Preparation method for nonlinear optical crystal continuous-wave frequency converter, and operating method

Abstract

The present invention relates to a preparation method for a trigonal nonlinear optical crystal frequency converter and an operating method. The frequency converter has a first group of parallel surfaces and a second group of parallel surfaces, wherein the first group of parallel surfaces is parallel to a c-axis and an a-axis of an optical crystal, an angle α is formed between a normal of the second group of parallel surfaces and the c-axis, and an included angle β is formed between the projection of the normal on an a-b plane and the a-axis; and θPM having a relation shown in Formula (1) and Formula (2) is a phase matching angle of the nonlinear optical crystal satisfying an incident fundamental frequency light and an emergent variable frequency light, where θB=arctann, which is a Brewster angle, n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light, and the second group of parallel surfaces includes a light transmission input surface and a light transmission output surface of the frequency converter. The present invention provides, on the basis of a thin sheet-like primary crystal, a preparation method for a nonlinear optical crystal frequency converter having a length in a light transmission direction as large as possible and suitable for continuous wave application and an operating method.

Description

Fabrication and Operation Methods of Nonlinear Optical Crystal Continuous Wave Frequency Conversion Devices Technical Field

[0001] This disclosure relates to the field of artificial crystal optical devices, and in particular to the technique of fabricating Brewster frequency doubling devices using thin-film nonlinear optical uniaxial crystals. Background Technology

[0002] Deep ultraviolet (UV) lasers (wavelengths less than 200 nm) have extremely important applications in information technology, scientific research, and national defense. Multi-stage frequency doubling technology using nonlinear optical crystals is currently the most effective way to realize UV ​​lasers, with the final stage frequency-doubling crystal being called a UV nonlinear optical crystal. Currently, only potassium fluoroborylate (KBe₂BO₃F₂, abbreviated as KBBF) and rubidium fluoroborylate (RbBe₂BO₃F₂, abbreviated as RBBF) meet the requirements for UV nonlinear optical crystals, enabling direct frequency doubling to output lasers with wavelengths less than 200 nm. In particular, it can achieve a sixth-harmonic (177.3 nm) output of Nd:YAG lasers, used for the development of advanced scientific instruments, and a fourth-harmonic tuning output of Ti:Sapphire lasers, with the 193 nm laser having important applications in chip manufacturing. The overall performance of KBBF crystals is significantly better than that of RBBF crystals.

[0003] KBBF crystals exhibit a strong layered growth habit, resulting in a complete primary crystal that is approximately a thin, hexagonal sheet. The normal direction of the large facet of this hexagon is the crystallographic c-axis (also known as the crystal c-axis or c-axis). The hexagon is a plane formed by the crystal's ab directions. The a-axis and b-axis are two equivalent crystallographic axes, forming a 120° angle (physically including three equivalent a-axiss), and both the a-axis and b-axis are perpendicular to the c-axis. The crystal can grow to several centimeters along the a-axis and b-axis directions, while the thickness d along the c-axis direction does not exceed 10 mm, and generally does not exceed 5 mm. To facilitate the description of the crystal's physical properties, this paper establishes a physical rectangular coordinate system according to the following rules: by convention, the crystal c-axis direction is defined as the Z-axis (hereinafter, the c-axis is the Z-axis), the a-axis direction is defined as the X-axis (hereinafter, the a-axis is the X-axis), and the direction perpendicular to the a-axis and c-axis is defined as the Y-axis, as shown in Figure 1. For a nonlinear optical crystal to achieve laser frequency doubling, the fundamental frequency light must travel along a specific phase-matching direction WO within the crystal. This direction is represented by the azimuth angle θ in a Cartesian coordinate system. PM And φ represent, where θ PM Let φ be the angle between the phase-matching direction and the Z-axis, and let φ be the angle between the projection of the phase-matching direction onto the XY plane (i.e., the ab plane) and the X-axis. KBBF crystal belongs to the trigonal crystal system; the c-axis is the optical axis of the crystal, and its structural symmetry determines its effective nonlinear coefficient d. eff The expression is d eff =d 11 cos(θ PMcos3φ, d 11 This is the nonlinear optical coefficient, approximately equal to 0.49 pm / V. It can be seen that the maximum effective nonlinear coefficient d is obtained when φ = 0°. eff That is, to obtain the maximum frequency doubling optical conversion efficiency.

[0004] Figure 2 schematically illustrates a first example of a prior art frequency-doubling optical device fabricated from a thin-film precursor crystal. For example, a cuboid ABCD-EFGH is cut from a hexagonal KBBF thin-film precursor crystal 10 along the phase-matching direction WO, with its four edges AE, BF, CG, and DH all parallel to the phase-matching direction WO. The two parallel end faces ABCD and EFGH are optically polished and used as the light-transmitting input and output surfaces, respectively, forming a frequency doubler 11 for pulsed laser frequency doubling output. The fundamental frequency laser travels along the phase-matching direction WO, enters the crystal from end face ABCD, and is continuously transformed into frequency-doubled light within the crystal. Finally, both the frequency-doubled light and the remaining fundamental frequency light exit the crystal from end face EFGH. Figure 3 shows the projection of the original crystal 10 from Figure 2 and the frequency multiplier 11 cut from it onto the XZ plane. In this example, L is the light transmission length of the frequency multiplier 11, h represents the distance between parallel surfaces ADHE and BCGF (also called the first side length of the light transmission surface), and k represents the distance between parallel surfaces ABFE and DCGH (also called the second side length of the light transmission surface), for example, AB = h, AD = k, and the side length represented by k is parallel to the XY plane. As can be seen from Figure 3, the light transmission length L is first limited by the crystal's thickness d in the Z direction, and secondly by the first side length h. Although the KBBF crystal has relatively large dimensions in the a and b directions, this does not help increase the light transmission length L. The maximum value of the light transmission length L... max d / cosθ PM At this point, h = 0. The length h of the first side of the light-transmitting surface of frequency multiplier 11 should not be too large, because the length L in the light-transmitting direction is given by the formula L = (d / cosθ). PM –h×tanθ PM It is determined that as the first side length h increases, the length L in the light transmission direction will decrease. Therefore, the side length h is generally chosen to be 1mm. To ensure the stability and lifespan of the frequency multiplier, a method of changing the incident point is often used. That is, after the laser works at a certain point on the light transmission surface for a certain period of time, it is moved to another point to continue working. This process is repeated until the entire area of ​​the light transmission surface has been traversed before a new frequency multiplier is installed. By reasonably selecting the size of the second side length k of the light transmission surface and changing the incident point along the direction of the second side length k, the requirement of changing the incident point during laser frequency multiplication can be met.

