Method for designing ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide

Abstract

The invention provides a method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide, which comprises the following steps of: carrying out modeling and mode analysis on the lithium niobate waveguide according to specific numerical values or specific conditions of various parameters of the preset lithium niobate waveguide so as to obtain an effective refractive index of a required mode; according to the effective refractive index, determining the phase mismatch amount required under different wavelengths so as to obtain the phase compensation amount required to be compensated for by the lithium niobate waveguide; according to the phase compensation amount, a preset required working frequency band and a target output spectrum, obtaining final required lithium niobate waveguide polarization period distribution by using an iteration method; the method has the following beneficial effects that the polarization period distribution of the lithium niobate waveguide can be automatically designed, and the problem of uneven efficiency of the traditional linear chirp polarization lithium niobate in ultra-wideband frequency doubling application is solved, so that the working bandwidth of frequency conversion of the lithium niobate waveguide can be widened to hundreds of nanometers or even thousands of nanometers.

Description

Technical Field

[0001] The present invention relates to the technical field of optical waveguide frequency conversion, and in particular to a method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide. Background Art

[0002] Ultra-wideband frequency conversion is a key approach to realizing new light sources such as ultra-wideband ultra-white light and visible-band optical frequency combs. Waveguide-based frequency conversion methods, such as frequency doubling and optical parametric amplification, offer numerous advantages, including low threshold power, high efficiency, and high integration. Furthermore, through dispersion structure design, they can address frequency conversion requirements across various frequency bands, from the ultraviolet to the mid-infrared. Although adjusting the waveguide structure allows for significant adjustments to the phase-matching band, thereby adjusting the operating frequency band, the phase-matching bandwidth of a single waveguide remains relatively narrow, resulting in a similarly narrow operating bandwidth, which cannot meet the requirements of ultra-wideband frequency conversion. Thin-film lithium niobate waveguides, however, effectively overcome this limitation. Quasi-phase matching can be achieved in lithium niobate by externally inverting the polarization of the ferroelectric domains. Consequently, the bandwidth of nonlinear conversion is no longer constrained by the inherent phase matching of the material. This approach, known as chirped-polarized lithium niobate, effectively addresses the need for frequency conversion with bandwidths on the order of tens of nanometers. However, for applications requiring even wider bandwidths of hundreds of nanometers, conventional chirped-polarized lithium niobate designs fail to account for the specific dispersion of the waveguide and the inherent differences in optical nonlinear efficiency across different frequency bands. Consequently, they are unable to guarantee flat conversion efficiency, reducing the quality of the resulting output spectrum. Therefore, a new lithium niobate waveguide design is needed to achieve ultra-broadband flat conversion efficiency. Summary of the Invention

[0003] In view of the shortcomings of the prior art described above, the purpose of the present invention is to provide a method for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling, so as to address the demand for wider-band applications of hundreds of nanometers. The design of conventional chirped-polarized lithium niobate does not correct for the specific dispersion of the waveguide and the differences in optical nonlinear efficiency in different frequency bands, and therefore cannot guarantee flat conversion efficiency, thereby reducing the quality of the final output spectrum.

[0004] To achieve the above-mentioned and other related purposes, the present invention provides the following technical solutions:

[0005] A method for designing an ultra-wideband frequency-doubling chirped-polarized lithium niobate waveguide comprises the following steps: obtaining specific values or conditions of various parameters of the lithium niobate and the lithium niobate waveguide that are preset; modeling the lithium niobate waveguide according to the specific values or conditions of the various parameters, performing mode analysis on the modeled lithium niobate waveguide, obtaining an effective refractive index of a desired mode based on the mode analysis results, and determining a wavelength dependence curve of an effective wave loss based on the effective refractive index; determining a required phase mismatch at different wavelengths based on the wavelength dependence curve of the effective wave loss, and determining a phase compensation amount required for the lithium niobate waveguide based on the phase mismatch;

[0006] A predetermined desired operating frequency band and target output spectrum are obtained; based on the phase compensation amount, the desired operating frequency band and the target output spectrum, an iterative method is used to obtain the final desired lithium niobate waveguide polarization period distribution, and an estimated value of the frequency doubling efficiency of each frequency is simulated based on the designed final desired lithium niobate waveguide polarization period distribution.

