Three-channel wavelength division multiplexer based on thin-film lithium niobate and wavelength division multiplexing method

Abstract

The invention discloses a three-channel wavelength division multiplexer based on thin film lithium niobate and a wavelength division multiplexing method, the device comprises three stages of filters which are connected in sequence, and each filter comprises a multi-mode combined waveguide, a bent gradient waveguide, a multi-mode waveguide grating and a straight waveguide, the multimode waveguide grating adopts a silicon Bragg grating heterogeneous integrated lithium niobate waveguide structure, the device receives multi-wavelength fundamental mode optical signals, filtering is completed, conversion from TE0 to three high-order modes is achieved, and optical signals with different wavelengths are sequentially obtained in three channels. The wavelength division multiplexer has the advantages of high extinction ratio, low channel crosstalk and small channel space.

Description

Technical Field

[0001] This invention relates to the field of integrated optics, and in particular to a three-channel wavelength division multiplexer and wavelength division multiplexing method based on thin-film lithium niobate. Background Technology

[0002] With the increasing data traffic, high demands are now placed on the capacity and efficiency of optical communication systems, and integrated photonics technology is key to addressing this challenge. Over the past decade, silicon-on-insulator (SiI) platforms have achieved significant success thanks to high refractive index contrast and mature fabrication processes. However, due to limitations inherent in silicon materials, such as high optical loss and lack of intrinsic electro-optic effects, research is increasingly focusing on lithium niobate-on-insulator (LNI) platforms to pursue lower-loss and higher-speed optical communication systems. Lithium niobate possesses excellent optoelectronic properties, including strong electro-optic effects, low loss, and strong nonlinearity. Combined with high refractive index contrast waveguide structures, it can realize miniaturized, highly integrated photonic chip devices. In applications requiring high speed and high capacity, wavelength division multiplexing (WDM) technology is one of the most common multiplexing technologies for improving the capacity of data communication links. There are two existing fabrication methods for thin-film lithium niobate WDM multiplexers. One method is to directly etch the lithium niobate waveguide. The drawback of this method is that etching lithium niobate is difficult, and because it uses sidewall etching, it can only achieve mode switching between TE0 and TE1. The other method is to heterogeneously integrate silicon nitride waveguides on thin-film lithium niobate and then etch the silicon nitride waveguides to achieve WDM and other functions. The drawback of this method is that most of the optical field is confined within the silicon nitride waveguide. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a three-channel wavelength division multiplexer and wavelength division multiplexing method based on thin-film lithium niobate, which avoids the direct etching of lithium niobate to form a grating, and at the same time uses different mode conversion to achieve multi-wavelength multiplexing, reducing crosstalk between different channels and improving the signal-to-noise ratio.

[0004] Technical solution: The present invention provides a three-channel wavelength division multiplexer based on thin-film lithium niobate, comprising three sequentially connected filters, including a first filter, a second filter and a third filter. The input of the first filter receives multi-wavelength fundamental mode optical signals, and the first to third filters respectively output three single-wavelength fundamental mode optical signals.

[0005] The first to third filters all include multimode combined waveguides, curved gradient waveguides, multimode waveguide Bragg gratings, and straight-through waveguides;

[0006] The multimode combined waveguide and the curved gradient waveguide together form an adiabatic coupling region, which is used to broaden the mode spot of the forward fundamental mode optical signal, and to convert the reflected single-wavelength optical signal into a fundamental mode optical signal and output it.

[0007] Multimode waveguide Bragg gratings are used to convert phase-matched wavelength fundamental mode optical signals into corresponding TE modes and reflect them to multimode combined waveguides and curved graded waveguides.

[0008] The through waveguide is used to transmit the fundamental mode optical signal that is not reflected by the multimode waveguide Bragg grating to the next stage filter or directly output.

[0009] Furthermore, the multimode combined waveguide includes a wide-gradient waveguide in the adiabatic coupling region, and the curved-gradient waveguide includes a narrow-gradient waveguide in the adiabatic coupling region. The positions of the wide-gradient waveguide in the adiabatic coupling region and the narrow-gradient waveguide in the adiabatic coupling region correspond to each other and together constitute the adiabatic coupling region.

