A method for producing a periodic domain erasure structure element based on potassium titanyl phosphate crystal and the product thereof

Abstract

The application discloses a preparation method of a periodic domain erasing structure element based on a potassium titanyl phosphate crystal and a product thereof, and the method comprises the following steps: a femtosecond laser strip-shaped light beam is used to vertically irradiate the potassium titanyl phosphate crystal along the z direction of the crystal, the ferroelectric domain of the femtosecond laser irradiation area is erased, and a domain erasing structure is generated; the femtosecond laser irradiation area is translated along the y direction of the potassium titanyl phosphate crystal, and a periodic domain erasing structure arranged along the y direction is obtained; the near-surface of the potassium titanyl phosphate crystal is ablated based on the femtosecond laser, and two air grooves which are parallel and penetrate through the periodic domain erasing structure are prepared; S3. Based on the air grooves, a waveguide region of a ridge waveguide is obtained; the two end faces perpendicular to the y direction of the crystal are subjected to optical polishing treatment, and finally, the potassium titanyl phosphate crystal periodic domain erasing structure element is obtained. The application can realize more rapid and efficient preparation of a periodic domain structure and nonlinear frequency conversion under a specific wavelength.

Description

Technical Field

[0001] The present invention relates to the technical field of optoelectronic device preparation, and more particularly to a method for preparing a periodic domain erasing structural element based on potassium titanyl phosphate crystals and a product thereof. Background Art

[0002] Potassium titanyl phosphate (KTiOPO4 or KTP) is a nonlinear ferroelectric crystal material with excellent performance. It has a large nonlinear optical coefficient (approximately 15 times that of KDP crystal), a wide spectral transmittance range, a large acceptance angle, a small walk-off angle, and stable chemical and mechanical properties. Potassium titanyl phosphate has important applications in laser frequency conversion, optical parametric oscillation, and amplification. By periodically manipulating the nonlinear optical coefficient (i.e., the ferroelectric crystal domain) of potassium titanyl phosphate crystals, nonlinear photonic crystal structures can be fabricated within the crystal, enabling efficient nonlinear frequency conversion at specific wavelengths.

[0003] Femtosecond lasers possess ultra-high peak power and ultra-short pulse widths. Their ultra-high peak power can stimulate a series of nonlinear interactions within transparent optical materials (such as multiphoton absorption, tunneling ionization, and avalanche breakdown). Their ultra-short pulse widths suppress the formation of heat-affected zones during processing, making femtosecond laser micro- and nanoscale processing possible. Femtosecond laser direct writing technology, characterized by high precision, rapid processing speed, and true three-dimensional processing, has been widely applied in fields such as biomedicine, materials engineering, and mechanical manufacturing.

[0004] At present, scientific researchers have discovered that by designing and optimizing femtosecond laser processing parameters, the ferroelectric domains of non-linear ferroelectric crystals can be erased at a specific point in the focal area of ​​the femtosecond laser. Then, based on the relevant phase matching theory, a periodic ferroelectric domain erasure structure can be prepared to achieve nonlinear frequency conversion at a specific wavelength.

[0005] Chinese patent document CN110568694B discloses a frequency converter based on a ridge-type lithium niobate single crystal thin film waveguide integrated with a periodic domain inversion structure and its fabrication method. This method uses femtosecond laser-induced nonlinear crystal ferroelectric domain inversion technology to fabricate a periodic domain inversion structure on a lithium niobate thin film (Lithium Niobate on Insulator, or LNOI). Using precision diamond knife cutting (two diamond knife cuts) or acid etching-assisted femtosecond laser direct writing (two laser scans), a ridge optical waveguide is fabricated on the lithium niobate thin film along the direction of the periodic structure. This allows for second harmonic generation at a specific wavelength within the waveguide. This method is simple to operate, low-cost, and high-quality.

