A nonlinear optical crystal and frequency converter for generating third harmonic laser light

Abstract

The application provides a nonlinear optical crystal and a frequency converter for generating third harmonic laser, and belongs to the field of optical technology. The nonlinear optical crystal comprises a centrosymmetric nonlinear optical crystal, and a periodic microstructure composed of a plurality of amorphous regions and a plurality of single crystal regions is arranged in the nonlinear optical crystal along a light transmission direction of the crystal. The amorphous regions are phase control units, and the third-order nonlinear coefficient in each amorphous region is zero. The single crystal regions are laser frequency conversion units, and the third-order nonlinear coefficient in each single crystal region is not zero. The application can realize efficient and stable output of third harmonic laser.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and more specifically to a nonlinear optical crystal and frequency converter for generating third-harmonic lasers. Background Technology

[0002] Nonlinear optical frequency conversion technology refers to shortening or lengthening the laser wavelength by utilizing the nonlinear response of materials to optics. There are two main categories of common nonlinear optical frequency conversion techniques: one is based on second-order nonlinear χ... (2) Techniques such as frequency doubling, sum frequency, difference frequency, and optical parametric oscillation in the process; another type is based on third-order nonlinear χ. (3) The process includes techniques such as direct triplet mixing and four-wave mixing.

[0003] Currently, third-harmonic lasers can be generated through a cascaded process based on second-order nonlinear frequency doubling and sum-frequency, and this technology has been widely used. However, this technique requires two second-order nonlinear optical crystals, and its complex structure and high cost limit the acquisition and application of high-efficiency lasers.

[0004] Direct third harmonic generation (ω+ω+ω→3ω) technology, based on a third-order nonlinear process, utilizes the upconversion effect of nonlinear crystals to increase the optical frequency, effectively shortening the laser wavelength by one-third. This significantly extends the laser wavelength and has important applications in lasers and nonlinear optics. The material used for direct third harmonic generation is a third-order nonlinear optical crystal. During the crystal's frequency conversion process, momentum conservation, i.e., phase matching, is a fundamental requirement for efficient optical frequency conversion. In centrosymmetric anisotropic crystals, based on the dispersion relation of birefringence, the refractive indices of the fundamental frequency (o-ray) or e-ray and the frequency-harmonic (e-ray) or o-ray are made equal at specific angles, achieving ooo-e or eee-o phase matching and thus efficient third harmonic laser output. Currently, direct third harmonic laser output has been achieved in crystals such as potassium titanium oxyphosphate, barium borate, and titanium dioxide based on birefringence phase matching. However, birefringence phase matching has a relatively small receiving bandwidth, only achieving phase matching conditions for specific wavelengths along specific angles, thus limiting the range of laser wavelengths obtainable. In addition, there are broadband third-harmonic generation techniques based on nonlinear metasurfaces, but these also suffer from low light-to-light conversion efficiency, poor wavelength tuning capability, and difficulty in practical application. The aforementioned "o-ray" refers to polarized light whose vibration direction is perpendicular to the principal plane formed by the crystal's optical axis and the incident direction. The aforementioned "e-ray" refers to polarized light whose vibration direction is parallel to the principal plane formed by the crystal's optical axis and the incident direction.

[0005] Centrosymmetric third-order nonlinear optical crystals, such as yttrium aluminum garnet (YAG), do not inherently exhibit second-order nonlinear effects, theoretically enabling direct third-harmonic generation. However, due to their optical isotropic nature, phase-matching conditions cannot be achieved, thus limiting their application to third-harmonic laser generation for a long time.

[0006] Periodic microstructure design is an effective means of altering the optical properties of crystalline materials. By introducing micrometer-scale periodic structures into uniform crystals, reciprocal lattice vectors are generated for phase compensation, thereby improving nonlinear conversion efficiency. Unlike previous methods of introducing phase gratings through polarization reversal in ferroelectric crystals such as lithium niobate, lithium tantalate, and potassium titanate phosphate, centrosymmetric nonlinear optical crystals do not possess ferroelectricity and cannot achieve periodic microstructures through polarization reversal.

[0007] Therefore, how to apply microstructure design technology to third-order nonlinear third-harmonic crystals to achieve laser frequency conversion is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] To address the shortcomings of existing nonlinear optical frequency conversion technologies and nonlinear optical materials, this invention provides a nonlinear optical crystal and frequency converter for generating third-harmonic lasers, which compensates for phase mismatch in the third-order nonlinear process and improves the third-harmonic conversion efficiency.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] The present invention first discloses a nonlinear optical crystal for generating third-harmonic laser, comprising a centrally symmetric nonlinear optical crystal, wherein the nonlinear optical crystal has a periodic microstructure composed of a number of amorphous regions and a number of single-crystal regions arranged periodically along the light transmission direction of the crystal.

