Preparation method and working method of nonlinear optical crystal continuous wave frequency conversion device

Abstract

The present invention relates to a preparation method and an operating method of a trigonal nonlinear optical crystal frequency conversion device. The frequency conversion device has a first group of parallel surfaces and a second group of parallel surfaces. The first group of parallel surfaces is parallel to the c-axis and the a-axis of the optical crystal. The normal of the second group of parallel surfaces forms an angle α with the c-axis, and the projection of the normal on the a-b surface and the a-axis forms an angle β. The relationship θ is shown in Formula 1 and Formula 2. PM The nonlinear optical crystal satisfies the phase matching angle between the incident fundamental frequency light and the outgoing frequency-converted light, θ B =arctann, where n is the Brewster angle, and n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light. The second set of parallel surfaces constitute the light-transmitting input and output surfaces of the frequency converter. The present invention provides a method for preparing and operating a nonlinear optical crystal frequency converter device based on a thin sheet-like primary crystal, having a maximum possible light-transmitting length and suitable for continuous-wave applications.

Description

Technical Field

[0001] The present disclosure relates to the field of artificial crystal optical devices, and in particular to a technology for preparing a Brewster frequency doubling device using a thin sheet-like nonlinear optical uniaxial crystal. Background Art

[0002] Deep ultraviolet (UV) lasers (wavelengths less than 200nm) have crucial applications in information technology, scientific research, and national defense. Multi-stage frequency conversion using nonlinear optical crystals is currently an effective way to achieve deep UV lasers. The final frequency-doubling crystal is called a deep UV nonlinear optical crystal. Currently, only potassium beryllium fluoroborate (KBe2BO3F2, KBBF) and rubidium beryllium fluoroborate (RbBe2BO3F2, RBBF) meet the requirements for deep UV nonlinear optical crystals. They can directly frequency-double and output lasers with wavelengths less than 200nm. In particular, they can achieve the sextupled (177.3nm) output of Nd:YAG lasers, which are used in the development of advanced scientific instruments, as well as the quadrupled bandwidth-tuned output of titanium sapphire lasers. 193nm lasers have important applications in chip manufacturing. The overall performance of KBBF crystals is significantly superior to that of RBBF crystals.

[0003] KBBF crystals have a severe lamellar growth habit, and the complete original crystals grown are approximately lamellar hexagons. The normal direction of the large face of the hexagon is the crystallographic c-axis (also called the crystal c-axis or c-axis), the hexagon is a plane formed by the ab direction of the crystal, the a-axis and the b-axis are two equivalent crystallographic axes, forming an angle of 120° (physically including 3 equivalent a-axes), and the a-axis and the b-axis are both perpendicular to the c-axis. The crystal can grow to several centimeters along the a-axis and b-axis directions, while the thickness d in the c-axis direction does not exceed 10 mm, and generally does not exceed 5 mm. In order to facilitate the description of the physical properties of the crystal, this article establishes a set of physical rectangular coordinate systems according to the following rules. By convention, the crystal c-axis direction is defined as the Z-axis (hereinafter referred to as the c-axis, namely the Z-axis), the a-axis direction is defined as the X-axis (hereinafter referred to as the a-axis, namely the X-axis), and the direction perpendicular to the a-axis and the c-axis is defined as the Y-axis, see Figure 1 In order to achieve laser frequency doubling output in nonlinear optical crystals, the fundamental frequency light must travel along a specific phase matching direction WO in the crystal. This direction is represented by the azimuth angle θ in the rectangular coordinate system. PM and φ, where θ PM is the angle between the phase matching direction and the Z axis, and φ is the angle between the projection of the phase matching direction on the XY plane (i.e., the ab plane) and the X axis. KBBF crystal belongs to the trigonal crystal system, and the c axis is the optical axis of the crystal. Its structural symmetry determines its effective nonlinear coefficient d eff The expression is d eff =d 11 cos(θ PM )cos3φ,d 11is the nonlinear optical coefficient, which is approximately equal to 0.49pm / V. It can be seen that the maximum effective nonlinear coefficient d can be obtained when φ=0° eff , that is, to obtain the maximum frequency-doubling light conversion efficiency.

[0004] Figure 2 The schematic diagram of the first prior art example of preparing a frequency-doubling optical device from a thin sheet crystal is shown. For example, a rectangular parallelepiped ABCD-EFGH is cut from a hexagonal KBBF thin sheet 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 to serve as the light-transmitting input and output surfaces, respectively, forming a frequency doubler 11 for pulsed laser frequency-doubling output. Fundamental laser light travels along the phase-matching direction WO, entering the crystal from end face ABCD, where it is continuously converted into frequency-doubled light. Finally, both the frequency-doubled light and the remaining fundamental light exit the crystal from end face EFGH. Figure 3 for Figure 2 Projection diagram of the original crystal 10 and the frequency doubler 11 cut from it on the XZ plane, where L is the length of the frequency doubler 11 in the light-transmitting direction, h represents the distance between the parallel surfaces ADHE and BCGF (also known as the first side length of the light-transmitting surface), and k represents the distance between the parallel surfaces ABFE and DCGH (also known as the second side length of the light-transmitting surface). For example, AB=h, AD=k, and the side length represented by k is parallel to the XY plane. Figure 3 It can be seen that the length L in the light-transmitting direction is first limited by the thickness d in the Z direction of the crystal, and secondly by the length h of the first side. Although the KBBF crystal has larger dimensions in the a and b directions, it does not help to increase the length L in the light-transmitting direction. The maximum value L of the light-transmitting direction length is max is d / cosθ PM , at this time h=0. The length h of the first side of the light-transmitting surface of the 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 ) determines that as the first side length h increases, the length L in the light-transmitting direction will decrease. Therefore, the side length h is generally selected to be 1mm. In order to ensure the stability and life of the frequency doubler, the method of changing the point of incidence is often used. That is, after the laser has worked at a certain point on the light-transmitting surface for a certain period of time, it is switched to another point to continue working. And so on. After the entire light-transmitting surface area has been rotated once, a new frequency doubler is replaced. By reasonably selecting the size of the second side length k of the light-transmitting surface and changing the point along the direction of the second side length k, the need for changing the point of incidence during the laser frequency doubling process can be met.

