Nonlinear optical crystal continuous wave frequency conversion device, working method and preparation method thereof

Abstract

The present invention relates to a trigonal nonlinear optical crystal frequency conversion device, wherein the frequency conversion device comprises a first group of parallel surfaces and a second group of parallel surfaces, wherein the first group of parallel surfaces is parallel to the c-axis of the optical crystal and forms a first angle β with the a-axis, and the second group of parallel surfaces is perpendicular to the first group of parallel surfaces and the normal of the second group of parallel surfaces forms a second angle α with the c-axis of the optical crystal, and the second angle α and the first angle β have a relationship θ shown in equations 1 and 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 fabricating a nonlinear optical crystal frequency converter device suitable for continuous-wave applications, having a maximum possible light-transmitting length, based on a thin sheet-like primary crystal.

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, national defense, and other fields. 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, abbreviated as KBBF) and rubidium beryllium fluoroborate (RbBe2BO3F2, abbreviated as RBBF) meet the requirements for deep UV nonlinear optical crystals. They can directly frequency-double and output lasers with wavelengths less than 200nm. Specifically, they can achieve sextuple-harmonic (177.3nm) output of Nd:YAG lasers and quadruple-harmonic bandwidth-tunable output of Ti:Sapphire lasers. The overall performance of KBBF crystals is significantly superior to that of RBBF crystals.

[0003] KBBF crystals have a severe layered 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, the a-axis direction is defined as 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 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 11 is 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-doubled optical power.

[0004] Figure 2 The schematic diagram of a first example of prior art for fabricating 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 2The frequency doubler 11 shown in the figure is improved. There are two technical solutions. The first is to coat the two light-transmitting surfaces ABCD and EFGH with an anti-reflection film, so that the fundamental frequency light can pass through these two light-transmitting surfaces with almost no loss. However, the laser damage threshold of the anti-reflection film is low, one to two orders of magnitude lower than the laser damage threshold of the crystal itself, and it is easily damaged by the laser. If the anti-reflection film is applied to the continuous wave frequency doubler of the KBBF crystal, the excellent characteristic of the KBBF crystal with a high laser damage threshold is not utilized, resulting in the anti-reflection film continuous wave frequency doubler being only suitable for low-power continuous wave laser frequency doubling.

[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 such a device unusable in practical use.

[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. A disadvantage of prism-coupled devices is that the presence of two additional interfaces between the KBBF crystal and the prism significantly reduces the device's overall optical damage threshold, by one to two orders of magnitude compared to the KBBF crystal itself. 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 typically only has a milliwatt output. Furthermore, the overall transmittance of prism-coupled devices is significantly lower than that of pure crystal devices. This is because the two optical interfaces are heterogeneous and cannot be perfect, subject to scattering and absorption, as well as unavoidable Fresnel reflections caused by the difference in refractive index between the crystal and prism materials. This low overall transmittance limits the use of prism-coupled devices for continuous-wave laser output, particularly short-wavelength continuous-wave laser output.

[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 group of parallel surfaces and a second group of parallel surfaces, the first group of parallel surfaces is parallel to the c-axis of the optical crystal and forms a first angle β with the a-axis, the second group of parallel surfaces is perpendicular to the first group of parallel surfaces and the normal of the second group of parallel surfaces forms a second angle α with the c-axis of the optical crystal, and the second angle α and the first angle β have 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 transmittance and 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 incident plane formed by the fundamental frequency light and the normal of the second set of parallel surfaces is perpendicular to the first set of parallel surfaces, and the polarization direction of the fundamental frequency light is perpendicular to the c-axis of the optical crystal. After entering the crystal, the projection of the light on the plane perpendicular to the c-axis is parallel to the a-axis. In fact, at this time, the light in the crystal travels in the phase matching direction.

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

[0028] 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:

[0029] 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 group of parallel surfaces, with the incident surface being perpendicular to the first parallel surface, the first incident fundamental frequency light is incident on the crystal along the Brewster angle, the second fundamental frequency light is collinear with the first fundamental frequency light after entering the crystal, and the projections of the two light rays on a plane perpendicular to the c-axis in the crystal are parallel to the a-axis, wherein the wavelength of the first incident fundamental frequency light is greater than the wavelength of the second incident fundamental frequency light, and the Brewster angle is determined by the first incident fundamental frequency light.

