Nonlinear optical device based on ammonium fluoroborate crystal and manufacturing method and application thereof

Abstract

The application discloses a nonlinear optical device based on ammonium fluoroborate crystal and a manufacturing method and application thereof, and the manufacturing method comprises the following steps: a, measuring the frequency doubling coefficient of the ammonium fluoroborate crystal; b, determining the effective frequency doubling coefficient formula of the ammonium fluoroborate crystal cut according to the phase matching angle; c, measuring the principal refractive index of the ammonium fluoroborate crystal at multiple wavelengths from deep ultraviolet to infrared, and fitting to obtain a Sellmeier equation; d, obtaining the type I or type II phase matching curve of the ammonium fluoroborate crystal by using the Sellmeier equation in step c, and determining the phase matching point of the ammonium fluoroborate crystal; and e, performing orientation, cutting, polishing of two light transmission surfaces and medium film plating on the ammonium fluoroborate crystal according to the phase matching direction to obtain a prism-free nonlinear optical device of the ammonium fluoroborate crystal. The nonlinear optical device can be obtained without the aid of a prism, and has a relatively high output power.

Description

Technical Field

[0001] This invention relates to the fields of laser and nonlinear optics, specifically to a nonlinear optical device based on ammonium fluoroborate crystal, its fabrication method, and its application. Background Technology

[0002] Compact and efficient deep ultraviolet (DUV) light sources in the 100–200 nm range (corresponding to photon energies of 12.4–6.2 eV) are crucial for a wide range of applications, including advanced spectroscopy, quantum research, and semiconductor lithography. Multi-stage frequency conversion using nonlinear optical crystals is currently an effective method for realizing deep ultraviolet lasers, with the final stage frequency-doubling crystal being called a deep ultraviolet nonlinear optical crystal. Currently, only potassium fluoroborylate (KBe₂BO₃F₂, abbreviated as KBBF) and rubidium fluoroborylate (RbBe₂BO₃F₂, abbreviated as RBBF) meet the requirements for deep ultraviolet nonlinear optical crystals. They can be combined with prisms to output lasers with wavelengths less than 200 nm through frequency doubling, particularly achieving the sixth harmonic (177.3 nm) output of Nd:YAG lasers and the fourth harmonic (193 nm) output of Ti:Sapphire lasers.

[0003] However, KBBF and RBBF crystals also face the following bottlenecks: Both crystals exhibit a severe layered growth habit, making growth along the c-direction difficult and prone to dissociation, resulting in small crystal sizes that cannot be directly used to fabricate nonlinear optical devices. Currently, when KBBF and RBBF crystals are used as components of laser frequency doubling devices, prisms are required for coupling, but the output power limitation remains difficult to overcome. To date, there are no reports of obtaining ultraviolet / deep ultraviolet lasers through direct multi-stage frequency doubling using prism-free frequency doubling devices, especially for 193nm and 177.3nm output. Summary of the Invention

[0004] To address the aforementioned problems, this invention aims to provide a nonlinear optical device based on ammonium fluoroborate crystal, its fabrication method, and its application. This nonlinear optical device can be obtained without the aid of a prism and possesses high output power, making it suitable for obtaining ultraviolet or deep ultraviolet lasers.

[0005] The technical solution adopted in this invention is as follows: a method for fabricating a nonlinear optical device based on ammonium fluoroborate crystal, comprising the following steps:

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

[0007] The doubling coefficient of ammonium fluoroborate crystal under type I phase matching was measured using the Mark stripe method, and a non-zero effective doubling coefficient was obtained.

[0008] d 32 =±2.8×d 36 (KDP);

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

[0010] b. Based on the propagation in the corresponding XY plane, determine the formula for the effective frequency doubling coefficient of the ammonium fluoroborate crystal cut at the phase-matching angle:

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

[0012] c. The principal refractive index of ammonium fluoroborate crystal at multiple wavelengths from deep ultraviolet to infrared was determined using the minimum deviation angle method, and the Sellmeier equation was obtained by fitting the results.

[0013]

[0014] Where n x n y n z λ is the principal axis refractive index of ammonium fluoroborate crystal, and λ is the incident wavelength in μm.

[0015] d. Using the Sellmeier equation from step c, the phase matching curves of ammonium fluoroborate crystal (Type I or II) are obtained by computer program calculation, and the phase matching point of ammonium fluoroborate crystal is determined by combining the octave coefficients measured in step a.

