Periodic domain erasure structural element preparation method based on potassium titanyl phosphate crystal and product of periodic domain erasure structural element preparation method

Abstract

The invention discloses a method for preparing a periodic domain erasure structural element based on a potassium titanyl phosphate crystal and a product thereof, and the method comprises the steps: erasing a ferroelectric domain in a femtosecond laser irradiation region by using a femtosecond laser strip-shaped beam which vertically enters the potassium titanyl phosphate crystal along the z direction of the crystal, and generating a domain erasure structure; translating a femtosecond laser irradiation area along the y direction of the potassium titanyl phosphate crystal to obtain a periodic domain erasing structure arranged along the y direction; ablating the near surface of the potassium titanyl phosphate crystal based on femtosecond laser to prepare two parallel air slots penetrating through the periodic domain erasing structure; s3, obtaining a guided wave region of the ridge-type optical waveguide based on the air slot; and performing optical polishing treatment on two end surfaces perpendicular to the y direction of the crystal body to finally obtain the periodic domain erasure structural element of the potassium titanyl phosphate crystal. According to the invention, the periodic domain structure can be prepared more quickly and efficiently, and nonlinear frequency conversion under a specific wavelength can be realized.

Description

Technical Field

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

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

[0003] Femtosecond lasers have ultra-high peak power and ultra-short pulse width. The ultra-high peak power of femtosecond lasers can stimulate a series of nonlinear interaction processes (such as multiphoton absorption, tunneling ionization, and avalanche breakdown) inside transparent optical materials. The ultra-short pulse width of femtosecond lasers can suppress the formation of heat-affected zones during processing, making femtosecond laser micro-nanoscale processing possible. Femtosecond laser direct writing technology has the characteristics of high processing accuracy, fast processing speed, and true three-dimensional processing. It has been widely used in fields such as biomedicine, materials engineering, and mechanical manufacturing.

[0004] At present, 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 fixed point in the focal area of ​​the femtosecond laser, and then a periodic ferroelectric domain erasure structure can be prepared based on the relevant phase matching theory 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 a preparation method thereof. The method is based on femtosecond laser-induced nonlinear crystal ferroelectric domain inversion technology, and a periodic domain inversion structure is prepared on a lithium niobate film (Lithium Niobate on Insulator, or LNOI). A ridge-type optical waveguide is prepared on the lithium niobate film along the direction of the periodic structure arrangement using precision diamond knife cutting technology (diamond knife cutting twice in total) or acid etching assisted femtosecond laser direct writing technology (laser scanning twice in total), thereby realizing second harmonic generation at a specific wavelength in the waveguide. The 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 on-chip high-integration frequency conversion photonic device preparation and integration, and can realize the generation of frequency-doubled signals at specific wavelengths, it is difficult to meet the related requirements for 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 solution:

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

[0011] S1. Erasing the ferroelectric domain of the femtosecond laser irradiated region in the potassium titanyl phosphate crystal by vertically incident a femtosecond laser strip beam along the z direction of the crystal to generate a domain erasing structure; and translating the femtosecond laser irradiated region along the y direction of the potassium titanyl phosphate crystal to obtain a periodic domain erasing 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 penetrate the periodic domain erasing structure;

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

[0014] S4. Perform optical polishing on the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal, and 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 a 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 by using a spatial light modulator, and the femtosecond laser strip beam is converged through a microscope objective to obtain a femtosecond laser strip beam vertically incident on a potassium titanyl phosphate crystal along a z direction of the crystal.

[0017] Furthermore, in step S1 of the method of the present invention, in the generated periodic domain erasing structure, 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 a lateral interval between two parallel air grooves is 12 μm.

[0019] On the other hand, the present invention discloses a periodic domain erasing 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 erasing structure arranged along the y-direction is arranged inside the potassium titanyl phosphate crystal, a ridge-type optical waveguide structure penetrating the periodic domain erasing structure is arranged on the surface of the potassium titanyl phosphate crystal, and 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-period internal 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 structural element disclosed in the present invention, the bottom of the ridge-type optical waveguide structure is provided with a femtosecond laser-induced refractive index reduction type notch 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 structural 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 structural element is obtained based on the preparation method of the periodic domain erasing structural element based on potassium titanyl phosphate crystal described in any one of the present invention.