[0006] However, this method uses femtosecond laser-induced ferroelectric domain inversion technology to prepare periodic ferroelectric domain structures. This technology requires precise control of the femtosecond laser energy to be very close to but not exceeding the damage threshold of the ferroelectric crystal, and relies on the thermoelectric field to induce the ferroelectric crystal domain near the laser focus to reverse (the nonlinear coefficient changes from 1 to -1). The original optical nonlinearity of the single crystal can still be maintained in the ferroelectric domain inversion region, and the stability requirements of the processing parameters and experimental conditions are relatively strict. In addition, this method is based on multiple scanning technology to control the quasi-phase matching period size and duty cycle, resulting in a long time for processing hundreds or thousands of periodic domain structures. Finally, although this method can realize the preparation and integration of highly integrated frequency conversion photonic devices on-chip and can generate frequency-doubled signals at specific wavelengths, it is difficult to meet the related requirements of high power and high stability of laser frequency conversion devices in industrial production.

[0007] Therefore, how to provide a frequency conversion structural element with relatively loose requirements on processing parameters and experimental conditions, shorter processing time, high stability and stronger resistance to light damage (i.e., tolerance to higher and stronger lasers), and realize frequency conversion at a specific wavelength, is a problem that technical personnel in this field urgently need to solve. Summary of the Invention

[0008] In view of this, the present invention provides a method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal and a product thereof, which are used to at least solve some of the technical problems in the background technology.

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

[0010] On the one hand, the present invention discloses a method for preparing a periodic domain erasure structure element based on potassium titanyl phosphate crystal, comprising the following steps:

[0011] S1. Erasing the ferroelectric domains in the femtosecond laser irradiated region of the potassium titanyl phosphate crystal by applying a femtosecond laser strip beam perpendicular to the z-direction of the crystal to generate a domain-erased structure; and translating the femtosecond laser irradiated region along the y-direction of the potassium titanyl phosphate crystal to obtain a periodic domain-erased structure arranged along the y-direction;

[0012] S2. ablating the near surface of the potassium titanyl phosphate crystal based on a femtosecond laser and scanning along the y-direction of the potassium titanyl phosphate crystal to prepare two parallel air grooves that pass through the periodic domain erase structure;

[0013] S3. Based on a femtosecond laser, a plurality of parallel refractive index-lowering modified grooves are scanned along the y direction of the potassium titanyl phosphate crystal between two parallel air slots to obtain a waveguide region of a ridge-type optical waveguide;

[0014] S4. Optically polish the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal to finally obtain a potassium titanyl phosphate crystal periodic domain erasing structure element.

[0015] Furthermore, in step S1 of the method of the present invention, a femtosecond laser strip beam perpendicularly incident on the potassium titanyl phosphate crystal along the z-direction of the crystal is obtained by the following steps:

[0016] A femtosecond laser Gaussian beam is shaped into a femtosecond laser strip beam using a spatial light modulator, and the femtosecond laser strip beam is converged through a microscope objective lens to obtain a femtosecond laser strip beam that is vertically incident on a potassium titanyl phosphate crystal along the z direction of the crystal.

[0017] Furthermore, in step S1 of the method of the present invention, in the periodic domain erasing structure generated, the duty cycle of the single-period intra-domain erasing structure is 0.5.

[0018] Furthermore, in step S2 of the method of the present invention, each air groove prepared has a groove depth of 16 μm and a span of 2 μm, and the lateral spacing between two parallel air grooves is 12 μm.

[0019] On the other hand, the present invention discloses a periodic domain erasure structure element based on potassium titanyl phosphate crystal, comprising: a potassium titanyl phosphate crystal, wherein the light transmission direction of the potassium titanyl phosphate crystal is along the y-direction of the crystal, a periodic domain erasure structure arranged along the y-direction is provided inside the potassium titanyl phosphate crystal, and a ridge-type optical waveguide structure penetrating the periodic domain erasure structure is provided on the surface of the potassium titanyl phosphate crystal, wherein the light transmission direction of the ridge-type optical waveguide structure is along the y-direction of the potassium titanyl phosphate crystal.

[0020] Preferably, in the periodic domain erasing structure element disclosed in the present invention, in the periodic domain erasing structure, the duty cycle of the single-cycle intra-domain erasing structure is 0.5.

[0021] Preferably, in the periodic domain erase structure element disclosed in the present invention, the potassium titanyl phosphate crystal is a z-cut potassium titanyl phosphate crystal.

[0022] Preferably, in the periodic domain erasing structure element disclosed in the present invention, the bottom of the ridge-type optical waveguide structure is provided with a femtosecond laser-induced refractive index-lowering groove with a width of 2 μm and a depth of 4 μm.

