Nonlinear optical device based on ammonium fluoborate crystal, manufacturing method and application

Abstract

The invention discloses a nonlinear optical device based on an ammonium fluoroborate crystal and a manufacturing method and application thereof. The manufacturing method comprises the following steps: a, measuring a crystal frequency multiplication coefficient of the ammonium fluoroborate; b, determining an effective frequency multiplication coefficient formula of the ammonium fluoborate crystal cut according to the phase matching angle; c, measuring the main refractive indexes of the ammonium fluoborate crystal under a plurality of wavelengths from deep ultraviolet to infrared, and fitting to obtain a Sellmeier equation; d, adopting the Sellmeier equation in the step c to obtain an ammonium fluoroborate crystal I or II type phase matching curve, and determining an ammonium fluoroborate crystal phase matching point; and e, orienting and cutting the ammonium fluoroborate crystal according to a phase matching direction, polishing two light-passing surfaces, and plating a dielectric film to obtain the prism-free ammonium fluoroborate crystal nonlinear optical device. According to the invention, the nonlinear optical device can be obtained without a prism, and the nonlinear optical device has high output power.

Description

Technical Field

[0001] The present invention relates to the technical fields of lasers and nonlinear optics, and particularly relates to a nonlinear optical device based on an ammonium fluoroborate crystal, a manufacturing method thereof, and an application thereof. Background Art

[0002] Compact and efficient deep ultraviolet (DUV) light sources in the range of 100 - 200 nm (corresponding to photon energies of 12.4 - 6.2 eV) are crucial for a wide range of applications such as advanced spectroscopy, quantum research, and semiconductor lithography. The multi-stage frequency conversion technology of nonlinear optical crystals is currently an effective way to achieve deep ultraviolet lasers, and the frequency doubling crystal in the last stage is called a deep ultraviolet nonlinear optical crystal. Currently, only potassium beryllium fluoroborate (KBe 2 BO 3 F 2 , abbreviated as KBBF) and rubidium beryllium fluoroborate (RbBe 2 BO 3 F 2 , abbreviated as RBBF) meet the conditions of deep ultraviolet nonlinear optical crystals, and can combine with a prism to output laser light with a wavelength less than 200 nm through frequency doubling. In particular, the sixth harmonic generation (177.3 nm) output of Nd:YAG laser and the fourth harmonic generation (193 nm) output of titanium sapphire laser can be achieved.

[0003] However, KBBF and RBBF crystals also face the following bottlenecks: both crystals have a serious layered growth habit in structure, are difficult to grow along the c direction, are prone to dissociation, resulting in small crystal sizes and being unable to directly manufacture nonlinear optical devices. Currently, when KBBF and RBBF crystals are used as one of the components of a laser frequency doubling device, a prism must be used for coupling, but the restriction of the output power is difficult to improve. Currently, there has been no report on directly obtaining ultraviolet / deep ultraviolet lasers through prism-free frequency doubling devices, especially the 193 nm and 177.3 nm outputs. Summary of the Invention

[0004] In view of the above problems, the present invention aims to provide a nonlinear optical device based on an ammonium fluoroborate crystal, a manufacturing method thereof, and an application thereof, which can obtain a nonlinear optical device without the aid of a prism and has a high output power for obtaining ultraviolet or deep ultraviolet lasers.

[0005] The technical solution adopted by the present invention is as follows: A manufacturing method of a nonlinear optical device based on an ammonium fluoroborate crystal is carried out according to the following steps:

[0006] a. Measuring the frequency doubling coefficient of the ammonium fluoroborate crystal;

[0007] Using the Maker fringe method to measure the frequency doubling coefficient of the ammonium fluoroborate crystal at type-I phase matching, and obtaining 1 non-zero and effective frequency doubling coefficient;

[0008] d 32 = ±2.8 × d 36 (KDP);

[0009] where d 36 (KDP) = 0.39 pm / V;

[0010] b. Determine the effective second - harmonic generation coefficient formula of the ammonium fluoroborate crystal cut at the phase - matching angle according to the propagation in the corresponding XY plane:

