Super-continuum spectrum light source based on lithium niobate film waveguide and manufacturing method of super-continuum spectrum light source

Abstract

The invention relates to the technical field of integrated nonlinear optics and lasers, in particular to a super-continuum spectrum light source based on a lithium niobate film waveguide and a manufacturing method thereof.The super-continuum spectrum light source comprises a pump light source, an incident optical fiber and a waveguide module, and the pump light source is connected with the input end of the waveguide module through the incident optical fiber and then outputs converted light; the waveguide module comprises a lithium niobate waveguide with a preset length in the light propagation direction, the lithium niobate waveguide is divided into a connection unpolarized waveguide section and a chirp polarization waveguide section in the light propagation direction, and the unpolarized waveguide section is arranged close to the input end of the waveguide module. Segmented design is adopted, dispersion management, waveguide size determination, modulation instability suppression and spectrum broadening flatness guaranteeing are carried out on unpolarized waveguide sections through group velocity dispersion, a chirp polarization period is designed for a required spectrum range through chirp polarization waveguide sections, full-spectrum coverage is carried out, and therefore the waveguide miniaturization and integration high-performance light source is achieved.

Description

Technical Field

[0001] This application relates to the fields of integrated nonlinear optics and laser technology, and in particular to a supercontinuum light source based on lithium niobate thin film waveguide and its fabrication method. Background Technology

[0002] Supercontinuum light sources, as special light sources with broad spectrum, high brightness, and spatial coherence, are generated primarily through various nonlinear effects (such as self-phase modulation, cross-phase modulation, and four-wave mixing) induced by the interaction of strong laser pulses with nonlinear media. These effects broaden the narrowband pump light into an ultra-wide continuous spectrum output covering the ultraviolet to mid-infrared range. With the rapid development of precision optical measurement, biomedical imaging, spectroscopic analysis, and optical communication, higher demands are being placed on the bandwidth, power density, and device integration of supercontinuum light sources. Currently, common supercontinuum light source generation technologies mainly rely on the combination of high-power femtosecond lasers and nonlinear optical fibers, utilizing the nonlinear effects excited by the high-intensity localized optical field in the fiber to achieve spectral broadening. In addition, there are also methods that use bulk nonlinear crystals to broaden the spectral range through cascaded nonlinear processes.

[0003] The method of generating supercontinuum light sources by using femtosecond pulsed light to interact with nonlinear optical fibers has the following problems: 1) Limited spectral coverage: the transparent window of silica optical fibers is mostly limited to the visible to near-infrared band, making it difficult to efficiently extend to the mid- and far-infrared regions; 2) Low efficiency: the χ² and χ³ coefficients of traditional nonlinear materials (such as silicon nitride and optical fibers) are low, requiring high pump energy, and dispersion management is complex, which can easily lead to spectral jumps and deterioration of flatness; 3) The length of optical fiber devices usually needs to be on the order of meters to accumulate sufficient nonlinear effects, which limits the compactness of the system.

[0004] Methods for generating supercontinuum based on bulk crystals are mainly limited by walk-off effects and weak optical field constraints in bulk materials, requiring complex angle tuning, resulting in large system size and low conversion efficiency. Supercontinuum generated based on lithium niobate thin films often employs lens-fiber coupling of nanometer-level thin films, but the mode mismatch between its submicron mode field and ordinary single-mode fiber leads to high coupling loss. Furthermore, this method is usually suitable for laboratory testing, and it is difficult to package it into an integrated fiber optic device, resulting in poor reliability.

[0005] Therefore, how to improve the design of the supercontinuum light source generation structure to effectively reduce the light source volume while improving its performance has become an urgent problem to be solved. Summary of the Invention

[0006] This application provides a supercontinuum light source based on a lithium niobate thin-film waveguide and its fabrication method, in order to solve the problem of how to improve the design of the supercontinuum light source generation structure to effectively reduce the light source volume while improving the light source performance.

[0007] In a first aspect, this application provides a supercontinuum light source based on a lithium niobate thin film waveguide, comprising: a pump light source, an incident optical fiber, and a waveguide module, wherein the pump light source is connected to the input end of the waveguide module through the incident optical fiber, and the output end of the waveguide module is used to output the converted light;

[0008] The waveguide module includes a lithium niobate waveguide of a preset length along the light propagation direction. The lithium niobate waveguide is divided into an unpolarized waveguide section and a chirped polarized waveguide section along the light propagation direction. The unpolarized waveguide section is connected to the chirped polarized waveguide section. The unpolarized waveguide section is located near the input end of the waveguide module. The unpolarized waveguide section is used to broaden the spectrum of the input light to obtain broadened input light. The chirped polarized waveguide section is used to convert the broadened input light to output supercontinuum light.

[0009] In one embodiment, the lithium niobate waveguide is further provided with a single-polarized waveguide segment along the light propagation direction. The single-polarized waveguide segment is used to convert the ultraviolet light in the broadened input light to output the ultraviolet light in the supercontinuum.

[0010] In one embodiment, the pump light source has a center wavelength of 1568 nm, a pulse width of 70 fs, a repetition frequency of 100 MHz, and a spectral 3dB bandwidth of 46 nm; the waveguide module is a ridge waveguide with a film thickness of 3 μm, a ridge width of 3 μm, an etching depth of 1.5 μm, a sidewall tilt angle of 70°, and a waveguide length of 20 mm; the output supercontinuum range is 390 nm-2400 nm.

