Nonlinear optical crystal for generating frequency tripling laser and frequency converter

Abstract

The invention provides a nonlinear optical crystal for generating triple frequency laser and a frequency converter, and belongs to the technical field of optics. The nonlinear optical crystal comprises a centrosymmetric nonlinear optical crystal body, and a periodic microstructure formed by periodically arranging a plurality of amorphous state areas and a plurality of single crystal areas is arranged in the nonlinear optical crystal body in the light transmission direction of the crystal. The amorphous regions serve as phase regulation units, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal areas serve as laser frequency conversion units, and the third-order nonlinear coefficient in each single crystal area is not zero. According to the invention, efficient and stable output of triple frequency laser can be realized.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and more particularly to a nonlinear optical crystal and a frequency converter for generating triple frequency laser. Background Art

[0002] Nonlinear optical frequency conversion technology refers to shortening or lengthening the laser wavelength through the nonlinear response of materials to optics. There are two common types of nonlinear optical frequency conversion. One is based on the second-order nonlinear χ (2) The other is based on the third-order nonlinear χ (3) Direct frequency tripling and four-wave mixing techniques in the process.

[0003] At present, tripled frequency lasers can be produced through a cascade process based on second-order nonlinear frequency doubling and sum frequency, and are widely used. However, this technology requires two second-order nonlinear optical crystals, and has a complex structure and high cost, which limits the acquisition and application of high-efficiency lasers.

[0004] The direct frequency tripling (ω+ω+ω→3ω) technology based on the third-order nonlinear process uses the up-conversion effect of nonlinear crystals to increase the optical frequency, thus shortening the laser wavelength by one third. It can significantly expand the laser wavelength and has important application value in the field of laser and nonlinear optics. The material for direct frequency tripling is a third-order nonlinear optical crystal. In the frequency conversion process of the crystal, momentum conservation, i.e., phase matching condition, is the basic requirement for efficient optical frequency conversion. In a centrosymmetric anisotropic crystal, based on the dispersion relation of the birefringence effect, the refractive index of the fundamental frequency light o or e light and the frequency doubled light e or o light is equal under a special cutting angle, thereby achieving phase matching of ooo-e or eee-o and achieving efficient triple frequency laser output. At present, based on birefringence phase matching, direct triple frequency laser output has been achieved in potassium titanyl phosphate, barium borate, titanium dioxide and other crystals. However, the receiving bandwidth of birefringence phase matching is small, and phase matching conditions of specific wavelengths can only be achieved along a specific angle, and the range of available laser wavelengths is limited. In addition, there is also a broadband triple frequency generation technology based on nonlinear metasurfaces, but it also has problems such as low light-to-light conversion efficiency, poor wavelength tuning ability, and difficulty in practical application. The above-mentioned "o light" refers to polarized light whose vibration direction is perpendicular to the main plane formed by the crystal optical axis and the incident direction. The above-mentioned "e light" refers to polarized light whose vibration direction is parallel to the main plane formed by the crystal optical axis and the incident direction.

[0005] Centrosymmetric third-order nonlinear optical crystals, represented by yttrium aluminum garnet, do not have interference from second-order nonlinear effects and can theoretically achieve direct triple frequency generation. However, due to its optical isotropy, it is impossible to achieve phase matching conditions, so it has not been used for triple frequency laser generation for a long time.

[0006] Periodic microstructure design is an effective means to change the optical properties of crystal materials. By introducing a micrometer-level periodic structure into a uniform crystal, a reciprocal lattice vector is generated for phase compensation, thereby improving the nonlinear conversion efficiency. Unlike the previous introduction of phase gratings in ferroelectric crystals such as lithium niobate, lithium tantalate, and potassium titanyl phosphate through polarization reversal technology, centrosymmetric nonlinear optical crystals do not have ferroelectricity and cannot achieve periodic microstructures through polarization reversal.

[0007] Therefore, how to apply microstructure design technology to third-order nonlinear triple frequency crystals to achieve laser frequency conversion is an urgent problem that technicians in this field need to solve. Summary of the invention

[0008] In view of the shortcomings of existing nonlinear optical frequency conversion technology and nonlinear optical materials, the present invention provides a nonlinear optical crystal and frequency converter for generating tripled frequency laser, which are used to compensate for the phase mismatch in the third-order nonlinear process and improve the tripled frequency conversion efficiency.

[0009] In order to achieve the above object, the present invention adopts the following technical solution:

[0010] The present invention firstly discloses a nonlinear optical crystal for generating triple frequency laser, comprising a centrally symmetric nonlinear optical crystal, wherein a periodic microstructure composed of a plurality of amorphous regions and a plurality of single crystal regions arranged periodically is arranged inside the nonlinear optical crystal along the light transmission direction of the crystal;

[0011] Among them, the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal region serves as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single crystal region is not zero.

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

[0013] Preferably, in the above nonlinear optical crystal, each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or an ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal is changed 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 in the periodic microstructure, no external conditions are applied during the preparation process, and a single crystal state and a non-zero third-order nonlinear coefficient are maintained.