[0011] is the angle between the projection direction of the light - wave propagation direction in the XY plane and the X - axis, where the crystal principal axis is the Z - axis;

[0012] c. Use the minimum - deviation angle method to measure the principal refractive indices of the ammonium fluoroborate crystal at multiple wavelengths from deep ultraviolet to infrared, and fit to obtain the Sellmeier equation:

[0013]

[0014] where n x , n y , n z are the principal refractive indices of the ammonium fluoroborate crystal, λ is the incident wavelength, with the unit of μm;

[0015] d. Adopt the Sellmeier equation in step c, calculate through a computer program to obtain the type - I or type - II phase - matching curve of the ammonium fluoroborate crystal, and then combine with the second - harmonic generation coefficient measured in step a to determine the phase - matching point of the ammonium fluoroborate crystal;

[0016] e. Orient, cut, polish the two light - passing surfaces and coat a dielectric film on the ammonium fluoroborate crystal according to the phase - matching direction to obtain a non - linear optical device of the ammonium fluoroborate crystal.

[0017] Furthermore, in step e, the length, width and height of the ammonium borate crystal before cutting are not less than 5 mm × 5 mm × 10 mm, and the light - passing length of the cut ammonium fluoroborate crystal is 0.1 - 10 mm.

[0018] Furthermore, in step e, the cut ammonium fluoroborate crystal has angles θ, 0° < θ < 90°, θ is the angle between the light - wave propagation direction and the crystal principal axis, is the angle between the projection direction of the light - wave propagation direction in the XY plane and the X - axis, where the crystal principal axis is the Z - axis.

[0019] Further, in step a, the Maker fringe method is used to measure the second harmonic generation coefficient of the ammonium fluoroborate crystal at the standard type-I phase matching in the 1.064 μm band.

[0020] Further, in step c, the minimum deviation angle method is used to measure the principal refractive indices of the ammonium fluoroborate crystal at 14 wavelengths between 193 nm and 1014 nm, and the Sellmeier equation is obtained by fitting.

[0021] The present invention also discloses a nonlinear optical device based on an ammonium fluoroborate crystal prepared by the manufacturing method of the nonlinear optical device based on an ammonium fluoroborate crystal as described above.

[0022] Further, the nonlinear optical device based on the ammonium fluoroborate crystal is a type-I second harmonic generation device, a type-II second harmonic generation device, a sum frequency device, a difference frequency device or an optical parametric amplification device of the ammonium fluoroborate crystal.

[0023] The present invention also discloses the laser output of a nonlinear optical device based on an ammonium fluoroborate crystal as described above: using a Nd-ion doped YAG, YLF, YAP, LuVO 4 , YVO 4 , GdVO 4 laser or a titanium sapphire laser as a light source, using a type-I GTP-KTP, type-I KTP, type-I KTA, type-I LBO, type-I BBO or type-I CLBO as a second harmonic generation device, and at least one beam of second harmonic light generated by the second harmonic generation device directly enters the nonlinear optical device of the ammonium fluoroborate crystal, so as to generate second harmonic laser output, sum frequency laser output, difference frequency laser output and optical parametric amplification laser output.

[0024] Advantages of the present invention:

[0025] (1) By using the ammonium fluoroborate crystal for cutting, nonlinear optical devices at different wavelengths can be fabricated. Among them, lasers with wavelengths less than 200 nm, especially 193 nm and 177.3 nm lasers, can be output through direct second harmonic generation without the need for a prism; while the existing KBBF and RBBF are limited by the crystal size and must rely on a prism to make up for their structural deficiencies.

[0026] (2) When applied to the output of lasers with wavelengths less than 200 nm, the prism is omitted, the preparation process of the laser is simplified, and the manufacturing cost is reduced; at the same time, due to the good mechanical properties of the ammonium fluoroborate crystal, it is not easy to break, and it is easy to prepare, and high-power deep ultraviolet second harmonic generation output can be realized, making the realization of deep ultraviolet second harmonic generation laser simpler and facilitating the application of deep ultraviolet second harmonic generation lasers.