[0016] e. Orient, cut, polish the two light-transmitting surfaces of the ammonium fluoroborate crystal according to the phase matching direction, and deposit a dielectric film to obtain an ammonium fluoroborate crystal nonlinear optical device.

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

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

[0019] Further, in step a, the harmonic coefficient of the ammonium fluoroborate crystal at standard Type I phase matching in the 1.064 μm band is measured using the Maker stripe method.

[0020] Further, in step c, the principal refractive index of ammonium fluoroborate crystal at 14 wavelengths from 193 nm to 1014 nm is determined using the minimum deviation angle method, and the Sellmeier equation is obtained by fitting the results.

[0021] The present invention also discloses a nonlinear optical device based on ammonium fluoroborate crystal, which is prepared by the above-described method for fabricating a nonlinear optical device based on ammonium fluoroborate crystal.

[0022] Furthermore, the nonlinear optical devices based on ammonium fluoroborate crystals are ammonium fluoroborate crystal type I frequency doubling devices, type II frequency doubling devices, sum frequency devices, difference frequency devices, or optical parametric amplifier devices.

[0023] This invention also discloses a laser output of a nonlinear optical device based on ammonium fluoroborate crystal as described above: using an Nd-doped YAG, YLF, YAP, LuVO4, YVO4, GdVO4 laser or a Ti:sapphire laser as the light source, and using a 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, at least one frequency-doubled beam generated by the frequency doubling device directly enters the ammonium fluoroborate crystal nonlinear optical device, thereby generating frequency-doubled laser output, sum-frequency laser output, difference-frequency laser output and optical parametric amplified laser output.

[0024] The beneficial effects of this invention are:

[0025] (1) By cutting ammonium fluoroborate crystals, nonlinear optical devices with different wavelengths can be fabricated. Among them, lasers with wavelengths less than 200nm can be output by direct frequency doubling, especially 193nm and 177.3nm lasers, without the need for prisms. However, existing KBBF and RBBF are limited by crystal size and must be compensated for by prisms to make up for their structural deficiencies.

[0026] (2) When applied to lasers with output wavelengths less than 200nm, the prism is eliminated, simplifying the laser fabrication process and reducing manufacturing costs. At the same time, due to the good mechanical properties and the fact that ammonium fluoroborate crystals are not easily broken and are easy to fabricate, high-power deep ultraviolet frequency doubling output can be achieved, making the realization of deep ultraviolet frequency doubling lasers simpler and beneficial to the application of deep ultraviolet frequency doubling lasers.

[0027] (3) By measuring the crystal frequency doubling coefficient, determining the effective frequency doubling coefficient formula, measuring the principal refractive index, and combining the type I phase matching curve and the frequency doubling coefficient to determine the phase matching point with high frequency doubling conversion efficiency, the ammonium fluoroborate crystal is then oriented, cut, polished on both optical surfaces and coated with a dielectric film according to the phase matching direction to obtain a nonlinear optical device. A unique manufacturing method suitable for ammonium fluoroborate crystal nonlinear optical devices has been developed, enabling ammonium fluoroborate crystal to be directly used as a frequency doubling device for laser output with an output wavelength of less than 200nm without the need for a prism. This is the first of its kind in China and abroad. Attached Figure Description

[0028] Figure 1 This is a design diagram of the ammonium fluoroborate crystal frequency doubling device of the present invention;

[0029] Figure 2 The refractive index dispersion curve of the ammonium fluoroborate crystal frequency doubling device of the present invention is shown below.

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

[0031] Figure 4 A schematic diagram showing the output of a sixth-harmonic laser using the ammonium fluoroborate crystal obtained in this invention;

[0032] Figure 5 This is a diagram of the ammonium fluoroborate crystal hexafold device 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 polarizing 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 Implementation

[0034] The present invention will be further described below with reference to the embodiments and accompanying drawings:

[0035] Example 1

[0036] A method for fabricating a nonlinear optical device based on ammonium fluoroborate crystal, comprising the following steps:

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

[0038] The harmonics coefficient of ammonium fluoroborate crystal at standard Type I phase matching in the 1.064 μm band was measured using the Maker fringe method, and a non-zero effective harmonics coefficient was obtained.