[0023] The present invention also discloses a potassium titanyl phosphate crystal frequency converter, comprising a near-infrared pulse laser, a half-wave plate, a fiber-coupled input end, a potassium titanyl phosphate crystal periodic domain erasing structure element, a fiber-coupled receiving end, and a filter, which are sequentially arranged along the light beam transmission direction. The potassium titanyl phosphate crystal periodic domain erasing structure element is obtained based on the preparation method of the periodic domain erasing structure element based on potassium titanyl phosphate crystal according to any one of the present inventions.

[0024] Compared with the prior art, the application provides a preparation method of a periodic domain erasure structure element based on a potassium titanyl phosphate crystal and a product thereof, and has the following beneficial effects:

[0025] The application utilizes the shaped femtosecond laser "strip" beam to prepare the periodic domain erasure structure in the near-surface of the z-cut potassium titanyl phosphate crystal, and by designing and optimizing the width of the femtosecond laser "strip" beam, the duty cycle of the domain erasure structure in a single period reaches 0.5, so that the periodic domain structure can be prepared more quickly and efficiently under the condition of satisfying phase matching, and the nonlinear frequency conversion under a specific wavelength is realized.

[0026] The application integrates the femtosecond laser direct writing ridge-type optical waveguide structure in the periodic domain erasure structure induced by the femtosecond laser, and can greatly improve the efficiency of nonlinear frequency conversion; the application adopts the femtosecond laser induced ferroelectric domain erasure technology, in which the thermal effect of the focused femtosecond laser in the ferroelectric crystal is small and can be ignored, at the laser focal point, the nonlinearity of the ferroelectric crystal can be completely erased (the nonlinear coefficient changes from 1 to 0) or partially erased (the nonlinear coefficient is greater than 0 and less than 1), and in the domain erasure area, the perfect single crystal becomes completely amorphous (complete erasure) or partially amorphous (partial erasure). Compared with the femtosecond laser induced domain inversion (nonlinear coefficient changes from 1 to -1) technology, the application has relatively loose requirements for processing parameters and experimental conditions (complete erasure or partial erasure), and is more easy to prepare the periodic ferroelectric domain structure and realize the nonlinear frequency conversion; in addition, the application selects the z-cut potassium titanyl phosphate crystal material platform, and the nonlinear crystal material has the characteristics of wide transmission spectrum range, strong anti-light damage ability, large nonlinear coefficient and stable chemical properties, and is more conducive to the preparation of high-power, high-efficiency and high-stability frequency conversion devices.

[0027] The frequency conversion device based on the periodic domain erasure structure of the potassium titanyl phosphate crystal has the characteristics of simple manufacturing process, fast preparation speed, high output power, high conversion efficiency, good stability, high integration and good beam quality, and can realize nonlinear frequency conversion under a specific wavelength, and will have wide application in the fields of laser technology, integrated optics and nonlinear optics. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0029] Figure 1 A top view of a frequency conversion element based on a periodic domain erasing structure of potassium titanyl phosphate crystals provided by the present invention;

[0030] Figure 2 This is a front view of a frequency conversion element based on a periodic domain erasing structure of potassium titanyl phosphate crystals provided by the present invention;

[0031] Figure 3 This is a left view of a frequency conversion element based on a periodic domain erasing structure of potassium titanyl phosphate crystals provided by the present invention;

[0032] Figure 4 A schematic diagram of a periodic domain erasing structure of potassium titanyl phosphate crystal provided by the present invention;

[0033] Figure 5 A schematic diagram of a method for preparing a frequency conversion element based on a periodic domain erasing structure of potassium titanyl phosphate crystals provided by the present invention;

[0034] Figure 6 A schematic structural diagram of a frequency converter based on a periodic domain erase structure of potassium titanyl phosphate crystal provided by the present invention;

[0035] Among them, 1- ferroelectric domain erasure structure, 2- femtosecond laser ablated crystal near-surface air groove, 3- z-cut potassium titanyl phosphate crystal, 4- femtosecond laser induced potassium titanyl phosphate crystal refractive index reduced modified region, 5- femtosecond laser direct writing ridge optical waveguide structure, 6- near-infrared pulse laser, 7- half-wave plate, 8- fiber-coupled input end, 9- fiber-coupled receiving end, 10- filter. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0037] Terminology Notes:

[0038] Duty cycle: The ratio of the width of the erased area within a single cycle to the overall width of the single cycle.