[0027] (3) By measuring the crystal doubling coefficient, determining the formula for the effective doubling coefficient, measuring the principal refractive index, and combining with the type-I phase matching curve and doubling coefficient to determine the phase matching point with a higher doubling conversion efficiency, and then orienting, cutting, polishing the two optical surfaces and coating a dielectric film on the ammonium fluoroborate crystal according to the phase matching direction to obtain a nonlinear optical device, an original manufacturing method applicable to the nonlinear optical device of ammonium fluoroborate crystal is developed, enabling the ammonium fluoroborate crystal to be directly used as a doubling device and successfully applied to the laser output with an output wavelength less than 200 nm, without the need to rely on a prism, which is a first in both domestic and international arenas. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a design drawing of the doubling device of ammonium fluoroborate crystal of the present invention;

[0029] Figure 2 It is the refractive index dispersion curve of the doubling device of ammonium fluoroborate crystal of the present invention;

[0030] Figure 3 It is the type-I phase matching curve of the doubling device of ammonium fluoroborate crystal of the present invention;

[0031] Figure 4 It is a schematic diagram of the output of the sixth harmonic generation laser by the ammonium fluoroborate crystal obtained by the present invention;

[0032] Figure 5 It is the sixth harmonic generation device diagram of ammonium fluoroborate crystal of the present invention.

[0033] Among them, 1 is a laser, 2 is a half-wave plate, 3 is a lens system, 4 is a polarization beam splitter, 5 is an optical trash can, 6 is a plane window, 7 is an ammonium fluoroborate crystal, 8 is a calcium fluoride prism, and 9 is an energy meter. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The present invention will be further described below through embodiments in combination with the drawings:

[0035] Embodiment 1

[0036] A manufacturing method of a nonlinear optical device based on ammonium fluoroborate crystal is carried out according to the following steps:

[0037] a. Measuring the crystal doubling coefficient of ammonium fluoroborate.

[0038] Using the Maker fringe method to measure the doubling coefficient of ammonium fluoroborate crystal at the standard type-I phase matching in the 1.064 μm band, and obtaining 1 non-zero effective doubling coefficient.

[0039] d 32 = ±2.8 × d 36 (KDP);

[0040] Among them, d 36(KDP) = 0.39 pm / V.

[0041] b. Determine the effective second harmonic generation coefficient formula of the ammonium fluoroborate crystal cut at the phase matching angle according to the propagation in the corresponding XY plane:

[0042] is the angle between the projection direction of the light wave propagation direction in the XY plane and the X-axis, where the crystal principal axis is the Z-axis.

[0043] c. Use the minimum deviation angle method to measure the principal refractive indices of the ammonium fluoroborate crystal at 14 wavelengths between 193 nm and 1014 nm, and fit to obtain the Sellmeier equation:

[0044]

[0045] where n x , n y , n z are the principal axis refractive indices of the ammonium fluoroborate crystal, and λ is the incident wavelength, with the unit of μm.

[0046] As Figure 2 shows its dispersion curve.

[0047] d. Adopt the Sellmeier equation in step c, and through computer program calculation, respectively obtain the type-I phase matching curve of the ammonium fluoroborate crystal (as Figure 3 shown), and then combine with the second harmonic generation coefficient measured in step a to determine the phase matching point with higher fourth harmonic conversion efficiency of the ammonium fluoroborate crystal (θ = 90°, ). θ is the angle between the light wave propagation direction and the crystal principal axis, is the angle between the projection direction of the light wave propagation direction in the XY plane and the X-axis, where the crystal principal axis is the Z-axis.

[0048] e. Orient, cut, polish the two optical surfaces and coat the dielectric film on the ammonium fluoroborate crystal according to the phase matching direction to obtain a prism-free fourth harmonic generation device of the ammonium fluoroborate crystal.

