Second harmonic generating crystal

Abstract

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

Description

[0001] priority

[0002] This application claims priority to U.S. Patent Application Serial No. 16 / 801,857, filed on February 26, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention generally relates to optical devices for generating second-harmonic radiation. More particularly, it relates to optical nonlinear crystals for generating ultraviolet radiation through harmonic conversion of visible wavelength radiation and separating the ultraviolet radiation from residual visible radiation. Background Technology

[0004] In laser devices used to provide ultraviolet (UV) radiation, UV radiation is typically generated by harmonic conversion of visible wavelength radiation in an optical nonlinear crystal. Visible radiation is usually generated by second harmonic conversion of near-infrared (NIR) wavelength radiation produced in a solid-state laser (e.g., an optically pumped semiconductor (OPS) laser).

[0005] For example, in a common arrangement for generating continuous-wave UV radiation, visible radiation is generated via intracavity second harmonic conversion of NIR radiation within a solid-state laser. The visible radiation is coupled into an impedance-matched resonant enhancement cavity for visible radiation. An optical nonlinear crystal within the resonant cavity then converts the visible radiation into UV radiation via type-I second harmonic conversion. Using an OPS solid-state laser, UV radiation with wavelengths of 266 nanometers (nm) or smaller can be generated. The difficulty with type-I second harmonic conversion lies in the lack of inherent spatial separation between the generated UV radiation and the residual visible radiation.

[0006] Typically, UV radiation is guided out of the resonant cavity by a dichroic mirror with a thin-film dielectric coating. This dichroic mirror can be used as one of the resonant mirrors, reflecting visible radiation and transmitting UV radiation. Alternatively, the dichroic mirror can be a separate intracavity element that transmits visible radiation and reflects UV radiation out of the resonant cavity.

[0007] A specific limitation of the aforementioned laser devices is the damage to the thin-film dielectric coating caused by UV radiation. This limitation becomes even more problematic with shorter wavelengths of UV radiation. While intracavity elements can be translated to remove the damaged area from the radiation and expose the original area, such translation significantly increases the complexity and cost of the laser device. Furthermore, even undamaged coatings typically suffer losses due to manufacturing variations in layer thickness or absorption by the coating material. This loss reduces the efficiency of resonant cavities generated using intracavity harmonics and ultimately shortens the lifespan of these cavities.

[0008] A device for overcoming these limitations is described in U.S. Patent No. 10,474,004, assigned to the assignee of this invention, the full disclosure of which is incorporated herein by reference. An uncoated birefringent prism receives visible and UV radiation generated by an optically nonlinear crystal. The visible and UV radiation have orthogonal linear polarization. The birefringent crystal has an inner surface oriented at a Brewster angle for visible radiation and at total internal reflection for UV radiation. The birefringent crystal is an additional element with optical surfaces that must be oriented with relatively high precision. Both visible and ultraviolet radiation must pass through both surfaces of the birefringent crystal, resulting in some unavoidable reflection losses.

[0009] There is a need for a laser device that generates UV radiation and separates it from visible radiation without exposing any coating to UV radiation. Preferably, the generation and separation are accomplished while adding minimal optical components, optical surfaces, complexity, and cost to the laser device. Invention Overview

[0011] In one aspect, an optical device according to the invention for converting radiation having a fundamental wavelength into radiation having a second harmonic wavelength comprises a first crystal made of an optically nonlinear material having a principal crystal axis. The principal crystal axis of the first crystal is oriented to convert the fundamental radiation into second harmonic radiation via second harmonic generation. A second crystal is provided, made of the same optically nonlinear material as the first crystal. The first and second crystals are joined together along a first planar interface. The first planar interface is tilted relative to the fundamental radiation. The principal crystal axes of the first and second crystals are angularly separated from each other and have reflection symmetry around the first planar interface. Therefore, the coefficients of thermal expansion of the first and second crystals are matched along the first planar interface.