[0007] In one embodiment of the present invention, the parameters of the lithium niobate include the tangent direction, doping, film thickness, substrate and cover layer of the lithium niobate, and the parameters of the lithium niobate waveguide include etching depth, etching tilt angle, length and width.

[0008] In one embodiment of the present invention, the lithium niobate waveguide is modeled according to the specific values or specific conditions of the various parameters, and a mode analysis is performed on the modeled lithium niobate waveguide, and the effective refractive index of the desired mode is obtained according to the mode analysis results. The steps include: using a finite element analysis method in Comsol software to model the lithium niobate waveguide according to the specific values or specific conditions of the various parameters, performing a parametric scan on the wavelength of light, and using the mode analysis function of the software to perform a mode analysis on the modeled lithium niobate waveguide to obtain the effective refractive index of the desired mode.

[0009] In one embodiment of the present invention, determining the curve of effective wave loss dependence on wavelength based on the effective refractive index includes: fitting the simulated data points according to the effective refractive index to obtain a curve of effective refractive index dependence on wavelength, and obtaining a curve of effective wave loss dependence on wavelength based on the curve of effective refractive index dependence on wavelength.

[0010] In one embodiment of the present invention, a mode analysis is performed on the modeled lithium niobate waveguide, and the mode volume size of the desired mode is also obtained based on the mode analysis results. The mode volume size is obtained by the ratio of the square of the full surface integral of the square of the electric field magnitude to the full surface integral of the fourth power of the electric field magnitude.

[0011] In one embodiment of the present invention, the final desired lithium niobate waveguide polarization period distribution is obtained according to the phase compensation amount, the required operating frequency band and the target output spectrum using an iterative method, including: obtaining a preset constant chirp rate of the lithium niobate waveguide, and determining an initial lithium niobate waveguide polarization period distribution according to the constant chirp rate; iteratively processing the initial lithium niobate waveguide polarization period distribution according to the phase compensation amount, the required operating frequency band and the target output spectrum, and obtaining the final desired lithium niobate waveguide polarization period distribution according to the iterative processing result.

[0012] In one embodiment of the present invention, the iterative processing of the initial lithium niobate waveguide polarization period distribution according to the phase compensation amount, the required operating frequency band, and the target output spectrum, and obtaining the final required lithium niobate waveguide polarization period distribution according to the iterative processing result, includes: calculating a frequency-doubled output according to a constant chirp rate of the lithium niobate waveguide using an approximate analytical solution, comparing the difference between the frequency-doubled output and the target output spectrum, and adjusting the constant chirp rate according to the difference to compensate for the difference;

[0013] A new lithium niobate waveguide polarization period distribution is calculated based on the adjusted chirp rate, and a new frequency-doubled output is calculated again using an approximate analytical solution. The difference between the new frequency-doubled output and the target output spectrum is compared, and the adjusted chirp rate is adjusted again based on the difference to compensate for the difference. The iteration is repeated until the calculated frequency-doubled output is consistent with the target output spectrum. The lithium niobate waveguide polarization period distribution obtained at this time is the final desired lithium niobate waveguide polarization period distribution.

[0014] In one embodiment of the present invention, in simulating the polarization period distribution of the lithium niobate waveguide required by the design to obtain the estimated value of the frequency doubling efficiency of each frequency, a fast theoretical calculation value and a slow numerical simulation can be selected.