[0011] The non-polarized waveguide section is 5 mm long and performs spectral broadening on the 1568 nm pulse light, resulting in a 10 dB bandwidth of 600 nm. The chirped polarized waveguide section is 15 mm long and has a chirped period of 32.3 μm to 2.4 μm.

[0012] Secondly, this application provides a method for fabricating the waveguide module in a supercontinuum light source based on a lithium niobate thin-film waveguide as described in the first aspect, comprising:

[0013] The emitted light parameters of the pump light source are obtained. Based on the emitted light parameters and the constraint that the group velocity dispersion tends to zero, the first size of the unpolarized waveguide section is determined. Based on the emitted light parameters and the first size, the initial bandwidth after spectral broadening at the reference decibel is determined.

[0014] Based on the initial bandwidth, the order bandwidth corresponding to each order is determined, and based on the order bandwidth of each order, the second dimension of the chirped polarized waveguide segment is determined.

[0015] The waveguide module is modeled based on the first and second dimensions, and the first effective refractive index corresponding to different wavelengths in the supercontinuum is determined based on the modeling simulation.

[0016] Based on the first effective refractive index corresponding to different wavelengths and combined with the phase mismatch equation, the polarization period corresponding to different wavelengths is determined, the polarization period corresponding to the largest wavelength in the supercontinuum is determined as the start period, and the polarization period corresponding to the smallest wavelength in the supercontinuum is determined as the end period.

[0017] The waveguide module is fabricated based on the first dimension, the second dimension, the start period, and the end period.

[0018] In one embodiment, determining the first dimension of the unpolarized waveguide segment based on the emitted light parameters and the constraint that the group velocity dispersion tends to zero includes:

[0019] Define the initial dimensions of the unpolarized waveguide section, and determine the scanning wavelength range based on the emitted light parameters;

[0020] The second effective refractive index at different wavelengths within the scanning wavelength range is calculated using simulation. Based on the second effective refractive index at different wavelengths, the group velocity dispersion at different wavelengths is calculated using the following formula:

[0021] ,

[0022] Where β2 is the group velocity dispersion, N eff The second effective refractive index, For wavelength, For waveguide width, The speed of light in a vacuum;

[0023] Based on the group velocity dispersion at different wavelengths, plot the group velocity dispersion as a function of wavelength. Based on the curve, adjust the waveguide width so that the group velocity dispersion at the output wavelength approaches 0, thus obtaining the final waveguide width.

[0024] Based on the emitted light parameters and the group velocity dispersion corresponding to the emitted light wavelength, the nonlinear coefficient during propagation is determined, and based on the nonlinear coefficient and the peak power of the emitted light parameters, the nonlinear action length is determined.

[0025] The waveguide length is determined based on the nonlinear action length, and the waveguide length is greater than the nonlinear action length.

[0026] In one embodiment, fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes:

[0027] Based on the start period, the end period, and the second dimension, a first period distribution is determined using a linearly decreasing method, and a first mask is fabricated based on the first period distribution.

[0028] Based on the first dimension and the second dimension, the unpolarized waveguide segment and the chirped polarized waveguide segment are fabricated using the first mask to obtain the waveguide module.

[0029] In one embodiment, fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes:

[0030] Based on the start period, the end period, and the second dimension, and in conjunction with the chirped polarization period distribution function, the chirped constant is determined; based on the chirped constant, the second period distribution is determined; and based on the second period distribution, the second mask is fabricated.

[0031] Based on the first and second dimensions, the unpolarized waveguide segment and the chirped polarized waveguide segment are fabricated using the second mask to obtain the waveguide module.

[0032] In one embodiment, after determining the first periodic distribution or the second periodic distribution, the method further includes:

[0033] The first periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized first periodic distribution.

[0034] The step of fabricating the first mask based on the first periodic distribution includes:

[0035] Based on the apodization-induced first periodic distribution, a first mask is fabricated; or

[0036] The second periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized second periodic distribution.

[0037] The step of fabricating the second mask based on the second periodic distribution includes:

[0038] A second mask is made based on the apodization of the second periodic distribution.

[0039] In one embodiment, before fabricating the waveguide module according to the first size, the second size, the start period, and the end period, the method further includes:

[0040] Obtain the required frequency-doubled optical power and the normalized conversion efficiency of the corresponding output light wavelength, and determine the fundamental frequency optical power of the output frequency-doubled light corresponding to the ultraviolet band from the output light parameters;

[0041] The third dimension of the single-polarized waveguide segment is determined based on the required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power.

[0042] The process of fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes:

[0043] The waveguide module is fabricated based on the first dimension, the second dimension, the start period, the end period, and the third dimension.

[0044] In one embodiment, determining the third dimension of the single-polarized waveguide segment based on the required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power includes:

[0045] The required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power are input into the length estimation model, and the length of the single-polarized waveguide segment is output as the third dimension.

[0046] The length estimation model is as follows:

[0047] ;

[0048] in, The fundamental frequency optical power, The required frequency-doubled optical power, The normalized transformation efficiency is... The length of the single-polarized waveguide segment is given.