[0049] Use a Q-switched Nd:YAG laser as the light source, use a type-I KTP as the second harmonic generation device, the laser emits infrared laser in the 1.064 μm band, the second harmonic light generated by the KTP second harmonic generation device directly enters the fourth harmonic generation device of the ammonium fluoroborate crystal for fourth harmonic conversion after passing through the filter, so as to generate a fourth harmonic 266 nm laser output with an energy of 1 W, and the conversion efficiency is up to 6.4% at most.

[0050] The phase matching direction during the cutting of the ammonium fluoroborate crystal is the type-I direction, the optical surfaces of the type-I fourth harmonic generation device of the ammonium fluoroborate crystal are square, the cross-sectional size is 4 mm × 5 mm, and the thickness in the optical propagation direction is 2 mm.

[0051] Example 2

[0052] In this example, the method steps a - c are carried out according to Example 1.

[0053] d. Using the Sellmeier equation in step c, through computer program calculation, the type - I phase - matching curves of ammonium fluoborate crystals are obtained respectively. Then, in combination with the second - harmonic generation coefficient measured in step a, the phase - matching points with relatively high fourth - harmonic conversion efficiency of ammonium fluoborate crystals are determined (θ = 90°, ).

[0054] e. Orient, cut, polish the two light - passing surfaces and coat a dielectric film on the ammonium fluoborate crystal in the phase - matching direction to obtain a prism - less fourth - harmonic device of ammonium fluoborate crystal.

[0055] As Figure 4 shown, at room temperature, the laser 1 emits a laser of 354.7 nm. After passing through the half - wave plate 2, being focused by the lens system 3 and the polarization beam splitter 4, it is incident on the fourth - harmonic device 7 of ammonium fluoborate crystal for fourth - harmonic conversion, thereby generating a fourth - harmonic laser output of 177.3 nm with a power of 4.8 mJ, and the conversion efficiency is up to 7.9% at most. In the optical path, an optical trash can 5, a plane window 6, a calcium fluoride prism 8, and an energy meter 9 are also provided.

[0056] As Figure 5 shown, the phase - matching direction during the cutting of the ammonium fluoborate crystal is the type - I direction. The light - passing surface of the type - I fourth - harmonic device of ammonium fluoborate crystal is square, the cross - section size is 4 mm × 5 mm, and the thickness in the light - passing direction is 4 mm.

[0057] Example 3

[0058] In this example, the method steps a - c are carried out according to Example 1.

[0059] d. Using the Sellmeier equation in step c, through computer program calculation, the type - I phase - matching curves of ammonium fluoborate crystals are obtained respectively. Then, in combination with the second - harmonic generation coefficient measured in step a, the phase - matching points with relatively high fourth - harmonic conversion efficiency of ammonium fluoborate crystals are determined (θ = 90°, ).

[0060] e. Orient, cut, polish the two light - passing surfaces and coat a dielectric film on the ammonium fluoborate crystal in the phase - matching direction to obtain a prism - less fourth - harmonic device of ammonium fluoborate crystal.

[0061] Using a titanium-doped sapphire laser as the light source and a type-I BBO crystal as the frequency doubling device; the laser emits laser light in the 773 nm band, and the second harmonic light generated by the BBO frequency doubling device directly enters the ammonium fluoroborate crystal quadrupling device through a filter for quadrupling conversion, thereby generating a quadrupled 193 nm laser output with an energy of 50 mW, and the conversion efficiency is up to 6% at most.

[0062] The phase matching direction during the cutting of the ammonium fluoroborate crystal is the type-I direction. The light passing surface of the type-I ammonium fluoroborate crystal quadrupling device is square, with a cross-sectional size of 4 mm × 5 mm and a thickness of 2 mm in the light passing direction.

[0063] Example 4

[0064] In this example, the method steps a to c are carried out according to Example 1.