[0015] As described above, the method of the present invention for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling has the following beneficial effects: starting from the effective refractive index of the lithium niobate waveguide, the present invention calculates the required phase mismatch at different wavelengths, combines the lithium niobate waveguide polarization period distribution pre-assumed or calculated in the previous step, calculates the frequency doubling output result, compares the difference between the frequency doubling output result and the target output spectrum, and adjusts the local chirp rate according to the difference to further obtain a new lithium niobate waveguide polarization period distribution. After repeated iterations, the final required lithium niobate waveguide polarization period distribution is obtained, thereby automatically designing the polarization period distribution of the lithium niobate waveguide, solving the problem of uneven efficiency of traditional linear chirped-polarized lithium niobate in ultra-wideband frequency doubling applications, broadening the working bandwidth of the lithium niobate waveguide frequency conversion to hundreds or even thousands of nanometers, while greatly simplifying the design process of the lithium niobate waveguide and reducing the design cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Shown is a schematic diagram of the overall process of the method for designing an ultra-wideband frequency-doubling chirped-polarized lithium niobate waveguide disclosed in an embodiment of the present invention;

[0017] Figure 2 Shown is a mode field distribution diagram at a wavelength of 1500 nm in the method for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling disclosed in an embodiment of the present invention;

[0018] Figure 3 A schematic diagram showing the effective refractive index at various wavelengths in the method for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling disclosed in an embodiment of the present invention;

[0019] Figure 4 The diagram shows the polarization period distribution of the lithium niobate waveguide ultimately required in the method for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling disclosed in an embodiment of the present invention;

[0020] Figure 5 Shown is a comparison diagram of output pulses before and after optimization in the method for designing a chirped-polarized lithium niobate waveguide for ultra-wideband frequency doubling disclosed in an embodiment of the present invention;

[0021] Figure 6 Shown is an output comparison diagram of the polarization period distribution designed by the present invention and the traditional linear chirp (chirp rate is constant) in the method for designing ultra-wideband frequency doubling chirped polarized lithium niobate waveguide disclosed in an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. It should be noted that the following embodiments and features in the embodiments can be combined with each other unless there is a conflict.

[0023] The present invention relates to a method for designing ultra-wideband frequency-doubling chirped polarized lithium niobate waveguides, and also a method for automatically designing the waveguide polarization period based on the waveguide mode field effective refractive index and the required operating frequency band, thereby widening the operating bandwidth of the lithium niobate waveguide frequency conversion to hundreds or even thousands of nanometers. The process is as follows: Figure 1 As shown, the details are as follows:

[0024] Step 101: Obtain specific values or specific conditions of various parameters of lithium niobate and lithium niobate waveguide that are preset.

[0025] Specifically, the various parameters of the lithium niobate include the tangent, doping, film thickness, substrate and covering layer of the lithium niobate, and the various parameters of the lithium niobate waveguide include etching depth, etching tilt angle, length and width, wherein the lithium niobate waveguide is made of lithium niobate and has a long strip shape.

[0026] More specifically, the lithium niobate tangent, doping, film thickness, substrate and cover layer conditions, and the lithium niobate waveguide structure, such as the etching depth, etching tilt angle, length and width of the ridge straight waveguide, are set. The lithium niobate doping refers to the commonly used undoped lithium niobate and 5% magnesium oxide doped lithium niobate. For other special lithium niobates, the refractive index curve and second-order nonlinear coefficient need to be directly set. The lithium niobate tangent is for conventional tangents such as X-cut or Z-cut. The calculation defaults to using the largest d33 second-order nonlinear component of lithium niobate for subsequent analysis, that is, analyzing the TE00 mode of X-cut lithium niobate or the TM00 mode of Z-cut lithium niobate. The film thickness is for lithium niobate films of 5 microns or less. Thicker films can be regarded as lithium niobate bulk materials and the refractive index curve of the lithium niobate bulk material is directly used. The spot diameter is additionally input, and the mode analysis in step 102 is skipped. The lithium niobate waveguide structure is not limited to ridge straight waveguides.

[0027] Step 102: Model the lithium niobate waveguide according to the specific values or specific conditions of each parameter, and perform mode analysis on the modeled lithium niobate waveguide. According to the mode analysis results, the effective refractive index of the required mode is obtained, and the dependence curve of the effective wave loss on the wavelength is determined according to the effective refractive index.