[0049] The technical advantages achieved by this application compared to the prior art are as follows: The light source of this application includes a pump light source, an incident optical fiber, and a waveguide module. The pump light source is connected to the input end of the waveguide module through the incident optical fiber, and the output end of the waveguide module is used to output the converted light. The waveguide module includes a lithium niobate waveguide of a preset length along the light propagation direction. The lithium niobate waveguide is divided into an unpolarized waveguide section and a chirped polarized waveguide section along the light propagation direction. The unpolarized waveguide section is connected to the chirped polarized waveguide section. The unpolarized waveguide section is located close to the input end of the waveguide module. The unpolarized waveguide section is used to broaden the spectrum of the input light to obtain broadened input light. The chirped polarized waveguide section is used to convert the broadened input light to output supercontinuum light. The waveguide module obtains the emitted light parameters of the pump source, and based on these parameters and the constraint that the group velocity dispersion tends to zero, determines the first dimension of the unpolarized waveguide section. Based on the emitted light parameters and the first dimension, it determines the initial bandwidth at a reference decibel level after spectral broadening. Based on the initial bandwidth, it determines the order bandwidth corresponding to each order, and based on the order bandwidth of each order, it determines the second dimension of the chirped polarized waveguide section. Based on the first and second dimensions, it models the waveguide module and determines the first effective refractive index corresponding to different wavelengths in the supercontinuum based on modeling and simulation. Based on the first effective refractive index corresponding to different wavelengths, and combined with the phase mismatch equation, it determines the polarization period corresponding to different wavelengths, determining the polarization period corresponding to the largest wavelength in the supercontinuum as the start period and the polarization period corresponding to the smallest wavelength in the supercontinuum as the termination period. Based on the first dimension, the second dimension, the start period, and the termination period, the waveguide module is fabricated. The segmented design utilizes group velocity dispersion for dispersion management in the unpolarized waveguide segment to determine the waveguide size, suppress modulation instability, and ensure the flatness of spectral broadening. The chirped polarized waveguide segment is designed with chirped polarization periods for the required spectral range to achieve full spectral coverage, thereby realizing a high-performance light source with miniaturized and integrated waveguides. Attached Figure Description

[0050] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0051] Figure 1 This is a schematic diagram of a supercontinuum light source based on a lithium niobate thin-film waveguide provided in Embodiment 1 of this application;

[0052] Figure 2This is a schematic diagram of the polarization period distribution in a waveguide module provided in Embodiment 1 of this application;

[0053] Figure 3 This is a schematic diagram of an optimized polarization period distribution in a waveguide module provided in Embodiment 2 of this application;

[0054] Figure 4 This is a cross-sectional schematic diagram of a waveguide module provided in Embodiment 3 of this application;

[0055] Figure 5 This is a schematic diagram of the packaging of a waveguide module provided in Embodiment 4 of this application;

[0056] Figure 6 This is a flowchart illustrating a manufacturing method provided in Embodiment 5 of this application;

[0057] Figure 7 This is a comparison chart of the tuning curves without apodization and with apodization provided in Embodiment 5 of this application. Detailed Implementation

[0058] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0059] like Figure 1 The diagram shown is a schematic representation of a supercontinuum light source based on a lithium niobate thin-film waveguide according to Embodiment 1 of this application. The supercontinuum light source includes a pump source, an incident optical fiber, and a waveguide module. The pump source is connected to the input end of the waveguide module via the incident optical fiber, and the output end of the waveguide module is used to output the converted light. Figure 1 A spectrometer is connected to the output of the waveguide module, which is suitable for detecting the generated supercontinuum light source or providing the supercontinuum light source for a spectrometer that needs to test samples. The supercontinuum range of this application is 390nm-2400nm.

[0060] The waveguide module of this application includes a lithium niobate waveguide of a preset length along the light propagation direction. The lithium niobate waveguide is divided into an unpolarized waveguide section and a chirped polarized waveguide section along the light propagation direction. The unpolarized waveguide section is connected to the chirped polarized waveguide section. The unpolarized waveguide section is located close to the input end of the waveguide module. The unpolarized waveguide section is used to broaden the spectrum of the input light to obtain broadened input light. The chirped polarized waveguide section is used to convert the broadened input light to output supercontinuum light.

[0061] The pulsed pump source can be a common 1560nm femtosecond pulsed laser with a pulse width of 150fs, a repetition rate of 100MHz, a pulse energy of 20nJ, and a spectral bandwidth of 46nm. The incident fiber can be a single-mode polarization-maintaining PM1550 fiber or a fluoride fiber with a mode matcher. These parameters can be set according to specific requirements and are not limited here.

[0062] Lithium niobate is a ferroelectric crystal material with strong second- and third-order nonlinear optical effects. In the optical band, the transmission range of lithium niobate is 400-4500nm, covering the ultraviolet to mid-infrared region, making it an ideal material for broadband supercontinuum generation.

[0063] like Figure 2 The diagram shown is a schematic representation of the polarization period distribution in a waveguide module according to Embodiment 1 of this application. It can be seen that... Figure 2 The entire structure is a lithium niobate waveguide. Figure 2 The left side is the unpolarized waveguide section, and the right side is the chirped polarized waveguide section. The left side is the input of the lithium niobate waveguide, and the right side is the output of the lithium niobate waveguide.

[0064] This application adopts a segmented design. The unpolarized waveguide segment uses group velocity dispersion for dispersion management, determines the waveguide size, suppresses modulation instability, and ensures the flatness of spectral broadening. The chirped polarized waveguide segment designs the chirped polarization period for the required spectral range to achieve full spectral coverage, thereby realizing a high-performance light source with miniaturized and integrated waveguides.