[0005] For continuous-wave laser frequency doubling, the higher the transmittance of the frequency doubler for the fundamental frequency laser, the better, ideally approaching 100%. Therefore, improvements are needed to the frequency doubler 11 shown in Figure 2, typically through two technical solutions. The first is to coat the two light-transmitting surfaces ABCD and EFGH with an anti-reflection coating, allowing the fundamental frequency light to pass through these surfaces with near-loss. However, the anti-reflection coating has a low laser damage threshold, one to two orders of magnitude lower than the crystal's own laser damage threshold, making it easily damaged by the laser. Applying the anti-reflection coating to a KBBF crystal continuous-wave frequency doubler essentially negates the excellent characteristic of KBBF crystal's high laser damage threshold. Therefore, continuous-wave frequency doublers coated with anti-reflection coatings are generally only suitable for low-power continuous-wave laser frequency doubling.

[0006] The second technical solution is to align both light-transmitting surfaces according to the Brewster angle θ. B Cutting, θ B =arctann, where n is the refractive index corresponding to the wavelength of the incident fundamental frequency light in the nonlinear optical crystal. When the polarization direction of the fundamental frequency light lies within the incident plane (the plane formed by the incident laser and the normal to the light-transmitting surface is the incident plane), the fundamental frequency light can pass through the light-transmitting input surface and the light-transmitting output surface almost without loss. Figure 4-6 schematically shows a second example of the prior art, which uses a thin-film primary crystal to prepare a light with a Brewster angle θ. B A schematic diagram of the frequency multiplier 12 is shown in Figure 2. The two light-transmitting surfaces ABCD and EFGH of the frequency multiplier ABCD-EFGH are further aligned according to the Brewster angle θ. B The process involves cutting away two right-angled triangular prisms from the cuboid ABCD-EFGH to form a right parallelepiped AD'HE'-BC'GF', resulting in frequency multiplier 12. The light-transmitting surfaces of frequency multiplier 12 are ABC'D' and E'F'GH. As shown in the projection diagram of Figure 6, the dashed lines represent light rays passing through surfaces with a Brewster angle θ. B The light travels through the crystal, and the length of the light transmission direction becomes L'. This scheme inevitably reduces the length of the light transmission direction while increasing the incident laser transmittance. As shown in Figures 5 and 6, the length of the light transmission direction L' = Lk × ctgθ B Given a fixed optical transmission direction length L, the larger the distance k between surfaces ABF'E' and D'C'GH, the shorter the optical transmission direction length L'. That is, in Brewster-cut devices, the optical transmission direction length is limited not only by the crystal's Z-direction thickness d and the first side length h, but also by the second side length k. As mentioned earlier, to meet the frequency multiplier switching requirements, since the distance h between parallel surfaces AD'HE' and BC'GF' is limited, the value of the distance k between parallel surfaces ABF'E' and D'C'GH needs to be increased. However, the wider the distance k, the shorter the optical transmission direction length; that is, the optical transmission direction length and device width cannot be simultaneously maximized. The frequency multiplication efficiency is proportional to the square of the optical transmission direction length; a too-short optical transmission direction length directly affects the frequency multiplication conversion efficiency.

[0007] In summary, due to the extreme thinness of KBBF crystals, the ability to directly cut frequency doubling devices with sufficiently long light transmission lengths from the original crystal is severely limited, making it difficult to achieve practical applications for such devices.

[0008] The practical application of KBBF crystals currently relies on sandwich-structured devices fabricated using a so-called prism coupling technique, detailed in Chinese patent application CN1381930A. Due to the addition of two optically bonded interfaces between the KBBF crystal and the prism, these heterogeneous interfaces cannot be perfect, inevitably resulting in scattering and absorption, as well as unavoidable Fresnel reflection due to the difference in refractive indices between the crystal and prism materials. Therefore, prism-coupled devices have two drawbacks: first, the device's resistance to optical damage is significantly reduced, by one to two orders of magnitude compared to KBBF crystals themselves; second, the overall transmittance of prism-coupled devices is also much lower than that of pure crystal devices, limiting their use for continuous-wave laser output, especially short-wavelength continuous-wave laser output. Therefore, prism-coupled devices can only be used for low-power laser output; for example, the 177.3nm laser currently used in advanced scientific instruments is generally only in the milliwatt range. Regarding continuous-wave laser output, the shortest wavelength currently achievable with prism-coupled devices is 171nm. For shorter output wavelengths, the overall transmittance of prism-coupled devices is still not high enough.

[0009] Therefore, there is a need to provide a method for fabricating frequency conversion devices with high transmittance and high continuous wave laser output power using thin-film nonlinear optical crystals. Summary of the Invention

[0010] To address the aforementioned technical problems, one aspect of the present invention provides a trigonal nonlinear optical crystal frequency conversion device, wherein...

[0011] The frequency converter has a first set of parallel surfaces and a second set of parallel surfaces. The first set of parallel surfaces is parallel to the c-axis and a-axis of the optical crystal. The normal of the second set of parallel surfaces forms a first angle α with the c-axis of the optical crystal, and the projection of the normal onto the ab-plane forms a second angle β with the a-axis, as shown in Equations 1 and 2.

[0012] θ PM For the nonlinear optical crystal to satisfy the phase matching angle between the incident fundamental frequency light and the output frequency-converted light,

[0013] θ B =arctann, where Brewster's angle is given, and n is the refractive index of the incident fundamental frequency light in the nonlinear optical crystal.

[0014] The second set of parallel surfaces are the light-transmitting input surface and the light-transmitting output surface of the frequency converter.

[0015] This invention utilizes a thin-film primary crystal with a small c-axis in a trigonal nonlinear optical crystal to prepare a continuous wave frequency converter. While increasing the width of the light transmission surface, it can maintain the longest possible length of the light transmission direction, thus possessing both the characteristics of the light transmission length direction and the width of the light transmission surface, thereby achieving high frequency conversion efficiency.

[0016] Preferably, the nonlinear optical crystal uses the parallel surfaces perpendicular to the c-axis of the sheet-like primary crystal (i.e., the two large parallel faces of the sheet-like primary crystal) as its third set of parallel surfaces. Thus, the thickness of the trigonal sheet-like nonlinear optical crystal primary crystal is utilized to the maximum extent during the fabrication of the frequency converter device.