[0028] Specifically, the lithium niobate waveguide is modeled using the finite element analysis method in the COMSOL software according to the specific values or specific conditions of each parameter, a parametric sweep is performed on the wavelength of light, and the mode analysis function of the software is used to perform mode analysis on the modeled lithium niobate waveguide to obtain the effective refractive index of the desired mode. The effective refractive index is then fitted to the simulated data points to obtain a curve of the effective refractive index's dependence on wavelength. Based on the curve of the effective refractive index's dependence on wavelength, a curve of the effective wave loss's dependence on wavelength is obtained.

[0029] In practical applications, the finite element analysis method in the COMSO software is used to model the cross section of the lithium niobate waveguide, and a parametric sweep is performed on the wavelength of light. The mode analysis function of the software is used to obtain the effective refractive index and mode volume size of the E-light fundamental mode propagation mode in the waveguide at various wavelengths. The effective refractive index is fitted to the simulated data points to obtain the wavelength dependence curve of the effective refractive index. eff (λ), that is, the dispersion curve, the result is divided by the wavelength to further obtain the dependence curve of effective wave loss on wavelength β eff (λ)=2πn eff (λ) / λ.

[0030] It should also be noted that the COMSOL software uses free triangle mesh modeling by default, using a very fine mesh for the lithium niobate waveguide and a finer mesh for the bottom and cover layer; the mode volume size is obtained by default from the ratio of the square of the full surface integral of the square of the electric field magnitude to the full surface integral of the fourth power of the electric field magnitude.

[0031] Step 103: Determine the required phase mismatch at different wavelengths based on the wavelength dependence curve of the effective wave loss, determine the phase compensation amount required for the lithium niobate waveguide based on the phase mismatch, and then obtain the preset required working frequency band and target output spectrum.

[0032] Specifically, the upper and lower limits of the required working frequency band and the target output spectrum (such as flattening) are set, and the effective wavelength loss dependence curve β obtained in step 102 is used to calculate the wavelength dependence curve β. eff (λ), the required phase mismatch Δβ(λ3) at different wavelengths is calculated, which is equal to the double frequency wave mismatch minus twice the fundamental frequency wave mismatch Δβ(λ3) = β eff (λ3)-2β eff (λ3 / 2), λ3 is the wavelength of the doubled frequency light. In order to compensate for the phase mismatch, the required polarization period is 2π multiplied by the inverse of the phase mismatch, that is, Λ(λ3)=2π / Δβ(λ3). The calculation formula for the phase compensation here is for the wavelength. In the subsequent step 104, an iterative method will be used to obtain the final required waveguide polarization period distribution Λ(z) according to the required operating frequency band.

[0033] Step 104 : According to the phase compensation amount, the required operating frequency band and the target output spectrum, an iterative method is used to obtain the final required polarization period distribution of the lithium niobate waveguide.

[0034] Specifically, a preset constant chirp rate of the lithium niobate waveguide, which can also be said to be a constant chirp rate, is obtained, and an initial lithium niobate waveguide polarization period distribution is determined based on the constant chirp rate; then, the initial lithium niobate waveguide polarization period distribution is iteratively processed based on the phase compensation amount, the required operating frequency band, and the target output spectrum, and the final required lithium niobate waveguide polarization period distribution is obtained based on the iterative processing result.

[0035] More specifically, the lithium niobate waveguide structure is set as a chirped polarization waveguide with a constant chirp rate, and an approximate analytical solution is used to calculate a frequency-doubled output based on the constant chirp rate of the lithium niobate waveguide. The difference between the frequency-doubled output and the target output spectrum is compared, and the constant chirp rate is adjusted based on the difference to compensate for the difference. A new lithium niobate waveguide polarization period distribution is calculated based on the adjusted chirp rate, and an approximate analytical solution is used again to calculate a new frequency-doubled output. The difference between the new frequency-doubled output and the target output spectrum is compared, and the adjusted chirp rate is adjusted again based on the difference to compensate for the difference. Iteration is repeated until the calculated frequency-doubled output is consistent with the target output spectrum, and the lithium niobate waveguide polarization period distribution Λ(z) obtained at this time is the final desired lithium niobate waveguide polarization period distribution.