[0065] like Figure 3 The diagram shown is a schematic representation of an optimized polarization period distribution in a waveguide module according to Embodiment 2 of this application. The lithium niobate waveguide also includes a single-polarization waveguide segment along the light propagation direction. Figure 3 On the far right, the single-polarized waveguide section is used to convert the ultraviolet light in the broadened input light to output the ultraviolet light in the supercontinuum, which helps to improve the conversion efficiency of this ultraviolet band.

[0066] In one embodiment, the pump light source has a center wavelength of 1568 nm, a pulse width of 70 fs, a repetition frequency of 100 MHz, and a spectral 3dB bandwidth of 46 nm; the waveguide module is a ridge waveguide with a film thickness of 3 μm, a ridge width of 3 μm, an etching depth of 1.5 μm, a sidewall tilt angle of 70°, and a waveguide length of 20 mm; the output supercontinuum range is 390 nm-2400 nm; wherein, the non-polarized waveguide section is 5 mm long, and the 1568 nm pulse light is spectrally broadened, resulting in a 10dB bandwidth of 600 nm; the chirped polarized waveguide section is 15 mm long, and the chirped period is 32.3 μm-2.4 μm.

[0067] like Figure 4 The diagram shown is a cross-sectional view of a waveguide module provided in Embodiment 3 of this application. The waveguide module uses a lithium niobate waveguide with a ridge structure. The film thickness, ridge width, etching depth, and sidewall tilt angle of the ridge waveguide are designed according to requirements. The above parameters are all part of the waveguide dimensions. Of course, the waveguide dimensions also include the waveguide length, etc. In this application, a Z-cut lithium niobate film with a thickness of 3 μm is used, the substrate is silicon with a thickness of 0.5 mm, and the buffer layer is silicon dioxide with a thickness of 2 μm. The ridge waveguide structure is fabricated using a dry etching process. After etching, the waveguide is then polished on the sidewalls to reduce the sidewall roughness and reduce the transmission loss of the waveguide. Polishing is also performed at both ends of the waveguide to reduce the end face roughness and reduce the fiber coupling loss.

[0068] The waveguide with the above parameters can generate ultraviolet-near-infrared supercontinuum output, and the final average supercontinuum output efficiency is 15.8%.

[0069] Furthermore, the above-mentioned light source structure is also applicable in the 2.5µm-4.5µm band and above, requiring only adjustment of the corresponding parameters. The polarization period increases with increasing wavelength. For achieving a wider spectrum, higher-order harmonics (fifth harmonics, sixth harmonics) can be added to achieve spectral coverage. The same steps are used to determine the corresponding polarization period by wavelength, and the chirped polarization length is determined according to power requirements and wavelength coverage. Furthermore, for higher-order harmonics, the conversion efficiency can be improved by increasing the number of polarization periods in that band, while for lower-order harmonics, the conversion efficiency can be reduced by deleting polarization periods, thereby ensuring the flatness of the full-spectrum output.

[0070] like Figure 5 The diagram shown is a packaging schematic of a waveguide module provided in Embodiment 4 of this application. This waveguide achieves chip integration through fiber coupling at the incident end. Furthermore, for the 3μm band, the incident fiber can be replaced with fluoride fiber. In one embodiment, the integrated waveguide can also be equipped with a temperature control box for temperature control, reducing the impact of temperature on the waveguide conversion efficiency.

[0071] like Figure 6 The diagram shown is a flowchart of a fabrication method provided in Embodiment 5 of this application. This fabrication method is used to fabricate the waveguide module in the aforementioned supercontinuum light source based on lithium niobate thin film waveguides, and may include the following steps:

[0072] Step S601: Obtain the output light parameters of the pump source. Based on the output light parameters and the constraint that the group velocity dispersion tends to zero, determine the first size of the unpolarized waveguide section. Based on the output light parameters and the first size, determine the initial bandwidth at the reference decibel after spectral broadening.

[0073] In this application, the lithium niobate waveguide is designed in a segmented manner, consisting of two segments. The first segment of the waveguide is not polarized, and its main function is to achieve initial spectral broadening through third-order nonlinear effects. Its main design points include: using COMSOL simulation to calculate the group velocity dispersion (GVD) of the waveguide near a wavelength of 1560 nm (i.e., the output wavelength of the pump source), and optimizing and adjusting the waveguide size of this segment (i.e., the first size) to make it near-zero dispersion (β2≈0), with the aim of suppressing the smoothness after modulation instability.

[0074] In one embodiment, the first dimension of the unpolarized waveguide segment is determined based on the emitted light parameters and the constraint that the group velocity dispersion tends to zero, including:

[0075] Define the initial dimensions of the unpolarized waveguide section and determine the scanning wavelength range based on the output light parameters;

[0076] The second effective refractive index at different wavelengths within the scanning wavelength range is simulated and calculated. Based on the second effective refractive index at different wavelengths, the group velocity dispersion at different wavelengths is calculated using the following formula:

[0077] ,

[0078] Where β2 is the group velocity dispersion, N eff The second effective refractive index, For wavelength, For waveguide width, The speed of light in a vacuum;

[0079] Based on the group velocity dispersion at different wavelengths, plot the group velocity dispersion as a function of wavelength. Based on the curve, adjust the waveguide width to make the group velocity dispersion at the output wavelength approach 0, thus obtaining the final waveguide width.