[0010] Furthermore, the curved gradient waveguide also includes a front 180° arc-shaped curved waveguide and a rear 90° arc-shaped curved waveguide, with the front 180° arc-shaped curved waveguide, the thermally adiabatic coupling region narrow gradient waveguide, and the rear 90° arc-shaped curved waveguide connected in sequence; the single-wavelength fundamental mode optical signal is output from the lower end of the front 180° arc-shaped curved waveguide.

[0011] Furthermore, the multimode combined waveguide, the curved gradient waveguide, and the straight waveguide all adopt a lithium niobate ridge waveguide structure.

[0012] Furthermore, the multimode waveguide Bragg grating adopts a multimode lithium niobate ridge waveguide structure, on which a silicon Bragg grating is deposited. By adjusting the number of structures, apodization intensity, and grating period of the silicon Bragg grating, the first to third filters are respectively matched with different wavelengths.

[0013] Furthermore, the silicon Bragg grating adopts a rectangular silicon Bragg grating structure, with the rectangles arranged in an antisymmetric distribution.

[0014] Furthermore, the multimode waveguide Bragg grating includes a silicon dioxide substrate, a thin-film lithium niobate layer, and a silicon load layer. The thin-film lithium niobate layer is bonded to the upper surface of the silicon dioxide substrate. A lithium niobate ridge waveguide is formed by first etching the thin-film lithium niobate layer. The silicon load layer is deposited on the upper surface of the thin-film lithium niobate layer. A subwavelength Bragg grating structure is formed by second etching the silicon load layer.

[0015] The present invention discloses a three-channel wavelength division multiplexing method based on thin-film lithium niobate, characterized in that wavelength division multiplexing is performed using the aforementioned three-channel wavelength division multiplexer, comprising the following steps:

[0016] The multi-wavelength fundamental mode optical signal is input into the first filter. The multi-mode waveguide Bragg grating of the first filter converts the first wavelength fundamental mode optical signal into TE1 mode. The TE1 mode optical signal is reflected to the multi-mode combined waveguide and the curved gradient waveguide of the first filter and then converted into the first wavelength fundamental mode optical signal and output.

[0017] The unreflected fundamental mode optical signal is transmitted to the second filter through the straight waveguide of the first filter. The multimode waveguide Bragg grating of the second filter converts the second wavelength fundamental mode optical signal into TE2 mode. The TE2 mode optical signal is reflected to the multimode combined waveguide and the curved gradient waveguide of the second filter and then converted into the second wavelength fundamental mode optical signal and output.

[0018] The unreflected fundamental mode optical signal is transmitted to the third filter through the straight waveguide of the second filter. The multimode waveguide Bragg grating of the third filter converts the fundamental mode optical signal of the third wavelength into the TE3 mode. The TE3 mode optical signal is reflected to the multimode combined waveguide and the curved gradient waveguide of the third filter and then converted into the fundamental mode optical signal of the third wavelength and output.

[0019] The electronic device of the present invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the computer program is loaded onto the processor, it implements the three-channel wavelength division multiplexing method based on thin-film lithium niobate.

[0020] The computer program product of the present invention includes a computer program that, when executed by a processor, implements the three-channel wavelength division multiplexing method based on thin-film lithium niobate.

[0021] Beneficial effects: Compared with the prior art, the advantages of the present invention are: (1) The present invention uses thin-film lithium niobate material, which retains the strong electro-optic effect, low loss and strong nonlinear photoelectric properties of lithium niobate material, while using a high refractive index difference optical waveguide structure to achieve high device integration; (2) The present invention uses heterogeneous integration of silicon Bragg grating on thin-film lithium niobate, which avoids direct etching of lithium niobate, overcomes the problem that it is difficult to directly etch lithium niobate into grating with small linewidth, and improves the design freedom; (3) The present invention has a three-way based on thin-film lithium niobate. The wavelength division multiplexer (WDM) achieves both fundamental mode conversion to different higher-order modes and filtering through multimode waveguide gratings. It utilizes multimode combined waveguides and curved tapered waveguides for adiabatic coupling, and cascades three filters to ultimately achieve three-channel WDM multiplexing. By designing multimode waveguide gratings with different structures, mode conversion from the fundamental mode to different higher-order modes is introduced during filtering, reducing inter-channel crosstalk. Furthermore, by introducing an apodization design and adjusting the grating period, duty cycle, and total number of grating structures, a WDM with narrow operating bandwidth and high extinction ratio is obtained. In summary, this invention provides a WDM with narrow operating bandwidth, high extinction ratio, and low inter-channel crosstalk, offering advantages such as high integration, low process requirements, and low loss. Attached Figure Description

[0022] Figure 1 This is a structural diagram of the three-channel wavelength division multiplexer of the present invention.