[0036] Where Λ(z) represents the polarization period of the chirped polarized lithium niobate waveguide at the length z position, then the phase compensation amount G provided by the waveguide at the length z position is m (z) = 2π / Λ(z), where the phase compensation amount is calculated based on the chirp rate D of the waveguide at each position along the length. c (z) can be expressed as the derivative of the phase compensation with respect to the length: Further considering the propagation behavior of ultra-wideband light in a chirped polarization material with a length of L, the frequency-doubled output spectrum in the present invention can be described by the following approximate analytical solution: In the formula, In the formula represents the electric field intensity of the frequency-doubled light with wavelength λ3 at the end point L of the waveguide, The fundamental frequency light with wavelength λ1 is a known quantity at the starting point of the waveguide. The value of is related to the polarization structure of the waveguide, and its meaning is When , the phase compensation amount provided by the chirped polarization waveguide just compensates the frequency doubling process from λ3 / 2 to λ3, that is,

[0037] Using this approximate analytical solution, we can directly obtain The frequency-doubled output spectrum of the waveguide with a polarization periodic structure of Λ(z) under the input is obtained without the need to use complex numerical simulation to slowly obtain the result of the frequency-doubled output, and the formula directly indicates the chirp rate It is a key parameter for regulating the frequency doubling efficiency.

[0038] It should also be noted that, if the calculated polarization period Λ(λ) in step 104 is smaller than the minimum size allowed by the manufacturing process, the required polarization period can be calculated using n-order phase matching according to the process limitations, i.e., using Λ n(λ)=nΛ(λ), where n is an odd number for phase compensation. The calculation in step 104 only considers the influence of the set n-order phase matching. Also, the waveguide polarization period designed in step 104 defaults to changing from large to small, that is, the phase mismatch of the long wave is compensated first, and then the phase mismatch of the short wave is compensated.

[0039] Step 105 , simulating the final desired polarization period distribution of the lithium niobate waveguide obtained by design to obtain an estimated value of the frequency doubling efficiency at each frequency.

[0040] Specifically, in the frequency doubling efficiency estimation values of various frequencies obtained by simulating the final desired lithium niobate waveguide polarization period distribution obtained by design, fast theoretical calculation values and slow numerical simulations can be selected. In this embodiment, the fast theoretical calculation values are used by default.

[0041] In practical applications, the above steps are explained in detail with examples, taking the flattened output application as an example: (1) Frequency doubling of ultrashort pulses in lithium niobate nanowaveguides:

[0042] Step 1: Parameter input: Set the material to an x-cut lithium niobate ridge straight waveguide with a film thickness of 710nm, an etching depth of 360nm, a width of 1800nm, and a length of 5mm. The expected operating frequency band is 600nm to 900nm, and the maximum and minimum chirp rates should differ within 10 times, making the range as wide as possible. The input pulse is a Gaussian pulse with a central wavelength of 1500nm and a pulse width of 15fs.

[0043] Step 2: Mode analysis: Use COMSO software to analyze the mode of the nanowaveguide, and the mode field distribution at a wavelength of 1500nm is as follows Figure 2 As shown, the effective refractive index at each wavelength is Figure 3 As shown, the effective refractive index curve equation obtained by using a 10th-order polynomial fitting is: eff (λ[nm])=2.18878549508774+0.110893957758470λ+0.0122159796241235λ 2 +0.0136342650075203λ 3 -0.000812889133266928λ 4 +0.00603786956562744λ 5 -0.00262952408692764λ 6 -0.00163464367771174λ 7 +0.000936542365633835λ 8 +0.000683403637380663λ 9-0.000293263580053048λ 10 , we can further get the dependence curve of effective wave loss on wavelength β eff (λ)=2πn eff (λ) / λ, and the phase mismatch curve Δβ(λ3)=β eff (λ3)-2β eff (λ3 / 2);