[0017] Preferably, the frequency converter is a continuous-wave frequency converter. The method of preparing frequency converters using thin-film trigonal crystals of the present invention is particularly suitable for preparing deep-violet band continuous-wave frequency converters using thin-film trigonal nonlinear optical crystals such as KBBF. The resulting frequency converter has a much longer light transmission direction length than that of traditional prism-free frequency converters. Compared to currently used prism-coupled devices, this method solves the problem of reduced overall optical damage threshold and overall transmittance caused by the addition of two extra optical adhesive interfaces, thereby providing a continuous-wave frequency converter with high-power continuous-wave laser output.

[0018] The frequency converter is a frequency multiplier, and the optical crystal is a negative uniaxial crystal with a refractive index n that is the refractive index of the o-light.

[0019] or,

[0020] The frequency conversion device is a sum-frequency device, the optical crystal is a negative uniaxial crystal, and the refractive index n is the o-ray refractive index of the first incident fundamental frequency light, wherein the wavelength of the first incident fundamental frequency light is greater than the wavelength of the second incident fundamental frequency light.

[0021] Preferably, the nonlinear optical crystal is a KBBF crystal, an RBBF crystal, a γ-BBF crystal, or other trigonal nonlinear optical crystals.

[0022] Preferably, the interplanar spacing of the second set of parallel surfaces is no greater than d / cosα, where d is the thickness of the plate-like crystal.

[0023] A second aspect of the present invention provides a method for operating a nonlinear optical crystal frequency converter, wherein if the frequency converter is a frequency multiplier, the method includes...

[0024] A continuous-wave fundamental frequency light is incident on the nonlinear optical crystal from one of the second set of parallel surfaces at Brewster's angle. The fundamental frequency light and the normals of the second set of parallel surfaces form the incident plane, such that the angle between the normal of this incident plane and the c-axis is (90-α), and the polarization direction of the fundamental frequency light lies within the incident plane (therefore, the polarization direction of the fundamental frequency light within the crystal is perpendicular to the c-axis of the optical crystal). The projection of the fundamental frequency light in the nonlinear optical crystal onto the plane perpendicular to the c-axis is parallel to the a-axis.

[0025] Frequency-doubled light is emitted from another of the second set of parallel surfaces from the nonlinear optical crystal.

[0026] Since the fundamental frequency light must be parallel to the first set of planes after entering the crystal, it is easy to determine from which direction (left or right) the fundamental frequency light enters the nonlinear optical crystal at Brewster angle.

[0027] A third aspect of the present invention provides a method for operating a nonlinear optical crystal frequency converter, wherein if the frequency converter is a sum-frequency converter, the method includes...

[0028] A first incident fundamental frequency light (with the longer wavelength) and a second incident fundamental frequency light are incident on the nonlinear optical crystal from one of the second set of parallel surfaces. The first incident fundamental frequency light enters the crystal along Brewster's angle. The first fundamental frequency light and the normal of the second set of parallel surfaces form the incident plane, such that the angle between the normal of the incident plane and the c-axis is (90-α), and the polarization direction of the first fundamental frequency light is located in the incident plane (therefore, the polarization direction of the first fundamental frequency light in the crystal is perpendicular to the c-axis of the optical crystal). The projection of the first fundamental frequency light in the nonlinear optical crystal onto the plane perpendicular to the c-axis is parallel to the a-axis. The second fundamental frequency light, after entering the crystal, is collinear with the first fundamental frequency light.

[0029] The sum-frequency light is emitted from another of the second set of parallel surfaces from the nonlinear optical crystal.

[0030] Since the fundamental frequency light must be parallel to the first set of planes after entering the crystal, it is easy to determine from which direction the fundamental frequency light enters the nonlinear optical crystal at the Brewster angle—that is, from one of the second set of parallel surfaces, or one of the parallel inclined surfaces. A fourth aspect of the present invention provides a method for fabricating a continuous-wave nonlinear optical crystal frequency converter, the method comprising:

[0031] Take the plate-shaped primary crystal of a trigonal nonlinear optical crystal;

[0032] Determine Brewster's angle θ B =arctann, where n is the refractive index of the incident fundamental frequency light in the nonlinear optical crystal;

[0033] Determining the phase matching angle θ based on the wavelengths of the incident fundamental light and the emitted frequency-converted light from a nonlinear optical crystal. PM ;

[0034] The Brewster angle θ B and phase matching angle θ PM The first included angle β and the second included angle α are determined according to Equations 1 and 2.

[0035] The nonlinear optical crystal is a trigonal crystal system, and the first set of parallel surfaces are cut parallel to the c-axis and a-axis of the optical crystal.

[0036] Cut a second set of parallel surfaces such that the normal to the surface forms a first angle α with the c-axis of the optical crystal, and the projection of the normal onto the ab-plane forms a second angle β with the a-axis, thus obtaining the frequency converter.

[0037] Preferably, the interplanar spacing of the second set of parallel surfaces is no greater than d / cosα, where d is the thickness of the plate-like primary crystal, i.e., the thickness in the c-axis direction. Beneficial effects

[0038] This invention utilizes the properties of nonlinear optical crystals to determine the phase matching angle and Brewster angle, calculates and determines the cutting position of a thin-film nonlinear optical crystal, and provides a nonlinear optical crystal frequency conversion device that can combine the maximum light transmission direction length and device width and is suitable for continuous wave laser applications for thin-film trigonal nonlinear optical crystals. It also provides a convenient and rapid method for fabricating such a nonlinear optical crystal frequency conversion device, as well as a working method for realizing frequency conversion.

[0039] The nonlinear optical crystal cutting method of the present invention does not sacrifice the length of the light transmission direction in order to cut Brewster's angle, unlike conventional techniques. The nonlinear optical crystal cutting method of the present invention can increase the width of the cut optical device while maintaining the length of the light transmission direction unchanged. Therefore, a frequency converter with high conversion efficiency continuous-wave laser output can be obtained using a thin-film optical crystal. Compared to currently used prism-coupled devices, this method solves the problem of reduced overall optical damage threshold and overall transmittance caused by the addition of two extra optical adhesive interfaces, thereby providing a continuous-wave frequency converter with high-power continuous-wave laser output.

[0040] The nonlinear optical crystal frequency converter obtained according to the present invention has both the phase matching angle and Brewster angle characteristics of the nonlinear optical crystal in its light-transmitting input surface and light-transmitting output surface. The length of the light-transmitting direction can be several times that of frequency converters prepared by conventional processing methods, and users can easily determine that the fundamental frequency light enters the crystal from the inclined parallel surface. Attached Figure Description

[0041] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0042] Figure 1 shows a schematic diagram of the rectangular coordinate system for KBBF crystal physics.