[0023] Figure 2 This is a planar schematic diagram of the first filter in the three-channel wavelength division multiplexer of the present invention.

[0024] Figure 3 This is a schematic diagram of the cross-section of the lithium niobate ridge waveguide of the first filter of the present invention.

[0025] Figure 4 This is a cross-sectional schematic diagram of the multimode waveguide grating structure of the first filter of the present invention.

[0026] Figure 5 This is a planar schematic diagram of the second filter in the three-channel wavelength division multiplexer of the present invention.

[0027] Figure 6 This is a planar schematic diagram of the third filter in the three-channel wavelength division multiplexer of the present invention.

[0028] Figure 7 This is a schematic diagram illustrating the working principle of the three-channel wavelength division multiplexer of the present invention.

[0029] Figure 8 The image shows the simulation results of the transmission spectrum of the three-channel wavelength division multiplexer according to an embodiment of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0031] like Figure 1 As shown, the three-channel wavelength division multiplexer based on thin-film lithium niobate includes three sequentially connected filters: a first filter, a second filter, and a third filter. The input of the first filter receives multi-wavelength fundamental mode optical signals, and the first to third filters output three single-wavelength fundamental mode optical signals, respectively. The signal input of the first filter is the signal input of the three-channel wavelength division multiplexer. The signal pass-through of the first filter is connected to the signal input of the second filter, and the signal pass-through of the second filter is connected to the signal input of the third filter. The output of the third filter is a free end.

[0032] The first, second, and third filters have the same components; the first filter will be used as an example for explanation below. Figure 2 As shown, the first filter includes a multimode combined waveguide 1, a curved gradient waveguide 2, a multimode waveguide Bragg grating 3, and a straight-through waveguide 4. The signal input terminal of the multimode combined waveguide 1 is the signal input terminal of the first filter. The signal output terminal of the multimode combined waveguide 1 is connected to the signal input terminal of the multimode waveguide Bragg grating 3. The signal output terminal of the multimode waveguide Bragg grating 3 is connected to the signal input terminal of the straight-through waveguide 4. The signal output terminal of the straight-through waveguide 4 is the signal straight-through terminal of the first filter. The curved gradient waveguide 2 is placed below and close to the multimode combined waveguide 1.

[0033] Specifically, the multimode combined waveguide 1 is formed by sequentially connecting an input waveguide 1-1, a thermally coupled wide-gradient waveguide 1-2, and an output waveguide 1-3. The input end of the input waveguide 1-1 is the signal input end of the multimode combined waveguide 1. The output end of the input waveguide 1-1 is connected to the input end of the thermally coupled wide-gradient waveguide 1-2. The output end of the thermally coupled wide-gradient waveguide 1-2 is connected to the input end of the output waveguide 1-3. The output end of the output waveguide 1-3 is the signal output end of the multimode combined waveguide 1.

[0034] Specifically, the curved gradient waveguide 2 is composed of a front 180° arc-shaped curved waveguide 2-1, a thermally adiabatic coupling region narrow gradient waveguide 2-2, and a rear 90° arc-shaped curved waveguide 2-3 connected sequentially. The upper end of the front 180° arc-shaped curved waveguide 2-1 is connected to one end of the thermally adiabatic coupling region narrow gradient waveguide 2-2, and the other end of the thermally adiabatic coupling region narrow gradient waveguide 2-2 is connected to the upper end of the rear 90° arc-shaped curved waveguide 2-3. The lower end of the front 180° arc-shaped curved waveguide 2-1 is the signal download end of the first filter. In the first filter, the lower end of the front 180° arc-shaped curved waveguide is the first signal download end of the wavelength division multiplexer; in the second filter, the lower end of the front 180° arc-shaped curved waveguide is the second signal download end of the wavelength division multiplexer; and in the third filter, the lower end of the front 180° arc-shaped curved waveguide is the third signal download end of the wavelength division multiplexer, outputting fundamental mode optical signals of different wavelengths respectively.

[0035] Furthermore, the wide-gradient waveguide 1-2 and the narrow-gradient waveguide 2-2 in the adiabatic coupling region are arranged close to each other and have the same length, thus coupling occurs, so that the wide-gradient waveguide 1-2 and the narrow-gradient waveguide 2-2 in the adiabatic coupling region form an adiabatic coupling region.