[0065] d. Using the Sellmeier equation in step c, through computer program calculation, the type-I or type-II phase matching curve of the ammonium fluoroborate crystal is obtained, and then combined with the second harmonic generation coefficient measured in step a to determine the phase matching point with a relatively high quadrupling conversion efficiency of the ammonium fluoroborate crystal (θ = 90°, )

[0066] e. Orient, cut, polish the two light passing surfaces and coat a dielectric film on the ammonium fluoroborate crystal according to the phase matching direction to obtain a type-II phase device of the ammonium fluoroborate crystal without a prism.

[0067] Using a Q-switched Nd:YAG laser as the light source and a type-I KTP as the frequency doubling device; the laser emits infrared laser light in the 1.064 μm band, and the second harmonic light generated by the KTP frequency doubling device directly enters the type-II ammonium fluoroborate crystal quadrupling device through a filter for quadrupling conversion, thereby generating a quadrupled 266 nm laser output with an energy of 900 mW, and the conversion efficiency is up to 7% at most.

[0068] The phase matching direction during the cutting of the ammonium fluoroborate crystal is the type-II direction. The light passing surface of the type-II ammonium fluoroborate crystal quadrupling device is square, with a cross-sectional size of 5 mm × 5 mm and a thickness of 2 mm in the light passing direction.

[0069] The chemical formula of the above ammonium fluoroborate crystal is NH 4 B 4 O 6 F (abbreviation: ABF), with a molecular weight of 176.28, belonging to the orthorhombic crystal system, space group Pna2 1 , and the unit cell parameters

[0070] The nonlinear optical device prepared by the manufacturing method of the nonlinear optical device based on ammonium fluoroborate crystal is a type-I frequency doubling device or a type-II frequency doubling device of ammonium fluoroborate crystal. The main application of the nonlinear optical device based on ammonium fluoroborate crystal is to generate frequency-doubled laser output, and the generated frequency-doubled laser output is as follows:

[0071] Using Nd-ion-doped YAG, YLF, YAP, LuVO 4 、YVO 4 、、GdVO 4 laser or titanium sapphire laser as the light source, using type-I GTP-KTP, type-I KTP, type-I KTA, type-I LBO, type-I BBO or type-I CLBO as the frequency doubling device, the frequency-doubled light generated by the frequency doubling device directly enters the above-mentioned ammonium fluoroborate crystal nonlinear optical device, thereby generating nonlinear optical effect laser output.

[0072] In step e, the ammonium fluoroborate crystal is grown by the single crystal vapor phase growth method, which includes the following steps:

[0073] S1. Raw material preparation;

[0074] Weigh two or more compounds containing NH 4 , B, O, and F in a metering ratio of 1:4:6:1 as raw materials. After mixing evenly, add a transport agent to obtain an initial mixture (fluoroborate ammonium polycrystal can also be used as the raw material), and load the initial mixture into the crystallization kettle;

[0075] S2. Suspending the seed crystal or screening out the seed crystal by spontaneous nucleation;

[0076] Suspending the seed crystal: Suspend the seed crystal at the top of the crystallization kettle, then seal it. Put the sealed crystallization kettle into a vertical tube furnace or a molten salt furnace. There is a temperature gradient zone in the vertical tube furnace or the molten salt furnace. The temperature gradient zone includes a high-temperature zone, a low-temperature zone, and a crystal growth zone. By adjusting the temperature field, control the temperature of the high-temperature zone at 300 - 550 °C, the temperature of the low-temperature zone at 20 - 300 °C, and the temperature of the crystal growth zone at 300 - 400 °C, and keep it warm for 1 - 5 days;

[0077] Screening out the seed crystal by spontaneous nucleation includes two methods: geometric elimination between grains and temperature screening;

[0078] Geometric elimination between grains: Design the top of the crystallization kettle as a conical shape with a taper of 30 - 40%. Put the crystallization kettle into a vertical tube furnace or a molten salt furnace. There is a temperature gradient zone in the vertical tube furnace or the molten salt furnace. The temperature gradient zone includes a high-temperature zone, a low-temperature zone, and a crystal growth zone. By adjusting the temperature field, control the temperature of the high-temperature zone at 300 - 550 °C, the temperature of the low-temperature zone at 20 - 300 °C, and the temperature of the crystal growth zone at 300 - 400 °C, and keep it warm for 1 - 10 days to screen out the seed crystal;