[0005] For continuous wave laser frequency doubling, the higher the transmittance of the frequency doubling device to the fundamental frequency laser, the better, preferably close to 100%. Figure 2There are generally two technical solutions for improving the frequency doubler 11 shown. 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 two surfaces with near-zero loss. However, the laser damage threshold of the anti-reflection coating is low, one to two orders of magnitude lower than that of the crystal itself, making it easily damaged by laser light. If an anti-reflection coating is applied to a continuous-wave frequency doubler based on a KBBF crystal, the excellent characteristic of the KBBF crystal, its high laser damage threshold, is not utilized. Therefore, continuous-wave frequency doublers coated with an anti-reflection coating can generally only be used for frequency doubling low-power continuous-wave lasers.

[0006] The second technical solution is to adjust the two light-transmitting surfaces according to the Brewster angle θ B Cutting, θ B =arctann, where n is the refractive index of the nonlinear optical crystal corresponding to the wavelength of the incident fundamental frequency light. When the polarization direction of the fundamental frequency light lies within the incident plane (the plane formed by the incident laser and the normal of the light-passing surface is the incident plane), the fundamental frequency light can pass through the light-passing input and output surfaces with almost no loss. Figure 4-6 Schematically shows the second example of the prior art, which is prepared from a thin plate-shaped original crystal with a Brewster angle θ B The schematic diagram of the frequency multiplier 12 is as follows. Figure 2 The two light-passing surfaces ABCD and EFGH of the frequency doubler ABCD-EFGH are further adjusted according to the Brewster angle θ. B Cutting is to cut off two right triangular prisms from the rectangular parallelepiped ABCD-EFGH to form a right parallelepiped AD'HE'-BC'GF', thereby obtaining the frequency multiplier 12. The light-transmitting surfaces of the frequency multiplier 12 are ABC'D' and E'F'GH. Figure 6 As shown in the projection diagram, the dotted line represents the light with Brewster angle θ B The length of the light-passing direction is L'. This solution inevitably reduces the length of the light-passing direction while improving the transmittance of the incident laser. Figure 5 and Figure 6 As shown, the length in the light direction L'=Lk×ctgθ B, when the original light-transmitting length L is determined, the larger the distance k between the surfaces ABF'E' and D'C'GH, the shorter the light-transmitting length L'. That is, in a Brewster angle-cut device, the light-transmitting length is limited not only by the crystal thickness d in the Z direction and the first side length h, but also by the second side length k. As mentioned above, in order to meet the needs of frequency doubling point switching, since the distance h between the parallel surfaces AD'HE' and BC'GF' is limited, it is necessary to increase the value of the distance k between the parallel surfaces ABF'E' and D'C'GH. However, the wider the distance k, the shorter the light-transmitting length, that is, the light-transmitting length and the device width cannot be achieved at the same time. The frequency doubling efficiency is proportional to the square of the light-transmitting length. A too short light-transmitting length directly affects the frequency doubling conversion efficiency.

[0007] In summary, because the KBBF crystal is too thin, directly cutting a frequency-doubling device with a sufficiently long length in the light-transmitting direction from the original crystal is greatly limited, making it difficult to make such a device practical.

[0008] Currently, the practical application of KBBF crystals relies on sandwich-structured devices fabricated using a technique known as prism coupling. See Chinese patent application CN1381930A for details. Due to the presence of two optical-resin interfaces between the KBBF crystal and the prism, these heterogeneous interfaces cannot be perfect, inevitably subject to scattering and absorption, as well as unavoidable Fresnel reflection due to the refractive index difference between the crystal and prism materials. Consequently, prism-coupled devices have two drawbacks. First, the device's optical damage threshold is significantly reduced, exceeding one to two orders of magnitude below that of the KBBF crystal itself. Second, the overall transmittance of prism-coupled devices is also significantly lower than that of pure crystal devices, limiting their use for continuous-wave laser output, particularly short-wavelength continuous-wave laser output. Consequently, 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 typically only in the milliwatt range. The shortest wavelength currently achievable with continuous-wave lasers using prism-coupled devices is 171nm. For even shorter output wavelengths, the overall transmittance of prism-coupled devices is insufficient.

[0009] Therefore, it is necessary to provide a method for preparing a frequency conversion device with high transmittance and high continuous wave laser output power by using a thin sheet-like nonlinear optical crystal. Summary of the Invention

[0010] In order to solve the above technical problems, one aspect of the present invention provides a trigonal nonlinear optical crystal frequency conversion device, wherein

[0011] The frequency conversion device has a first set of parallel surfaces and a second set of parallel surfaces. The first set of parallel surfaces are 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 on the ab plane forms a second angle β with the a-axis, having the relationship shown in Formula 1 and Formula 2.

[0012]

[0013]

[0014] θ PM The nonlinear optical crystal satisfies the phase matching angle between the incident fundamental frequency light and the output frequency-converted light,

[0015] θ B =arctann, is the Brewster angle, n is the refractive index of the incident fundamental frequency light of the nonlinear optical crystal,

[0016] The second group of parallel surfaces is the light input surface and the light output surface of the frequency conversion device.

[0017] The present invention utilizes a continuous wave frequency converter prepared from a thin sheet-shaped original crystal of a trigonal nonlinear optical crystal with a smaller c-axis direction, which can increase the width of the light-transmitting surface while maintaining the longest possible length in the light-transmitting direction, that is, it has both light-transmitting length direction and light-transmitting surface width characteristics, thereby achieving high frequency conversion efficiency.

[0018] Preferably, the nonlinear optical crystal uses the parallel surfaces of the thin lamellar original crystal perpendicular to the c-axis (i.e., the two parallel large faces of the thin lamellar original crystal) as its third set of parallel surfaces. Thus, during the process of manufacturing the frequency conversion device, the thickness of the trigonal thin lamellar nonlinear optical crystal original crystal is maximized.