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

[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 a 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] The present invention also discloses a method for preparing a nonlinear optical crystal for generating triple frequency laser, comprising the following steps:

[0020] preparing a centrosymmetric nonlinear optical crystal;

[0021] A periodic microstructure consisting of a plurality of amorphous regions and a plurality of single crystal regions arranged periodically 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 an ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal is changed from a uniform and complete atomic arrangement to a disordered atomic arrangement, thereby erasing the third-order nonlinear coefficient in the amorphous region;

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

[0024] In addition, the present invention also discloses a periodic microstructured triple frequency laser frequency converter, comprising a fundamental frequency light source, a focusing system, a centrosymmetric third-order nonlinear optical crystal and a filter arranged in sequence along the laser transmission direction;

[0025] Among them, a periodic microstructure consisting of a number of amorphous regions and a number of single crystal regions arranged periodically is arranged inside the nonlinear optical crystal along the light transmission direction of the crystal; the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal region serves as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single crystal region is not zero.

[0026] Preferably, in the above triple frequency laser frequency converter, the output end of the filter is also connected to a photodetector, and the photodetector is used to convert the optical signal into an electrical signal.

[0027] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a nonlinear optical crystal and a frequency converter for generating tripled frequency laser, which have the following beneficial effects:

[0028] The present invention can realize efficient and stable output of triple frequency laser.

[0029] The third-order nonlinear laser frequency conversion device involved in the present invention has the advantages of simple and compact structure, high conversion efficiency and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

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

[0032] Figure 2 Schematic diagram of the preparation of periodic microstructures by direct writing through femtosecond laser processing provided in an embodiment of the present invention.

[0033] Figure 3 A schematic structural diagram of a periodic microstructured tripled frequency laser frequency converter provided in an embodiment of the present invention.

[0034] Figure 4 Output spectra of 1030nm pump fundamental frequency light and 343.3nm triple frequency laser provided in an embodiment of the present invention, wherein the abscissa is wavelength (nm) and the ordinate is relative intensity.

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

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] The present invention first discloses a nonlinear optical crystal for generating triple frequency laser, including a centrally symmetric nonlinear optical crystal, such as Figure 1 As shown, the nonlinear optical crystal has a periodic microstructure composed of a number of amorphous regions 002 and a number of single crystal regions 001 arranged periodically along the light transmission direction of the crystal; wherein the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region 002 is zero; and the single crystal region serves as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single crystal region 001 is not zero.

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

[0039] In the above nonlinear optical crystal, each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or an ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal is changed 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;

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

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

[0042] In the present 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] The present invention also discloses a method for preparing a nonlinear optical crystal for generating triple frequency laser, comprising the following steps:

[0044] preparing a centrosymmetric nonlinear optical crystal;

[0045] A periodic microstructure consisting of a number of amorphous regions and a number of single crystal regions arranged periodically is prepared along the light transmission direction of the crystal inside the nonlinear optical crystal;

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

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

[0048] like Figure 2 As shown, a schematic diagram of preparing a periodic microstructure by direct writing through femtosecond laser processing is given, and the preparation of a nonlinear optical crystal microstructure is achieved by using a femtosecond laser 003 used for direct writing.

[0049] The embodiment of the present invention also discloses a triple frequency 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 symmetrical third-order nonlinear optical crystal 3 and a filter 4 which are sequentially arranged along the laser transmission direction;

[0050] Among them, a periodic microstructure consisting of a number of amorphous regions and a number of single crystal regions arranged periodically is arranged inside the nonlinear optical crystal 3 along the light transmission direction of the crystal; the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal region serves as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single crystal region is not zero.

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

[0052] The “microstructure period” refers to the sum of the lengths of the ordered and disordered regions in one period. The microstructure period Λ=L m +L n , L m is the width of the ordered region consisting of the unprocessed part of the crystal in the light passing direction, L n It is the width of the disordered region formed by the processed part of the crystal in the light-transmitting direction.

[0053] The tripled frequency laser frequency converter of the present invention using periodic microstructures of different crystals is further described below through a plurality of different specific embodiments.

[0054] Example 1

[0055] The triple frequency laser frequency converter of the periodic microstructure disclosed in Example 1 is as follows Figure 3 As shown, the fundamental frequency light source 1, the focusing system 2 and the direct triple frequency nonlinear frequency conversion crystal 3 are arranged in sequence. The fundamental frequency pump source 1 is an ytterbium ion fiber femtosecond laser with an emission wavelength of 1030nm, the laser focusing system 2 is a focusing mirror with a focal length of 5cm, and the nonlinear optical crystal 3 is yttrium aluminum garnet Y3Al5O 12 Crystal, yttrium aluminum garnet crystal has a crystal length of 5mm and a light-transmitting surface of 2*2mm 2 The crystal is polished on six sides and perpendicular to the light transmission direction. The periodic microstructure disclosed in the present invention is written into the crystal by femtosecond laser processing. The period of each microstructure is Λ=5.28μm. After the pump light passes through the processing area in the crystal, a 343.3nm triple frequency ultraviolet laser output can be achieved. Its wavelength is as follows Figure 4 shown.

