beam 20 on output surface 26. Again, in the depicted embodiment, optical system 10 is made of BBO. The fundamental wavelength is 426 nm, and the second-harmonic wavelength is 213 nm. Fundamental beam 18 and remaining fundamental beam 24 have diameters of approximately 330 μm near internal interface 16. Angle γ is selected to be 12.5°. Angle θ is 106.7°, which is equivalent to an angle of 73.3° with respect to fundamental beam 18 to achieve phase matching, but with the z-axis tilted in the opposite direction relative to the fundamental beam propagation. Angle θ' is 48.3° to achieve symmetry of the major crystal axes. Changing the z-axis tilt direction in first crystal 12 causes the spatial exit of the second-harmonic beam on its opposite side. The opposite boundary ray of this second harmonic beam 20 diverges from the fundamental beam 18 at an angle of approximately 2.9°. After refraction at the internal interface 16, the second harmonic beam converges toward the remaining fundamental beam at an angle of approximately 6.4°.

[0033] Together, Figures 6A and 6B demonstrate how a nominally arbitrary angle γ can be selected to manipulate the mutual separation of the remaining fundamental and second-harmonic beams. However, a compromise exists between the reflection of the remaining fundamental beam from internal interfaces and the separation of the beams at the output surface. Alternatively, the angle γ can have any value between 0° and the maximum angle corresponding to the maximum allowable dispersion imparted to the beam while maintaining symmetry between the major crystal axes (tilt angle φ = tilt angle φ'). Preferably, the major crystal axes of the bonded crystals are separated from each other by an angle 2φ of at least 2° to minimize second-harmonic conversion in the second crystal. More preferably, the major crystal axes are separated from each other by an angle of at least 5°, and most preferably by an angle of at least 12°. For BBO, this is the mutual angular separation of the z-axes (crystallographic c-axes) of the two crystals, while the x-axis (one of the crystallographic a-axes) is common to both crystals.

[0034] The optical system of the present invention may be made of other optically nonlinear materials suitable for second harmonic generation, including lithium triborate (LBO) and cesium lithium borate (CLBO). The angle θ in the first crystal 12 is derived from the requirements of phase matching and energy conservation, respectively, which is

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[0035] Both equations (1) and (2) are F =n SH Since we require (θ), the phase matching angle θ at the first crystal 12 can be determined from equations (3) and (4).

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[0036] Referring to FIG. 2A, by applying Snell's law at the output surface 26, the divergence angle Δα and refractive index of the second harmonic beam can be calculated for a selected angle θ′.

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[0037] Referring to FIG. 2B, by applying Snell's law at the internal interface 16, the divergence angle δ can also be calculated for a selected angle θ′ and a corresponding angle γ.

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[0038] Alternatively, angle θ′ and corresponding angle γ can be determined to achieve a desired divergence angle Δα between the remaining fundamental beam and the second harmonic beam propagating away from the optical system of the present invention. For simplicity, only results relating to angle γ are provided here, while Equation 11 is simply a rearrangement of Equation 8.

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[0039] Returning to the example above, the fundamental wavelength is 426 nm and the second harmonic wavelength is 213 nm. For BBO, at a temperature of 200°C, the refractive index is n x (426)=n y (426)=1.686, n z (426)=1.561, n x (213)=n y (213)=1.850, and n z (213) = 1.671. At 426 nm, the internal Brewster angle is calculated to be β F =30.67°, the external Brewster angle is α F = 59.32°. The angle θ is 73.3° to achieve phase matching in the first crystal. In the above example, to achieve symmetry of the major crystal axes, the angle γ is selected to be 29.25°, the angle θ' is 48.2°, the tilt angle is φ = φ' = 12.55°, and the divergence angle is Δα = 2.3°. The refractive index of the second harmonic beam is n in the first crystal. SH (θ)=1.684, and in the second crystal, n SH (θ')=1.725.