[0039] Frequency doubling: A nonlinear frequency conversion process that doubles the frequency of fundamental light.

[0040] Example 1

[0041] The first embodiment of the present invention discloses a periodic domain erasing structure element based on potassium titanium phosphate crystal, which is used to achieve frequency conversion, such as Figures 1-4 , comprising:

[0042] z-cut potassium titanyl phosphate crystal 3 material platform, the light transmission direction of the crystal is along the y direction of the crystal;

[0043] The z-cut potassium titanyl phosphate crystal is provided with a femtosecond laser-induced periodic ferroelectric domain erasure structure 1 arranged along the y direction;

[0044] A femtosecond laser direct-written ridge-type optical waveguide structure 2 is arranged along the arrangement direction of the periodic ferroelectric domain erasure structure 1, and the light transmission direction of the ridge-type optical waveguide is along the y direction of the potassium titanyl phosphate crystal.

[0045] In this embodiment, the duty cycle of the domain erasure structure in a single period approaches 0.5, and the closer the duty cycle is to 0.5, the higher the conversion efficiency is. When the duty cycle of the domain erasure structure 1 in a single period is 0.5, the conversion efficiency is the highest.

[0046] And the duty cycle approaching 0.5 can ensure that the frequency conversion efficiency is as high as possible and the output laser power is as large as possible; in a room temperature environment, the period of the first-order quasi-phase matching in the nonlinear frequency conversion process is:

[0047]

[0048] Wherein, Λ is the quasi-phase matching period size, λ ω is the wavelength of the incident fundamental frequency light, n ω and n 2ω are the refractive index values of the material under the conditions of the fundamental frequency light and the frequency-doubled light; the refractive index of the potassium titanyl phosphate crystal z direction under a certain incident fundamental frequency light wavelength λ ω is calculated by the dispersion equation, specifically:

[0049] n 2 z = 3.3134 + 0.05694 / (λ 2 -0.05658) - 0.01682λ 2 .

[0050] In order to further implement the above technical solutions, the number of the femtosecond laser-induced periodic domain erasure structure arranged along the y direction is ≥300.

[0051] In order to further implement the above technical solutions, the ridge-type optical waveguide that guides light along the y direction of the potassium titanyl phosphate crystal has two parallel air slots arranged along the x direction with a size of 2μm(x)*16μm(z), and the transverse interval (distance along the x direction of the crystal) of the two parallel air slots is 12μm. The ridge-type optical waveguide region 5 is between the two parallel air slots, and 5 femtosecond laser-induced refractive index reduction notches with a size of 2μm(x)*4μm(z) are arranged in the bottom region between the two air slots to generate the ridge-type optical waveguide.

[0052] In order to further implement the above technical solution, the length of the femtosecond laser-induced domain erasure structure along the z-direction of the potassium titanyl phosphate crystal is 10 μm.

[0053] In order to further implement the above technical solution, the six sides of the potassium titanyl phosphate crystal were optically polished, and a femtosecond laser was incident along the z-direction of the potassium titanyl phosphate crystal to induce a domain erasure structure. The incident near-infrared fundamental frequency light and the emitted frequency-doubled laser were transmitted along the y-direction of the potassium titanyl phosphate crystal.

[0054] Furthermore, the element size is 10(x)*10(y)*2(z)mm 3 , the tangent direction is z-direction, and the light transmission direction is along the y direction.

[0055] Example 2

[0056] like Figure 5 As shown, Example 2 discloses a method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal, comprising the following steps:

[0057] S1. A femtosecond laser Gaussian beam is shaped into a femtosecond laser strip beam using a spatial light modulator. After being converged by a microscope objective, the beam is incident vertically on a potassium titanyl phosphate crystal along the z-direction of the crystal to erase the ferroelectric domains in the femtosecond laser-irradiated area, generating a single-cycle internal domain erased structure. The duty cycle of the single-cycle internal domain erased structure is then controlled by adjusting the width of the femtosecond laser strip beam, ultimately resulting in a periodic domain erased structure arranged along the y-direction.