[0080] Based on the output light parameters and the group velocity dispersion of the corresponding output light wavelength, the nonlinear coefficient during propagation is determined, and based on the nonlinear coefficient and the peak power of the output light parameters, the nonlinear action length is determined.

[0081] The waveguide length is determined based on the nonlinear action length, and the waveguide length is greater than the nonlinear action length.

[0082] Assuming the target wavelength is 1560 nm, the initial waveguide dimensions are 3 μm in height and 3 μm in width, and the sidewall tilt angle is 70°, COMSOL is used to calculate the effective refractive index N at different wavelengths. eff The scan step size is 0.1 nm, and the scan range is 1550-1570 nm, depending on the wavelength and N. effBased on the data, the group velocity dispersion (β2) of the waveguide is calculated using the following formula:

[0083] ,

[0084] For N eff The data is smoothed to avoid numerical noise. The curve of group velocity dispersion versus wavelength is plotted based on the calculation results. By adjusting the waveguide size, β2 at a wavelength of 1560nm is brought close to 0.

[0085] Specifically, if the group velocity dispersion β2 calculated based on the initial dimensions is >0 (normal dispersion), the waveguide width can be reduced gradually in 0.1 μm increments. If the group velocity dispersion β2 calculated based on the initial dimensions is <0 (anomalous dispersion), the waveguide width can be increased. However, in actual simulations, it's important to note that width changes may introduce higher-order modes, so a comprehensive consideration is necessary. When the center wavelength is in the near-zero dispersion region (… At this point, the dispersion effect is minimal, and the spectral broadening is dominated by the SPM, resulting in a faster broadening speed. Furthermore, there is no need to overcome the dispersion effect, and the required pulse energy is lower, resulting in a broad and smooth spectrum.

[0086] Once the width and other dimensions of the unpolarized waveguide section are determined, after the initial pulse light is coupled to the waveguide via the optical fiber, the initial spectral unfolding process in the first unpolarized section is dominated by self-phase modulation. In the nonlinear medium, pulse propagation is described by the nonlinear Schrödinger equation (NLSE):

[0087] ,

[0088] in, Let α be the electric field envelope, β2 be the loss term, and β2 be the group velocity dispersion, which mainly affects the pulse time-domain bandwidth compression. This is a nonlinear term that primarily affects spectral broadening. This is the electric field mode. Self-phase modulation mainly originates from the Kerr effect, and the phase change depends on... Intensity modulation in the time domain is converted into frequency broadening, increasing the spectral width. It is uniformly distributed in the time domain, with small phase differences between different frequency components, resulting in smoother spectral broadening. The nonlinear coefficient and the peak power of the input pulse light can determine the nonlinear action length. The pulse light's spectrum broadens to its maximum after passing through this length; therefore, the length of the first waveguide segment needs to be greater than the nonlinear action length.

[0089] Step S602: Determine the order bandwidth corresponding to each order based on the initial bandwidth, and determine the second size of the chirped polarized waveguide segment based on the order bandwidth of each order.

[0090] The second waveguide is designed with chirped periodic polarization. Through quasi-phase matching, second-order nonlinear effects (second, third, and fourth harmonics) and cascaded third-order nonlinear effects (such as cross-phase modulation XPM, four-wave mixing, etc.) are used to continuously extend the spectrum to the ultraviolet-mid-infrared region (i.e., 400nm-4500nm in this application). Specifically, this part can also be equivalently calculated into three segments: the first segment is the second harmonic, generated by frequency doubling of the pump light; the second segment is the third harmonic, generated by the sum of the second harmonic light and the pump light; and the third segment is the fourth harmonic, generated by the re-frequency doubling of the second harmonic light.

[0091] For example, the length L1 of the first unpolarized waveguide segment is determined using a 10dB bandwidth as a reference. Step S601 determines the initial bandwidth of the spectral broadening after the pulsed light source passes through the first waveguide segment. Based on the set harmonics, the bandwidth of each order is calculated progressively. For example, if the initial bandwidth is 100nm, the second harmonic bandwidth is 50nm, and the fourth harmonic bandwidth is 25nm. Combining each order bandwidth, the length L2 of the second chirped polarized waveguide segment is determined. In this application, the difference in size between the unpolarized and chirped polarized waveguide segments lies only in the waveguide length; the width, thickness, etc., can be set to be the same, or they can be set differently depending on requirements.

[0092] Step S603: Model the waveguide module according to the first and second dimensions, and determine the first effective refractive index corresponding to different wavelengths in the supercontinuum based on the modeling simulation.

[0093] Among them, COMSOL is used to model the lithium niobate waveguide. Based on the first and second dimensions, the overall structure of the lithium niobate waveguide can be formed. Based on this modeled structure, the effective refractive index corresponding to different wavelengths can be obtained by simulating the full wavelength in the supercontinuum.

[0094] Step S604: Based on the first effective refractive index corresponding to different wavelengths and combined with the phase mismatch equation, determine the polarization period corresponding to different wavelengths, determine the polarization period corresponding to the maximum wavelength in the supercontinuum as the start period, and determine the polarization period corresponding to the minimum wavelength in the supercontinuum as the end period.