[0043] Figure 2-3 shows a schematic diagram of a first example of using sheet-like nonlinear optical crystals to fabricate frequency conversion devices in the prior art.

[0044] Figure 4-6 shows a schematic diagram of a second example of the fabrication of a continuous wave frequency converter using a sheet-like nonlinear optical crystal in the prior art.

[0045] Figures 7-8 show schematic diagrams of the continuous wave frequency conversion device fabricated using sheet-like nonlinear optical crystals according to the present invention.

[0046] Figure 9-11 shows the working principle diagram of the continuous wave frequency converter according to the present invention. Detailed Implementation

[0047] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and preferred embodiments. The various parts in the drawings are not drawn to scale. It should be understood that the preferred embodiments of the present invention are illustrative rather than limiting, and the scope of protection of the present invention will be defined by the claims.

[0048] This invention provides a novel method for fabricating nonlinear optical crystal frequency converters by cutting crystals. For thin-film trigonal nonlinear optical crystals, the length of the light transmission direction of the frequency converter can be kept constant while increasing the width of the cut frequency converter, thereby enabling the fabrication of frequency converter optical devices with high conversion efficiency continuous wave laser output using thin-film optical crystals.

[0049] The first embodiment of the present invention provides a method for fabricating frequency conversion optical devices using a thin-film primary crystal of a nonlinear optical crystal, comprising:

[0050] The crystal axis is determined by taking the plate-shaped primary crystal of the trigonal nonlinear optical crystal. For example, the thickness direction of the plate-shaped primary crystal is generally the c-axis of the optical crystal.

[0051] Determine Brewster's angle θ B =arctann, where n is the refractive index of the incident fundamental frequency light in the nonlinear optical crystal; as a specific example, the nonlinear optical crystal is a negative uniaxial crystal, and the refractive index n is the refractive index of the o-ray;

[0052] Determining the phase matching angle θ based on the wavelengths of the incident fundamental light and the emitted frequency-converted light from a nonlinear optical crystal. PM ;

[0053] The Brewster angle θ B and phase matching angle θ PM The first included angle α and the second included angle β are determined according to Equations 1 and 2.

[0054] A first set of parallel surfaces S1 and S2 are cut from the sheet-like primary crystal parallel to the c-axis and a-axis, resulting in a strip. The four edges of this set of parallel surfaces are all parallel to one of the three equivalent a-axis. The height of this strip is the height of the sheet-like primary crystal along the c-axis, and the width can be relatively wide, for example, 4-12 mm, as shown in Figure 7. It can be understood that due to the three-fold rotational symmetry of the trigonal crystal system, the first set of parallel surfaces S1 and S2 in Figure 7 can actually be cut in three ways; any one of the three a-axis can be chosen to be parallel to its four edges. Those skilled in the art can select the appropriate method of using the optical crystal frequency converter after cutting based on the specific positional relationship between the cutting plane and the a-axis during crystal cutting to achieve the technical solution of this invention. For the sake of simplicity, further details are omitted here.

[0055] Cut a second set of parallel surfaces A1B1F1E1 and D1C1G1H1, also known as parallel inclined planes or inclined planes, such that the angle between G1F1 and G1H1 is (90-β), and the normal of surface A1B1F1E1 forms a first angle α with the c-axis. Naturally, the projection of the normal of the parallel inclined plane onto the XY plane forms a second angle β with the a-axis. Thus, a parallelepiped B1C1G1F1-A1D1H1E1 is cut out, as shown in Figure 8. To ensure that the laser is only incident and emitted from these parallel inclined planes, the interplanar spacing l of the parallel inclined planes cannot be greater than d / cosα, where d is the thickness of the sheet-like optical crystal along the c-axis.

[0056] The second set of parallel inclined surfaces A1B1F1E1 and D1C1G1H1, cut above, are the light-transmitting input and output surfaces of the obtained frequency converter, respectively. Polishing these parallel surfaces to meet the optical-grade requirements of optical devices completes the fabrication of the entire frequency converter; the preparation steps are very simple. The frequency converter fabricated in this way is a parallelepiped B1C1G1F1-A1D1H1E1. The first set of parallel surfaces B1C1G1F1 / A1D1H1E1 is perpendicular to the large parallel surface A1B1C1D1 / E1F1G1H1. The second set of parallel inclined surfaces A1B1F1E1 / D1C1G1H1 are inclined about the first set of parallel surfaces and the large parallel surface, respectively. Since the fundamental frequency light must be parallel to the first set of planes after entering the crystal, it is easy to determine the fundamental frequency light incident surface of the frequency converter. That is, one of the parallel inclined surfaces is used as the light-transmitting surface to allow the fundamental frequency light to enter the nonlinear optical crystal at a Brewster angle, facilitating the use of the frequency converter.

[0057] As a specific example, the nonlinear optical crystal uses the parallel surface perpendicular to the c-axis of the sheet-like primary crystal as its third set of parallel surfaces. For sheet-like trigonal nonlinear optical crystals with a thickness d not exceeding 10 mm in the c-axis direction, the frequency conversion device processing method provided by this invention can obtain a nonlinear optical crystal frequency conversion device with the maximum optical transmission direction length and suitable for continuous wave laser applications. In other words, compared with the prior art, under the condition of limited primary crystal material thickness, the frequency conversion device can be cut to have both the maximum possible optical transmission direction length and a sufficiently wide switching width, solving the problem that the prior art cannot use sheet-like optical crystals to prepare frequency conversion devices that simultaneously have optical transmission direction length and device width.

[0058] As a specific example, the nonlinear optical crystal is a KBBF crystal, an RBBF crystal, a γ-BBF crystal, or other trigonal nonlinear optical crystals.

[0059] According to the method of the present invention, after determining the angles of the first included angle α and the second included angle β, during the processing, the sheet-like original crystal is continuously cut in the manner of cutting the first set of parallel surfaces as described above, and multiple strip-shaped crystals can be obtained; then, the strip-shaped crystals are continuously cut in the manner of cutting the second set of parallel surfaces, and multiple nonlinear optical crystal frequency conversion devices of the present invention can be obtained quickly and easily.

[0060] The following uses a 193nm continuous wave frequency converter with KBBF crystal as an example to illustrate the fabrication method of the frequency conversion optical device of the present invention.

[0061] Take a thin sheet-like primary KBBF crystal and determine its crystal axis (c-axis) and a-axis. The thickness direction of the KBBF crystal is the c-axis of this optical crystal.