[0058] S2. Using a microscope objective, the original femtosecond laser Gaussian beam is focused onto a potassium titanyl phosphate crystal. Based on the processing parameters for femtosecond laser ablation of air grooves near the surface of the potassium titanyl phosphate crystal, two parallel air grooves are scanned along the arrangement direction of the periodic ferroelectric domain structure, i.e., the y-direction of the potassium titanyl phosphate crystal. Based on the processing parameters for femtosecond laser-induced refractive index reduction modification within the potassium titanyl phosphate crystal, five parallel refractive index reduction modification grooves are scanned along the arrangement direction of the periodic ferroelectric domain structure, i.e., the y-direction of the potassium titanyl phosphate crystal, at the bottom of the two parallel air grooves. This generates a femtosecond laser-induced refractive index reduction modification region 4 of the potassium titanyl phosphate crystal, thereby forming the waveguide region of the ridge optical waveguide.

[0059] In this step, the refractive index-lowering modified score is placed at the bottom of the area between the two air slots. If the air slots are trenches running from left to right, then the refractive index-lowering modified score also runs from left to right. Each score is 4 microns long and 2 microns wide. Five scores are closely spaced, with 2 microns between each score, to precisely fill the gap between the two parallel air slots.

[0060] S3. Optically polish the two y-end faces of the potassium titanyl phosphate crystal, i.e., the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal, to obtain a potassium titanyl phosphate crystal periodic domain erasing structure element.

[0061] For a laser with a specific wavelength, the present invention sets a specific strip beam width. Then, with a period equal to twice the width of the strip beam and a duty cycle of 0.5, hundreds or even thousands of domain-erased structures are fabricated along the transmission direction of the "laser with a specific wavelength," achieving efficient frequency conversion. To achieve frequency conversion for other wavelengths, the strip beam width must be calculated and readjusted, and a periodic structure with a period equal to twice the width of the strip beam must be fabricated to achieve frequency conversion.

[0062] In practical applications, the microscope objective has a magnification of 50 and a numerical aperture of 0.67.

[0063] To further implement the above technical solution, in step S1, the femtosecond laser pulse width is 400 fs, the wavelength is 1031 nm, the repetition frequency is 25 kHz, the single pulse energy is 4 μJ, and the femtosecond laser scanning speed is 3 mm / s;

[0064] In step S2, the processing parameters for femtosecond laser ablation of air grooves near the surface of the potassium titanyl phosphate crystal are: femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 12 μJ, and femtosecond laser scanning speed of 0.5 mm / s. The processing parameters for femtosecond laser-induced refractive index reduction modification within the potassium titanyl phosphate crystal are: femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 0.7 μJ, and femtosecond laser scanning speed of 5 mm / s.

[0065] Example 3

[0066] like Figure 6 As shown, the present invention also discloses a potassium titanyl phosphate crystal frequency converter, which is based on a potassium titanyl phosphate crystal periodic domain erasing structural element and includes a near-infrared pulse laser 6, a half-wave plate 7, a fiber-coupled input end 8, a potassium titanyl phosphate crystal periodic domain erasing structural element, a fiber-coupled receiving end 9 and a filter 10 arranged in sequence along the light beam transmission direction.

[0067] In this embodiment, the working principle of a potassium titanyl phosphate crystal frequency converter is as follows: a near-infrared pulse laser is used as a pump source to generate linearly polarized laser light, and the polarization direction of the pulse laser is adjusted to be consistent with the z-axis direction of the potassium titanyl phosphate crystal by rotating the half-wave plate (the reason is that the maximum nonlinear coefficient d of the potassium titanyl phosphate crystal can be used to generate linearly polarized laser light). 33, then the laser is coupled into the periodic domain-erased structure element containing the ridge optical waveguide through the fiber coupling input end, the fiber coupling receiving end collects the pump light and signal light (such as frequency-doubled light) emitted from the ridge optical waveguide together, and finally the pump light is cut off and the signal light is transmitted through the optical filter, so that the laser frequency conversion function at a specific wavelength can be realized.