[0056] Example 2

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

[0058] Example 3

[0059] The overall structure of the periodic microstructure tripled frequency laser frequency converter disclosed in Example 3 is similar to that in Example 1, except that the pump source is an erbium ion fiber femtosecond laser with an emission wavelength of 1550nm, the nonlinear frequency conversion crystal used is an yttrium vanadate YVO4 crystal, each microstructure period is designed to be Λ=7.37μm, and the pump light can achieve 516.7nm tripled frequency green laser output after passing through the processing area in the crystal. Other conditions are consistent with those described in Example 1.

[0060] Example 4

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

[0062] Example 5

[0063] The overall structure of the periodic microstructured tripled frequency laser frequency converter disclosed in Example 5 is similar to that of Example 1, except that the pump source is an erbium ion fiber femtosecond laser with an emission wavelength of 1550nm, the nonlinear frequency conversion crystal used is a potassium tantalate niobate KTaO3 crystal, the designed microstructure period is Λ=3.29μm, and the pump light can achieve 516.7nm tripled frequency green laser output after passing through the processing area in the crystal. Other conditions are consistent with those described in Example 1.

[0064] Example 6

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

[0066] Example 7

[0067] The overall structure of the periodic microstructured tripled frequency laser frequency converter disclosed in Example 7 is similar to that of Example 1, except that the pump source is an ultraviolet picosecond laser with an emission wavelength of 355 nm, the nonlinear frequency conversion crystal used is a lithium fluoride LiF crystal, the designed microstructure period is Λ=0.64 μm, and the pump light can achieve 118.3 nm extreme ultraviolet laser output after passing through the processing area in the crystal. Other conditions are consistent with those described in Example 1.

[0068] Example 8

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

[0070] Example 9

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

[0072] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0073] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one 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 present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A nonlinear optical crystal for generating triple frequency laser, characterized in that: It comprises a centrally symmetrical nonlinear optical crystal, wherein a periodic microstructure composed of a plurality of amorphous regions and a plurality of single crystal regions arranged periodically is arranged inside the nonlinear optical crystal along the light transmission direction of the crystal; Among them, the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal region serves as a 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 tripled frequency laser according to claim 1, characterized in that: The phase difference of laser transmission between the single crystal region and the amorphous region in one period of the periodic microstructure is: m is an integer.

3. The nonlinear optical crystal for generating tripled frequency laser according to claim 1, characterized in that: Each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or an ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal is changed 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; In each of the single crystal regions in the periodic microstructure, no external conditions are applied during the preparation process, and a single crystal state and a non-zero third-order nonlinear coefficient are maintained.

4. The nonlinear optical crystal for generating tripled frequency laser according to claim 1, characterized in that: The nonlinear optical crystal includes Y3Al5O 12 crystal, Y2O3 crystal, diamond crystal, CaF2 crystal, LiF crystal, Ba(NO3)2 crystal, KTaO3 crystal, YVO4 crystal or KY(WO4)2 crystal.

5. The nonlinear optical crystal for generating tripled frequency laser according to claim 1, characterized in that: The cross section of the nonlinear optical crystal is square or circular.

6. The nonlinear optical crystal for generating tripled frequency laser according to claim 5, characterized in that: When the cross section of the nonlinear optical crystal is square, the side length of the square cross section is 2-10 mm.

7. The nonlinear optical crystal for generating tripled frequency laser according to claim 5, characterized in that: When the cross section of the nonlinear optical crystal is circular, the radius of the circular cross section is 2-10 mm.

8. A method for preparing a nonlinear optical crystal for generating a triple frequency laser, characterized in that: The following steps are involved: preparing a centrosymmetric nonlinear optical crystal; A periodic microstructure consisting of a plurality of amorphous regions and a plurality of single crystal regions arranged periodically is prepared inside the nonlinear optical crystal along the light transmission direction of the crystal; Each of the amorphous regions in the periodic microstructure is prepared by applying a femtosecond laser or an ion beam to destroy the lattice structure in the nonlinear optical crystal, so that the lattice structure in the nonlinear optical crystal is changed from a uniform and complete atomic arrangement to a disordered atomic arrangement, thereby erasing the third-order nonlinear coefficient in the amorphous region; In each of the single crystal regions in the periodic microstructure, no external conditions are applied during the preparation process, and a single crystal state and a non-zero third-order nonlinear coefficient are maintained.

9. A periodic microstructured triple frequency laser frequency converter, characterized in that: It includes a fundamental frequency light source, a focusing system, a centrally symmetrical third-order nonlinear optical crystal and a filter which are sequentially arranged along the laser transmission direction; Among them, a periodic microstructure consisting of a number of amorphous regions and a number of single crystal regions arranged periodically is arranged inside the nonlinear optical crystal along the light transmission direction of the crystal; the amorphous region serves as a phase control unit, and the third-order nonlinear coefficient in each amorphous region is zero; the single crystal region serves as a laser frequency conversion unit, and the third-order nonlinear coefficient in each single crystal region is not zero.

10. The triple frequency laser frequency converter of the periodic microstructure according to claim 9, characterized in that: The output end of the filter is also connected to a photodetector, and the photodetector is used to convert the optical signal into an electrical signal.

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