[0024] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal and a product thereof, which has the following beneficial effects:

[0025] The present invention uses a shaped femtosecond laser "strip" beam to prepare a periodic domain erasing structure on the near surface of a z-cut potassium titanyl phosphate crystal. By designing and optimizing the width of the femtosecond laser "strip" beam, the duty cycle of the single-period internal domain erasing structure reaches 0.5. Under the condition of satisfying phase matching, the periodic domain structure can be prepared more quickly and efficiently and nonlinear frequency conversion at a specific wavelength can be achieved.

[0026] The present invention integrates a ridge-type optical waveguide structure directly written by a femtosecond laser into a periodic domain erasure structure induced by a femtosecond laser, which can greatly improve the efficiency of nonlinear frequency conversion. The present invention adopts a femtosecond laser-induced ferroelectric domain erasure technology, in which the thermal effect generated by the focused femtosecond laser inside the ferroelectric crystal is small and can be ignored. At the laser focus, the nonlinearity of the ferroelectric crystal may 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). In the domain erasure area, the perfect single crystal becomes completely amorphous (completely erased) or partially amorphous (partially erased). Compared with the femtosecond laser induced domain inversion (nonlinear coefficient changes from 1 to -1) technology, this technology has relatively loose requirements on processing parameters and experimental conditions (complete erasure or partial erasure is possible), and is easier to prepare periodic ferroelectric domain structures and realize nonlinear frequency conversion; in addition, the present invention uses a z-cut potassium titanyl phosphate crystal material platform, which has the characteristics of wide transmission spectrum range, strong resistance to light damage, large nonlinear coefficient, stable chemical properties, etc., which is more conducive to the preparation of high-power, high-efficiency and high-stability frequency conversion devices;

[0027] The frequency converter based on the periodic domain erasure structure of potassium titanyl phosphate crystal disclosed in the present invention 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 at a specific wavelength. It will have a wide range of applications in the fields of laser technology, integrated optics, nonlinear optics, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

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

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

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

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

[0035] Among them, 1- ferroelectric domain erasure structure, 2- near-surface air grooves of the crystal ablated by femtosecond laser, 3- z-cut potassium titanyl phosphate crystal, 4- femtosecond laser-induced refractive index reduction modified region of potassium titanyl phosphate crystal, 5- ridge-type optical waveguide structure written by femtosecond laser, 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 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] Terminology Note:

[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 increases the frequency of fundamental light by a factor of 2.

[0040] Embodiment 1

[0041] Embodiment 1 of the present invention discloses a periodic domain erasing structure element based on potassium titanium phosphate crystal, which is used to realize frequency conversion, such as Figure 1-Figure 4 ,include:

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

[0043] A femtosecond laser-induced periodic ferroelectric domain erasing structure 1 arranged along the y direction is provided inside the z-cut potassium titanyl phosphate crystal;

[0044] A ridge-type optical waveguide structure 2 directly written by a femtosecond laser is provided along the arrangement direction of the periodic ferroelectric domain erasing 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 single-cycle intra-domain erase structure approaches 0.5. The closer the duty cycle is to 0.5, the higher the conversion efficiency. When the duty cycle of the single-cycle intra-domain erase structure 1 is 0.5, the conversion efficiency reaches the highest.

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

[0047]

[0048] Where Λ is the quasi-phase matching period, 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 fundamental frequency light and double frequency light respectively; the dispersion equation is used to calculate the wavelength λ of a certain incident fundamental frequency light ω The refractive index of potassium titanyl phosphate crystal in the z direction is:

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

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

[0051] In order to further implement the above technical scheme, a ridge-type optical waveguide that guides light along the y-direction of the potassium titanyl phosphate crystal has two parallel air grooves arranged along the x-direction with a size of 2μm(x)*16μm(z), and the lateral spacing (the distance along the x-direction of the crystal) of the two parallel air grooves is 12μm. The ridge-type optical waveguide region 5 is between the two parallel air grooves, and the bottom area between the two air grooves is provided with 5 femtosecond laser-induced refractive index-reducing grooves with a size of 2μm(x)*4μm(z) to generate a 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 are optically polished, and a femtosecond laser is incident along the z direction of the potassium titanyl phosphate crystal to induce a domain erasure structure, and the incident near-infrared fundamental frequency light and the emitted frequency-doubled laser are transmitted along the y direction of the potassium titanyl phosphate crystal.