[0012] In another aspect, a laser device according to the invention for generating a laser radiation beam at a second harmonic wavelength includes: a laser that delivers the laser radiation beam at a fundamental wavelength twice the second harmonic wavelength; an impedance-matched resonant enhancement cavity for receiving the fundamental laser beam; the resonant enhancement cavity being defined by a plurality of cavity mirrors having high reflectivity at the fundamental wavelength and arranged to guide the fundamental laser beam along a closed path within the resonant enhancement cavity; and optics provided within the closed beam path of the resonant enhancement cavity. The optics include a first crystal and a second crystal made of the same optical nonlinear material. The optical nonlinear material has a principal crystal axis. The principal crystal axis of the first crystal is oriented to convert laser radiation at the fundamental wavelength to laser radiation at the second harmonic wavelength via second harmonic generation. The first and second crystals are joined together along a planar interface inclined relative to the fundamental laser beam. The principal crystal axes of the first and second crystals are separated from each other by an angle of at least 2°. The principal crystal axes of the first and second crystals have reflective symmetry about the planar interface. Therefore, the coefficients of thermal expansion of the first and second crystals are matched along the planar interface.

[0013] In another aspect, an optical device according to the invention for converting radiation having a first fundamental wavelength and radiation having a second fundamental wavelength into radiation having a total frequency wavelength includes a first crystal made of an optically nonlinear material having a principal crystal axis. The principal crystal axis of the first crystal is oriented to convert the first fundamental radiation and the second fundamental radiation into total frequency radiation by generating the total frequency. A second crystal is provided, made of the same optically nonlinear material as the first crystal. The first and second crystals are joined together along a planar interface. The planar interface is tilted relative to the first and second fundamental radiations. The principal crystal axes of the first and second crystals are angularly separated from each other and have reflection symmetry around the planar interface. Therefore, the coefficients of thermal expansion of the first and second crystals are matched along the planar interface. Brief description of the attached diagram

[0015] The accompanying drawings, which are included in and form part of this specification, schematically illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of preferred embodiments given below, serve to explain the principles of the invention.

[0016] Figure 1A It is a perspective view. Figure 1B It is a floor plan, and Figure 1C This is a schematic side view illustrating a preferred embodiment of an optical device according to the invention, which is used to convert a fundamental radiation beam into a second harmonic radiation beam and separate two beams, including a first crystal bonded to a second crystal along a planar interface, the separated beams exiting the optical device through an uncoated output surface.

[0017] Figure 2A It is a cross-sectional plan view, and Figure 2B It is shown schematically. Figure 1A-1C A cross-sectional side view showing further details of the optical components.

[0018] Figure 3 It is shown Figure 1A-1C A magnified cross-sectional side view showing further details of the optical components.

[0019] Figure 4A It is a cross-sectional plan view, and Figure 4B This is a schematic cross-sectional side view illustrating another preferred embodiment of the optical device according to the invention, similar to... Figure 1A-1C Optical devices, but including a third crystal bonded to the first crystal along another planar interface.

[0020] Figure 5 This is an illustrative representation of the manufacture according to the present invention. Figure 4A and 4B A side view of the method for constructing optical devices.

[0021] Figure 6A yes Figure 1A-1C An example of an optical device has a master crystal axis and a planar interface tilted at a selected angle to maximize the separation between the fundamental and second harmonic radiation beams on the output surface.

[0022] Figure 6B yes Figure 1A-1C An example of an optical device has a principal crystal axis and a planar interface tilted at a selected angle to maximize the overlap of the fundamental and second harmonic radiation beams on the output surface.

[0023] Figure 7A It is a perspective view. Figure 7B It is a floor plan, and Figure 7C This is a schematic side view illustrating yet another preferred embodiment of the optical device according to the invention, similar to... Figure 1A-1C The optical device has a fundamental radiation beam that exits through an uncoated output surface, while the second harmonic radiation beam exits through another uncoated output surface.

[0024] Figure 8 An embodiment of a laser system according to the present invention is schematically illustrated, including a laser, a resonant enhancement cavity, and... Figure 1A-1C Optical devices. Invention Details

[0026] Referring now to the accompanying drawings, where the same parts are indicated by the same numbers. Figure 1A-1C A preferred embodiment of the optical device 10 according to the present invention is illustrated schematically. Figure 1AThis is a perspective view of optical component 10. Figure 1B It is a floor plan, and Figure 1C This is a side view. The outer edge of the optical device 10 is emphasized by a thick line in the figure. The optical device 10 includes a first crystal 12 and a second crystal 14 made of the same transparent optical nonlinear material. The first crystal 12 and the second crystal 14 are joined together along an in-plane interface 16.