[0019] Preferably, the frequency conversion device is a continuous wave frequency conversion device. The method of the present invention for preparing a frequency conversion device by cutting a thin sheet of rhombic crystals is particularly suitable for preparing a deep-violet continuous wave frequency conversion device using a thin sheet of rhombic nonlinear optical crystals, such as KBBF. The resulting frequency conversion device has a light-transmitting length that is much longer than that of a conventional prism-free frequency conversion device. Compared to currently used prism-coupled devices, this method solves the problem of reducing the overall optical damage threshold and overall transmittance of the device due to the addition of two additional optical-resin interfaces, thereby providing a continuous wave frequency conversion device with high-power continuous wave laser output.

[0020] The frequency conversion device is a frequency doubling device, the optical crystal is a negative uniaxial crystal, and the refractive index n is the refractive index of o light;

[0021] or,

[0022] The frequency conversion device is a sum frequency device, the optical crystal is a negative uniaxial crystal, the refractive index n is the refractive index of the o light 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.

[0023] Preferably, the nonlinear optical crystal is a KBBF crystal, a RBBF crystal, a γ-BBF crystal or other trigonal nonlinear optical crystals.

[0024] 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.

[0025] The second aspect of the present invention provides a method for operating a nonlinear optical crystal frequency conversion device. If the frequency conversion device is a frequency doubling device, the method includes:

[0026] A continuous wave fundamental frequency light is incident on the nonlinear optical crystal from one of the second set of parallel surfaces of the nonlinear optical crystal along the Brewster angle, the fundamental frequency light and the normal of the second set of parallel surfaces constitute an incident plane, 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 is located within the incident plane (the polarization direction of the fundamental frequency light in the crystal is perpendicular to the c-axis of the optical crystal), and the projection of the fundamental frequency light in the nonlinear optical crystal on a plane perpendicular to the c-axis is parallel to the a-axis.

[0027] The frequency-doubled light exits the nonlinear optical crystal from another one of the second set of parallel surfaces.

[0028] 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 the Brewster angle.

[0029] The third aspect of the present invention provides a method for operating a nonlinear optical crystal frequency conversion device. If the frequency conversion device is a sum frequency device, the method includes:

[0030] A first incident fundamental frequency light (the one 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 is incident on the crystal along the Brewster angle, the first fundamental frequency light and the normal of the second set of parallel surfaces form an incident plane, 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 within the incident plane (so that 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 on a plane perpendicular to the c-axis is parallel to the a-axis, and the second fundamental frequency light is collinear with the first fundamental frequency light after entering the crystal.

[0031] The sum frequency light exits the nonlinear optical crystal from another one of the second set of parallel surfaces.

[0032] 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, i.e., from which one of the second set of parallel surfaces, i.e., parallel inclined surfaces, the fundamental frequency light enters the nonlinear optical crystal at the Brewster angle. A fourth aspect of the present invention provides a method for preparing a continuous-wave nonlinear optical crystal frequency conversion device, comprising:

[0033] Taking a plate-shaped original crystal of a trigonal nonlinear optical crystal;

[0034] Determine Brewster's angle θ B =arctann, n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light;

[0035] Determine the phase matching angle θ based on the wavelength of the incident fundamental frequency light and the output frequency-converted light of the nonlinear optical crystal PM ;

[0036] The Brewster angle θ B and phase matching angle θ PM , determine the first angle β and the second angle α according to equations 1 and 2;

[0037]

[0038]

[0039] The nonlinear optical crystal is a trigonal crystal system, and a first set of parallel surfaces is cut parallel to the c-axis and a-axis of the optical crystal;

[0040] A second set of parallel surfaces is cut so that the normal lines form a first angle α with the c-axis of the optical crystal, and the projection of the normal lines on the ab plane forms a second angle β with the a-axis, thereby obtaining the frequency conversion device.

[0041] 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 original crystal, ie, the thickness in the c-axis direction.

[0042] Beneficial effects

[0043] The present invention utilizes the properties of nonlinear optical crystals to determine the phase matching angle and the Brewster angle, calculates and determines the cutting position of a thin-sheet nonlinear optical crystal, and provides a nonlinear optical crystal frequency conversion device for thin-sheet rhombic nonlinear optical crystals that has both a maximum light-transmitting length and a device width and is suitable for continuous-wave laser applications. It also provides a method for conveniently and quickly preparing the nonlinear optical crystal frequency conversion device and a working method for achieving frequency conversion.

[0044] The nonlinear optical crystal cutting method of the present invention eliminates the need to sacrifice optical length in order to achieve the Brewster angle, as is the case with conventional techniques. This method can increase the width of the resulting optical device while maintaining the same optical length. This allows the use of thin sheets of optical crystal to produce a frequency-converting device with high-efficiency continuous-wave laser output. Compared to currently used prism-coupled devices, this method overcomes the issue of reduced overall optical damage threshold and transmittance due to the presence of two additional optical-resin interfaces, thereby providing a continuous-wave frequency-converting device with high-power continuous-wave laser output.

[0045] The nonlinear optical crystal frequency conversion device obtained according to the present invention has a light-transmitting input surface and a light-transmitting output surface that have both the phase matching angle and Brewster angle characteristics of the nonlinear optical crystal. The length of the light-transmitting direction can be several times that of the frequency conversion device prepared by traditional processing methods, and the user can easily determine that the fundamental frequency light enters the crystal from the inclined parallel surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The specific embodiments of the present disclosure will be further described in detail below with reference to the accompanying drawings.

[0047] Figure 1 Shows a schematic diagram of the KBBF crystal physics rectangular coordinate system.

[0048] Figure 2-3 A schematic diagram showing a first example of fabricating a frequency conversion device using a sheet-like nonlinear optical crystal in the prior art.

[0049] Figure 4-6 A schematic diagram showing a second example of fabricating a continuous wave frequency conversion device using a sheet-like nonlinear optical crystal in the prior art.

[0050] Figure 7-8 A schematic diagram showing the present invention's use of a sheet-like nonlinear optical crystal to prepare a continuous wave frequency conversion device.