[0040] 7A-7C schematically illustrate yet another preferred embodiment of an optical system 80 according to the present invention. FIG. 7A is a perspective view of the optical system 80, FIG. 7B is a plan view, and FIG. 7C is a side view. The optical system 80 is similar to the optical system 10 of FIGS. 1A-1C, but has a first output surface 82 and a second output surface 84. The first output surface 82 is positioned at an internal Brewster angle β of the fundamental radiation to minimize reflection losses for the remaining fundamental beam 24 emerging from the second crystal 14. F The second output surface 84 is oriented at an internal Brewster angle β of the second harmonic radiation to minimize reflection losses for the second harmonic beam 20 emerging from the second crystal 14. SH An exemplary impingement location 86 of the remaining fundamental beam on the first surface 82 and a corresponding exemplary impingement location 88 of the second harmonic beam on the second surface 84 are shown in FIG. 7A.

[0041] The second output surface 84 essentially eliminates the internally reflected beam 28 depicted in FIG. 1B, and all of the second harmonic radiation generated in the first crystal 12 emerges from the second surface 84. Another advantage of the optical system 80 is that the second harmonic beam is highly divergent from the remaining fundamental beam after emerging from the second surface 84. The optical system 30 can be similarly fabricated with a second output surface. The incidence locations available on the input surface are then located along a diagonal when translating the optical system.

[0042] FIG. 8 schematically illustrates a preferred embodiment of a laser system 90 according to the present invention for generating a beam of second-harmonic laser radiation 20. The laser system 90 includes a laser 92 that delivers a beam of fundamental laser radiation 18 to an impedance-matched resonant enhanced cavity 94, here defined by four cavity mirrors 96, 98, 100, and 102. The four cavity mirrors are highly reflective at the fundamental wavelength and are arranged to direct the fundamental laser beam along a closed path within the resonant enhanced cavity 94. The fundamental laser beam 18 is coupled into the resonant enhanced cavity through the cavity mirror 96. The cavity mirror 98 is mounted to a piezoelectric (PZT) transducer 104 for adjusting the optical length of the closed beam path. The PZT transducer 104 requires an electrical driver (not shown). Impedance matching is achieved by continuous adjustment of the optical length of the resonant enhanced cavity.

[0043] Laser system 90 also includes optical system 10 located in the closed beam path of resonant enhanced cavity 94 to partially convert fundamental laser beam 18 into second harmonic laser beam 20. Cavity mirror 102 is positioned and arranged to reflect the fundamental laser beam and allow the divergent second harmonic laser beam to propagate out of the resonant enhanced cavity. The inventive optical system, which spatially separates the laser beams, eliminates the need for an additional intracavity dichroic mirror to direct the second harmonic laser beam out of the resonant enhanced cavity. Either optical system 10, optical system 30, or optical system 80 can be incorporated into laser system 90.

[0044] The optical system of the present invention described herein above is particularly useful for Type I second harmonic generation. However, those skilled in the art will recognize that equivalent optical systems can be fabricated for other optical nonlinear processes, such as Type II second harmonic generation and sum frequency generation. In Type II second harmonic generation, two photons having an orthogonal linear polarization to the fundamental wavelength are converted into respective photons having a second harmonic wavelength and an ordinary linear polarization. In sum frequency generation, two photons having different fundamental wavelengths are converted into photons having a sum frequency wavelength. That is, the first fundamental wavelength λ F1and radiation having a second fundamental wavelength λ F2 The radiation having a sum frequency wavelength λ SF Here, the "fundamental wavelength" refers to a wavelength that is longer than the "sum frequency wavelength." These wavelengths are closely related.

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[0045] Sum frequency generation can be a Type I process with a common fundamental polarization, or a Type II process with different fundamental polarizations. Note that second harmonic generation is a special case of sum frequency generation, with only one fundamental wavelength and usually only one fundamental beam.