[0039] d 32 =±2.8×d 36 (KDP);

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

[0041] b. Based on the propagation in the corresponding XY plane, determine the formula for the effective frequency doubling coefficient of the ammonium fluoroborate crystal cut at the phase-matching angle:

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

[0043] c. The principal refractive index of ammonium fluoroborate crystal at 14 wavelengths from 193 nm to 1014 nm was determined using the minimum deflection angle method, and the Sellmeier equation was obtained by fitting the results.

[0044]

[0045] Where n x n y n z λ is the principal axis refractive index of the ammonium fluoroborate crystal, and λ is the incident wavelength in μm.

[0046] like Figure 2 Its dispersion curve is shown.

[0047] d. Using the Sellmeier equation from step c, the type I phase-matching curves of ammonium fluoroborate crystals were obtained through computer calculations (e.g., ...). Figure 3 As shown), and combined with the harmonic coefficients measured in step a, the phase matching point with higher fourth harmonic conversion efficiency of ammonium fluoroborate crystal (θ = 90°) is determined. θ is the angle between the direction of light wave propagation and the principal axis of the crystal. It is the angle between the projection of the light wave propagation direction onto the XY plane and the X-axis, where the crystal principal axis is the Z-axis.

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

[0049] Using a Q-switched Nd:YAG laser as the light source and a type I KTP as the frequency doubling device, infrared laser light in the 1.064μm band is incident on the laser. The second-harmonic light generated by the KTP frequency doubling device passes through a filter and then directly enters the ammonium fluoroborate crystal fourth-harmonic device for fourth-harmonic conversion, thereby generating a 1W fourth-harmonic 266nm laser output with a maximum conversion efficiency of 6.4%.

[0050] The phase matching direction during the cutting of ammonium fluoroborate crystal is the type I direction. The light-transmitting surface of the type I fourth harmonic device of ammonium fluoroborate crystal is square, with a cross-sectional size of 4mm×5mm and a thickness of 2mm in the light-transmitting direction.

[0051] Example 2

[0052] In this embodiment, steps a to c are performed according to embodiment 1.

[0053] d. Using the Sellmeier equation from step c, the type I phase-matching curves of ammonium fluoroborate crystal are obtained through computer calculation. Combined with the harmonic coefficients measured in step a, the phase-matching point with higher sixth-harmonic conversion efficiency of the ammonium fluoroborate crystal (θ = 90°) is determined. ).

[0054] e. Orient, cut, polish the two light-transmitting surfaces of the ammonium fluoroborate crystal according to the phase matching direction, and deposit a dielectric film to obtain a prism-free ammonium fluoroborate crystal sixth-harmonic device.

[0055] like Figure 4 As shown, at room temperature, laser 1 emits a 354.7nm laser beam, which, after being focused by a half-wave plate 2, a lens system 3, and a polarizing beam splitter 4, is incident on an ammonium fluoroborate crystal sixth-harmonic generation device 7 for sixth-harmonic conversion, thereby generating a 177.3nm sixth-harmonic laser output with a power of 4.8mJ and a maximum conversion efficiency of 7.9%. The optical path also includes an optical trash can 5, a planar window 6, a calcium fluoride prism 8, and an energy meter 9.

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

[0057] Example 3

[0058] In this embodiment, steps a to c are performed according to embodiment 1.

[0059] d. Using the Sellmeier equation from step c, the type I phase-matching curves of ammonium fluoroborate crystal are obtained through computer calculation. Combined with the harmonic coefficients measured in step a, the phase-matching point with higher fourth-harmonic conversion efficiency of the ammonium fluoroborate crystal (θ = 90°) is determined. ).

[0060] e. Orient, cut, polish the two light-transmitting surfaces of the ammonium fluoroborate crystal according to the phase matching direction, and deposit a dielectric film to obtain a prism-free ammonium fluoroborate crystal fourth harmonic device.

[0061] A Ti:sapphire laser is used as the light source, and a type I BBO crystal is used as the frequency doubling device. The laser incident with 773nm wavelength light is generated by the BBO frequency doubling device, and after passing through a filter, it directly enters the ammonium fluoroborate crystal fourth frequency doubling device for fourth frequency conversion, thereby generating a 50mW fourth frequency doubling 193nm laser output with a conversion efficiency of up to 6%.

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

[0063] Example 4

[0064] In this embodiment, steps a to c are performed according to embodiment 1.