[0036] Furthermore, the rear 90° arc-shaped curved waveguide 2-3 is used to separate the coupling waveguide from the upper main waveguide (multimode combined waveguide 1) to avoid interfering with the transmission of the fundamental mode optical signal in the main waveguide.

[0037] Specifically, the signal input terminal of the multimode waveguide Bragg grating 3 is connected to the output terminal of the multimode combined waveguide 1, and the signal output terminal of the multimode waveguide Bragg grating 3 is connected to the signal output terminal of the through waveguide 4.

[0038] Specifically, the through waveguide 4 is formed by connecting the through tapered waveguide 4-1 and the through output waveguide 4-2. The input end of the through tapered waveguide 4-1 is the input end of the through waveguide 4, the output end of the through tapered waveguide 4-1 is connected to the input end of the through output waveguide 4-2, and the output end of the through output waveguide 4-2 is the signal output end of the through waveguide 4.

[0039] like Figure 3As shown, the multimode combined waveguide 1, the curved gradient waveguide 2, and the straight waveguide 4 of the first filter, the second filter, and the third filter all adopt a lithium niobate ridge waveguide structure, including a silicon dioxide substrate 5 and a thin film lithium niobate layer 6, wherein the thin film lithium niobate layer 6 is bonded to the upper surface of the silicon dioxide substrate 5, and the ridge waveguide is formed by etching on the thin film lithium niobate layer 6.

[0040] like Figure 4 As shown, the multimode waveguide Bragg gratings 3 of the first to third filters all employ silicon Bragg gratings deposited on a multimode lithium niobate ridge waveguide. These include a silicon dioxide substrate 7, a thin-film lithium niobate layer 8, and a silicon loading layer 9. The thin-film lithium niobate layer 8 is bonded to the upper surface of the silicon dioxide substrate 7, and undergoes a first etching to form the lithium niobate ridge waveguide. The silicon loading layer 9 is deposited on the upper surface of the thin-film lithium niobate layer 8, and undergoes a second etching to form a subwavelength Bragg grating structure. Compared to directly etching lithium niobate, silicon material has a mature processing technology and is easier to fabricate into different subwavelength structures.

[0041] Furthermore, the multimode waveguide Bragg gratings 3 of the first to third filters all adopt rectangular silicon Bragg grating structures, with the rectangular arrangement in an antisymmetric distribution. The grating period of the antisymmetric distribution satisfies the phase matching condition of the following formula (1). The rectangular grating structures are of the same size, and the width design adopts a Gaussian apodized variation to suppress sidelobes and improve the signal-to-noise ratio. The rectangular width variation A Gaussian distribution that satisfies the following formula (2):

[0042] (1)

[0043] (2)

[0044] in, The effective refractive index of the TE0 mode. The effective refractive index of the corresponding mode, For the filter wavelength, For the grating period, The maximum rectangle width in the design. For toe-cutting strength, The number of structures in the grating. , This represents the total number of grating structures. For the first filter, Effective refractive index of TE1 mode For the second filter, Effective refractive index of TE2 mode For the third filter, Effective refractive index of TE3 mode .

[0045] Furthermore, such as Figure 5 and Figure 6 As shown, for the first to third filters, it is important to note that the straight-through tapered waveguide 4-1 of the first filter and the straight-through tapered waveguide 4-3 of the second filter gradually widen from left to right. After connecting to the straight-through output waveguide 4-2 of the first filter and the straight-through output waveguide 4-4 of the second filter, they can be used for connection between filters. The straight-through tapered waveguide 4-5 of the third filter gradually narrows from left to right. Connecting to the straight-through output waveguide 4-6 of the third filter, it serves as the straight-through end of the signal, and the straight-through output waveguide 4-6 of the third filter is a single-mode waveguide. Meanwhile, the multimode waveguide Bragg grating 3 of the first filter has a double-row anti-symmetric distribution, and its width supports TE1 mode transmission; the multimode waveguide Bragg grating 3 of the second filter has a triple-row anti-symmetric distribution, and its width supports TE2 mode transmission; and the multimode waveguide Bragg grating 3 of the third filter has a quadruple-row anti-symmetric distribution, and its width supports TE3 mode transmission.

[0046] like Figure 7 As shown, the working process of the three-channel wavelength division multiplexer of the present invention is as follows.