[0044] Step 3: Calculate the polarization period distribution: 1) Initially set the waveguide polarization period distribution to a constant chirp rate D c , the corresponding waveguide polarization period distribution is Λ(0)=Δβ(λ3=900nm)=5.092μm, D c =4.825e7m -2 ;

[0045] 2) Calculate the new polarization period distribution and obtain the frequency-doubled output spectrum based on the approximate analytical solution in the principle. Flattening application requires that the frequency-doubled output spectrum be a constant, that is, thereby remember get Right now get

[0046] In order to ensure appropriate output efficiency, the flattening requirement stipulates that the maximum chirp rate should be within 10 times the minimum value, so that |A const The value size and working bandwidth range of | are Further get Also because Can get By fitting the result with a polynomial, we can get the adjusted waveguide structure:

[0047] 3) Use the polarization period distribution obtained in step 2) to replace the original polarization period distribution, return to step 2) and iterate 5 times. The final polarization period distribution Λ(z)=Λ (5) (z) Figure 4 As shown in the figure, the output pulse comparison before and after optimization is as follows: Figure 5 As shown, in Figure 5 In the figure, the solid line is the approximate analytical solution, and the dotted line is the rigorous simulation result.

[0048] (2) Frequency doubling of continuous light in lithium niobate bulk crystals:

[0049] Step 1: Parameter input: Set the material to 5cm lithium niobate bulk material, and the goal is to have a flat frequency-doubled efficiency for continuous light with a unit intensity of 1300nm to 2300nm;

[0050] Step 2: Mode analysis: The effective refractive index of lithium niobate bulk is the refractive index of the material itself for e-light, i.e. We can further obtain the dependence curve of effective wave loss on wavelength β eff (λ)=2πn eff (λ) / λ, and the phase mismatch curve Δβ(λ3)=β eff (λ3)-2β eff (λ3 / 2);

[0051] Step 3: Calculate the polarization period distribution: 1) Since the input light is a continuous light with unit light intensity, is a constant and has nothing to do with the waveguide structure, and the flattening requirement is simplified to Right now get Thus there is Also because Can get By fitting the result with a polynomial, the final waveguide structure can be obtained as follows:

[0052] Figure 6 Comparing the output of the polarization period distribution designed by the present invention with that of the traditional linear chirp (chirp rate is constant), it can be seen that the lithium niobate with a gradient of polarization chirp rate designed according to the present invention can achieve nearly flat frequency doubling efficiency for the fundamental frequency light of 1300nm to 2300nm, while the lithium niobate produced using the traditional linear chirp method also has the obvious problem of high efficiency for short waves and low efficiency for long waves.

[0053] In summary, the present invention starts from the effective refractive index of the lithium niobate waveguide, calculates the required phase mismatch at different wavelengths, combines the lithium niobate waveguide polarization period distribution pre-assumed or calculated in the previous step, calculates the frequency doubling output result, compares the difference between the frequency doubling output result and the target output spectrum, and adjusts the local chirp rate according to the difference to further obtain a new lithium niobate waveguide polarization period distribution. After repeated iterations, the final required lithium niobate waveguide polarization period distribution is obtained, thereby automatically designing the polarization period distribution of the lithium niobate waveguide, solving the problem of uneven efficiency of traditional linear chirped polarized lithium niobate in ultra-wideband frequency doubling applications, broadening the working bandwidth of lithium niobate waveguide frequency conversion to hundreds or even thousands of nanometers, while greatly simplifying the design process of the lithium niobate waveguide and reducing the design cost.

[0054] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Any equivalent modifications or variations made by persons skilled in the art without departing from the spirit and technical concepts disclosed herein shall be encompassed by the claims of the present invention.