[0030] The sum frequency light exits the nonlinear optical crystal from another one of the second set of parallel surfaces.

[0031] A fourth aspect of the present invention provides a method for preparing a continuous-wave nonlinear optical crystal frequency conversion device, the method comprising:

[0032] Taking a plate-shaped original crystal of a trigonal nonlinear optical crystal;

[0033] Determine Brewster's angle θ B =arctann, n is the refractive index of the nonlinear optical crystal corresponding to the incident fundamental frequency light;

[0034] 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 ;

[0035] 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;

[0036]

[0037]

[0038] The nonlinear optical crystal is a trigonal crystal system, and a first group of parallel surfaces are cut parallel to the c-axis of the optical crystal, and the first group of parallel surfaces forms a first angle β with the a-axis of the optical crystal;

[0039] A second set of parallel surfaces is cut perpendicularly to the first set of parallel surfaces so that the normals of the second set of parallel surfaces form a second angle α with the c-axis direction of the optical crystal, thereby obtaining the frequency conversion device.

[0040] 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.

[0041] Beneficial effects

[0042] The present invention calculates and determines the cutting position of a thin-sheet nonlinear optical crystal by utilizing the phase matching angle and Brewster angle determined by the properties of the nonlinear optical crystal. For the thin-sheet rhombic nonlinear optical crystal, a nonlinear optical crystal frequency conversion device is provided, which has both the maximum light transmission direction length and the device width and is suitable for continuous wave laser applications. A method for conveniently and quickly preparing the nonlinear optical crystal frequency conversion device is also provided.

[0043] The nonlinear optical crystal cutting method of the present invention eliminates the need to specifically target the Brewster angle when cutting thin nonlinear optical crystals, thus eliminating the need to sacrifice optical length in the light-transmitting direction to achieve the Brewster angle, as is the case with conventional techniques. The nonlinear optical crystal cutting method of the present invention increases the width of the resulting optical device while maintaining the same optical length. This allows the use of thin optical crystals to produce frequency-converting devices with high-conversion-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 transmittance and high-power continuous-wave laser output.

[0044] The nonlinear optical crystal frequency conversion device obtained according to the present invention has a light input surface and a light output surface that have both the phase matching angle and Brewster angle characteristics of the nonlinear optical crystal, and the light direction length can be several times that of the frequency conversion device prepared by traditional processing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The specific embodiments of the present disclosure will be further described in detail below with reference to the accompanying drawings.

[0046] Figure 1 Shows a schematic diagram of the KBBF crystal physics rectangular coordinate system.

[0047] Figure 2-3A schematic diagram showing a first example of fabricating a frequency conversion device using a sheet-like nonlinear optical crystal in the prior art.

[0048] 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.

[0049] 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.

[0050] Figure 9-11 FIG. 1 shows a working principle diagram of a continuous wave frequency conversion device according to the present invention. DETAILED DESCRIPTION

[0051] 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.

[0052] 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.

[0053] 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:

[0054] 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.

[0055] 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;

[0056] 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 ;

[0057] 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;

[0058]

[0059]

[0060] The first set of parallel surfaces S1 and S2 are cut from the thin plate original crystal parallel to the crystal c-axis to obtain a long strip. The four edges of this set of parallel surfaces all form a first angle β with respect to any one of the three equivalent a-axes. The height of this strip is the height of the thin plate original crystal on 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 system, although Figure 7 The first set of parallel surfaces S1 and S2 form a first angle β with respect to the a-axis. The first set of parallel surfaces S1 and S2 may also form an angle -β with respect to the a-axis, or (60° -β). Those skilled in the art can select a method for using the optical crystal frequency converter after cutting based on the specific positional relationship between the cutting plane and the a-axis when cutting the crystal to implement the technical solution of the present invention. For the purpose of brevity, this will not be described in detail here.

[0061] A second set of parallel surfaces A1B1F1E1 and D1C1G1H1 are cut perpendicular to the first set of parallel surfaces S1 and S2, also called parallel bevels or bevels. Naturally, the projection of the normal of the parallel bevels on the XY plane forms a first angle β with the a-axis, and the normal direction of the parallel bevels forms a second angle α with the c-axis of the crystal, thus cutting out a right 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.