[0065] d. Using the Sellmeier equation from step c, calculate the type I or II phase matching curves of ammonium fluoroborate crystal through a computer program. Then, combine this with the harmonic coefficients measured in step a to determine the phase matching point (θ = 90°) with higher fourth harmonic conversion efficiency of the ammonium fluoroborate crystal. ).

[0066] e. Orient, cut, polish the two light-transmitting surfaces of the ammonium fluoroborate crystal according to the phase matching direction, and deposit a dielectric film to obtain a prism-free ammonium fluoroborate crystal type II phase device.

[0067] A Q-switched Nd:YAG laser is used as the light source, and a type I KTP is used as the frequency doubling device. The laser is incident with infrared laser light in the 1.064μm band. The second-harmonic light generated by the KTP frequency doubling device passes through a filter and then directly enters a type II ammonium fluoroborate crystal fourth-harmonic device for fourth-harmonic conversion, thereby generating a 900mW fourth-harmonic 266nm laser output with a maximum conversion efficiency of 7%.

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

[0069] The above-mentioned ammonium fluoroborate crystal has the chemical formula NH4B4O6F (abbreviated as ABF), a molecular weight of 176.28, belongs to the orthorhombic crystal system, has a space group of Pna21, and a unit cell parameter of...

[0070] The nonlinear optical device fabricated using the method for ammonium fluoroborate crystal is a type I or type II frequency doubling device based on ammonium fluoroborate crystal. The main application of this ammonium fluoroborate crystal-based nonlinear optical device is the generation of frequency-doubled laser output, as shown below:

[0071] Using Nd-doped YAG, YLF, YAP, LuVO4, YVO4, GdVO4 lasers or Ti:sapphire lasers as the light source, and 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 aforementioned ammonium fluoroborate crystal nonlinear optical device, thereby producing nonlinear optical effect laser output.

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

[0073] S1. Raw material preparation;

[0074] Weigh two or more compounds containing NH4, B, O, and F as raw materials according to a stoichiometric ratio of 1:4:6:1, mix them thoroughly, add a transport agent to obtain an initial mixture (ammonium fluoroborate polycrystalline material can also be used as raw material), and load the initial mixture into a crystallization vessel.

[0075] S2. Seed crystals are selected by suspension or spontaneous nucleation.

[0076] Suspended seed crystal: Suspend the seed crystal at the top of the crystallization vessel, then seal it. Place the sealed crystallization vessel into a vertical tube furnace or molten salt furnace. The vertical tube furnace or molten salt furnace is equipped with a temperature gradient zone, which includes 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℃, the temperature of the low-temperature zone is controlled at 20-300℃, and the temperature of the crystal growth zone is controlled at 300-400℃. The temperature is maintained for 1-5 days.

[0077] Spontaneous nucleation screening of seed crystals includes two methods: geometric elimination between grains and temperature screening.

[0078] Geometric elimination of grains: The top of the crystallization vessel is designed as a cone with a taper of 30-40%. The crystallization vessel is placed in a vertical tube furnace or molten salt furnace. The vertical tube furnace or molten salt furnace is equipped with a temperature gradient zone, which includes 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℃, the temperature of the low-temperature zone is controlled at 20-300℃, and the temperature of the crystal growth zone is controlled at 300-400℃. The temperature is maintained for 1-10 days to screen out seed crystals.

[0079] Temperature screening: The crystallization vessel is placed in a vertical tube furnace or molten salt furnace, which is equipped 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℃, the temperature of the low temperature zone is controlled at 20-300℃, and the temperature of the crystal growth zone is controlled at 300-400℃. After holding at this temperature for 1-10 days, temperature oscillation is performed in the crystal growth zone to screen out seed crystals.

[0080] S3, crystal growth;

[0081] The crystallization vessel is dynamically heated within a temperature gradient zone. This dynamic heating involves independently raising or lowering the temperatures of the high-temperature and low-temperature zones to dynamically control the temperature gradient of the crystal growth zone, ensuring it matches the temperature gradient required for each stage of the crystal growth process. The raw material crystallizes in the crystal growth zone under the transport agent, resulting in large-sized ammonium fluoroborate crystals.

[0082] Preferably, the mass percentage of the transport agent in the initial mixture is 0-80%; the transport agent is N2, H2O, HF, H3BO3, NH3·H2O, NH4F, NH4Cl, NH4Br, (NH4)2CO3, NH4HCO3 or (NH4)2SO4.