[0051] Figure 9-11 FIG. 1 shows a working principle diagram of a continuous wave frequency conversion device according to the present invention. DETAILED DESCRIPTION

[0052] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings in conjunction with preferred embodiments of the present invention. The various parts in the accompanying drawings are not drawn to scale. It should be understood that the preferred embodiments of the present invention are illustrative and not restrictive, and the scope of protection of the present invention will be defined by the claims.

[0053] The present invention provides a novel method for cutting crystals to produce nonlinear optical crystal frequency converters. For thin, rhombohedral nonlinear optical crystals, the width of the resulting frequency converter can be increased while maintaining the same length in the light transmission direction. This allows the use of thin optical crystals to produce frequency converters with high-conversion-efficiency continuous-wave laser output.

[0054] The first embodiment of the present invention provides a method for preparing a frequency conversion optical device using a thin sheet of a nonlinear optical crystal, comprising:

[0055] A plate-shaped original crystal of a trigonal nonlinear optical crystal is used to determine the crystal axis. For example, the thickness direction of the plate-shaped original crystal is generally the c-axis of the optical crystal.

[0056] Determine Brewster's angle θ B =arctann, n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light; as a specific example, the nonlinear optical crystal is a negative uniaxial crystal, and the refractive index n is the refractive index of o light;

[0057] Determine the phase matching angle θ based on the wavelength of the incident fundamental frequency light and the output frequency-converted light of the nonlinear optical crystal PM ;

[0058] The Brewster angle θ B and phase matching angle θ PM , determine the first angle α and the second angle β according to equations 1 and 2;

[0059]

[0060]

[0061] The first set of parallel surfaces S1 and S2 are cut from the thin plate original crystal parallel to the crystal c-axis and a-axis to obtain a long strip. The four edges of this set of parallel surfaces are parallel to one of the three equivalent a-axes. The height of this strip is the height of the thin plate original crystal along the c-axis. The width can be cut wider, for example 4-12 mm. Figure 7 It can be understood that due to the three-dimensional rotational symmetry of the trigonal crystal system, Figure 7 There are actually three ways to cut the first set of parallel surfaces S1 and S2: any one of the three a-axes can be made parallel to its four edges. Those skilled in the art can select a method for using the optical crystal converter after cutting based on the specific positional relationship between the cutting plane and the a-axis during crystal cutting to implement the technical solution of the present invention. For the sake of simplicity, this will not be detailed here.

[0062] Cut a second set of parallel surfaces A1B1F1E1 and D1C1G1H1, also called parallel bevels or bevels, so that the angle between G1F1 and G1H1 is (90-β), and the normal of A1B1F1E1 forms a first angle α with the c-axis. Naturally, the projection of the normal of the parallel bevels on the XY plane forms a second angle β with the a-axis, thus cutting out a parallelepiped B1C1G1F1-A1D1H1E1, see Figure 8 To ensure that the laser is incident and emitted only on these parallel inclined surfaces, the interplanar spacing l of the parallel inclined surfaces cannot be greater than d / cosα, where d is the thickness of the sheet optical crystal along the c-axis.

[0063] The second set of parallel inclined surfaces A1B1F1E1 and D1C1G1H1 cut above serve as the light-passing input and light-passing output surfaces of the resulting frequency converter device, respectively. Polishing this set of parallel surfaces to the optical grade required for an optical device completes the entire frequency converter device, a process that is remarkably simple. In the thus-fabricated frequency converter device, the parallelepiped B1C1G1F1-A1D1H1E1 is characterized by a first set of parallel surfaces B1C1G1F1 / A1D1H1E1 perpendicular to the larger parallel surface A1B1C1D1 / E1F1G1H1. The second set of parallel inclined surfaces A1B1F1E1 / D1C1G1H1 is angled relative to the first set of parallel surfaces and the larger parallel surface, respectively. Since fundamental frequency light must be parallel to the first set of planes upon entering the crystal, the fundamental frequency light incident plane of the frequency converter device can be easily determined. By using one of the parallel inclined surfaces as the light-passing plane, the fundamental frequency light is injected into the nonlinear optical crystal at the Brewster angle, facilitating the use of the frequency converter device.

[0064] As a specific example, the nonlinear optical crystal uses the parallel surfaces of the thin sheet-like original crystal perpendicular to the c-axis as its third set of parallel surfaces. For a thin sheet-like trigonal nonlinear optical crystal with a thickness d along the c-axis of no more than 10 mm, the frequency conversion device processing method provided by the present invention can produce a nonlinear optical crystal frequency conversion device with a maximum light-transmitting length and suitable for continuous-wave laser applications. In other words, compared to the prior art, given the limited thickness of the original crystal material, the frequency conversion device cut out has both the maximum possible light-transmitting length and a sufficiently wide switching point width, solving the problem of the prior art in which thin sheet-like optical crystals cannot be used to produce frequency conversion devices with both a light-transmitting length and a device width.

[0065] As a specific example, the nonlinear optical crystal is a KBBF crystal, a RBBF crystal, a γ-BBF crystal or other trigonal nonlinear optical crystals.

[0066] According to the method of the present invention, after determining the angles of the first angle α and the second 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, so that a plurality of strip crystals can be obtained; then, the strip crystals are continuously cut in the manner of cutting the second set of parallel surfaces, so that a plurality of nonlinear optical crystal frequency conversion devices of the present invention can be quickly and easily obtained.

[0067] The following describes the method for preparing the frequency conversion optical device of the present invention by taking a 193 nm continuous wave frequency converter of a KBBF crystal as an example.

[0068] The crystal axes c and a are determined by taking a KBBF thin-sheet original crystal 10. The thickness direction of the KBBF crystal is the c-axis of the optical crystal.

[0069] First, determine the Brewster angle. KBBF crystal is a negative uniaxial crystal. The 386nm fundamental frequency light is o light in the crystal. The Brewster angle θ B =arctann, where n is the refractive index of o light corresponding to the wavelength of the fundamental frequency light of the KBBF crystal. o Substitute @386nm=1.4934 into the above formula and we get θ B =arctan1.4934=56.19°.