[0062] The second set of parallel inclined surfaces A1B1F1E1 and D1C1G1H1 obtained above are the light input and light output surfaces of the resulting frequency converter device, respectively. Polishing this set of parallel surfaces to meet the optical grade requirements of the optical device completes the entire frequency converter device. The preparation steps are very simple.

[0063] 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.

[0064] 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.

[0065] According to the method of the present invention, after determining the angles of the first angle β and the second angle α, during the processing, the sheet-like original crystal is continuously cut in the manner of cutting the first group 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 group of parallel surfaces, so that a plurality of nonlinear optical crystal frequency conversion devices of the present invention can be quickly and easily obtained.

[0066] 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.

[0067] 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.

[0068] 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°.

[0069] Then determine the second angle α. The value of the second 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°.

[0070] Finally, the second angle β is determined. According to formula 2, after calculation, β=42.5° is obtained.

[0071] After determining the above parameters, the specific cutting method is detailed as follows:

[0072] Cut the thin KBBF crystal twice parallel to the c-axis (vertical) along the direction of β angle away from the a-axis to obtain a set of parallel surfaces S1 and S2. Cut out a long strip, the four edges of which are all at 42.5° with 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.

[0073] Then, a set of parallel inclined surfaces A1B1F1E1 and D1C1G1H1 are cut perpendicular to a set of parallel surfaces S1 and S2 and at an angle α between the normal and the c-axis to form a right parallelepiped B1C1G1F1-A1D1H1E1 for preparing a nonlinear optical crystal frequency conversion device 31, see Figure 8 The 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 first 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 original crystal along the c-direction, to ensure that the laser is incident and emitted only on these parallel inclined planes.

[0074] 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.

[0075] 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 incident plane is perpendicular to the first set of parallel surfaces, so the first parallel surface, the second parallel surface, and the incident plane are mutually perpendicular planes. The angle between the normal to the incident plane and the c-axis of the optical crystal is the complementary angle of the second angle α. The polarization direction of the fundamental frequency light is perpendicular to the c-axis of the optical crystal, so that the projection of the light on the plane perpendicular to the c-axis after entering the crystal is parallel to the a-axis. As a result, the fundamental frequency light enters the nonlinear optical crystal as o-light and travels along the phase-matching direction. Because 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 of the first set of parallel surfaces B1C1G1F1 (side surface). It can be seen that the three light segments, 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 multiplier is a parallelepiped, and only a part of the area is actually used, that is, Figure 10The cuboid labeled C1D1JK-MNE1F1 has two light-transmitting surfaces (also called light windows)—C1D1JK and MNE1F1. The length of the first side of the light window, C1K, is h. For convenience, the cuboid C1D1JK-MNE1F1 does not need to be specifically cut out.

[0076] 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. Based on the calculated result of a first angle β, two cuts are first made parallel to the c-axis to produce a first set of parallel surfaces S1 and S2, yielding a strip-shaped crystal blank. A second set of parallel inclined surfaces is then cut perpendicular to the first set of parallel surfaces at a second angle α from this strip. The normals of the second set of parallel inclined surfaces form a second angle α with the crystal's c-axis, resulting in a parallelepiped. The projection of the normals onto a plane perpendicular to the crystal's c-axis forms the first angle β with the crystal's a-axis. After polishing the parallel inclined surfaces, a nonlinear optical crystal continuous-wave frequency converter is obtained. After determining the two angles, multiple strip-shaped crystal blanks can be cut, and multiple parallel inclined surfaces can be cut from each blank. This maximizes the utilization of the original crystal and directly yields multiple, highly consistent continuous-wave frequency converter devices. The method is simple to manufacture.

[0077] 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.

[0078] 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 10The 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 D1C1G1H1 and the light output surface A1B1F1E1. Figure 3 Calculation: l = d / cosα-h×tanα = 4.786mm. Figure 11 , the actual length of the light direction L1=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.

[0079] 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.

[0080] 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 inclined surface C1D1H1G1 form an incident surface. If the incident surface is perpendicular to the side surface, the angle between the normal of the incident surface and the c-axis of the optical crystal must be the complementary angle of the second 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 light rays in the crystal on a plane perpendicular to the c-axis are parallel to the a-axis. The sum frequency light exits the nonlinear optical crystal through the other inclined surface B1A1E1F1.