[0095] Among them, through the phase mismatch equation:

[0096] ,

[0097] The polarization period corresponding to different wavelengths can be obtained through calculation. , where k s Let k be the fundamental frequency wave vector. p Let k be the pump light wave vector. i For the idler frequency light wave vector, such as the harmonic, k s and k pBoth are pump light, k i It is frequency-doubled light; for sum frequency, k s For fundamental frequency light, k p For pump light, k i For sum-frequency light, its calculation formula is: Where n is the refractive index, λ is the wavelength, and the polarization period corresponding to the maximum wavelength is the starting period. The polarization period corresponding to the minimum wavelength is the termination period. .

[0098] From the start period to the end period, a periodic distribution can be formed throughout the entire chirped polarized waveguide segment by directly linearly decreasing the period. Alternatively, it can be based on the polarization periodic distribution function. Determine the chirp constant. Then, the periodic distribution in the chirped polarized waveguide segment is determined based on the chirped constant, so as to facilitate the fabrication of a mask for generating the corresponding chirped polarized waveguide segment.

[0099] Step S605: Based on the first dimension, the second dimension, the start period, and the end period, the waveguide module is fabricated.

[0100] Once the dimensions are known, the length, thickness, and width of the waveguide module can be determined. Referring to the description of Embodiment 3 above, after knowing the start period and the end period, the chirp period can be determined using the method described in step S604 above. The resulting period distribution is used to fabricate a mask, and the chirped polarized waveguide segment is fabricated using the mask, thereby forming a complete lithium niobate waveguide.

[0101] In one embodiment, a waveguide module is fabricated based on a first dimension, a second dimension, a start period, and an end period, comprising:

[0102] Based on the start period, end period, and second dimension, the first period distribution is determined using a linear decreasing method, and the first mask is fabricated based on the first period distribution.

[0103] Based on the first and second dimensions, an unpolarized waveguide segment and a chirped polarized waveguide segment are fabricated using a first mask to obtain a waveguide module.

[0104] This linear decreasing method can effectively reduce the difficulty of mask production and increase the success rate.

[0105] In one embodiment, a waveguide module is fabricated based on a first dimension, a second dimension, a start period, and an end period, comprising:

[0106] Based on the start period, end period, and second dimension, and combined with the chirped polarization period distribution function, the chirped constant is determined. Based on the chirped constant, the second period distribution is determined. Based on the second period distribution, the second mask is fabricated.

[0107] Based on the first and second dimensions, using a second mask, unpolarized waveguide segments and chirped polarized waveguide segments are fabricated to obtain the waveguide module.

[0108] Among them, using the chirp constant to determine the periodic distribution can effectively improve the waveguide conversion efficiency.

[0109] In one embodiment, after determining the first periodic distribution or the second periodic distribution, the method further includes:

[0110] The first periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized first periodic distribution.

[0111] Based on the first periodic distribution, the first mask is fabricated, including:

[0112] Based on the first periodic distribution after apodization, create the first mask; or

[0113] The second periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized second periodic distribution.

[0114] Based on the second periodic distribution, a second mask is fabricated, including:

[0115] Based on the second periodic distribution after apodization, a second mask is made.

[0116] To eliminate fluctuations in the conversion efficiency curve caused by chirped polarization, apodization is applied to the local period. A positive hyperbolic tangent apodization function is introduced to smooth out the peak fluctuations, as shown below:

[0117] ,

[0118] ,

[0119] In the formula, 'a' represents the apodization rate, which mainly characterizes the strength of the apodization curve. By modulating the polarization duty cycle at both ends of the waveguide through the apodization function, a flat output of the conversion efficiency curve can be achieved. Taking a 150-1600nm broadband frequency doubling waveguide as an example, ... Figure 7 The image shows a comparison of the tuning curves before and after apodization.

[0120] In one embodiment, before fabricating the waveguide module according to the first dimension, the second dimension, the start period, and the end period, the method further includes:

[0121] Obtain the required frequency-doubled optical power and the normalized conversion efficiency of the corresponding output wavelength, and determine the fundamental frequency optical power of the corresponding ultraviolet band frequency-doubled light from the output light parameters;

[0122] The third dimension of the single-polarized waveguide segment is determined based on the required frequency-doubled optical power, normalized conversion efficiency, and fundamental frequency optical power.

[0123] Based on the first dimension, the second dimension, the start period, and the end period, a waveguide module is fabricated, including:

[0124] The waveguide module is fabricated based on the first dimension, the second dimension, the start period, the end period, and the third dimension.

[0125] In the unpolarized waveguide section, spectral broadening is positively correlated with pulse energy. Theoretically, the higher the pulse energy, the wider the output spectral broadening. However, after the frequency doubling of the spectrum in the chirped polarized waveguide section, its conversion efficiency in the ultraviolet band is still low. This is related to the strong dispersion in the ultraviolet band. Furthermore, due to apodization processing, the duty cycle at the end of the polarization period is very low, which also leads to a decrease in conversion efficiency in this band.

[0126] Here, redesigning the waveguide chip and adding a polarization period for the ultraviolet band helps improve the conversion efficiency of this band. For example, calculating the corresponding polarization period for the ultraviolet band and adding a single polarization period of a specific length for the ultraviolet band at the end of the chirped polarization region can improve the conversion efficiency of the ultraviolet band.