[0070] Then determine the first angle α. The value of the first angle α is determined by the above formula 1. For the frequency doubling of the 386nm incident light to 193nm, the phase matching angle θ PM =55.4°. According to formula 1, the angle α is calculated to be 46.9°.

[0071] Finally, the second angle β is determined. According to formula 2, after calculation, β=42.5° is obtained.

[0072] After determining the above parameters, the specific cutting method is detailed as follows:

[0073] Cut the thin KBBF crystal twice along the a-axis parallel to the c-axis (vertical) to obtain a set of parallel surfaces S1 and S2. Cut out a long strip, whose four edges are parallel to the a-axis. Figure 7 The height of this strip is the thickness of the KBBF crystal in the c-axis direction, and the width can be cut wider, for example, 6 mm, which will be used as the second side length (width) of the light-transmitting surface of the frequency conversion device.

[0074] Then, the first set of parallel surfaces S1 and S2 are rotated 42.5° (i.e., the angle between G1F1 and G1H1 is 90-β=47.5°) and a set of parallel inclined surfaces A1B1F1E1 and D1C1G1H1 are cut at an angle α between the normal and the c-axis to form a parallelepiped B1C1G1F1-A1D1H1E1 for preparing a nonlinear optical crystal frequency conversion device 31, see Figure 8The angle between the normals of the inclined planes A1B1F1E1 and D1C1G1H1 and the c-axis of the KBBF crystal is α, that is, the angle between the parallel inclined planes and the XY plane is α. At the same time, the projection of the normals of these parallel inclined planes on 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 thin KBBF crystal along the c-direction, to ensure that the laser is incident and emitted only on these parallel inclined planes.

[0075] The inclined surfaces A1B1F1E1 and D1C1G1H1 are polished as the light-passing surfaces to meet the optical grade requirements of the optical device, thus completing the production of the 193nm continuous wave frequency converter of the KBBF crystal.

[0076] The nonlinear optical crystal frequency converter of the present invention can be used as a frequency multiplier. Figure 9-11 The fundamental frequency light is incident on the frequency doubler along the Brewster angle from any one of the optical surfaces, such as D1C1G1H1, and exits the frequency doubler from the other optical surface, A1B1F1E1. The incident fundamental frequency light and the normal to the optical surface D1C1G1H1 form the incident plane. The angle between the incident plane normal and the c-axis of the optical crystal is the complementary angle of the first angle α, and the fundamental frequency light's polarization lies within the incident plane. Thus, 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 ways to inject the fundamental frequency light that satisfy these conditions. A common approach is to select one that ensures that the projection of the fundamental frequency light onto the ab plane after entering the crystal is parallel to the a-axis (i.e., the first parallel plane of the device within the crystal). Consequently, upon entering the nonlinear optical crystal, the fundamental frequency light is entirely o-light and travels in the phase-matching direction. Since the phase-matching condition is met, the incident fundamental frequency light is gradually converted into frequency-doubled light. Figure 9 This is a projection of the device along the c-axis. Arrows indicate the directions of incident and outgoing light rays, and dashed lines represent the paths of light rays through the crystal. It can be seen that the dotted lines project parallel to the a-axis in the XY plane, satisfying the φ = 0 phase-matching direction of the KBBF crystal. Figure 10 This is the projection along the second set of parallel surfaces D1C1G1H1. It can be seen that the three light rays, the incident light, the light in the crystal and the outgoing light, are in the same plane. This plane is the coincident incident surface and the outgoing surface. Since the two light-transmitting surfaces are parallel, the incident surface and the outgoing surface coincide. This plane is perpendicular to the light-transmitting input surface D1C1G1H1 and the light-transmitting output surface A1B1F1E1. The angle between the normal of the incident surface and the c-axis is the complementary angle of the second angle α (90°-α). It should be noted that this frequency doubler is a parallelepiped, and only a part of the area is actually used, that is, Figure 10 The parallelepiped labeled C1D1JK-MNE1F1 has two actual light-transmitting surfaces (also called light-transmitting windows)—C1D1JK and MNE1F1. The length of the first side C1K of the light-transmitting window is h. For convenience, the parallelepiped C1D1JK-MNE1F1 does not need to be specifically cut out.

[0077] The present invention discloses a method for fabricating a frequency converter device using a thin sheet of nonlinear optical crystal. The method utilizes two existing large surfaces (two parallel surfaces perpendicular to the c-axis) of a thin sheet of nonlinear optical crystal. First, two cuts are made parallel to the c-axis and a-axis to produce a first set of parallel surfaces S1 and S2, forming a strip-shaped crystal blank. A second set of parallel inclined surfaces is then cut from this strip, rotated at an angle β relative to the parallel surfaces S1 and S2, with the normal forming an angle α with the c-axis, to form a parallelepiped. The projections of the normals of the second set of parallel inclined surfaces onto the ab plane form a second angle β with the a-axis of the crystal. After polishing the parallel inclined surfaces, a nonlinear optical crystal continuous-wave frequency converter is obtained. After determining the two angles, the present invention can cut multiple strips of crystal blanks, and then cut multiple parallel inclined surfaces from each blank. This maximizes the use of the original crystal and directly produces multiple, highly consistent continuous-wave frequency converter devices. The method is simple to manufacture.

[0078] The frequency conversion device obtained by the method of the present invention has a light-transmitting length in the direction of light, which is not affected by the increase in the width of the light-transmitting surface, and is much longer than the light-transmitting length of the continuous wave frequency doubling device obtained by traditional cutting. The following calculations illustrate the comparison of the light-transmitting length using a 193nm continuous wave frequency doubling device made of a KBBF crystal as an example. PM =55.4°,θ B =56.19°. For the traditional cutting technology to produce continuous wave frequency doubler, taking the thickness of thin KBBF original crystal d = 4mm as an example, Figure 3 The length of one side AB of the middle light-transmitting surface is h = 1mm, and the length of the other side AD is k = 6mm. To ensure that the change point requirement is met, the length in the light-transmitting direction is L = (d / cosθ PM -h×tanθ PM )=5.595mm. Further, according to Figure 6 After cutting two parallel inclined planes according to the Brewster angle, the length in the light direction becomes further shortened, L'=Lk×ctgθ B =5.595-6×ctg56.19°=1.578mm. If k continues to increase, the length in the light transmission direction will continue to decrease.