[0079] Temperature screening: Place the crystallization kettle in a vertical tube furnace or molten salt furnace, which is provided with a temperature gradient zone, including a high temperature zone, a low temperature zone and a crystal growth zone. By adjusting the temperature field, the temperature of the high temperature zone is controlled at 300-550°C, the temperature of the low temperature zone is controlled at 20-300°C, and the temperature of the crystal growth zone is controlled at 300-400°C. After keeping the temperature for 1-10 days, perform temperature oscillation in the crystal growth zone to screen out the seed crystals.

[0080] S3, crystal growth;

[0081] The crystallization kettle is dynamically controlled and heated in the temperature gradient zone. The dynamic controlled heating is: the temperature gradient of the crystal growth zone is dynamically controlled by independently heating or cooling the temperature of the high temperature zone and the low temperature zone to match the temperature gradient required in each stage of the crystal growth process; the raw material is crystallized in the crystal growth zone under the carrier, and large-sized ammonium fluoroborate crystals are obtained after the crystal growth.

[0082] Preferably, the mass percentage of the transmission agent in the initial mixture is 0-80%; the transmission agent is N 2 , H 2 O, HF, H 3 BO 3 NH 3 ·H 2 O、NH 4 F. NH 4 Cl, NH 4 Br, (NH 4 ) 2 CO 3 NH 4 HCO 3 or (NH 4 ) 2 SO 4 .

[0083] The crystallization kettle is a platinum crucible, an iridium crucible, a ceramic crucible, a quartz tube or a stainless steel crucible with a sealing structure.

[0084] The specific operation of dynamic heating control is: program the high temperature zone at a rate of 2-5°C / h, program the low temperature zone at a rate of 5-10°C / h for 5-10 hours, and then program the low temperature zone at a rate of 15-20°C / h.

[0085] The temperature gradient in the crystal growth zone is 1 to 40°C / cm; the temperature gradient in the initial stage of crystal growth is 1 to 30°C / cm; when the crystal grows to a size greater than 3mm in at least one dimension, the temperature gradient is 1 to 25°C / cm; when the crystal grows to a size greater than 10mm in at least one dimension, the temperature gradient is 1 to 40°C / cm.

[0086] The high-temperature zone, low-temperature zone, and homogeneous crystal growth zone include at least 1 independent heating zone, facilitating zone-wise heating or cooling.

[0087] The ammonium fluoroborate crystal is grown by the single-crystal vapor growth method, enabling the production of large-sized crystals for non-prismatic nonlinear optical devices. It is significantly different from traditional crystal growth methods, mainly manifested in:

[0088] 1. The vapor phase method is adopted, and seed crystals are suspended or selected by temperature oscillation and crucible shape, thus facilitating the growth of large-sized and structurally complete ABF crystals; 2. Temperature control is ingeniously applied, and large-sized and high-quality ABF single crystals are obtained by adjusting the temperature gradient at different stages; 3. The layer growth habit of ABF crystals can be overcome, enabling the crystals to grow into large-sized bulk crystals (the size in at least one dimension is greater than or equal to 30 mm). The positive effect of the present invention is to grow ABF crystals with high optical quality having a size of 30 mm × 20 mm × 10 mm or above; 4. The crystals can be cut and processed into frequency doubling devices according to the phase matching angle direction, paving the way for their large-scale use and industrialization.

[0089] The ABF single crystal of the present invention has a stable structure and excellent frequency doubling performance; the full width at half maximum of high-resolution X-ray diffraction is less than 40″, indicating that the crystal has a high crystallization quality; the ultraviolet-visible-near infrared spectrophotometer shows that the transmittance of the unpolished a-plane is above 85% in the 1500 - 200 nm wavelength band; the Maker fringe method test shows that ABF has a large nonlinear optical frequency doubling response.