[0011] In this system, the amorphous region serves as the phase modulation unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single-crystal region serves as the laser frequency conversion unit, and the third-order nonlinear coefficient in each single-crystal region is not zero.

[0012] Preferably, in the above-mentioned nonlinear optical crystal, the phase difference of laser transmission between the single-crystal region and the amorphous region within one period of the periodic microstructure is: m is an integer.

[0013] Preferably, in the above-mentioned nonlinear optical crystal, each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal changes from a uniform and complete atomic arrangement to a disordered atomic arrangement, thereby reducing the third-order nonlinear coefficient in the amorphous region to zero.

[0014] In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

[0015] Preferably, the nonlinear optical crystal comprises Y3Al5O 12 Crystals, Y2O3 crystals, diamond crystals, CaF2 crystals, LiF crystals, Ba(NO3)2 crystals, KTaO3 crystals, YVO4 crystals, or KY(WO4)2 crystals.

[0016] Preferably, the cross-section of the nonlinear optical crystal is square or circular.

[0017] Preferably, when the cross-section of the nonlinear optical crystal is square, the side length of the square cross-section is 2-10 mm.

[0018] Preferably, when the cross-section of the nonlinear optical crystal is circular, the radius of the circular cross-section is 2-10 mm.

[0019] This invention also discloses a method for preparing a nonlinear optical crystal for generating third-harmonic laser, comprising the following steps:

[0020] Prepare a centrosymmetric nonlinear optical crystal;

[0021] A periodic microstructure consisting of several amorphous regions and several single-crystal regions is prepared inside the nonlinear optical crystal along the light transmission direction of the crystal.

[0022] Each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, thereby changing the lattice structure in the nonlinear optical crystal from a uniform and complete atomic arrangement to a disordered atomic arrangement, and thus erasing the third-order nonlinear coefficients in the amorphous regions.

[0023] In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

[0024] In addition, the present invention also discloses a third-harmonic laser frequency converter with a periodic microstructure, comprising a fundamental frequency source, a focusing system, a centrally symmetric third-order nonlinear optical crystal, and a filter arranged sequentially along the laser transmission direction;

[0025] The nonlinear optical crystal has a periodic microstructure composed of several amorphous regions and several single-crystal regions arranged periodically along the light transmission direction. The amorphous regions serve as phase modulation units, and the third-order nonlinear coefficient in each amorphous region is zero. The single-crystal regions serve as laser frequency conversion units, and the third-order nonlinear coefficient in each single-crystal region is not zero.

[0026] Preferably, in the above-mentioned third-harmonic laser frequency converter, the output end of the filter is further connected to a photodetector, which is used to convert optical signals into electrical signals.

[0027] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a nonlinear optical crystal and frequency converter for generating third-harmonic laser, which has the following beneficial effects:

[0028] This invention enables efficient and stable output of third-harmonic laser.

[0029] The third-order nonlinear laser frequency conversion device involved in this invention has the advantages of simple and compact structure and high conversion efficiency. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0031] Figure 1 A schematic diagram of the internal periodic microstructure of a nonlinear optical crystal provided in an embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram of periodic microstructures prepared by direct writing via femtosecond laser processing, as provided in an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram of a periodic microstructure tripled frequency laser frequency converter provided in an embodiment of the present invention.

[0034] Figure 4 The output spectra of the 1030nm pump fundamental frequency light and the 343.3nm third harmonic laser provided in the embodiments of the present invention are shown, where the horizontal axis is wavelength (nm) and the vertical axis is relative intensity.

[0035] In the figure, 001-single crystal region, 002-amorphous region, 003-femtosecond laser for direct writing, 1-pump fundamental frequency source, 2-focusing system, 3-centrosymmetric third-order nonlinear optical crystal, 4-filter, 5-photodetector. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] This invention first discloses a nonlinear optical crystal for generating third-harmonic lasers, including a centrosymmetric nonlinear optical crystal, such as... Figure 1 As shown, the nonlinear optical crystal has a periodic microstructure arranged along the light transmission direction of the crystal, consisting of several amorphous regions 002 and several single-crystal regions 001 arranged periodically. The amorphous regions serve as phase modulation units, and the third-order nonlinear coefficient in each amorphous region 002 is zero. The single-crystal regions serve as laser frequency conversion units, and the third-order nonlinear coefficient in each single-crystal region 001 is not zero.