Claims

1. A method for designing a chirped polarized lithium niobate waveguide for ultra-wideband frequency doubling, characterized in that: The following steps are involved: Obtaining specific values or specific conditions of various parameters of the lithium niobate and lithium niobate waveguide that are preset; Modeling the lithium niobate waveguide according to the specific values or specific conditions of the various parameters, performing mode analysis on the modeled lithium niobate waveguide, obtaining an effective refractive index of a desired mode according to the mode analysis results, and determining a wavelength dependence curve of an effective wave loss according to the effective refractive index; Determining the required phase mismatch amount at different wavelengths according to the wavelength dependence curve of the effective wave loss, and determining the phase compensation amount required for the lithium niobate waveguide according to the phase mismatch amount; Obtaining the preset required working frequency band and target output spectrum; According to the phase compensation amount, the required operating frequency band and the target output spectrum, an iterative method is used to obtain the final required lithium niobate waveguide polarization period distribution, and the estimated value of the frequency doubling efficiency of each frequency is simulated based on the final required lithium niobate waveguide polarization period distribution obtained by design.

2. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 1, characterized in that: The various parameters of the lithium niobate include the tangent direction, doping, film thickness, substrate and cover layer of the lithium niobate, and the various parameters of the lithium niobate waveguide include etching depth, etching tilt angle, length and width.

3. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 1, characterized in that: Modeling the lithium niobate waveguide according to the specific values or specific conditions of the various parameters, performing mode analysis on the modeled lithium niobate waveguide, and obtaining the effective refractive index of the desired mode according to the mode analysis results, includes: The lithium niobate waveguide is modeled using the finite element analysis method in the Comsol software according to the specific values or specific conditions of the various parameters, a parametric sweep is performed on the wavelength of light, and a mode analysis function of the software is used to perform mode analysis on the modeled lithium niobate waveguide to obtain the effective refractive index of the desired mode.

4. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 3, wherein: Determining a curve of dependence of effective wave loss on wavelength according to the effective refractive index includes: The effective refractive index is fitted to the simulated data points according to the effective refractive index to obtain a curve of dependence of the effective refractive index on the wavelength, and the effective wavelength loss is obtained according to the curve of dependence of the effective refractive index on the wavelength.

5. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 4, characterized in that: After performing mode analysis on the modeled lithium niobate waveguide, the mode volume size of the required mode is also obtained based on the mode analysis results. The mode volume size is obtained by the ratio of the square of the full surface integral of the square of the electric field magnitude to the full surface integral of the fourth power of the electric field magnitude.

6. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 1, characterized in that: The method of obtaining the final desired polarization period distribution of the lithium niobate waveguide according to the phase compensation amount, the required operating frequency band, and the target output spectrum by an iterative method includes: Obtaining a preset constant chirp rate of the lithium niobate waveguide, and determining an initial polarization period distribution of the lithium niobate waveguide according to the constant chirp rate; The initial lithium niobate waveguide polarization period distribution is iteratively processed according to the phase compensation amount, the required operating frequency band and the target output spectrum, and the final required lithium niobate waveguide polarization period distribution is obtained according to the iterative processing result.

7. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 6, characterized in that: The iterative processing of the initial lithium niobate waveguide polarization period distribution according to the phase compensation amount, the required operating frequency band, and the target output spectrum, and obtaining the final required lithium niobate waveguide polarization period distribution according to the iterative processing result, includes: Calculating a frequency-doubled output using an approximate analytical solution based on a constant chirp rate of the lithium niobate waveguide, comparing a difference between the frequency-doubled output and a target output spectrum, and adjusting the constant chirp rate based on the difference to compensate for the difference; Calculating a new polarization period distribution of the lithium niobate waveguide based on the adjusted chirp rate, again calculating a new frequency-doubled output using an approximate analytical solution, comparing the new frequency-doubled output with a target output spectrum, and further adjusting the adjusted chirp rate based on the difference to compensate for the difference; Iterations are repeated until the calculated frequency-doubled output is consistent with the target output spectrum. At this time, the obtained lithium niobate waveguide polarization period distribution is the final required lithium niobate waveguide polarization period distribution.

8. The method for designing an ultra-wideband frequency-doubling chirped polarized lithium niobate waveguide according to claim 1, characterized in that: In the above-mentioned simulation of the polarization period distribution of the lithium niobate waveguide required finally obtained according to the design to obtain the estimated value of the frequency doubling efficiency of each frequency, a fast theoretical calculation value and a slow numerical simulation can be selected.

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