Claims

1. A method for manufacturing a nonlinear optical device based on ammonium fluoroborate crystal, characterized in that: Proceed as follows: a. Measurement of crystal harmonic coefficient of ammonium fluoroborate; The SHG coefficient of ammonium fluoroborate crystal in type Ⅰ phase matching was measured by Maker fringe method, and a non-zero effective SHG coefficient was obtained. d 32 N±2.8×d 36 (KDP) Among them, d 36 (KDP) = 0.39 pm / V; b. According to the propagation in the corresponding XY plane, the formula for the effective frequency doubling coefficient of the ammonium fluoroborate crystal cut according to the phase matching angle is determined: It is the angle between the projection direction of the light wave propagation direction in the XY plane and the X axis, where the principal axis of the crystal is the Z axis; c. The principal refractive index of ammonium fluoroborate crystal at multiple wavelengths from deep ultraviolet to infrared was measured using the minimum deflection angle method, and the Sellmeier equation was fitted: where n x , n y , n z is the principal axis refractive index of the ammonium fluoroborate crystal, λ is the incident wavelength, in μm; d. Using the Sellmeier equation in step c, a computer program is used to calculate a phase matching curve of the ammonium fluoroborate crystal, and then the phase matching point of the ammonium fluoroborate crystal is determined in combination with the frequency doubling coefficient measured in step a; e. Orienting, cutting, polishing the two-pass optical surfaces of the ammonium fluoroborate crystal according to the phase matching direction and coating it with a dielectric film to obtain a prism-free nonlinear optical device.

2. The method for manufacturing a nonlinear optical device based on ammonium fluoroborate crystal according to claim 1, characterized in that: In step e, the length, width and height of the ammonium borate crystal before cutting are not less than 5 mm×5 mm×10 mm, and the light transmission length of the ammonium fluoroborate crystal after cutting is 0.1 to 10 mm.

3. The method for manufacturing a nonlinear optical device based on ammonium fluoroborate crystal according to claim 2, characterized in that: In step e, the cut ammonium fluoroborate crystal has θ, Angle, 0°<θ<90°, θ is the angle between the propagation direction of the light wave and the principal axis of the crystal, It is the angle between the projection direction of the light wave propagation direction in the XY plane and the X-axis, where the principal axis of the crystal is the Z-axis.

4. The method for manufacturing a nonlinear optical device based on ammonium fluoroborate crystal according to claim 1, characterized in that: In step a, the Maker fringe method is used to measure the frequency doubling coefficient of the ammonium fluoroborate crystal in the standard type I phase matching in the 1.064 μm band.

5. The method for manufacturing a nonlinear optical device based on ammonium fluoroborate crystal according to claim 1, characterized in that: In step c, the principal refractive index of the ammonium fluoroborate crystal at 14 wavelengths between 193 nm and 1014 nm is measured using a minimum deflection angle method, and a Sellmeier equation is obtained by fitting.

6. A nonlinear optical device based on ammonium fluoroborate crystal made by the method for making a nonlinear optical device based on ammonium fluoroborate crystal as claimed in any one of claims 1 to 5.

7. The nonlinear optical device based on ammonium fluoroborate crystal according to claim 6, characterized in that: The nonlinear optical device is an ammonium fluoroborate crystal type I frequency doubling device, type II frequency doubling device, sum frequency device, difference frequency device or optical parametric amplifier device.

8. An application of a nonlinear optical device based on ammonium fluoroborate crystal as claimed in claim 6 or 7, characterized in that: Using Nd ion-doped YAG, YLF, YAP, LuVO4, YVO4, GdVO4 laser or titanium sapphire laser as the light source, and using Class I GTP-KTP, Class I KTP, Class I KTA, Class I LBO, Class I BBO or Class I CLBO as the frequency doubling device, at least one beam of frequency doubling light generated by the frequency doubling device directly enters the ammonium fluoroborate crystal nonlinear optical device, thereby generating frequency doubling laser output, sum frequency laser output, difference frequency laser output and optical parametric amplification laser output.

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