[0038] By controlling the lengths of the amorphous and single-crystal regions in the periodic microstructure, the phase mismatch in the third-harmonic laser generation process can be compensated, thereby making up for the phase difference between the fundamental frequency light and the third-harmonic light and improving the conversion efficiency of the third-harmonic process. The relationship between the optical phase Φ and the length L of each period in the periodic microstructure is Φ = 2πnL / λ, where π is pi, λ is the laser wavelength, and n is the refractive index of the nonlinear optical crystal at wavelength λ and frequency ω. Therefore, when light propagates a length L in the crystal, the phase difference between the fundamental frequency laser ω and the third-harmonic laser 3ω is... By setting the phase difference between laser transmission in the single-crystal region and the amorphous region within a period to φ = (2m + 1)π, where m is an integer, the corresponding microstructure length L can be obtained. The width L of the single-crystal region... m =0.01-50μm, width L of the amorphous region n =0.01-50μm.

[0039] In the aforementioned nonlinear optical crystal, each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, thereby changing the lattice structure in the nonlinear optical crystal from a uniform and complete atomic arrangement to a disordered atomic arrangement, and thus reducing the third-order nonlinear coefficient in the amorphous region to zero.

[0040] In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

[0041] The nonlinear optical crystal proposed in this invention comprises Y3Al5O 12Crystals, Y2O3 crystals, diamond crystals, CaF2 crystals, LiF crystals, Ba(NO3)2 crystals, KTaO3 crystals, YVO4 crystals, or KY(WO4)2 crystals.

[0042] In this invention, the cross-section of the nonlinear optical crystal is square or circular. When the cross-section of the nonlinear optical crystal is square, the side length of the square cross-section is 2-10 mm. When the cross-section of the nonlinear optical crystal is circular, the radius of the circular cross-section is 2-10 mm.

[0043] This invention also discloses a method for preparing a nonlinear optical crystal for generating third-harmonic laser, comprising the following steps:

[0044] Prepare a centrosymmetric nonlinear optical crystal;

[0045] A periodic microstructure consisting of several amorphous regions and several single-crystal regions is prepared along the light transmission direction inside a nonlinear optical crystal.

[0046] Each amorphous region in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, thereby changing the lattice structure in the nonlinear optical crystal from a uniform and complete atomic arrangement to a disordered atomic arrangement, and thus erasing the third-order nonlinear coefficients in the amorphous region.

[0047] In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

[0048] like Figure 2 As shown, a schematic diagram of the preparation of periodic microstructures by direct writing using a femtosecond laser is presented. The preparation of nonlinear optical crystal microstructures is achieved by using a femtosecond laser 003 for direct writing.

[0049] This invention also discloses a third-harmonic laser frequency converter with a periodic microstructure, such as... Figure 3 As shown, it includes a fundamental frequency light source 1, a focusing system 2, a centrally symmetric third-order nonlinear optical crystal 3, and a filter 4 arranged sequentially along the laser transmission direction;

[0050] The nonlinear optical crystal 3 has a periodic microstructure composed of several amorphous regions and several single-crystal regions arranged periodically along the light transmission direction. The amorphous regions serve as phase modulation units, and the third-order nonlinear coefficient in each amorphous region is zero. The single-crystal regions serve as laser frequency conversion units, and the third-order nonlinear coefficient in each single-crystal region is not zero.

[0051] Furthermore, in the aforementioned triplet laser frequency converter, the output end of the filter 4 is also connected to a photodetector 5, which is used to convert the optical signal into an electrical signal.

[0052] The "microstructure periodicity" refers to the sum of the lengths of the ordered and disordered regions within a period, where the microstructure period Λ = L. m +L n L m L is the width of the ordered region formed by the unprocessed portion of the crystal along the light transmission direction. n It is the width of the disordered region formed by the processing part in the light transmission direction of the crystal.

[0053] The following describes the third-harmonic laser frequency converter of the present invention, which uses periodic microstructures of different crystals, in further detail through several different specific embodiments.