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

[0055] Embodiment 2

[0056] like Figure 5 As shown, Example 2 discloses a method for preparing a periodic domain erasing structure element based on potassium titanium 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 vertically incident on a potassium titanyl phosphate crystal along the z-direction of the crystal to erase the ferroelectric domains in the femtosecond laser irradiated area and generate a single-period internal domain erase structure. The duty cycle of the single-period internal domain erase structure is controlled by adjusting the width of the femtosecond laser strip beam, and finally a periodic domain erase structure arranged along the y-direction is obtained.

[0058] S2. Use a microscope objective to focus the original femtosecond laser Gaussian beam onto the potassium titanyl phosphate crystal. Based on the processing parameters of femtosecond laser ablation of air grooves near the surface of the potassium titanyl phosphate crystal, two parallel air grooves are prepared by scanning 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 of the femtosecond laser induced refractive index reduction modification inside the potassium titanyl phosphate crystal, five parallel refractive index reduction modification scratches are prepared by scanning at the bottom of the two parallel air grooves along the arrangement direction of the periodic ferroelectric domain structure, i.e., the y direction of the potassium titanyl phosphate crystal, to generate a femtosecond laser induced refractive index reduction modification region 4 of the potassium titanyl phosphate crystal, thereby forming a waveguide region of the ridge-type optical waveguide.

[0059] In this step, the refractive index reduction type modified scratch is set at the bottom of the middle area of ​​the two air grooves. If the air groove is a groove running from left to right, then the refractive index reduction type modified scratch is also running from left to right. A single scratch is 4 microns long and 2 microns wide. The five scratches are closely arranged, and the scratches are spaced 2 microns apart from each other, which can just fill the gap between the bottoms of the two parallel air grooves.

[0060] S3. Optically polish the two y-end faces of the potassium titanyl phosphate crystal, that is, 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 of a certain wavelength, the present invention sets a specific strip beam width. Then, with a period of twice the width of the strip beam, the duty cycle reaches 0.5, and hundreds or thousands of domain erasing structures are prepared along the transmission direction of the "laser of a certain wavelength", so that efficient frequency conversion can be achieved. If frequency conversion of other wavelengths is to be achieved, it is necessary to calculate and readjust the width of the strip beam, and prepare a periodic structure with a period of twice the width of the strip beam to achieve frequency conversion.

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

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

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

[0065] Embodiment 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 a linearly polarized laser, 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). 33, thereby improving the frequency conversion efficiency), and then the laser is coupled into the periodic domain erasing 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 doubling light) emitted from the ridge optical waveguide together, and finally filters the light through the filter to isolate the pump light and transmit the signal light, thereby realizing the laser frequency conversion function at a specific wavelength.

[0068] The frequency converter is easy to operate. It only needs to turn on the near-infrared pulse laser and rotate the half-wave plate to adjust the direction of the incident polarized light to be consistent with the z direction of the potassium titanyl phosphate crystal (corresponding to the maximum nonlinear optical coefficient d 33 ), it is possible to realize efficient nonlinear frequency conversion under the wavelength laser in the ridge optical waveguide. The advantages of the present invention are mainly reflected in the following three aspects: first, by using the shaped "strip" beam of the femtosecond laser, a periodic domain erasure structure can be quickly and efficiently prepared, and nonlinear frequency conversion of lasers of specific wavelengths can be realized; in addition, by using 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 the femtosecond laser), the transmission loss of the device can be reduced and the output performance of the frequency conversion device can be improved; finally, the frequency converter is based on the z-cut potassium titanyl phosphate crystal material platform, which has a large nonlinear optical coefficient, strong resistance to light damage, and stable mechanical properties, making it easier to realize a high-efficiency, high-power, and high-stability nonlinear frequency conversion process.

[0069] Embodiment 4

[0070] Prepare a 1550nm "double frequency" frequency converter, specifically:

[0071] (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 "bar" beam using a spatial light modulator. After being converged by a microscope objective (magnification of 50, numerical aperture of 0.67), the beam was vertically incident into 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 "strip" beam, the duty cycle of the single-period internal domain erasing structure is set to 0.5, thereby 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 1550nm is 25.64μm, so the width of the femtosecond laser "strip" beam used is set to 25.64μm, and with the help of temperature control technology, the periodic domain erasing structure can be used to realize the 1550nm "frequency doubling" conversion process;