[0068] The frequency converter is simple to operate. Only the near-infrared pulsed laser is turned on, and the direction of the incident linearly polarized light is adjusted to be consistent with the z direction of the potassium titanyl phosphate crystal (corresponding to the maximum nonlinear optical coefficient d 33 ) by rotating the half-wave plate, and high-efficiency nonlinear frequency conversion of the laser at the wavelength in the ridge optical waveguide can be realized. The advantages of the present application mainly lie in the following three aspects: first, the periodic domain-erased structure can be quickly and efficiently prepared by using the shaped femtosecond laser "strip" beam, and the nonlinear frequency conversion of the laser at a specific wavelength can be realized; second, the ridge optical waveguide directly written by the femtosecond laser (the air groove formed by femtosecond laser ablation is combined with the refractive index reduction modification induced by femtosecond laser) can reduce the transmission loss of the device and improve the output performance of the frequency conversion device; finally, the frequency converter is based on the z-cut potassium titanyl phosphate crystal material platform, which has large nonlinear optical coefficient, strong light damage resistance and stable mechanical performance, and is more easy to realize the nonlinear frequency conversion process with high efficiency, high power and high stability.

[0069] Example Four

[0070] A "frequency-doubled" frequency converter at 1550 nm is prepared, specifically:

[0071] (1) A z-cut potassium titanyl phosphate crystal is selected, a common femtosecond laser Gaussian beam (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition frequency of 25 kHz, single pulse energy of 4 μJ, and femtosecond laser scanning speed of 3 mm / s) is shaped into a femtosecond laser "strip" beam by using a spatial light modulator, and after converging through a microscope objective (magnification of 50 and numerical aperture of 0.67), the femtosecond laser "strip" beam is perpendicularly incident on the potassium titanyl phosphate crystal along the z direction of the crystal, and the ferroelectric domains in the irradiation area of the femtosecond laser are erased. By adjusting the width of the femtosecond laser "strip" beam, the duty cycle of the domain-erased structure in a single period is 0.5, so that a periodic domain-erased structure based on first-order quasi-phase matching is obtained; the theoretical calculation value of the first-order quasi-phase matching period corresponding to the 1550 nm wavelength is 25.64 μm, and therefore the width of the femtosecond laser "strip" beam is set to 25.64 μm, and the temperature control technology is used, so that the "frequency-doubled" conversion process at 1550 nm can be realized by using the periodic domain-erased structure;

[0072] (2) The experimental conditions for forming air grooves by femtosecond laser ablation on the near-surface of potassium titanyl phosphate crystals (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 12 μJ, and femtosecond laser scanning speed of 0.5 mm / s) were used. Two air grooves with a spacing of 12 μm were prepared along the direction of the periodic structure arrangement (i.e., the y direction of the potassium titanyl phosphate crystals). The dimensions of the two parallel air grooves were 2 μm (x)*16 μm (z). Using femtosecond laser-induced refractive index reduction modification conditions in potassium titanyl phosphate crystals (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 0.7 μJ, and femtosecond laser scanning speed of 5 mm / s), five refractive index reduction grooves with a size of 2 μm (x) by 4 μm (z) were prepared at the bottom of two parallel air slots along the direction of the periodic structure arrangement (i.e., the y-direction of the potassium titanyl phosphate crystal), forming a waveguide region for the ridge-type optical waveguide, thereby reducing transmission loss and improving the "frequency doubling" conversion efficiency of the frequency converter;

[0073] (3) Optically polishing the two y-end faces of the potassium titanyl phosphate crystal (i.e., the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal), polishing off approximately 0.5 mm of each face, and then cleaning the sample with a mixed solution of ethanol and acetone to obtain a polished surface;

[0074] (4) A near-infrared pulse laser (central wavelength 1550 nm) is integrated with a half-wave plate, a fiber coupling system, a potassium titanyl phosphate periodic domain erasing structure element, and a filter to achieve efficient "frequency doubling" conversion at a wavelength of 1550 nm. By rotating the half-wave plate to adjust the polarization direction of the incident fundamental frequency light so that it is consistent with the z direction of the potassium titanyl phosphate crystal, the maximum nonlinear coefficient d of the potassium titanyl phosphate crystal can be fully utilized. 33 , which helps to improve the efficiency of nonlinear frequency conversion; the filter used blocks the 1550nm fundamental frequency light and transmits the 775nm double frequency light.