[0027] The first crystal 12 is oriented and arranged to convert a fundamental radiation beam 18 with a fundamental wavelength into a second harmonic radiation beam 20 with a second harmonic wavelength via Type I second harmonic generation. Two photons from the fundamental beam 18 are converted into one photon from the second harmonic beam 20. For energy conservation, the fundamental wavelength is twice the second harmonic wavelength. In Type I second harmonic generation, the fundamental radiation has ordinary linear polarization, while the second harmonic radiation has superordinary linear polarization.

[0028] Here, "orientation" refers to the orientation of the principal crystal axis relative to the polarization plane and the propagation direction of the fundamental beam 18 to achieve momentum conservation or "phase matching." Phase matching utilizes the birefringence of optically nonlinear materials and is necessary for effective second harmonic generation. The principal crystal axis of the second crystal 14 has a different orientation relative to the fundamental beam 18. Therefore, crystal 14 has no phase matching and does not produce significant second harmonic radiation.

[0029] Here, the fundamental beam 18 enters the optics 10 through the uncoated input surface 22, radiating at the internal Brewster angle β of the fundamental wave. F Orientation to minimize reflection loss. Another arrangement would have an input surface 22 perpendicularly incident on the fundamental beam 18 and an anti-reflection coating on the input surface 22 to minimize reflection loss. The fundamental beam 18 is partially converted into a second harmonic beam 20 as it propagates through the first crystal 12. The second harmonic beam and the residual fundamental radiation beam 24 are... Figure 1B It is incident on the internal interface 16 at a perpendicular angle in the plane, and Figure 1C The beam is incident on the inner interface 16 at a non-perpendicular angle γ in the plane. The optical nonlinear material has different refractive indices for orthogonally polarized beams. The ordinary polarized fundamental beam has the same refractive index in each crystal, while the super-orthogonally polarized second harmonic beam has a different refractive index, which causes the second harmonic beam to be refracted away from the residual fundamental beam at the inner interface 16.

[0030] Both the residual fundamental beam 24 and the second harmonic beam 20 exit the optics 10 through the uncoated output surface 26 at an internal Brewster angle β relative to the residual fundamental beam. FOrientation is achieved to minimize reflection loss at the fundamental wavelength. The input surface 22 and output surface 26 are parallel, ensuring that the entire optics 10 does not cause angular displacement of the fundamental radiation; that is, the fundamental beam 18 and the residual fundamental beam 24 propagate in the same direction outside the optics 10. The different refractive indices of the orthogonally polarized second harmonic and the residual fundamental beam cause these beams to be refracted at different angles by the output surface 26, generating additional internal reflection beams 28 at the second harmonic wavelength, such as... Figure 1B As shown, the refracted residual fundamental beam 24 and second harmonic beam 20 diverge and spatially separate as they propagate away from the output surface 26 of the optics 10.

[0031] Figure 2A and 2B This is a cross-sectional view schematically showing further details of the optical device 10. Figure 2A It is a floor plan, and Figure 2B These are side views. For clarity, the internal beams are omitted from these drawings. Figure 2A The outer Brewster angle α of fundamental beam 18 is represented. F and the external angle α of the second harmonic beam 20 emitted from the output surface 26 SH These beams of light are at an angle Δα. Figure 2A In the plane and at an angle δ Figure 2B Offset in the plane.

[0032] The principal crystal axes x, y, and z are oriented in the first crystal 12 and the second crystal 14, as follows: Figure 2A and 2B As shown, the principal crystal axes are symmetrically oriented with respect to the internal interface 16. Specifically, the principal crystal axes exhibit reflection symmetry around the internal interface 16. A particular advantage of this symmetrical arrangement is that the thermal expansion coefficients of the first and second crystals match in all directions of the internal interface.

[0033] When bonding crystals together, matching their coefficients of thermal expansion is important. Crystals can be bonded using commercially available polymer adhesives. For example, a photocurable optical adhesive is available from Nolan Products, Inc., Cranbury, New Jersey. These adhesives are nominally transparent over a relatively wide wavelength range, cure at room temperature, and provide a degree of compliance when stress is applied to the adhesive by heating. However, such an adhesive layer is still weakly absorbent and degraded by high-power ultraviolet radiation. If the crystals have different coefficients of thermal expansion, sufficient heating by light radiation or other means will cause the bond to fail.