[0079] If the technology of the present invention is used, the first angle α = 46.9° and the second angle β = 42.5° are obtained. Figure 10 The projection length of the light-transmitting direction on the paper (perpendicular to the light-transmitting input surface D1C1G1H1 and the light-transmitting output surface A1B1F1E1) is the surface distance l between the two light-transmitting surfaces. Figure 3 Calculation: l = d / cosα-h×tanα = 4.786mm. Figure 11, the actual length of the light direction L'=l / sinθ B =5.759mm. This length is much longer than the length of 1.578mm in the light-transmitting direction of an ordinary continuous wave frequency doubler, reaching 3.6 times of the latter. The reason is that the length of the light-transmitting direction of the device of the present invention will not be affected by the increase in the width k of the light-transmitting surface. According to the frequency conversion device of the present invention, the light-transmitting surface width can be selected according to the size of the original crystal or according to the working needs of the frequency converter, for example, 4mm-12mm. Therefore, the nonlinear optical crystal frequency conversion device obtained by the preparation method of the present invention can achieve high frequency conversion efficiency, meet the actual power requirements and the requirements for the use of continuous wave frequency converters, and expand the applicability and application range of deep ultraviolet laser frequency conversion devices.

[0080] As a specific embodiment, the continuous wave frequency conversion device needs to be placed in a resonant cavity with an incident cavity mirror and an output cavity mirror. The incident fundamental frequency light is only emitted from the incident cavity mirror into the nonlinear optical crystal frequency converter. After being emitted from the frequency converter, it is reflected by the output cavity mirror and re-injected into the nonlinear optical crystal frequency converter, thereby realizing intra-cavity circulation, which is repeated continuously and resonance enhancement is achieved by locking the cavity length; the frequency conversion light is only emitted from the resonant cavity once from the output cavity mirror.

[0081] As a specific embodiment, the nonlinear optical crystal frequency converter of the present invention can be used as a sum frequency converter. During use, a first incident fundamental frequency light and a second incident fundamental frequency light are injected into the nonlinear optical crystal through one of the parallel inclined surfaces 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 surface C1D1H1G1 form an 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 enters the nonlinear optical crystal along the Brewster angle determined by the first incident fundamental frequency light. After entering the crystal, the second fundamental frequency light 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 is emitted from the nonlinear optical crystal through the other inclined surface B1A1E1F1.

[0082] The technical solution of the present invention is described in detail below with the help of examples.

[0083] Example 1

[0084] A continuous wave frequency doubling device with a wavelength of 193 nm was fabricated using a KBBF primary crystal with a thickness of 4 mm.

[0085] First, determine Brewster's angle.

[0086] The refractive index of o light of KBBF crystal at the fundamental frequency wavelength of 386nm is n o @386nm=1.4934, Brewster angle θ B =arctann o=arctan1.4934=56.19°.

[0087] Then, the first angle α is determined using Equation 1:

[0088]

[0089] The phase matching angle θ of the KBBF crystal when doubling the fundamental frequency light 386nm to the doubled frequency light 193nm PM =55.4°. According to formula 1, we can get α=46.9°.

[0090] Then, the second angle β is determined using Equation 2:

[0091]

[0092] After calculation, β=42.5°.

[0093] After determining the first angle α and the second angle β, the thin-sheet KBBF crystal is cut parallel to the a-axis and parallel to the c-axis to obtain a first set of parallel surfaces to obtain a cuboid. The height of this cuboid is the thickness of the thin-sheet crystal d = 4 mm, and the width is the spacing between the first set of parallel surfaces 6 mm.

[0094] Then, the first set of parallel surfaces S1 and S2 are rotated 42.5° and the second set of parallel inclined surfaces are cut at an angle of 46.9° (α) between the normal and the c-axis to obtain a parallelepiped. The interplanar spacing of the second set of parallel inclined surfaces is 4.7mm, which ensures that the length of the first side h of the light-transmitting surface reaches 1mm ( Figure 10 C1K in the figure); at the same time, the projection of the normal of this parallel inclined plane on the XY plane must satisfy the angle of 42.5° with the a-axis.

[0095] The parallel bevels are polished to meet the optical grade requirements of the frequency doubling device, thereby completing the production of the nonlinear optical crystal for the frequency doubling device with a very simple process. In the obtained frequency doubling device, a part of the parallel bevels is the light-transmitting surface of the frequency doubling device (such as Figure 10 As shown in the figure). According to the calculation above, the actual length in the light-transmitting direction is 5.759 mm.

[0096] When in use, the above device is placed in a resonant cavity, and a beam of continuous wave laser light with a wavelength of 386nm and a power of 4W is incident on the KBBF crystal frequency doubler from one of the second set of inclined planes along the Brewster angle through the incident cavity mirror. The incident light and the normal line of the inclined plane constitute the incident plane. At the same time, the angle between the normal line of the incident plane and the c-axis must be 43.1° (i.e., the complementary angle of the α angle), and the projection of the 386nm laser light on the ab plane in the crystal must be parallel to the a-axis. The polarization direction of the 386nm laser light is within the incident plane. After entering the crystal, the 386nm laser light advances in the phase matching direction and the polarization direction is perpendicular to the c-axis, meeting the phase matching condition, and gradually converting into a continuous wave 193nm frequency-doubled light and emitting the crystal. The remaining 386nm laser light is reflected by the cavity mirror, continues to oscillate in the cavity, and passes through the KBBF crystal multiple times, repeatedly, and achieves resonance enhancement by locking the cavity length. The newly generated 193nm laser is output once through the output cavity mirror, and 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 doubler.

[0097] Example 2

[0098] A 177.5nm continuous wave frequency doubling device was fabricated using KBBF primary crystal with a thickness of d=4mm.

[0099] First determine the Brewster angle.

[0100] The refractive index of o light of KBBF crystal at 355nm is n o @355nm=1.4974, Brewster angle θ B =arctann o =arctan1.4974=56.26°.