[0075] Prepare a 1064nm "frequency doubling" frequency converter, specifically:

[0076] (1) A z-cut potassium titanyl phosphate crystal was selected, and a commonly used femtosecond laser Gaussian beam (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 4 μJ, and femtosecond laser scanning speed of 3 mm / s) was shaped into a femtosecond laser "strip" beam using a spatial light modulator. After being converged by a microscope objective lens (magnification of 50, numerical aperture of 0.67), the beam was vertically incident on the potassium titanyl phosphate crystal along the z direction of the crystal to erase the ferroelectric domains in the femtosecond laser irradiated area. By adjusting the width of the femtosecond laser "bar" beam, the duty cycle of the single-cycle internal domain erasing structure is set to 0.5, thus obtaining a periodic domain erasing structure based on first-order quasi-phase matching. The theoretical calculated value of the first-order quasi-phase matching period corresponding to a wavelength of 1064nm is 9.29μm, so the width of the femtosecond laser "bar" beam used is set to 9.29μm. With the help of temperature control technology, this periodic domain erasing structure can realize the 1064nm "frequency doubling" conversion process.

[0077] (2) The experimental conditions for forming air grooves by femtosecond laser ablation on the near-surface of potassium titanyl phosphate crystals (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 12 μJ, and femtosecond laser scanning speed of 0.5 mm / s) were used. Two air grooves with a spacing of 12 μm were prepared along the direction of the periodic structure arrangement (i.e., the y direction of the potassium titanyl phosphate crystals). The dimensions of the two parallel air grooves were 2 μm (x)*16 μm (z). Using femtosecond laser-induced refractive index reduction modification conditions in potassium titanyl phosphate crystals (femtosecond laser pulse width of 400 fs, wavelength of 1031 nm, repetition rate of 25 kHz, single pulse energy of 0.7 μJ, and femtosecond laser scanning speed of 5 mm / s), five refractive index reduction grooves with a size of 2 μm (x) by 4 μm (z) were prepared at the bottom of two parallel air slots along the direction of the periodic structure arrangement (i.e., the y-direction of the potassium titanyl phosphate crystal), forming a waveguide region for the ridge-type optical waveguide, thereby reducing transmission loss and improving the "frequency doubling" conversion efficiency of the frequency converter;

[0078] (3) Optically polishing the two y-end faces of the potassium titanyl phosphate crystal (i.e., the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal), polishing off approximately 0.5 mm of each face, and then cleaning the sample with a mixed solution of ethanol and acetone to obtain a polished surface;

[0079] (4) A near-infrared pulse laser (central wavelength 1064 nm) is integrated with a half-wave plate, a fiber coupling system, a potassium titanyl phosphate periodic domain erasing structure element, and a filter to achieve efficient "frequency doubling" conversion at a wavelength of 1064 nm. By rotating the half-wave plate to adjust the polarization direction of the incident fundamental frequency light so that it is consistent with the z direction of the potassium titanyl phosphate crystal, the maximum nonlinear coefficient d of the potassium titanyl phosphate crystal can be fully utilized. 33 , which helps to improve the efficiency of nonlinear frequency conversion; the filter used blocks the 1064nm fundamental frequency light and transmits the 532nm doubled frequency light.

[0080] In this embodiment, two comparative examples are used to assemble frequency converters, and a comparative experiment is conducted with the frequency converter of the potassium titanyl phosphate crystal periodic domain erasing structure prepared in Example 4:

[0081] Comparative Example 1 is different from Example 4 in that no waveguide is added, and the frequency converter is assembled with reference to Example 4; Comparative Example 2 is different from Example 4 in that the duty cycle of the domain erasure area is 1 / 5, and the frequency converter is assembled with reference to Example 4.

[0082] In Comparative Example 1, due to the lack of the optical waveguide's binding effect on the light beam, the frequency doubling conversion efficiency and output power are significantly reduced; in Comparative Example 2, since the duty cycle of the periodic domain erasing structure is set far away from 0.5, the first-order quasi-phase matching condition is not met, and the frequency doubling signal is also significantly weakened.