[0034] Optical contact bonding is preferred when the bond is exposed to ultraviolet radiation, especially in high-power applications. Contact bonding forms a direct chemical bond between the crystals, eliminating the need for an intermediate adhesive layer, thus making it highly reliable and durable. Contact bonding is achieved by pressing very flat, polished surfaces together at relatively high temperatures. For example, temperatures exceeding 800°C are observed when bonding barium β-borate (BBO). Therefore, even small differences in the coefficients of thermal expansion can cause significant stress during cooling, leading to crystal separation or cracking. The difference in the coefficients of thermal expansion between BBO crystals along their orthogonal axes is ninefold. The optics of this invention have crystals made of the same material, whose orientation matches the coefficient of thermal expansion at the internal interface 16, enabling contact bonding at high temperatures without such failure due to thermal stress.

[0035] Another advantage of the optics 10 is that the fundamental beam 18 experiences minimal reflection at the internal interface 16 because the refractive index remains unchanged as a beam with normal polarization propagates through it. Minimizing the power loss of the fundamental beam is particularly important when the optics 10 is incorporated into a laser resonator or resonant enhancement cavity. The loss of the cyclic fundamental beam significantly reduces the efficiency of such a resonator, and the nonlinearity generated by the second harmonic amplifies the effect of this loss.

[0036] Figure 3 Is Figure 2B An enlarged view of the internal interface 16 depicted in the cross-sectional side view. The principal crystal axes of the first crystal 12 and the second crystal 14 are centered on the internal interface in the figure to emphasize their symmetry around the internal interface. The z-axis of the first crystal 12 is tilted at an angle φ relative to the internal interface, and the z-axis of the second crystal 14 is tilted at an angle φ′ relative to the internal interface in opposite directions. These tilt angles φ and φ′ are the same to match the coefficient of thermal expansion. The z-axis of the first crystal is tilted at an angle θ relative to the fundamental beam 18, chosen to achieve phase matching. The z-axis of the second crystal is tilted at a different angle θ′ relative to the fundamental beam 18. The second harmonic beam 20 is approximated to the internal angle paraxially in the plane of the figure. Refraction from the residual fundamental beam 24. Here, n F The refractive index of the fundamental wave in the two crystals, n SH (θ) is the refractive index of the second harmonic beam in the first crystal, and n SH (θ′) is the refractive index of the second harmonic beam in the second crystal.

[0037] Figure 4A and 4B Another preferred embodiment of the optical device 30 according to the present invention is illustrated schematically. Figure 4A It is a cross-sectional plan view of optical device 30, and Figure 4BThis is a cross-sectional side view of optical device 30. Optical device 30 is similar to optical device 10, but has 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 plane internal interface 34, preferably parallel to the internal interface 16. The principal crystal axis of the third crystal preferably has the same orientation as the responsive principal crystal axis of the second crystal. An uncoated input surface 36 is located on the third crystal of optical device 30 and at the internal Brewster angle β of the fundamental beam 18. F Orientation. The input surface 36 and the output surface 26 are preferably parallel.

[0038] The advantage of optics 30 over optics 10 is that the fundamental beam 18 propagates through the first crystal 12 along a constant path length L, independent of its incident position on the input surface 36. This constant path distance allows optics 30 to be translated without altering the overall efficiency of second harmonic generation. The parallel input surface 36 and output surface 26 allow optics 30 to be translated without moving any external beam. This translation can be used to extend the lifespan of optics 30, as areas or volumes degraded by UV radiation or otherwise damaged can be moved relative to the beam. Optics 30 can be translated in two dimensions, as indicated by the double-headed arrow T in the figure, making a two-dimensional array of incident positions available on the input surface. The maximum number of incident positions will depend on the diameter of the fundamental beam relative to the size of the optics.

[0039] Figure 5 This is a schematic side view of a method 40 according to the invention for efficiently and precisely manufacturing an optical device 30. A first preform 42 and a second preform 44 are joined together along a planar interface 46. A first preform 42 and a third preform 48 are joined together along parallel planar interfaces 50. The principal crystal axis of the first preform 42 is oriented relative to the parallel planar interfaces to provide phase matching for second harmonic generation. The second preform 44 and the third preform 48 have principal crystal axes symmetrically oriented relative to the principal crystal axis of the first preform 42, and are oriented to match the coefficients of thermal expansion in all directions along each parallel planar interface. A length L is selected to optimize the second harmonic generation efficiency. Angle γ is a nominally free parameter and can be selected based on considerations described below.