[0054] Example 1

[0055] Example 1 discloses a periodic microstructure third-harmonic laser frequency converter, such as Figure 3 As shown, the system consists of a fundamental frequency pump source 1, a focusing system 2, and a direct third harmonic nonlinear frequency conversion crystal 3 arranged sequentially. The fundamental frequency pump source 1 is a ytterbium-ion fiber femtosecond laser with an emission wavelength of 1030 nm. The laser focusing system 2 is a focusing lens with a focal length of 5 cm. The nonlinear optical crystal 3 is yttrium aluminum garnet (Y3Al5O3). 12 The yttrium aluminum garnet crystal has a length of 5 mm and a light-transmitting surface of 2*2 mm. 2 The crystal is polished on all six sides and perpendicular to the light transmission direction. Periodic microstructures, as disclosed in this invention, are fabricated inside the crystal using femtosecond laser processing. The period of each microstructure is Λ = 5.28 μm. Pump light passing through the processed region in the crystal can achieve 343.3 nm third-harmonic ultraviolet laser output, with a wavelength as follows: Figure 4 As shown.

[0056] Example 2

[0057] The overall structure of the frequency converter of the third harmonic laser with periodic microstructure disclosed in Example 2 is similar to that of Example 1. The difference is that the pump source is an erbium-ion fiber femtosecond laser with an emission wavelength of 1550nm, and the nonlinear frequency conversion crystal used is yttrium oxide (Y2O3) crystal. The period of each microstructure is designed to be Λ = 10.15μm. After the pump light passes through the processing area in the crystal, it can achieve 516.7nm third harmonic green laser output. Other conditions are the same as those described in Example 1.

[0058] Example 3

[0059] The overall structure of the frequency converter of the third harmonic laser with periodic microstructure disclosed in Example 3 is similar to that in Example 1. The difference is that the pump source is an erbium-ion fiber femtosecond laser with an emission wavelength of 1550nm, and the nonlinear frequency conversion crystal used is yttrium vanadate (YVO4) crystal. The period of each microstructure is designed to be Λ = 7.37μm. After the pump light passes through the processing area in the crystal, it can achieve a 516.7nm third harmonic green laser output. Other conditions are the same as those described in Example 1.

[0060] Example 4

[0061] The overall structure of the frequency converter of the third harmonic laser with periodic microstructure disclosed in Example 4 is similar to that in Example 1. The difference is that the pump source is an erbium-ion fiber femtosecond laser with an emission wavelength of 1550nm, the nonlinear frequency conversion crystal used is diamond crystal, and the designed microstructure period is Λ = 12.00μm. After the pump light passes through the processing area in the crystal, it can achieve 516.7nm third harmonic green laser output. Other conditions are the same as those described in Example 1.

[0062] Example 5

[0063] The overall structure of the frequency converter of the third harmonic laser with periodic microstructure disclosed in Example 5 is similar to that in Example 1. The difference is that the pump source is an erbium-ion fiber femtosecond laser with an emission wavelength of 1550nm, and the nonlinear frequency conversion crystal used is potassium tantalate niobate (KTaO3) crystal. The designed microstructure period is Λ = 3.29μm. After the pump light passes through the processing area in the crystal, it can achieve 516.7nm third harmonic green laser output. Other conditions are the same as those described in Example 1.

[0064] Example 6

[0065] The overall structure of the third-harmonic laser frequency converter with periodic microstructure disclosed in Example 6 is similar to that in Example 1. The difference is that the pump source is a neodymium ion picosecond laser with an emission wavelength of 1064nm, the nonlinear frequency conversion crystal used is calcium fluoride CaF2 crystal, the designed microstructure period is Λ = 20.27μm, and the pump light can achieve 355nm ultraviolet laser output after passing through the processing area in the crystal. Other conditions are the same as those described in Example 1.

[0066] Example 7

[0067] The overall structure of the third-harmonic laser frequency converter with periodic microstructure disclosed in Example 7 is similar to that in Example 1. The difference is that the pump source is an ultraviolet picosecond laser with an emission wavelength of 355nm, the nonlinear frequency conversion crystal used is a lithium fluoride (LiF) crystal, and the designed microstructure period is Λ = 0.64μm. After the pump light passes through the processing area in the crystal, it can achieve 118.3nm extreme ultraviolet laser output. Other conditions are the same as those described in Example 1.

[0068] Example 8

[0069] The overall structure of the frequency converter of the third harmonic laser with periodic microstructure disclosed in Example 8 is similar to that in Example 1. The difference is that the pump source is an erbium-ion fiber femtosecond laser with an emission wavelength of 1550nm, and the nonlinear frequency conversion crystal used is potassium ytterbium tungstate KY(WO4)2 crystal. The designed microstructure period is Λ = 7.32μm. After the pump light passes through the processing area in the crystal, it can achieve 516.7nm third harmonic green laser output. Other conditions are the same as those described in Example 1.