[0062] First, determine the Brewster angle. KBBF crystal is a negative uniaxial crystal; the fundamental frequency light at 386nm is o-ray in the crystal. Therefore, the Brewster angle θ is... B =arctann, where n is the o-ray refractive index corresponding to the fundamental wavelength of the KBBF crystal. o Substituting @386nm=1.4934 into the above equation, we get θ B =arctan1.4934 = 56.19°.

[0063] Then, the first included angle α is determined. The value of the first included angle α is determined by Equation 1 above. For frequency doubling of 386nm incident light to 193nm, the phase matching angle θ... PM =55.4°. According to Equation 1, angle α = 46.9° is calculated.

[0064] Finally, the second included angle β was determined. According to Equation 2, after calculation, β = 42.5° was obtained.

[0065] After determining the above parameters, the specific cutting method is detailed below:

[0066] Two cuts are made along the a-axis direction of the KBBF crystal, parallel to the c-axis (vertical), to obtain a set of parallel surfaces S1 and S2. A long strip is then cut out, with all four edges parallel to the a-axis, as shown in Figure 7. The height of this strip is the thickness of the KBBF crystal along the c-axis direction, and the width can be cut relatively wide, for example, 6 mm. This width will then serve as the second side length (width) of the light-transmitting surface of the frequency converter.

[0067] Then, relative to the first set of parallel surfaces S1 and S2, rotate 42.5° (i.e., the angle between G1F1 and G1H1 is 90-β=47.5°) and cut a set of parallel inclined planes A1B1F1E1 and D1C1G1H1 at an angle α between the normal and the c-axis, to cut out a parallelepiped B1C1G1F1-A1D1H1E1, which is used to fabricate a nonlinear optical crystal frequency converter 31, as shown in Figure 8. The angle between the normal of inclined planes A1B1F1E1 and D1C1G1H1 and the c-axis of the KBBF crystal is α, that is, the angle between the parallel inclined plane and the XY plane is α. At the same time, the projection of the normal of this parallel inclined plane onto the XY plane satisfies the second angle β with the a-axis. The interplanar spacing l of the parallel inclined planes is less than d / cosα, where d is the thickness of the sheet-like KBBF precursor crystal along the c-direction, to ensure that the laser is only incident and emitted from this parallel inclined plane.

[0068] The inclined surfaces A1B1F1E1 and D1C1G1H1 were polished as light-transmitting surfaces to meet the optical grade requirements of optical devices, thus completing the fabrication of a 193nm continuous wave frequency converter with KBBF crystal.

[0069] The nonlinear optical crystal frequency converter of this invention can be used as a frequency multiplier. In use, as shown in Figures 9-11, the fundamental frequency light is incident into the frequency multiplier from one of the optional light-transmitting surfaces, such as D1C1G1H1, along the Brewster angle, and exits from the frequency multiplier from another light-transmitting surface, A1B1F1E1. The incident fundamental frequency light and the normal to the light-transmitting surface D1C1G1H1 constitute the incident surface, such that the angle between the normal to the incident surface and the c-axis of the optical crystal is the complementary angle of the first angle α, and the polarization direction of the fundamental frequency light lies within the incident surface. Therefore, the polarization direction of the fundamental frequency light within the KBBF crystal is perpendicular to the c-axis of the optical crystal. There are two fundamental frequency light incident methods that satisfy the above conditions. It is easy to choose one where the projection of the direction of the fundamental frequency light into the ab plane after entering the crystal is parallel to the a-axis (i.e., the fundamental frequency light within the crystal is parallel to the first parallel plane of the device). Thus, the fundamental frequency light entering the nonlinear optical crystal is entirely o-light and propagates along the phase-matching direction. Due to the satisfied phase-matching condition, the incident fundamental frequency light gradually transforms into frequency-doubled light. Figure 9 shows the projection of this device along the c-axis. The arrows indicate the directions of the incident and outgoing rays, and the dashed lines represent the ray paths within the crystal. It can be seen that the rays represented by the dashed lines in the crystal are projected parallel to the a-axis in the XY plane, satisfying the KBBF crystal phase-matching direction φ = 0. Figure 10 shows the projection along the second set of parallel surfaces D1C1G1H1. It can be seen that the incident ray, the ray in the crystal, and the outgoing ray are all in the same plane. This plane is the coincident incident and outgoing surfaces. Since the two light-transmitting surfaces are parallel, the incident and outgoing surfaces coincide. This plane is perpendicular to the light-transmitting input surface D1C1G1H1 and the light-transmitting output surface A1B1F1E1. The angle between the incident surface normal and the c-axis is the complementary angle (90° - α) of the second angle α. It should be noted that this frequency multiplier is a parallelepiped, but only a portion of it is actually used, namely the parallelepiped labeled C1D1JK-MNE1F1 in Figure 10. The actual light-transmitting surfaces (also called light-transmitting windows) are C1D1JK and MNE1F1, and the length of the first side of the light-transmitting window, C1K, is h. For convenience, it is not necessary to specifically cut out the parallelepiped C1D1JK-MNE1F1.

[0070] This invention utilizes a method for fabricating frequency converter devices using thin-film nonlinear optical crystals. It takes two large faces (two parallel surfaces perpendicular to the c-axis) of the pre-existing thin-film nonlinear optical crystal. First, two cuts are made parallel to the c-axis and a-axis to obtain the first set of parallel surfaces S1 and S2, resulting in a strip-shaped crystal blank. Then, on this strip-shaped crystal blank, a second set of parallel bevels is cut relative to the parallel surfaces S1 and S2 at an angle β, with the normal and c-axis forming an angle α, resulting in a parallelepiped. The projection of the normal of the second set of parallel bevels onto the ab plane forms a second angle β with the a-axis direction of the crystal. After polishing the parallel bevels, a nonlinear optical crystal continuous-wave frequency converter is obtained. By determining the two included angles, this invention allows for the cutting of multiple strip-shaped crystal blanks, and then multiple parallel bevels can be cut from each blank, maximizing the utilization of the pre-existing crystal and directly obtaining multiple continuous-wave frequency converter devices with good consistency. The fabrication method is simple.