[0040] By cutting the preform along cutting line 52, it is possible to... Figure 5Multiple optical devices 30 (two shown) are fabricated from combined preforms. The material within the first, second, and third preforms becomes the first, second, and third crystals of each optical device 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 devices are then polished to the desired optical quality. Method 40 minimizes the total volume of waste removed during the fabrication of optical devices 30 and minimizes the number of discrete operations required to fabricate a batch of optical devices. A similar method can be used to fabricate multiple optical devices 10 by cutting two oriented and combined preforms.

[0041] One precise method for fabricating preforms is to cut them from a large piece of optically nonlinear material. A first preform is cut at an angle γ along a predetermined planar interface 46. The cut surface is polished, and the cut portion becomes a second preform. The second preform is rotated 180 degrees, and the planar interface 46 is formed by contacting and bonding the first preform with the second preform. Similarly, a third preform is cut from the first preform along a predetermined planar interface 50, the cut surface is polished, the third preform is rotated 180 degrees, and then the third preform is contacted and bonded with the first preform to form the planar interface 50.

[0042] Figure 6A This is a cross-sectional side view illustrating an example of an optics device 10 with an angle γ, chosen to maximize the separation between the residual fundamental beam 24 and the second harmonic beam 20 on the output surface 26. For some optical nonlinear materials, it is known that when a surface is simultaneously exposed to both fundamental and UV radiation, the damage threshold is low and / or the degradation rate is high; these are 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 necessary to separate these beams. Another advantage is that the fundamental radiation does not propagate through areas on the output surface that have been degraded by UV radiation, which extends the lifespan of the optics in in-cavity applications.

[0043] Figure 6AA specific example of an optical device 10 made of BBO for operation at approximately 200°C is depicted. The fundamental wavelength is 426 nm and the second harmonic wavelength is 213 nm. The residual fundamental beam 24 and the second harmonic beam 20 are depicted by their boundary rays. The diameters of the fundamental beam 18 and the residual fundamental beam 24 are approximately 330 micrometers (μm) and are located near the internal interface 16. The angle γ is chosen to be 29.25°. The angle θ is 73.3° and the angle θ′ is 48.2° to achieve phase matching and symmetry of the principal crystal axis in the crystal 12. This example utilizes the spatial separation of the second harmonic beam from the fundamental beam within the first crystal 12 due to birefringence. The boundary ray on one side of the second harmonic beam diverges 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 residual fundamental beam at an angle of approximately 6.4°. The distance d between the internal interface 16 and the output surface 26 is about 3 millimeters (mm), which is sufficient to separate the beam on the output surface.

[0044] Figure 6B This is a cross-sectional side view showing another example of the optics 10, but here the angle γ is chosen to maximize the overlap between the residual fundamental beam 24 and the second harmonic beam 20 on the output surface 26. Again, in the specific example shown, the optics 10 is made of BBO. The fundamental wavelength is 426 nm, and the second harmonic wavelength is 213 nm. The diameters of the fundamental beam 18 and the residual fundamental beam 24 are approximately 330 nm, located near the inner interface 16. The angle γ is chosen to be 12.5°. The angle θ is 106.7°, which corresponds to an angle of 73.3° relative to the fundamental beam 18, to achieve phase matching, but the z-axis is tilted in the opposite direction relative to the fundamental beam propagation. The angle θ′ is 48.3° to achieve symmetry of the principal crystal axis. Changing the z-axis tilt direction in the first crystal 12 causes the second harmonic beam to move away from space on its opposite side. The boundary ray on this opposite side of the second harmonic beam 20 diverges from the fundamental beam 18 at an angle of approximately 2.9°. After refraction at 16 points on the internal interface, the second harmonic beam converges toward the residual fundamental beam at an angle of approximately 6.4°.