[0127] In one embodiment, determining the third dimension of the single-polarized waveguide segment based on the required frequency-doubled optical power, normalized conversion efficiency, and fundamental frequency optical power includes:

[0128] The required frequency-doubled optical power, normalized conversion efficiency, and fundamental frequency optical power are input into the length estimation model, and the length of the single-polarized waveguide segment is output as the third dimension.

[0129] The length estimation model is as follows:

[0130] ;

[0131] in, For fundamental frequency optical power, For the required frequency-doubled optical power, To normalize the conversion efficiency, This represents the length of the single-polarized waveguide segment.

[0132] The required length is calculated based on the requirements (such as a specification of 1nW / nm), using the following formula: Based on the known signal optical power and required frequency doubling optical power and the normalized conversion efficiency at that wavelength. The required length can then be determined. .

[0133] Furthermore, when calculating the length L2 of the second chirped polarized waveguide segment, the pulsed light, after passing through the first unpolarized waveguide segment, becomes a spectrally broadened pulsed light. For example, the initial bandwidth of 1568nm is 46nm, and after broadening, the bandwidth becomes 400nm, meaning the pulsed light spectrum is now 1368-1768nm. According to simulation calculations, the bandwidth of frequency doubling at the center wavelength of 1568nm (note that this is the bandwidth of frequency doubling, not the spectral bandwidth) is 0.65nm. Therefore, the normalized conversion efficiency of the 400nm bandwidth pulsed light after frequency doubling is 1 / (400 / 0.65) times that of single-wavelength frequency doubling. Thus, the normalized efficiency can be equivalent to 1 / (400 / 0.65) times the normalized efficiency of single-wavelength frequency doubling, and then substituted into the formula: The length l1 corresponding to this harmonic can be derived. The third harmonic is generated by the sum of the second harmonic and the pump light, and the formula used is [formula missing]. The length l2 corresponding to this harmonic can be derived from this formula. The lengths of the third and fourth harmonics can be calculated similarly to obtain l3. The sum of the three is the length L2. In addition, after the waveguide module is fabricated, the bandwidth change caused by the single-frequency wavelength to the broadband wavelength is used to calculate the equivalent conversion efficiency. Combined with the power of the input pulse light, the average conversion efficiency of the output supercontinuum is calculated.

[0134] This application takes an output supercontinuum range of 390nm-2400nm as an example. The pulsed pump light has a center wavelength of 1568nm, a pulse width of 70fs, a repetition frequency of 100MHz, and a spectral 3dB bandwidth of 46nm. The ridge waveguide design dimensions are: film thickness 3μm, ridge width 3μm, etching depth 1.5μm, sidewall tilt angle 70°, and waveguide length 20mm. The waveguide adopts a segmented design. The first segment is a non-polarized waveguide with a length of 5mm, which broadens the spectral width of the 1568nm pulsed light, resulting in a 10dB bandwidth of 600nm. The second segment is a chirped periodically polarized waveguide with a length of 15mm, designed with a chirped period... The polarization period of the region is 32.3µm-2.4µm. The polarization period at both ends of the region is apodized using the positive hyperbolic tangent function. Broadband 2nd to 4th harmonics are achieved through cascaded frequency doubling and frequency summing processes, thereby generating ultraviolet-near-infrared supercontinuum output. The final supercontinuum average output efficiency is 15.8%.

[0135] The unpolarized waveguide section utilizes group velocity dispersion for dispersion management, determines waveguide dimensions, suppresses modulation instability, and ensures the flatness of spectral broadening. The chirped polarized waveguide section determines the polarization period and chirp constant at the start and end positions based on the broadening wavelength, the required broadband range, and the waveguide length. In addition, apodization processing is applied to both ends of this region, using a positive hyperbolic tangent function to modulate the duty cycle at both ends of the chirped polarized region, varying it from 0% to 50%. Furthermore, a single-period polarization period of the short-wavelength end of the spectrum is added at the tail, which can appropriately improve its conversion efficiency at the end of the spectrum.

[0136] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A supercontinuum light source based on a lithium niobate thin-film waveguide, characterized in that, include: The system includes a pump source, an incident optical fiber, and a waveguide module. The pump source is connected to the input end of the waveguide module via the incident optical fiber, and the output end of the waveguide module is used to output the converted light. The waveguide module includes a lithium niobate waveguide of a preset length along the light propagation direction. The lithium niobate waveguide is divided into an unpolarized waveguide section and a chirped polarized waveguide section along the light propagation direction. The unpolarized waveguide section is connected to the chirped polarized waveguide section. The unpolarized waveguide section is located near the input end of the waveguide module. The unpolarized waveguide section is used to broaden the spectrum of the input light to obtain broadened input light. The chirped polarized waveguide section is used to convert the broadened input light to output supercontinuum light.

2. The supercontinuum light source based on lithium niobate thin-film waveguide according to claim 1, characterized in that, The lithium niobate waveguide is further provided with a single-polarized waveguide segment along the light propagation direction. The single-polarized waveguide segment is used to convert the ultraviolet light in the broadened input light and output the ultraviolet light in the supercontinuum.