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

[0084] The specific operation of dynamic heating control is as follows: the high temperature zone is programmed to increase the temperature at a rate of 2-5℃ / h, the low temperature zone is programmed to increase the temperature at a rate of 5-10℃ / h for 5-10 hours, and then programmed to decrease the temperature at a rate of 15-20℃ / h.

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

[0086] The high-temperature zone, low-temperature zone, and homogeneous crystal growth zone each include at least one independent heating zone, which facilitates zoned heating or cooling.

[0087] This ammonium fluoroborate crystal, grown using a single-crystal vapor phase method, can achieve large-size crystals for the fabrication of prism-free nonlinear optical devices. It differs significantly from traditional crystal growth methods, primarily in the following ways:

[0088] 1. The gas-phase method is adopted, and seed crystals are selected by suspending them or by using temperature oscillation and crucible shape, which facilitates the growth of large-sized and structurally complete ABF crystals; 2. Temperature control is cleverly applied, and large-sized, high-quality ABF single crystals are obtained by adjusting the temperature gradient at different stages; 3. The layered growth habit of ABF crystals can be overcome, enabling large-sized bulk crystal growth (at least one dimension is greater than or equal to 30mm). The positive effect of this invention is the growth of high-optical-quality ABF crystals with a size of 30mm×20mm×10mm or larger; 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 structure of this invention is stable and has excellent frequency doubling performance; the full width at half maximum (FWHM) of the high-resolution X-ray diffraction is less than 40″, indicating that the crystal has high crystal quality; the UV-Vis-NIR spectrophotometer shows that the transmittance of the unpolished a-piece is as high as 85% or more in the 1500-200nm band; the Maker fringe method test shows that the ABF has a large nonlinear optical frequency doubling response.

Claims

1. A method for fabricating a nonlinear optical device based on ammonium fluoroborate crystal, characterized in that, Follow these steps: a. Measurement of the crystal frequency doubling coefficient of ammonium fluoroborate; The doubling coefficient of ammonium fluoroborate crystal under type I phase matching was measured using the Mark stripe method, and a non-zero effective doubling coefficient was obtained. d 32 N±2.8×d 36 (KDP) Where, d 36 (KDP) = 0.39 pm / V; b. Based on the propagation in the corresponding XY plane, determine the formula for the effective frequency doubling coefficient of the ammonium fluoroborate crystal cut at the phase-matching angle: It is the angle between the projection of the light wave propagation direction onto the XY plane and the X-axis, where the crystal principal axis is the Z-axis; c. The principal refractive index of ammonium fluoroborate crystals at multiple wavelengths from deep ultraviolet to infrared was determined using the minimum deviation angle method, and the Sellmeier equation was obtained by fitting the results. Where n x n y n z λ is the principal axis refractive index of ammonium fluoroborate crystal, and λ is the incident wavelength in μm. d. Using the Sellmeier equation from step c, the phase matching curve of the ammonium fluoroborate crystal is obtained by computer program calculation, and then the phase matching point of the ammonium fluoroborate crystal is determined by combining the frequency doubling coefficients measured in step a. e. Orient, cut, polish the two light-transmitting surfaces of the ammonium fluoroborate crystal according to the phase matching direction, and deposit a dielectric film to obtain a prism-free nonlinear optical device.

2. The method for fabricating 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 5mm×5mm×10mm, and the light transmission length of the cut ammonium fluoroborate crystal is 0.1~10mm.

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

4. The method for fabricating a nonlinear optical device based on ammonium fluoroborate crystal according to claim 1, characterized in that: In step a, the harmonic coefficient of ammonium fluoroborate crystal at standard Type I phase matching in the 1.064 μm band is measured using the Maker stripe method.

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

6. A nonlinear optical device based on ammonium fluoroborate crystal, prepared by the method of fabricating a nonlinear optical device based on ammonium fluoroborate crystal as described in any one of claims 1-5.

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 described in claim 6 or 7, characterized in that: Using Nd-doped YAG, YLF, YAP, LuVO4, YVO4, GdVO4 lasers or Ti:sapphire lasers as light sources, and using type I GTP-KTP, type I KTP, type I KTA, type I LBO, type I BBO or type I CLBO as frequency doubling devices, at least one frequency-doubled beam generated by the frequency doubling device directly enters the ammonium fluoroborate crystal nonlinear optical device, thereby generating frequency-doubled laser output, sum-frequency laser output, difference-frequency laser output and optical parametric amplified laser output.

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