[0045] Figure 6A and 6BTogether, we demonstrate how a nominal free angle γ can be selected to manipulate the mutual separation of the residual fundamental beam and the second harmonic beam. However, there is a trade-off between reflecting the residual fundamental beam from the internal interface and separating the beam on the output surface. Otherwise, the angle γ can have any value between 0° and a maximum angle corresponding to the maximum acceptable dispersion imparted to the beam, while maintaining the symmetry between the principal crystal axes (tilt angle φ = tilt angle φ′). Preferably, the principal crystal axes of the combined crystals are separated from each other by an angle of at least 2° 2φ to minimize the second harmonic conversion in the second crystal. More preferably, the principal 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-axis (c-axis of crystallization) of the two crystals, while the x-axis (one of the a-axis of crystallization) is common to both crystals.

[0046] The optical device of the present invention can be made of other optical nonlinear materials suitable for second harmonic generation, including lithium triborate (LBO) and lithium cesium borate (CLBO). The angle θ in the first crystal 12 is derived from the requirements of phase matching and energy conservation, and can be expressed as:

[0047] k F +k F =k SH (1)

[0048]

[0049] Where k is the wave vector amplitude, and λ is the wavelength of the fundamental beam and the second harmonic beam. (Reference) Figure 3 The polarization-based beam at wavelength λ F The refractive index at a certain point and the orthogonally polarized second harmonic beam at wavelength λ SH The refractive indices at these locations are:

[0050] n F =n x (λ F (3)

[0051]

[0052] Equations (1) and (2) together require n F =n SH (θ), therefore the phase matching angle θ in the first crystal 12 can be determined by equations (3) and (4):

[0053]

[0054] refer to Figure 2A By applying Snell's law at the output surface 26, the divergence angle Δα and the refractive index of the second harmonic beam can be calculated for a selected angle θ′:

[0055] Δα=α SH -α F =sin -1 (n SH (θ′).sin(β F ))-α F (6)

[0056]

[0057] refer to Figure 2B By applying Snell's law at point 16 on the internal interface, the divergence angle δ can be calculated for a selected angle θ′ and the corresponding angle γ:

[0058]

[0059] δ=γ(n SH (θ′)-n SH (θ)) . (9)

[0060] Alternatively, angles θ′ and the corresponding angle γ can determine the desired divergence angle Δα between the residual fundamental beam and the second harmonic beam propagating away from the optics of this invention. For simplicity, only the result for angle γ is provided here, and Equation 11 is simply a rearrangement of Equation 8:

[0061]

[0062] θ′=180°-θ-2γ. (11)

[0063] Returning to the example above, the fundamental wavelength is 426 nm and the second harmonic wavelength is 213 nm. In BBO, at a temperature of 200°C, the refractive index is calculated as: n x (426)=n y (426) = 1.686, n z (426)=1.561n x (213)=n y (213) = 1.850, and n z (213) = 1.671. At 426 nm, the internal Brewster angle β F = 30.67° and the outer Brewster angle α F = 59.32°. Angle θ is 73.3° to achieve phase matching in the first crystal. For the specific example above, angle γ is chosen to be 29.25° and angle θ′ is 48.2° to achieve axial symmetry of the master crystal, tilt angle φ = φ′ = 12.55°, and divergence angle Δα = 2.3°. The refractive index n of the second harmonic beam in the first crystal is... SH(θ) = 1.684, and in the second crystal n SH (θ′)=1.725.

[0064] Figures 7A-7C Another preferred embodiment of the optical device 80 according to the present invention is illustrated schematically. Figure 7A This is a perspective view of optical component 80. Figure 7B It is a floor plan, and Figure 7C This is a side view. Optical component 80 is similar to... Figure 1A-1C The optical device 10 has a first output surface 82 and a second output surface 84. The first output surface 82 radiates at an internal Brewster angle β of the fundamental wave. F Orientation is chosen to minimize the reflection loss of the residual fundamental beam 24 emitted from the second crystal 14. The second output surface 84 is oriented at the internal Brewster angle β of the second harmonic radiation. SH Orientation to minimize reflection loss of the second harmonic beam 20 emitted from the second crystal 14. An exemplary incident position 86 of the residual fundamental beam on the first surface 82 and a corresponding exemplary incident position 88 of the second harmonic beam on the second surface 84 are... Figure 7A As shown in the image.