[0070] Example 9

[0071] The overall structure of the third-harmonic laser frequency converter with periodic microstructure disclosed in Example 9 is similar to that in Example 1. The difference is that the pump source is a ytterbium-ion fiber femtosecond laser with an emission wavelength of 1030 nm, and the nonlinear frequency conversion crystal used is barium nitrate Ba(NO3)2 crystal. The designed microstructure period is Λ = 1.62 μm. After the pump light passes through the processing area in the crystal, it can achieve 343.3 nm ultraviolet laser output. Other conditions are the same as those described in Example 1.

[0072] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0073] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A nonlinear optical crystal for generating third harmonic laser light, characterized by, This includes a centrosymmetric nonlinear optical crystal, wherein a periodic microstructure, consisting of several amorphous regions and several single-crystal regions arranged periodically along the light transmission direction of the crystal, is disposed within the nonlinear optical crystal. This periodic microstructure is used to compensate for phase mismatch in a third-order nonlinear third harmonic process. Within one period of the periodic microstructure, the phase difference between the laser transmission of the single-crystal region and the amorphous region is [missing information]. m is an integer; In this system, the amorphous region serves as the phase modulation unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single-crystal region serves as the laser frequency conversion unit, and the third-order nonlinear coefficient in each single-crystal region is not zero.

2. The nonlinear optical crystal for generating third harmonic laser according to claim 1, wherein, Each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, thereby changing the lattice structure in the nonlinear optical crystal from a uniform and complete atomic arrangement to a disordered atomic arrangement, and thus reducing the third-order nonlinear coefficient in the amorphous region to zero. In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

3. The nonlinear optical crystal for generating third harmonic laser according to claim 1, wherein, The nonlinear optical crystal includes Y3Al5O 12 crystal, a Y2O3 crystal, a diamond crystal, a CaF2 crystal, a LiF crystal, a Ba(N03)2 crystal, a KTa03 crystal, a YVO4 crystal, or a KY(WO4)2 crystal.

4. The nonlinear optical crystal for generating third harmonic laser according to claim 1, wherein, The cross-section of the nonlinear optical crystal is square or circular.

5. The nonlinear optical crystal for generating third harmonic laser light according to claim 4, wherein When the cross-section of the nonlinear optical crystal is square, the side length of the square cross-section is 2-10 mm.

6. The nonlinear optical crystal for generating third harmonic laser light according to claim 4, wherein When the cross-section of the nonlinear optical crystal is circular, the radius of the circular cross-section is 2-10 mm.

7. A method for producing a nonlinear optical crystal for generating a third harmonic laser, characterized by, Includes the following steps: Prepare a centrosymmetric nonlinear optical crystal; A periodic microstructure, consisting of several amorphous regions and several single-crystal regions arranged periodically along the light transmission direction, is fabricated inside the nonlinear optical crystal. This periodic microstructure is used to compensate for phase mismatch in a third-order nonlinear third harmonic process. Within one period of the periodic microstructure, the phase difference between laser transmission in the single-crystal region and the amorphous region is [missing information]. m is an integer; Each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or ion beam to destroy the lattice structure in the nonlinear optical crystal, thereby changing the lattice structure in the nonlinear optical crystal from a uniform and complete atomic arrangement to a disordered atomic arrangement, and thus erasing the third-order nonlinear coefficients in the amorphous regions. In each of the single-crystal regions of the periodic microstructure, no external conditions are applied during the preparation process, maintaining the single-crystal state and non-zero third-order nonlinear coefficients.

8. A three-octave microstructured laser frequency converter, characterized in that It includes a fundamental frequency light source, a focusing system, a centrally symmetric third-order nonlinear optical crystal, and a filter arranged sequentially along the laser transmission direction; The nonlinear optical crystal contains a periodic microstructure arranged along its light transmission direction, consisting of several amorphous regions and several single-crystal regions. This periodic microstructure compensates for phase mismatch in the third-order nonlinear third harmonic process. Within one period of this periodic microstructure, the phase difference between the laser transmission in the single-crystal region and the amorphous region is... m is an integer; the amorphous region is used as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single-crystal region is used as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single-crystal region is not zero.

9. A third harmonic generation laser frequency converter of a periodic microstructure according to claim 8, characterized in that, The output end of the filter is also connected to a photodetector, which is used to convert optical signals into electrical signals.

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