[0046] In summary, an optical system is disclosed that partially converts a fundamental beam into a second-harmonic beam and spatially separates the second-harmonic beam from the remaining fundamental beam. The optical system comprises two or three crystals made of the same optically nonlinear material bonded along planar interfaces. The major axes of the crystals have reflection symmetry about each planar interface, allowing for contact bonding of the crystals for high-power applications. The output surface of the optical system is at Brewster's angle relative to the fundamental beam to minimize reflection losses, and the output surface is uncoated to minimize optical damage. The angular separation of the second-harmonic beam and the remaining fundamental beam is determined by selecting the major crystal axes and the tilt angle of the planar interfaces relative to the fundamental beam. Importantly, these beams can be spatially separated on the output surface, further minimizing optical damage and extending the useful life of the optical system.

[0047] The present invention has been described above with reference to preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. 1. An optical system for converting radiation having a fundamental wavelength into radiation having a second harmonic wavelength, the optical system comprising: a first crystal made of an optically nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert fundamental radiation into second harmonic radiation by second harmonic generation; a second crystal made of the same optically nonlinear material as the first crystal, the first crystal and the second crystal being bonded together along a first planar interface, the first planar interface being inclined with respect to the fundamental radiation; and Equipped with 1. An optical system, wherein a major crystal axis of the first crystal and a major crystal axis of the second crystal have the same angle of separation with respect to the first planar interface and have mirror symmetry about the first planar interface, and wherein a coefficient of thermal expansion of the first crystal and a coefficient of thermal expansion of the second crystal are matched along the first planar interface.

2. 2. The optical system of claim 1, wherein a major crystal axis of the first crystal and a major crystal axis of the second crystal are oriented such that the fundamental radiation is not refracted and the second harmonic radiation is refracted at the first planar interface.

3. 3. The optical system of claim 1 or claim 2, wherein the fundamental radiation enters the optical system through an uncoated input surface at Brewster's angle relative to the fundamental radiation.

4. The optical system according to any one of claims 1 to 3, wherein the fundamental radiation exits the optical system through an uncoated output surface at Brewster's angle relative to the fundamental radiation.

5. 5. The optical system of claim 4, wherein an inclination angle of the first planar interface relative to the fundamental radiation and a distance between the first planar interface and the output surface are selected to separate the fundamental radiation from the second harmonic radiation on the output surface.

6. 5. The optical system of claim 4, wherein an angle of inclination of the first planar interface relative to the fundamental radiation and a distance between the first planar interface and the output surface are selected to cause the fundamental radiation and the second harmonic radiation to overlap on the output surface.

7. The optical system of any one of claims 4 to 6, wherein the second harmonic radiation exits the optical system through another uncoated output surface at Brewster's angle relative to the second harmonic radiation.

8. The optical system according to any one of claims 1 to 7, wherein the fundamental radiation enters and exits the optical system through a parallel uncoated surface at Brewster's angle relative to the fundamental radiation.

9. The optical system of any preceding claim, wherein the first crystal and the second crystal are bonded together by an optical contact bond.

10. 10. The optical system of claim 1, further comprising a third crystal made of the same optically nonlinear material as the first crystal and the second crystal, the third crystal being bonded to the first crystal along a second planar interface, the second planar interface being parallel to the first planar interface, the major crystal axes of the first crystal and the third crystal having the same angle of separation from the first planar interface and mirror symmetry with respect to the second planar interface, and the coefficients of thermal expansion of the first crystal and the third crystal being matched along the second planar interface.

11. The optical system of claim 10 , wherein the major crystal axes of the second crystal and the third crystal have the same orientation.

12. The optical system of claim 10 , wherein the first crystal and the third crystal are bonded together by an optical contact bond.

13. The optical system of any one of claims 1 to 12, wherein an angle of separation of a major crystal axis of the first crystal and a major crystal axis of the second crystal relative to the first planar interface is at least 5°.

14. 14. The optical system of claim 13, wherein an angle of separation of a major crystal axis of the first crystal and a major crystal axis of the second crystal relative to the first planar interface is at least 12 degrees.