[0101] Then, the first angle α is determined using Equation 1:

[0102]

[0103] The KBBF crystal doubles the fundamental frequency light 355nm to 177.5nm with a phase matching angle θ PM =64.3°. According to formula 1, we can get α=58.57°.

[0104] Then, the second angle β is determined using Equation 2:

[0105]

[0106] After calculation, β=38.05°.

[0107] After determining the first angle α and the second angle β, the thin-sheet KBBF crystal is cut parallel to the a-axis and parallel to the c-axis to obtain a first set of parallel surfaces to obtain a cuboid. The height of this cuboid is the thickness of the thin-sheet crystal d = 4 mm, and the width is the spacing between the first set of parallel surfaces 6 mm.

[0108] Then, relative to the first set of parallel surfaces S1 and S2, rotate 38.05° and cut the second set of parallel inclined surfaces at an angle of 58.57° (α) between the normal and the c-axis to obtain a parallelepiped. The surface spacing of the second set of parallel inclined surfaces is 6mm, which can ensure that the length of the first side h of the light-transmitting surface reaches 1mm ( Figure 10 C1K in the figure); at the same time, the projection of the normal of this parallel inclined plane on the XY plane must satisfy the angle of 38.05° with the a-axis.

[0109] Polishing this set of parallel bevels to meet the optical grade requirements of the frequency doubling device completes the production of the entire device, which is very simple. Part of the two parallel bevels is the light-transmitting surface of the frequency doubling device (such as Figure 10 shown).

[0110] according to Figure 10 The projection of the length of the light direction on the side of the frequency doubler is the surface distance l between the light input surface and the light output surface. Figure 3 Calculation, l = d / cosα-h×tanα, d = 4mm, h = 1mm and other values ​​​​substitute, we get l = 4 / cos58.57°-1×tan58.57°=6.034mm; according to Figure 11 , the actual length of the light direction L'=l / sinθ B =6.034 / sin56.26°=7.256mm.

[0111] When in use, the above device is placed in a resonant cavity, and a continuous wave laser with a wavelength of 355nm and a power of 4W is incident on the KBBF crystal from one of the second parallel inclined planes along the Brewster angle through the incident cavity mirror. The incident light and the normal line of the inclined plane constitute the incident plane. At the same time, the angle between the normal line of the incident plane and the c-axis must be 31.43° (i.e., the complementary angle of the α angle), and the projection of the 355nm laser on the ab plane in the crystal must be parallel to the a-axis. The polarization direction of the 355nm laser is within the incident plane. After entering the crystal, the 355nm laser moves along the phase matching direction and the polarization direction is perpendicular to the c-axis, meeting the phase matching condition, and gradually converting into a continuous wave 177.5nm frequency-doubled light, which is emitted from the crystal. The remaining 355nm laser is reflected by the cavity mirror, continues to oscillate in the cavity, and passes through the KBBF crystal multiple times, repeatedly, and achieves resonance enhancement by locking the cavity length. The 177.5nm laser is output once through the output cavity mirror, and the final output continuous wave 177.5nm laser power reaches about 10mW.

[0112] Example 3

[0113] A continuous wave sum frequency device with a sum frequency wavelength of 153.43 nm was fabricated using a KBBF primary crystal with a thickness of 4 mm.

[0114] Yb laser with a wavelength of 1074nm is used as the first fundamental frequency light, and 179nm laser with a six-harmonic frequency of the first fundamental frequency light is used as the second fundamental frequency light. The sum frequency process is that 1074nm laser and 179nm laser are summed to obtain 153.43nm laser.

[0115] First, determine the Brewster angle. Since the sum frequency process involves two beams of fundamental frequency light of different wavelengths, the refractive index is different and therefore the Brewster angle is also different. The present invention does not need to cut out a Brewster angle that satisfies both wavelengths at the same time. Taking the longer wavelength of 1074nm as the standard, the refractive index of o light of KBBF crystal at 1074nm is n o @1074nm=1.4713, Brewster angle θ B =arctann o =arctan1.4713=55.80°.

[0116] Then determine the angle α. The value of the angle α is given by the equation Determine that for the sum frequency of 1074nm laser and 179nm laser with sum frequency of 153.43nm, the phase matching angle θ of KBBF crystal is PM =52.1°. Calculated α = 42.3°.

[0117] Finally, determine the β angle, After calculation, β=45.3°.

[0118] After determining the above parameters, the thin KBBF crystal was cut parallel to the a-axis and parallel to the c-axis to obtain a first set of parallel surfaces to obtain a cuboid with a height of 4 mm and a width equal to the spacing between the first set of parallel surfaces of 6 mm.

[0119] Then, the first set of parallel surfaces S1 and S2 are rotated 45.3° and the second set of parallel inclined surfaces are cut at an angle of 42.3° (α) between the normal and the c-axis to obtain a parallelepiped. The interplanar spacing of the second set of parallel inclined surfaces is 4.4 mm, which ensures that the length of the first side h of the light-transmitting surface reaches 1.1 mm ( Figure 10 C1K in the figure); at the same time, the projection of the normal of this parallel inclined plane on the XY plane must satisfy the angle of 45.3° with the a-axis.

[0120] Polish the two parallel bevels to meet the optical grade requirements of the frequency doubling device, thus completing the production of the entire device. Part of the parallel bevel is the light-transmitting surface of the frequency doubling device (such as Figure 10 shown).

[0121] according to Figure 10 The projection of the length of the light direction on the side of the frequency converter is the surface distance l between the light input surface and the light output surface. Figure 3 Calculation, l = d / cosα-h×tanα, d = 4mm, h = 1.1mm and other values ​​​​substituted into, we get l = 4 / cos42.3°-1.1×tan42.3° = 4.407mm; according to Figure 11 , the actual length of the light direction L1=l / sinθ B =4.407 / sin55.80°=5.328mm.