[0081] The technical solution of the present invention is described in detail below with the help of examples.

[0082] Example 1

[0083] 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.

[0084] First, determine Brewster's angle.

[0085] 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°.

[0086] Then, the second angle α is determined using Equation 1:

[0087]

[0088] 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°.

[0089] Then, the first angle β is determined using Equation 2:

[0090]

[0091] After calculation, β=42.5°.

[0092] After determining the second angle α and the first angle β, the thin-plate KBBF crystal is cut along the direction parallel to the c-axis at a deviation of 42.5° (angle β) from the a-axis to obtain the first set of parallel surfaces, resulting in a cuboid with a height equal to the thickness of the thin-plate crystal d = 4 mm and a width equal to the spacing between the first set of parallel surfaces, 6 mm.

[0093] Then, perpendicular to the first set of parallel surfaces, parallel bevels are cut at an angle of 46.9° (α) between the normal and the c-axis to obtain a straight parallelepiped. The interplanar spacing of the parallel bevels is 4.7 mm, which ensures that the length of the first side h of the light-transmitting surface reaches 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 42.5° with the a-axis.

[0094] 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.

[0095] In use, the device is placed in a resonant cavity. A continuous-wave laser beam with a wavelength of 386nm and a power of 4W is incident on one of the inclined surfaces along the Brewster angle through the incident cavity mirror. The incident light and the normal to the inclined surface form the incident plane. The incident plane must be perpendicular to the first set of parallel surfaces. The angle between the normal to the incident plane and the c-axis must be 43.1° (the complementary angle of the α angle), and the polarization direction of the 386nm laser beam must be perpendicular to the c-axis. After entering the crystal, the 386nm laser beam travels along the phase-matching direction and gradually converts into continuous-wave 193nm frequency-doubled light, which then exits the crystal. The remaining 386nm laser beam is reflected by the cavity mirror and continues to oscillate within the cavity, passing through the KBBF crystal multiple times. This repetition achieves resonant enhancement by locking the cavity length. The newly generated 193nm laser beam is output as a single shot through the exit cavity mirror. The final output 193nm laser beam achieves a long-term stable power of 20mW, approximately 10 times the output power of existing prism-coupled device frequency doublers.

[0096] Example 2

[0097] A 177.5nm continuous wave frequency doubling device was fabricated using 4mm thick KBBF original crystal.

[0098] First determine the Brewster angle.

[0099] 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°.

[0100] Then, the second angle α is determined using Equation 1:

[0101]

[0102] 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°.

[0103] Then, the first angle β is determined using Equation 2:

[0104]

[0105] After calculation, β=38.05°.

[0106] After determining the second angle α and the first angle β, the thin-sheet KBBF crystal is cut along the direction parallel to the c-axis at a deviation of 38.05° (angle β) from the a-axis to obtain the first set of parallel surfaces, resulting in a cuboid with a height equal to the thickness of the thin-sheet crystal d = 4 mm and a width equal to the spacing between the first set of parallel surfaces, 6 mm.

[0107] Then, perpendicular to the first set of parallel surfaces, a set of parallel inclined planes are cut at an angle of 58.57° (α) between the normal and the c-axis to obtain a straight parallelepiped. The surface spacing of the parallel inclined planes is 6 mm, so that the length of the first side h of the light-transmitting surface can reach 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 satisfies the angle of 38.05° with the a-axis.

[0108] 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).

[0109] 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 L1=l / sinθ B =6.034 / sin56.26°=7.256mm.

[0110] When used, the device is placed in a resonant cavity. A 355nm continuous-wave laser beam with a power of 4W is incident on the KBBF crystal through the input cavity mirror along the Brewster angle from one of the inclined surfaces. The incident light and the normal to the inclined surface form the incident plane. The incident plane must also be perpendicular to the first set of parallel surfaces. This automatically creates an angle of 31.43° (the complementary angle of angle α) between the normal to the incident plane and the Z axis, and the polarization direction of the 355nm laser is perpendicular to the Z axis. After entering the crystal, the 355nm laser travels in the phase-matching direction and is gradually converted into a high-power continuous-wave 177.5nm frequency-doubled light, which then exits the crystal. The newly generated 177.5nm laser beam is output through the output cavity mirror. The remaining 355nm laser beam is reflected by the cavity mirror and continues to oscillate within the cavity, passing through the KBBF crystal multiple times. This process is repeated, achieving resonant enhancement by locking the cavity length. The 177.5nm laser beam is output once through the output cavity mirror, ultimately achieving a continuous-wave 177.5nm laser power of approximately 10mW.