[0071] The frequency converter obtained according to the method of this invention, for the point-switching continuous wave frequency converter, has a transmission direction length that is not affected by the increase in the width of the transmission surface, and the transmission direction length is much greater than that of the continuous wave frequency multiplier obtained by traditional cutting methods. The following uses a 193nm continuous wave frequency multiplier with KBBF crystal as an example to illustrate the comparison of transmission direction lengths. θ of the 193nm frequency multiplier with KBBF crystal... PM =55.4°, θ B =56.19°. For traditional techniques used to cut and fabricate continuous wave frequency multipliers, taking a thin KBBF primary crystal with a thickness of d = 4mm as an example, in Figure 3, the length of one side AB of the light-transmitting surface is h = 1mm, and the length of the other side AD is k = 6mm. To ensure the requirement of changing the point of contact is met, the length in the light-transmitting direction is L = (d / cosθ). PM -h×tanθ PM = 5.595mm. Furthermore, according to Figure 6, after cutting two parallel bevels at Brewster's angle, the length in the light transmission direction becomes even shorter, L' = Lk × ctgθ B =5.595 - 6 × ctg56.19° = 1.578 mm. If k continues to increase, the length in the light transmission direction will continue to decrease.

[0072] Using the technology of this invention, the first included angle α = 46.9° and the second included angle β = 42.5° are obtained. According to Figure 10, the projected length of the light transmission direction on the paper (perpendicular to the light input surface D1C1G1H1 and the light output surface A1B1F1E1) is the inter-surface distance l between the two light transmission surfaces. Following the calculation in Figure 3, l = d / cosα - h × tanα = 4.786 mm. According to Figure 11, the actual light transmission direction length L' = l / sinθ B= 5.759mm. This length is significantly greater than the 1.578mm light-transmitting length of a typical continuous-wave frequency multiplier, reaching 3.6 times the latter. This is because the light-transmitting length of the device of this invention is not affected by an increase in the light-transmitting surface width k. In the frequency conversion device of this invention, the light-transmitting surface width can be selected according to the original crystal size or the operating requirements of the frequency converter, for example, 4mm-12mm. Therefore, the nonlinear optical crystal frequency conversion device obtained by the preparation method of this invention can achieve high frequency conversion efficiency, meeting the power requirements of practical applications and the switching requirements of continuous-wave frequency converters, thus expanding the applicability and application range of deep ultraviolet laser frequency conversion devices.

[0073] In a specific embodiment, the continuous wave frequency converter needs to be placed in a resonant cavity with an incident cavity mirror and an exit cavity mirror. The incident fundamental frequency light enters the nonlinear optical crystal frequency converter only through the incident cavity mirror, and after exiting the frequency converter, it is reflected by the exit cavity mirror and re-enters the nonlinear optical crystal frequency converter. This achieves intracavity circulation, which is repeated continuously. Resonance enhancement is achieved by locking the cavity length. The frequency-converted light exits the resonant cavity only once through the exit cavity mirror.

[0074] As a specific embodiment, the nonlinear optical crystal frequency converter of the present invention can be used as a sum-frequency converter. In use, a first incident fundamental frequency light and a second incident fundamental frequency light are incident into the nonlinear optical crystal from one of the parallel inclined planes C1D1H1G1. The wavelength of the first incident fundamental frequency light is greater than the wavelength of the second incident fundamental frequency light. The incident fundamental frequency light and the normal of the inclined plane C1D1H1G1 form the incident surface, such that the angle between the normal of the incident surface and the c-axis of the optical crystal is the complementary angle of the first angle α. The first fundamental frequency light is incident into the nonlinear optical crystal along the Brewster angle, which is determined by the first incident fundamental frequency light. After the second fundamental frequency light enters the crystal, it is collinear with the first fundamental frequency light. The projections of the two fundamental frequency lights on the ab plane in the crystal are parallel to the a-axis. The sum-frequency light exits the nonlinear optical crystal from the other inclined plane B1A1E1F1.

[0075] The technical solution of the present invention will be described in detail below with the help of examples.

[0076] Example 1

[0077] A frequency doubling device with a wavelength of 193nm was fabricated using KBBF primary crystal with a thickness of d=4mm.

[0078] First, determine Brewster's angle.

[0079] The refractive index of KBBF crystal at the fundamental wavelength of 386nm is n. o @386nm=1.4934, Brewster's angle θ B =arctann o =arctan1.4934 = 56.19°.

[0080] Then, the first included angle α is determined using Equation 1.

[0081] The KBBF crystal frequency-doubles the fundamental light at 386nm to a frequency-doubled light at 193nm, with a phase matching angle θ. PM =55.4°. Solving equation 1, we get α = 46.9°.

[0082] Then, the second included angle β is determined using Equation 2.

[0083] Calculations show that β = 42.5°.

[0084] After determining the first included angle α and the second included angle β, the first set of parallel surfaces is cut from the sheet-like KBBF crystal parallel to the a-axis and parallel to the c-axis to obtain a cuboid. The height of this cuboid is the thickness of the sheet-like crystal d = 4 mm, and the width is the spacing of the first set of parallel surfaces 6 mm.

[0085] Then, rotate 42.5° relative to the first set of parallel surfaces S1 and S2 and cut the second set of parallel inclined planes at an angle of 46.9° (α) with the normal and the c-axis to obtain a parallelepiped. The interface spacing of the second set of parallel inclined planes is 4.7mm, which ensures that the first side length h of the light-transmitting surface reaches 1mm (C1K in Figure 10). At the same time, the projection of the normal of this parallel inclined plane onto the XY plane must satisfy the angle of 42.5° with the a-axis.

[0086] The parallel bevels were polished to meet the optical-grade requirements of the frequency doubler, thus enabling the fabrication of a nonlinear optical crystal for the frequency doubler using a very simple process. In the resulting frequency doubler, a portion of the parallel bevels serves as the light-transmitting surface of the frequency doubler (as shown in Figure 10). The actual light-transmitting length, calculated earlier, is 5.759 mm.

[0087] In use, the aforementioned device is placed in the resonant cavity. A 386nm wavelength, 4W continuous-wave laser beam is incident through the incident cavity mirror along Brewster's angle from one of the second set of inclined planes into the KBBF crystal frequency multiplier. The incident light and the normal to the inclined plane constitute the incident surface. Simultaneously, the angle between the normal to the incident surface and the c-axis must be 43.1° (i.e., the complementary angle of angle α), and the projection of the 386nm laser onto the ab plane in the crystal must be parallel to the a-axis. Since the polarization direction of the 386nm laser is within the incident surface, after entering the crystal, the 386nm laser travels along the phase-matching direction with its polarization direction perpendicular to the c-axis, satisfying the phase-matching condition. It gradually transforms into a 193nm continuous-wave frequency-doubled beam and exits the crystal. The remaining 386nm laser is reflected by the cavity mirror, continues to oscillate within the cavity, and passes through the KBBF crystal multiple times, repeating this process continuously. Resonance enhancement is achieved by locking the cavity length. The newly generated 193nm laser is output in a single pass through the exit cavity mirror. The final output 193nm laser has a long-term stable power of 20mW, which is about 10 times that of the existing prism coupling device frequency multiplier.