[0065] The second output surface 84 essentially eliminates... Figure 1B The internally reflected beam 28, and all second-harmonic radiation generated in the first crystal 12, exit from the second surface 84. Another advantage of the optics 80 is that the second-harmonic beam diverges significantly from the remaining fundamental beam after exiting from the second surface 84. The optics 30 can be similarly fabricated with a second output surface. When the optics are translated, the available incident positions on the input surface will be set diagonally.

[0066] Figure 8 A preferred embodiment of a laser system 90 according to the invention for generating a second-harmonic laser radiation beam 20 is schematically illustrated. The laser system 90 includes a laser 92 that delivers a fundamental laser radiation beam 18 to an impedance-matched resonant enhancement cavity 94, defined here by four cavity mirrors 96, 98, 100, and 102. The four cavity mirrors are highly reflective at the fundamental wavelength and are arranged to guide the fundamental laser beam along a closed path within the resonant enhancement cavity 94. The fundamental laser beam 18 is coupled into the resonant enhancement cavity via cavity mirror 96. Cavity mirror 98 is mounted on 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 continuously adjusting the optical length of the resonant enhancement cavity.

[0067] The laser system 90 also includes optics 10 located in the closed beam path of the resonant enhancement cavity 94 to partially convert the fundamental laser beam 18 into a second harmonic laser beam 20. A cavity mirror 102 is positioned and arranged to reflect the fundamental laser beam and allow the diverging second harmonic beam to propagate out of the resonant enhancement cavity. The spatially separated laser beam optics of the present invention eliminate the need for an additional intracavity dichroic mirror to guide the second harmonic laser beam out of the resonant enhancement cavity. Any of optics 10, 30, or 80 can be incorporated into the laser system 90.

[0068] The optical device of the present invention described above is particularly useful for type I second harmonic generation. However, those skilled in the art will recognize that equivalent optical devices can be fabricated for other optical nonlinear processes, such as type II second harmonic generation and total frequency generation. In type II second harmonic generation, two photons having a fundamental wavelength and orthogonal linear polarization are converted into each photon having a second harmonic wavelength and ordinary linear polarization. In total frequency generation, two photons with different fundamental wavelengths are converted into each photon having a total frequency wavelength, i.e., having a first fundamental wavelength λ. F1 Radiation and having a second fundamental wavelength λ F2 The radiation is converted into a wavelength with a total frequency λ. SF The radiation. Here, "fundamental wavelength" refers to a wavelength longer than the total frequency wavelength. These wavelengths have an approximate relationship:

[0069]

[0070] Total frequency generation can be a Type I process with a common fundamental polarization or a Type II process with different fundamental polarizations. It should be noted that second harmonic generation is a special case of total frequency generation, having only one fundamental wavelength and typically only one fundamental beam.

[0071] In summary, an optical device is disclosed that partially converts a fundamental beam into a second harmonic beam and spatially separates the second harmonic beam from the residual fundamental beam. The optical device of the present invention comprises two or three crystals made of the same optically nonlinear material bonded along planar interfaces. The principal axes of the crystals have reflective symmetry about each planar interface, enabling the crystals to be contact-bonded for high-power applications. The output surface of the optical device of the present invention is at a Brewster angle relative to the fundamental beam to minimize reflection loss, and the output surface is uncoated to minimize optical damage. The angular separation between the second harmonic beam and the residual fundamental beam is determined by selecting the tilt angles of the principal crystal axes and the planar interfaces relative to the fundamental beam. Importantly, these beams can be spatially separated on the output surface to further minimize optical damage and extend the lifespan of the optical device.

[0072] The present invention has been described above with reference to preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the appended claims.

Claims

1. An optical device for converting radiation having a fundamental wavelength into radiation having a second harmonic wavelength, comprising: The first crystal is made of an optical nonlinear material with a principal crystal axis, the principal crystal axis of the first crystal being oriented to convert fundamental radiation into second harmonic radiation through second harmonic generation; The second crystal is made of the same optical nonlinear material as the first crystal, and the first and second crystals are joined together along a first planar interface that is tilted relative to the fundamental radiation. The principal crystal axes of the first and second crystals are angularly separated and have reflective symmetry around the first planar interface, so the thermal expansion coefficients of the first and second crystals match along the first planar interface.