[0122] When in use, the device is placed in a resonant cavity, and the fundamental frequency light is a continuous wave laser with wavelengths of 1074nm and 179nm, respectively. The laser power of the first fundamental frequency light with a wavelength of 1074nm is 8W, and the second fundamental frequency light with a wavelength of 179nm is a sextuple frequency of the 1074nm laser. That is, after frequency doubling, sum frequency multiplication, and then frequency doubling again, the power is only 1mW. The 1074nm first fundamental frequency light passes through the incident cavity mirror and is incident on one of the second parallel inclined planes along the Brewster angle into the KBBF crystal. The incident light and the normal to the inclined plane form the incident plane. At the same time, the angle between the normal to the incident plane and the c-axis must be 47.7° (i.e., the complementary angle of the α angle), and the projection of the 1074nm laser on the ab plane in the crystal must be parallel to the a-axis. The polarization direction of the 1074nm laser is within the incident plane. After entering the crystal, the 1074nm laser travels in the phase-matching direction, with its polarization direction perpendicular to the c-axis, satisfying the phase-matching condition and undergoing repeated cyclic resonance enhancement within the cavity. The 179nm laser is incident on the inclined surface of the KBBF crystal at a near-Brewster angle and is collinear with the 1074nm laser within the crystal. After entering the crystal, the 1074nm and 179nm lasers travel in the phase-matching direction and are gradually sum-frequency converted into 153.43nm sum-frequency light, which is then emitted from the crystal. The newly generated 153.43nm laser light is output through the output cavity mirror, while the remaining 1074nm laser light is reflected by the cavity mirror, continues to oscillate within the cavity, and passes 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.

[0123] Obviously, the above embodiments of the present disclosure are merely examples for clearly illustrating the present disclosure, and are not intended to limit the implementation methods of the present disclosure. For those skilled in the art, other different forms of changes or modifications can be made based on the above descriptions. It is impossible to enumerate all the implementation methods here. Any obvious changes or modifications derived from the technical solutions of the present disclosure are still within the scope of protection of the present disclosure.

Claims

1. A trigonal nonlinear optical crystal frequency conversion device, characterized in that: The frequency conversion device has a first set of parallel surfaces and a second set of parallel surfaces, the first set of parallel surfaces are parallel to the c-axis and the 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 on the ab surface forms a second angle β with the a-axis, having the relationship shown in formula 1 and formula 2 θ PM The nonlinear optical crystal satisfies the phase matching angle between the incident fundamental frequency light and the output frequency-converted light, θ B =arctann, is the Brewster angle, n is the refractive index of the incident fundamental frequency light of the nonlinear optical crystal, The second group of parallel surfaces is the light input surface and the light output surface of the frequency conversion device.

2. The nonlinear optical crystal frequency conversion device according to claim 1, characterized in that: The nonlinear optical crystal uses the parallel surfaces of the plate-shaped crystal that are perpendicular to the c-axis of the optical crystal as its third group of parallel surfaces.

3. The nonlinear optical crystal frequency conversion device according to claim 1, characterized in that: The frequency conversion device is a continuous wave frequency conversion device.

4. The nonlinear optical crystal frequency conversion device according to claim 1, characterized in that: The frequency conversion device is a frequency doubling device, the optical crystal is a negative uniaxial crystal, and the refractive index n is the refractive index of o light; or, The frequency conversion device is a sum frequency device, the optical crystal is a negative uniaxial crystal, the refractive index n is the refractive index of the o light 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 according to claim 1, characterized in that: The nonlinear optical crystal is a KBBF crystal, a RBBF crystal, a γ-BBF crystal or other trigonal nonlinear optical crystals.

6. The nonlinear optical crystal frequency conversion device according to claim 2, characterized in that: The interplanar spacing of the second group of parallel surfaces is no greater than d / cosα, where d is the thickness of the plate-like crystal.

7. A method for operating a nonlinear optical crystal frequency conversion device according to claim 4, characterized in that: The frequency conversion device is a frequency multiplication device, 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 of the nonlinear optical crystal along the Brewster angle, the plane defined by the fundamental frequency light and the normal of the second set of parallel surfaces forming an incident plane, the angle between the normal of the incident plane and the c-axis is (90-α), the polarization direction of the fundamental frequency light is located within the incident plane, and the projection of the fundamental frequency light in the nonlinear optical crystal on the plane perpendicular to the c-axis is parallel to the a-axis. The frequency-doubled light exits the nonlinear optical crystal from another one of the second set of parallel surfaces.

8. A method for operating a nonlinear optical crystal frequency conversion device according to claim 4, characterized in that: The frequency conversion device is a sum frequency device, and the method includes: A first incident fundamental frequency light 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 is incident on the crystal along the Brewster angle, a plane defined by the first incident fundamental frequency light and the normal of the second set of parallel surfaces constitutes an incident plane, the angle between the normal of the incident plane and the c-axis is (90-α), and the polarization direction of the first incident fundamental frequency light is located within the incident plane, the projection of the first incident fundamental frequency light in the nonlinear optical crystal on a plane perpendicular to the c-axis is parallel to the a-axis, and the second incident fundamental frequency light is collinear with the first incident fundamental frequency light after entering the crystal. The sum frequency light exits the nonlinear optical crystal from another one of the second set of parallel surfaces.

9. A method for preparing a continuous wave nonlinear optical crystal frequency conversion device, characterized in that: The method includes: Determine Brewster's angle θ B =arctann, where n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light; Determine the phase matching angle θ based on the wavelength of the incident fundamental frequency light and the output frequency-converted light of the nonlinear optical crystal PM ; The Brewster angle θ B and phase matching angle θ PM , determine the first angle α and the second angle β according to equations 1 and 2; The nonlinear optical crystal is a trigonal crystal system, and a first set of parallel surfaces is cut parallel to the c-axis and a-axis of the optical crystal; A second set of parallel surfaces is cut so that the normal lines form a first angle α with the c-axis of the optical crystal, and the projection of the normal lines on the ab plane forms a second angle β with the a-axis, thereby obtaining the frequency conversion device.

10. The preparation method according to claim 9, characterized in that The nonlinear optical crystal is a plate-shaped crystal, and the interplanar spacing of the second group of parallel surfaces is no greater than d / cosα, where d is the thickness of the plate-shaped crystal.

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