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

[0073] (3) performing optical polishing on 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 about 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 pulsed 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. The polarization direction of the incident fundamental frequency light is adjusted by rotating the half-wave plate to keep it consistent with the z direction of the potassium titanyl phosphate crystal, thereby making full use of the maximum nonlinear coefficient d of the potassium titanyl phosphate crystal. 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 "double frequency" 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 "bar" beam using a spatial light modulator. After being converged by a microscope objective (magnification of 50, numerical aperture of 0.67), the beam was vertically incident into 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 "strip" beam, the duty cycle of the single-period internal domain erasing structure is set to 0.5, thereby 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 the 1064nm wavelength is 9.29μm, so the width of the femtosecond laser "strip" beam used is set to 9.29μm, and with the help of temperature control technology, the periodic domain erasing structure can be used to realize the 1064nm "frequency doubling" conversion process;

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

[0078] (3) performing optical polishing on 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 about 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. The polarization direction of the incident fundamental frequency light is adjusted by rotating the half-wave plate to keep it consistent with the z direction of the potassium titanyl phosphate crystal, so that 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 double frequency light.

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

[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 and does not meet the first-order quasi-phase matching conditions, the frequency doubling signal is also significantly weakened.

[0083] Experiments show that the present invention innovatively uses a shaped femtosecond laser "strip" beam to process a periodic domain erasure structure on a z-cut potassium titanyl phosphate crystal, and then based on the femtosecond laser-induced refractive index reduction modification conditions of the potassium titanyl phosphate crystal and the femtosecond laser ablation of near-surface air grooves in 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] 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.

[0085] 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 method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal, characterized in that: The following steps are involved: S1. Erasing the ferroelectric domain of the femtosecond laser irradiated region in the potassium titanyl phosphate crystal by vertically incident a femtosecond laser strip beam along the z direction of the crystal to generate a domain erasing structure; and translating the femtosecond laser irradiated region along the y direction of the potassium titanyl phosphate crystal to obtain a periodic domain erasing structure arranged along the y direction; 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 penetrate the periodic domain erasing structure; S3. Based on a femtosecond laser, a plurality of parallel refractive index-lowering modified notches are prepared by scanning along the y direction of the potassium titanyl phosphate crystal between two parallel air grooves to obtain a waveguide region of a ridge-type optical waveguide; S4. Perform optical polishing on the two end faces perpendicular to the y-direction of the potassium titanyl phosphate crystal, and finally obtain a potassium titanyl phosphate crystal periodic domain erasing structure element.

2. The method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 1, characterized in that: In step S1, a femtosecond laser strip beam vertically incident on a potassium titanyl phosphate crystal along the z direction of the crystal is obtained by the following steps: A 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 to obtain a femtosecond laser strip beam vertically incident on a potassium titanyl phosphate crystal along a z direction of the crystal.

3. The method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 1, characterized in that: In step S1, in the generated periodic domain erasing structure, the duty cycle of the single-period intra-domain erasing structure is 0.

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4. The method for preparing a periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 1, characterized in that: In step S2, each air groove prepared has a groove depth of 16 μm and a span of 2 μm, and a lateral interval between two parallel air grooves is 12 μm.

5. A periodic domain erasing structure element based on potassium titanium phosphate crystal, characterized in that: include: 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 erasing structure arranged along the y-direction is arranged inside the potassium titanyl phosphate crystal, a ridge-type optical waveguide structure penetrating the periodic domain erasing structure is arranged on the surface of the potassium titanyl phosphate crystal, and the light transmission direction of the ridge-type optical waveguide structure is along the y-direction of the potassium titanyl phosphate crystal.

6. A periodic domain erasing structural element based on potassium titanyl phosphate crystal according to claim 5, characterized in that: In the periodic domain erasing structure, the duty cycle of the single-period internal domain erasing structure is 0.

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7. A periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 5, characterized in that: The potassium titanyl phosphate crystal is a z-cut potassium titanyl phosphate crystal.

8. The periodic domain erasing structure element based on potassium titanyl phosphate crystal according to claim 5, 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.

9. A potassium titanyl phosphate crystal frequency converter, characterized in that: It comprises a near-infrared pulse laser, a half-wave plate, a fiber-coupled input end, a potassium titanyl phosphate crystal periodic domain erasing structural 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 structural element is obtained based on the preparation method of the periodic domain erasing structural element based on potassium titanyl phosphate crystal according to any one of claims 1 to 4.

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