2. The optical device according to claim 1, wherein, The principal crystal axes of the first and second crystals are oriented such that the fundamental wave radiation is not refracted and the second harmonic radiation is refracted at the first planar interface.

3. The optical device according to claim 1 or 2, wherein, The fundamental wave radiation enters the optics at a Brewster angle relative to the uncoated input surface of the fundamental wave radiation.

4. The optical device according to claim 1 or 2, wherein, The fundamental wave radiation exits the optical device at a Brewster angle relative to the uncoated output surface of the fundamental wave radiation.

5. The optical device according to claim 4, wherein, The tilt angle of the first planar interface relative to the fundamental radiation and the 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. The optical device according to claim 4, wherein, The tilt angle of the first planar interface relative to the fundamental radiation and the distance between the first planar interface and the output surface are selected to overlap the fundamental radiation with the second harmonic radiation on the output surface.

7. The optical device according to claim 4, wherein, The second harmonic radiation exits the optics at a Brewster angle relative to the additional uncoated output surface of the second harmonic radiation.

8. The optical device according to claim 1 or 2, wherein, The fundamental wave radiation enters and exits the optical device at a Brewster angle through a parallel, uncoated surface relative to the fundamental wave radiation.

9. The optical device according to claim 1 or 2, wherein, The first and second crystals are joined together by optical contact.

10. The optical device according to claim 1 or 2, further comprising a third crystal made of the same optical nonlinear material as the first and second crystals, the third crystal being bonded to the first crystal along a second planar interface parallel to the first planar interface, the principal crystal axes of the first and third crystals having angular separation from each other and having reflective symmetry around the second planar interface, such that the coefficients of thermal expansion of the first and third crystals are matched along the second planar interface.

11. The optical device according to claim 10, wherein, The principal crystal axes of the second and third crystals have the same orientation.

12. The optical device according to claim 10, wherein, The first and third crystals are joined together by optical contact.

13. The optical device according to claim 1 or 2, wherein, The principal crystal axes of the first and second crystals are spaced at least 5° apart.

14. The optical device according to claim 13, wherein, The principal crystal axes of the first and second crystals are spaced at least 12° apart.

15. The optical device according to claim 1 or 2, wherein, The second harmonic generation is a type I second harmonic generation.

16. The optical device according to claim 1 or 2, wherein, The optical nonlinear material is selected from the group consisting of barium β-borate (BBO), lithium triborate (LBO), and lithium cesium borate (CLBO).

17. The optical device according to claim 1 or 2, wherein, The fundamental radiation has a wavelength of approximately 426 nanometers, and the second harmonic radiation has a wavelength of approximately 213 nanometers.

18. A laser device for generating a laser radiation beam at a second harmonic wavelength, comprising: A laser that delivers a laser radiation beam at twice the fundamental wavelength of the second harmonic; An impedance-matched resonant enhancement cavity for receiving a fundamental laser beam, the resonant enhancement cavity being defined by a plurality of cavity mirrors having high reflectivity at the fundamental wavelength and being arranged to guide the fundamental laser beam along a closed path within the resonant enhancement cavity; and An optical device located within the closed beam path of the resonant enhancement cavity, the optical device comprising a first crystal and a second crystal made of the same optical nonlinear material having a principal crystal axis, the principal crystal axis of the first crystal being oriented to convert laser radiation at the fundamental wavelength into laser radiation at the second harmonic wavelength through second harmonic generation, the first and second crystals being joined together along a planar interface inclined relative to the fundamental laser beam; The principal crystal axes of the first and second crystals are separated by an angle of at least 2°, and the principal crystal axes of the first and second crystals have reflective symmetry around the planar interface. Therefore, the thermal expansion coefficients of the first and second crystals are matched along the planar interface.

19. The laser device according to claim 18, wherein, The fundamental laser beam enters the optical device through an uncoated input surface and exits the optical device through a parallel uncoated output surface, wherein the input and output surfaces are at a Brewster angle relative to the fundamental laser beam.

20. The laser device according to claim 19, wherein, The tilt angle of the planar interface relative to the fundamental laser beam and the 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. The laser device according to any one of claims 18 to 20, wherein, The first and second crystals are joined together by optical contact.

22. The laser device according to any one of claims 18 to 20, wherein, The second harmonic generation is a type I second harmonic generation.

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