[0088] Example 2

[0089] A 177.5nm continuous wave frequency doubling device was fabricated using KBBF primary crystal with a thickness of d=4mm.

[0090] First, determine Brewster's angle.

[0091] The refractive index of KBBF crystal at 355nm is n o @355nm=1.4974, Brewster's angle θ B =arctann o =arctan1.4974 = 56.26°.

[0092] Then, the first included angle α is determined using Equation 1.

[0093] KBBF crystals frequency-double the fundamental light from 355nm to 177.5nm with a phase-matching angle θ. PM =64.3°. Solving equation 1, we get α = 58.57°.

[0094] Then, the second included angle β is determined using Equation 2.

[0095] The calculated value is β = 38.05°.

[0096] After determining the first included angle α and the second included angle β, the first set of parallel surfaces is cut from the sheet-like KBBF crystal parallel to the a-axis and parallel to the c-axis to obtain a cuboid. The height of this cuboid is the thickness of the sheet-like crystal d = 4 mm, and the width is the spacing of the first set of parallel surfaces 6 mm.

[0097] Then, relative to the first set of parallel surfaces S1 and S2, rotate 38.05° and cut the second set of parallel inclined planes at an angle of 58.57° (α) with the normal and the c-axis to obtain a parallelepiped. The inter-face spacing of the second set of parallel inclined planes is 6mm, which ensures that the first side length h of the light-transmitting surface reaches 1mm (C1K in Figure 10). At the same time, the projection of the normal of this parallel inclined plane onto the XY plane must satisfy the angle of 38.05° with the a-axis.

[0098] Polish this set of parallel bevels to meet the optical-grade requirements of the frequency multiplier, thus completing the fabrication of the entire device, which is very simple. Part of the two parallel bevels is the light-transmitting surface of the frequency multiplier (as shown in Figure 10).

[0099] According to Figure 10, the projection of the light transmission direction length onto the side of the frequency multiplier is the inter-surface distance l between the light transmission input surface and the light transmission output surface. Following the calculation in Figure 3, l = d / cosα - h × tanα. Substituting d = 4mm, h = 1mm, etc., we get l = 4 / cos58.57° - 1 × tan58.57° = 6.034mm. Furthermore, according to Figure 11, the actual light transmission direction length L' = l / sinθ B =6.034 / sin56.26°=7.256mm.

[0100] In use, the aforementioned device is placed in the resonant cavity. A 355nm wavelength, 4W continuous-wave laser beam is incident on the KBBF crystal through an incident cavity mirror, along Brewster's angle, from one of the second parallel inclined planes. The incident light and the normal to the inclined plane constitute the incident surface. Simultaneously, the angle between the normal to the incident surface and the c-axis must be 31.43° (i.e., the complementary angle of angle α), and the projection of the 355nm laser onto the ab plane in the crystal must be parallel to the a-axis. Since the polarization direction of the 355nm laser is within the incident surface, after entering the crystal, the 355nm laser travels along the phase-matching direction and its polarization direction is perpendicular to the c-axis, satisfying the phase-matching condition. It gradually transforms into a 177.5nm frequency-doubled continuous-wave light and exits the crystal. The remaining 355nm laser is reflected by the cavity mirror, continues to oscillate within the cavity, and passes through the KBBF crystal multiple times, repeating this process continuously. Resonance enhancement is achieved by locking the cavity length. The 177.5nm laser is output in a single pass through the exit cavity mirror, and the final output continuous wave 177.5nm laser power reaches about 10mW.

[0101] Example 3

[0102] A continuous wave summing device with a summing wavelength of 153.43 nm was fabricated using KBBF precursor with a thickness of d = 4 mm.

[0103] A Yb laser with a wavelength of 1074nm is used as the first fundamental frequency light, and a 179nm laser, which is the sixth harmonic of the first fundamental frequency light, is used as the second fundamental frequency light. The frequency summation process is that the 1074nm laser and the 179nm laser are summed to obtain a 153.43nm laser.

[0104] First, determine the Brewster angle. Since the sum-frequency process involves two fundamental frequency beams of different wavelengths, their refractive indices differ, resulting in different Brewster angles. This invention does not require cutting a Brewster angle that simultaneously satisfies both wavelengths. Taking the longer wavelength of 1074 nm as a reference, the refractive index of the o-ray at 1074 nm in the KBBF crystal is n. o @1074nm=1.4713, Brewster's angle θ B =arctann o =arctan1.4713 = 55.80°.

[0105] Then determine angle α. The value of angle α is given by the equation... It is determined that the KBBF crystal phase matching angle θ is the sum frequency of the 1074nm laser and the 179nm laser with a sum frequency of 153.43nm. PM =52.1°. Calculation yields α = 42.3°.

[0106] Finally, determine the angle β. The calculated value is β = 45.3°.

[0107] After determining the above parameters, the first set of parallel surfaces is cut from the thin KBBF crystal parallel to the a-axis and parallel to the c-axis to obtain a cuboid. The height of this cuboid is 4mm and the width is the distance between the first set of parallel surfaces, which is 6mm.

[0108] Then, rotate 45.3° relative to the first set of parallel surfaces S1 and S2 and cut the second set of parallel inclined planes at an angle of 42.3° (α) with the normal and the c-axis to obtain a parallelepiped. The interplanar spacing of the second set of parallel inclined planes is 4.4 mm, which ensures that the first side length h of the light-transmitting surface reaches 1.1 mm (C1K in Figure 10). At the same time, the projection of the normal of this parallel inclined plane onto the XY plane must satisfy the angle of 45.3° with the a-axis.

[0109] Polish the two parallel bevels to meet the optical grade requirements of the frequency multiplier, thus completing the fabrication of the entire device. Part of the parallel bevels forms the light-transmitting surface of the frequency multiplier (as shown in Figure 10).

[0110] According to Figure 10, the projection of the light transmission direction length onto the side of the frequency converter is the inter-surface distance l between the light transmission input surface and the light transmission output surface. Following the calculation in Figure 3, l = d / cosα - h × tanα. Substituting d = 4 mm, h = 1.1 mm, etc., we get l = 4 / cos42.3° - 1.1 × tan42.3° = 4.407 mm. Furthermore, according to Figure 11, the actual light transmission direction length L1 = l / sinθ B =4.407 / sin55.80°=5.328mm.