[0111] Example 3

[0112] A 153.43nm continuous wave sum frequency device was fabricated using 4mm thick KBBF original crystal.

[0113] 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.

[0114] First, determine the Brewster angle. Since the sum frequency process involves two beams of fundamental frequency light with different wavelengths, the refractive index is different and therefore the Brewster angle is also different. Obviously, it is impossible 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°.

[0115] 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°.

[0116] Finally, determine the β angle, After calculation, β=45.3°.

[0117] After determining the above parameters, the thin KBBF crystal was cut along the direction of 45.3° (β angle) away from the a-axis to obtain the first set of parallel surfaces to obtain a cuboid with a height of 4 mm and a width of 6 mm.

[0118] Then, perpendicular to the first set of parallel surfaces, a set of parallel inclined planes are cut at an angle of 42.3° between the normal and the c-axis, with a plane spacing of 4.4 mm. This ensures that the length h of the first side of the light-transmitting surface reaches 1.1 mm ( Figure 10 C1K in the figure), and 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.

[0119] 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).

[0120] 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 3Calculation, 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.

[0121] During use, the device is placed in a resonant cavity, and the fundamental frequency light is continuous wave laser light with wavelengths of 1074nm and 179nm, respectively. The first fundamental frequency light with a wavelength of 1074nm has a laser power of 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, the power is only 1mW. The first fundamental frequency light of 1074nm is incident on one of the inclined surfaces of the KBBF crystal along the Brewster angle through the incident cavity mirror. The incident light and the normal of the inclined surface form the incident plane, and the incident plane is perpendicular to the first parallel surface. This naturally forms an angle of 47.7° (i.e., the complementary angle of angle α) with the c-axis, and the polarization direction of the laser is perpendicular to the c-axis, and the laser is repeatedly resonated and enhanced 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 gradually undergo sum-frequency conversion to 153.43nm sum-frequency light, which then exits the crystal. The newly generated 153.43nm laser light is output through the exit cavity mirror. The remaining 1074nm laser light is reflected by the cavity mirror and continues to oscillate within the cavity, passing through the KBBF crystal multiple times to continuously generate 153.43nm sum-frequency light. The final output continuous-wave 153.43nm laser power exceeds 0.1mW.

[0122] 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 group of parallel surfaces and a second group of parallel surfaces, the first group of parallel surfaces is parallel to the c-axis of the optical crystal and forms a first angle β with the a-axis, the second group of parallel surfaces is perpendicular to the first group of parallel surfaces and the normal of the second group of parallel surfaces forms a second angle α with the c-axis of the optical crystal, and the second angle α and the first angle β have 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 incident plane formed by the plane defined by the fundamental frequency light and the normal of the second set of parallel surfaces is perpendicular to the first set of parallel surfaces, and the polarization direction of the fundamental frequency light is perpendicular to the c-axis of the optical crystal, so that the projection of the light on the plane perpendicular to the c-axis after entering the crystal 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 group of parallel surfaces, with the incident surface being perpendicular to the first parallel surface. The first incident fundamental frequency light is incident on the crystal along the Brewster angle. After the second incident fundamental frequency light enters the crystal, it is collinear with the first incident fundamental frequency light. The projections of the two light rays on a plane perpendicular to the c-axis are parallel to the a-axis. The wavelength of the first incident fundamental frequency light is greater than the wavelength of the second incident fundamental frequency light, and the Brewster angle is determined by the first incident fundamental frequency light. 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 β and the second angle α according to equations 1 and 2; The nonlinear optical crystal is a trigonal crystal system, and a first group of parallel surfaces are cut parallel to the c-axis of the optical crystal, and the first group of parallel surfaces forms a first angle β with the a-axis of the optical crystal; A second set of parallel surfaces is cut perpendicularly to the first set of parallel surfaces so that the normals of the second set of parallel surfaces form a second angle α with the c-axis direction of the optical crystal, 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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