[0083] Experiments show that the present invention innovatively uses a shaped femtosecond laser "bar" beam to process a periodic domain erase structure on a z-cut potassium titanyl phosphate crystal. Then, based on the femtosecond laser-induced refractive index reduction modification conditions of the potassium titanyl phosphate crystal and the femtosecond laser ablation of air grooves near the surface of the potassium titanyl phosphate crystal, a ridge-type optical waveguide structure is prepared along the direction of the periodic structure arrangement, which can achieve efficient frequency conversion at a specific laser wavelength.

[0084] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0085] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily 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 is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a periodic domain erasure structure element based on potassium titanyl phosphate crystal, characterized in that: The following steps are involved: S1. Using the crystal z A femtosecond laser strip beam is incident vertically on the potassium titanyl phosphate crystal to erase the ferroelectric domain in the femtosecond laser irradiation area of ​​the potassium titanyl phosphate crystal to generate a domain erased structure; and the femtosecond laser irradiation area is translated along the y direction of the potassium titanyl phosphate crystal to obtain a domain erased structure. y A periodic domain erasing structure arranged in a direction; in the generated periodic domain erasing structure, the duty cycle of the domain erasing structure in a single period is 0.5; S2. ablate the near surface of the potassium titanyl phosphate crystal based on a femtosecond laser, and y Scanning in the direction of the first axis to prepare two parallel air grooves that pass through the periodic domain erasing structure; each air groove has a groove depth of 16 μm and a span of 2 μm, and the lateral spacing between the two parallel air grooves is 12 μm; S3. Based on the femtosecond laser, the potassium titanyl phosphate crystal is formed between two parallel air slots. y In the direction of scanning, a plurality of parallel refractive index-lowering modified grooves are prepared to obtain the waveguide region of the ridge-type optical waveguide; S4. Potassium Titanyl Phosphate Crystals y Optical polishing is performed on the two perpendicular end faces to finally obtain a potassium titanyl phosphate crystal periodic domain erasing structural element.

2. The method for preparing a periodic domain erasure structure element based on potassium titanyl phosphate crystal according to claim 1, characterized in that: In step S1, along the crystal z A femtosecond laser strip beam perpendicular to the potassium titanyl phosphate crystal is obtained by the following steps: The femtosecond laser Gaussian beam is shaped into a femtosecond laser strip beam by using a spatial light modulator, and the femtosecond laser strip beam is converged through a microscope objective lens to obtain a z A femtosecond laser strip beam is incident perpendicularly on a potassium titanyl phosphate crystal.

3. A periodic domain erasure structure element based on potassium titanyl phosphate crystal obtained by the method for preparing a periodic domain erasure structure element based on potassium titanyl phosphate crystal according to any one of claims 1 to 2, characterized in that: include: Potassium titanyl phosphate crystal, wherein the light transmission direction of the potassium titanyl phosphate crystal is along the crystal y The interior of the potassium titanyl phosphate crystal is provided with a y A periodic domain erasing structure is arranged in a y-direction, and a ridge-type optical waveguide structure penetrating the periodic domain erasing structure is provided on the surface of the potassium titanyl phosphate crystal. The light transmission direction of the ridge-type optical waveguide structure is along the y-direction of the potassium titanyl phosphate crystal.

4. The periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 3, characterized in that: The potassium titanyl phosphate crystals are z Cut potassium titanyl phosphate crystals.

5. The periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 3, characterized in that: The bottom of the ridge-type optical waveguide structure includes a plurality of refractive index-lowering grooves induced by femtosecond laser with a width of 2 μm and a depth of 4 μm.

6. A potassium titanyl phosphate crystal frequency converter, characterized in that: The invention comprises a near-infrared pulse laser, a half-wave plate, a fiber-coupled input end, a potassium titanyl phosphate crystal periodic domain erasing structure element, a fiber-coupled receiving end and a filter, which are sequentially arranged along the light beam transmission direction, wherein the potassium titanyl phosphate crystal periodic domain erasing structure element is obtained based on the preparation method of the periodic domain erasing structure element based on potassium titanyl phosphate crystal according to any one of claims 1-2.

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