[0111] In use, the aforementioned device is placed in a resonant cavity. The fundamental frequency light consists of continuous wave lasers with wavelengths of 1074 nm and 179 nm, respectively. The first fundamental frequency laser with a wavelength of 1074 nm has a power of 8 W, and the second fundamental frequency laser with a wavelength of 179 nm is a sixth harmonic of the 1074 nm laser, i.e., after frequency doubling, sum-frequency doubling, and then frequency doubling again, with a power of only 1 mW. The first fundamental frequency laser of 1074 nm is passed through an incident cavity mirror and incident into the KBBF crystal along Brewster's angle from one of the second parallel inclined planes. The incident light and the normal of the inclined plane constitute the incident surface. At the same time, the angle between the normal of the incident surface and the c-axis must be 47.7° (i.e., the complementary angle of angle α), and the projection of the 1074 nm laser on the ab plane in the crystal must be parallel to the a-axis. The 1074nm laser's polarization direction is within the incident plane. After entering the crystal, the 1074nm laser travels along the phase-matching direction, with its polarization direction perpendicular to the c-axis, satisfying the phase-matching condition. It then repeatedly resonates and enhances within the cavity. The 179nm laser is incident on the inclined plane of the KBBF crystal at a near Brewster angle and is collinear with the 1074nm laser within the crystal. Both the 1074nm and 179nm lasers travel along the phase-matching direction after entering the crystal and gradually convert into a 153.43nm sum-frequency light before exiting the crystal. The newly generated 153.43nm laser is output through the exit cavity mirror, while the remaining 1074nm laser is reflected by the cavity mirror, continuing to oscillate within the cavity and passing through the KBBF crystal multiple times, continuously generating 153.43nm sum-frequency light. The final output continuous-wave 153.43nm laser power reaches over 0.1mW.

[0112] Obviously, the above embodiments of this disclosure are merely examples for clearly illustrating this disclosure, and are not intended to limit the implementation of this disclosure. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all implementation methods here. Any obvious variations or modifications derived from the technical solutions of this disclosure are still within the protection scope of this disclosure.

Claims

1. A trigonal nonlinear optical crystal frequency converter, characterized in that, The variable frequency device has a first set of parallel surfaces and a second set of parallel surfaces, the first set of parallel surfaces is parallel to the optical crystal c-axis and a-axis, the normal of the second set of parallel surfaces has a first included angle of α with the optical crystal c-axis, and the projection of the normal on the a-b plane has a second included angle of β with the a-axis, having the relationship shown in formula 1 and formula 2 θ PM the nonlinear optical crystal satisfies a phase matching angle of incident fundamental light and output frequency converted light, θ B = arctan n, where n is the refractive index of the nonlinear optical crystal for the incident fundamental light, The second set of parallel surfaces are the light-transmitting input surface and the light-transmitting output surface of the frequency converter.

2. The nonlinear optical crystal frequency converter according to claim 1, characterized in that, The nonlinear optical crystal uses the parallel surface of the sheet-like crystal perpendicular to the c-axis of the optical crystal as its third set of parallel surfaces.

3. The nonlinear optical crystal frequency conversion device of claim 1, wherein, The frequency converter is a continuous wave frequency converter.

4. The nonlinear optical crystal frequency converter according to claim 1, characterized in that, The frequency converter is a frequency multiplier, and the optical crystal is a negative uniaxial crystal with a refractive index n that is the refractive index of the o-light. or, The frequency conversion device is a sum-frequency device, the optical crystal is a negative uniaxial crystal, and the refractive index n is the o-ray refractive index of the first incident fundamental frequency light, wherein the wavelength of the first incident fundamental frequency light is greater than the wavelength of the second incident fundamental frequency light.

5. The nonlinear optical crystal frequency conversion device of claim 1, wherein, The nonlinear optical crystal is a KBBF crystal, an RBBF crystal, a γ-BBF crystal, or other trigonal nonlinear optical crystals.

6. The nonlinear optical crystal frequency conversion device of claim 2, wherein, The interplanar spacing of the second set of parallel surfaces is no greater than d / cosα, where d is the thickness of the plate-like crystal.

7. A method of operating a nonlinear optical crystal frequency conversion device according to claim 4, wherein, The frequency converter is a frequency multiplier, and the method includes... A continuous-wave fundamental frequency light is incident on the nonlinear optical crystal from one of the second set of parallel surfaces at Brewster's angle. The fundamental frequency light and the normals of the second set of parallel surfaces form the incident plane, such that the angle between the normal of the incident plane and the c-axis is (90-α), and the polarization direction of the fundamental frequency light lies within the incident plane, so that the projection of the fundamental frequency light in the nonlinear optical crystal onto the plane perpendicular to the c-axis is parallel to the a-axis. Frequency-doubled light is emitted from another of the second set of parallel surfaces from the nonlinear optical crystal.

8. A method of operating a nonlinear optical crystal frequency conversion device according to claim 4, wherein, The frequency converter is a sum-frequency device, and the method includes: A first incident fundamental frequency beam and a second incident fundamental frequency beam are incident on the nonlinear optical crystal from one of the second set of parallel surfaces. The first incident fundamental frequency beam enters the crystal along Brewster's angle. The first fundamental frequency beam and the normal of the second set of parallel surfaces form the incident plane, such that the angle between the normal of the incident plane and the c-axis is (90-α), and the polarization direction of the first fundamental frequency beam lies within the incident plane. This ensures that the projection of the first fundamental frequency beam in the nonlinear optical crystal onto the plane perpendicular to the c-axis is parallel to the a-axis. The second fundamental frequency beam, after entering the crystal, is collinear with the first fundamental frequency beam. The sum-frequency light is emitted from another of the second set of parallel surfaces from the nonlinear optical crystal.

9. A method for fabricating a continuous-wave nonlinear optical crystal frequency converter, characterized in that, The method includes: determining the Brewster angle θ B = arctan n, n is the refractive index of the nonlinear optical crystal for the incident fundamental light Determining phase matching angle θ based on wavelength of incident fundamental light and exited frequency converted light of nonlinear optical crystal PM ; from the Brewster angle θ B and the phase matching angle θ PM determining the first and second included angles a and β from equations 1 and 2; The nonlinear optical crystal is a trigonal crystal system, and the first set of parallel surfaces are cut parallel to the c-axis and a-axis of the optical crystal. Cut a second set of parallel surfaces such that the normal to the surface forms a first angle α with the c-axis of the optical crystal, and the projection of the normal onto the ab-plane forms a second angle β with the a-axis, thus obtaining the frequency converter.

10. The method of claim 9, wherein, The nonlinear optical crystal is a sheet-like crystal, and the interplanar spacing of the second set of parallel surfaces is no greater than d / cosα, where d is the thickness of the sheet-like crystal.

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