3. The supercontinuum light source based on lithium niobate thin-film waveguide according to claim 1 or 2, characterized in that, The pump light source has a center wavelength of 1568 nm, a pulse width of 70 fs, a repetition frequency of 100 MHz, and a spectral 3dB bandwidth of 46 nm; the waveguide module is a ridge waveguide with a film thickness of 3 μm, a ridge width of 3 μm, an etching depth of 1.5 μm, a sidewall tilt angle of 70°, and a waveguide length of 20 mm; the output supercontinuum range is 390 nm-2400 nm. The non-polarized waveguide section is 5 mm long and performs spectral broadening on the 1568 nm pulse light, resulting in a 10 dB bandwidth of 600 nm. The chirped polarized waveguide section is 15 mm long and has a chirped period of 32.3 μm to 2.4 μm.

4. A method for fabricating the waveguide module in a supercontinuum light source based on a lithium niobate thin-film waveguide as described in any one of claims 1-3, characterized in that, include: The emitted light parameters of the pump light source are obtained. Based on the emitted light parameters and the constraint that the group velocity dispersion tends to zero, the first size of the unpolarized waveguide section is determined. Based on the emitted light parameters and the first size, the initial bandwidth after spectral broadening at the reference decibel is determined. Based on the initial bandwidth, the order bandwidth corresponding to each order is determined, and based on the order bandwidth of each order, the second dimension of the chirped polarized waveguide segment is determined. The waveguide module is modeled based on the first and second dimensions, and the first effective refractive index corresponding to different wavelengths in the supercontinuum is determined based on the modeling simulation. Based on the first effective refractive index corresponding to different wavelengths and combined with the phase mismatch equation, the polarization period corresponding to different wavelengths is determined, the polarization period corresponding to the largest wavelength in the supercontinuum is determined as the start period, and the polarization period corresponding to the smallest wavelength in the supercontinuum is determined as the end period. The waveguide module is fabricated based on the first dimension, the second dimension, the start period, and the end period.

5. The manufacturing method according to claim 4, characterized in that, The step of determining the first dimension of the unpolarized waveguide segment based on the emitted light parameters and the constraint that the group velocity dispersion tends to zero includes: Define the initial dimensions of the unpolarized waveguide section, and determine the scanning wavelength range based on the emitted light parameters; The second effective refractive index at different wavelengths within the scanning wavelength range is calculated using simulation. Based on the second effective refractive index at different wavelengths, the group velocity dispersion at different wavelengths is calculated using the following formula: ， Where β2 is the group velocity dispersion, N eff The second effective refractive index, For wavelength, For waveguide width, The speed of light in a vacuum; Based on the group velocity dispersion at different wavelengths, plot the group velocity dispersion as a function of wavelength. Based on the curve, adjust the waveguide width so that the group velocity dispersion at the output wavelength approaches 0, thus obtaining the final waveguide width. Based on the emitted light parameters and the group velocity dispersion corresponding to the emitted light wavelength, the nonlinear coefficient during propagation is determined, and based on the nonlinear coefficient and the peak power of the emitted light parameters, the nonlinear action length is determined. The waveguide length is determined based on the nonlinear action length, and the waveguide length is greater than the nonlinear action length.

6. The manufacturing method according to claim 4, characterized in that, The process of fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes: Based on the start period, the end period, and the second dimension, a first period distribution is determined using a linearly decreasing method, and a first mask is fabricated based on the first period distribution. Based on the first dimension and the second dimension, the unpolarized waveguide segment and the chirped polarized waveguide segment are fabricated using the first mask to obtain the waveguide module.

7. The manufacturing method according to claim 4, characterized in that, The process of fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes: Based on the start period, the end period, and the second dimension, and in conjunction with the chirped polarization period distribution function, the chirped constant is determined; based on the chirped constant, the second period distribution is determined; and based on the second period distribution, the second mask is fabricated. Based on the first and second dimensions, the unpolarized waveguide segment and the chirped polarized waveguide segment are fabricated using the second mask to obtain the waveguide module.

8. The manufacturing method according to claim 6 or 7, characterized in that, After determining the first periodic distribution or the second periodic distribution, the following steps are also included: The first periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized first periodic distribution. The step of fabricating the first mask based on the first periodic distribution includes: Based on the apodization-induced first periodic distribution, a first mask is fabricated; or The second periodic distribution is ablated using the positive hyperbolic tangent apodization function to obtain the apodized second periodic distribution. The step of fabricating the second mask based on the second periodic distribution includes: A second mask is made based on the apodization of the second periodic distribution.

9. The manufacturing method according to claim 4, characterized in that, Before fabricating the waveguide module according to the first size, the second size, the start period, and the end period, the method further includes: Obtain the required frequency-doubled optical power and the normalized conversion efficiency of the corresponding output light wavelength, and determine the fundamental frequency optical power of the output frequency-doubled light corresponding to the ultraviolet band from the output light parameters; The third dimension of the single-polarized waveguide segment is determined based on the required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power. The process of fabricating the waveguide module based on the first dimension, the second dimension, the start period, and the end period includes: The waveguide module is fabricated based on the first dimension, the second dimension, the start period, the end period, and the third dimension.

10. The manufacturing method according to claim 9, characterized in that, The step of determining the third dimension of the single-polarized waveguide segment based on the required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power includes: The required frequency-doubled optical power, the normalized conversion efficiency, and the fundamental frequency optical power are input into the length estimation model, and the length of the single-polarized waveguide segment is output as the third dimension. The length estimation model is as follows: ； in, The fundamental frequency optical power, The required frequency-doubled optical power, The normalized transformation efficiency is... The length of the single-polarized waveguide segment is given.

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