15. The optical system according to any one of claims 1 to 14, wherein the second harmonic generation is Type I second harmonic generation.

16. The optical system according to any one of claims 1 to 15, wherein the optically nonlinear material is selected from the group consisting of beta-barium borate (BBO), lithium triborate (LBO), and cesium lithium borate (CLBO).

17. The optical system of any preceding claim, wherein the fundamental radiation has a wavelength of about 426 nanometers and the second harmonic radiation has a wavelength of about 213 nanometers.

18. 1. A laser apparatus for producing a beam of laser radiation at a second harmonic wavelength, said laser apparatus comprising: a laser that delivers a beam of laser radiation at a fundamental wavelength that is twice the second harmonic wavelength; an impedance-matched resonant-enhanced cavity for receiving a fundamental laser beam, the resonant-enhanced cavity being defined by a plurality of cavity mirrors, the plurality of cavity mirrors being highly reflective at the fundamental wavelength and arranged to direct the fundamental laser beam along a closed path within the resonant-enhanced cavity; an optical system disposed in a closed beam path of the resonant enhanced cavity, the optical system including a first crystal and a second crystal made of the same optically nonlinear material, the optically nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert laser radiation at the fundamental wavelength into laser radiation at the second harmonic wavelength by second harmonic generation, the first crystal and the second crystal being bonded together along a planar interface that is tilted with respect to the fundamental laser beam; Equipped with a laser device, wherein an angle of separation of a major crystal axis of the first crystal and a major crystal axis of the second crystal with respect to the first planar interface is at least 2°, the major crystal axis of the first crystal and the major crystal axis of the second crystal have mirror symmetry with respect to the planar interface, and the coefficients of thermal expansion of the first crystal and the second crystal are matched along the planar interface.

19. 20. The laser apparatus of claim 18, wherein the fundamental laser beam enters the optical system through an uncoated input surface and exits the optical system through a parallel uncoated output surface, the input surface and the output surface being at a Brewster angle relative to the fundamental laser beam.

20. 20. The laser device of claim 19, wherein a tilt angle of the planar interface relative to the fundamental laser beam and a distance between the planar interface and the output surface are selected to separate the fundamental laser beam from the second harmonic laser beam on the output surface.

21. 21. The laser device of claim 18, wherein the first crystal and the second crystal are bonded together by an optical contact bond.

22. 22. The laser device according to claim 18, wherein the second harmonic generation is type I second harmonic generation.

23. 1. An optical system for converting radiation having a first fundamental wavelength and radiation having a second fundamental wavelength into radiation having a sum frequency wavelength, the optical system comprising: a first crystal made of an optically nonlinear material having a major crystal axis, the major crystal axis of the first crystal being oriented to convert first fundamental radiation and second fundamental radiation into sum frequency radiation by sum frequency generation; a second crystal made of the same optically nonlinear material as the first crystal, the first crystal and the second crystal being bonded together along a planar interface, the planar interface being inclined with respect to the first fundamental radiation and the second fundamental radiation; and Equipped with 1. An optical system, wherein a major crystal axis of the first crystal and a major crystal axis of the second crystal have the same angle of separation with respect to the first planar interface and have mirror symmetry about the planar interface, and wherein a coefficient of thermal expansion of the first crystal and a coefficient of thermal expansion of the second crystal are matched along the planar interface.

24. 24. The optical system of claim 23, wherein the first fundamental radiation and the second fundamental radiation exit the optical system through an uncoated output surface at Brewster's angles relative to the first fundamental radiation and the second fundamental radiation.

25. 25. The optical system of claim 24, wherein an angle of inclination of the planar interface relative to the first fundamental radiation and the second fundamental radiation and a distance between the planar interface and the output surface are selected to separate the first fundamental radiation and the second fundamental radiation from the sum frequency radiation on the output surface.

26. The optical system of any of claims 23 to 25, wherein the first crystal and the second crystal are bonded together by an optical contact bond.

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