[0047] Multiwavelength ( , , ...) The fundamental mode optical signal is input from the input terminal of the three-channel wavelength division multiplexer. After passing through the adiabatic coupling region of the multimode combined waveguide and the curved gradient waveguide of the first filter, the mode spot is broadened, while still maintaining the fundamental mode propagating to the right. It then enters the multimode waveguide Bragg grating, and the wavelength that satisfies the phase matching condition of the multimode waveguide Bragg grating ( The signal is reflected as TE1 mode and transmitted to the left. It passes through the adiabatic coupling region of the multimode combined waveguide of the first filter and the curved gradient waveguide. The TE1 mode evolves into TE0 mode in the curved gradient waveguide through adiabatic coupling and is output from the lower end of the first 180° arc waveguide of the first filter, becoming the first download signal.

[0048] Other wavelengths ( , The fundamental mode optical signal continues to propagate to the right, passes through the first filter's direct waveguide, and then enters the second filter. After passing through the adiabatic coupling region of the second filter, the mode spot is broadened again, while still maintaining the fundamental mode's rightward propagation. It then enters the multimode waveguide Bragg grating of the second filter, at a wavelength that satisfies the phase-matching condition of the multimode waveguide Bragg grating. The signal is reflected as TE2 mode and transmitted to the left, passing through the adiabatic coupling region of the second filter again. The TE2 mode evolves into TE0 mode in the curved gradient waveguide through adiabatic coupling and is output from the lower end of the first 180° arc waveguide of the second filter, becoming the second download signal.

[0049] Other wavelengths ( The fundamental mode optical signal continues to propagate to the right, passes through the direct waveguide of the second filter, and then enters the third filter. After passing through the adiabatic coupling region of the third filter, the mode spot is broadened, but the fundamental mode still propagates to the right. It then enters the multimode waveguide Bragg grating of the third filter, and the wavelength that satisfies the phase matching condition of the multimode waveguide Bragg grating (…) The signal is reflected as TE3 mode, passes through the adiabatic coupling region of the third filter to the left again, and evolves into TE0 mode in the curved gradient waveguide through adiabatic coupling. It is then output from the lower end of the first 180° arc waveguide of the third filter, becoming the third download signal.

[0050] The remaining fundamental mode optical signal is directly output at the through waveguide end of the third filter.

[0051] By selecting the period of the multimode waveguide Bragg gratings in the first to third filters respectively, and optimizing the apodization parameters, the total number of grating structures, and the size of the grating structures, a three-channel wavelength division multiplexer with low loss, low inter-channel crosstalk, high extinction ratio, and narrow channel space can be obtained.

[0052] The following is a specific simulation experiment to verify the three-channel wavelength division multiplexer described in this invention.

[0053] This experiment uses a thin-film lithium niobate nanowire waveguide based on lithium niobate-on-insulator (LNOI) material. The core layer is lithium niobate material with a thickness of 600 nm, the waveguide structure is etched to a depth of 300 nm, and the refractive index n0 = 2.21. e =2.14, the lower cladding material is silicon dioxide (SiO2) with a refractive index of 1.44, and the upper cladding is air.

[0054] In the first filter, both the input and output waveguides are multimode waveguides with widths of 1.5µm and 1.8µm, respectively. Figure 2 The vertical direction represents the waveguide width. The width of the wide-gradient waveguide in the adiabatic coupling region transitions from the input waveguide width to the output waveguide width. The widths of the front 180° circular arc waveguide and the rear 90° circular arc waveguide are 0.6µm and 0.4µm, respectively. The width of the narrow-gradient waveguide in the adiabatic coupling region gradually changes from the width of the front 180° circular arc waveguide to the width of the rear 90° circular arc waveguide. The wide-gradient and narrow-gradient waveguides in the adiabatic coupling region have the same length, 100µm. The inner diameter of the circular arc waveguide is 100µm, and the spacing between the wide-gradient and narrow-gradient waveguides in the adiabatic coupling region is 0.6µm. A multimode waveguide Bragg grating with a waveguide width of 1.8µm is used. FDTD simulations determine the grating period to be 400nm, the total number of structures to be 500, the apodization intensity to be 8, the grating height to be 50nm, and the center wavelength to be... The width of the through-through waveguide is 2.3µm, while the width of the tapered through-through waveguide gradually changes from 1.8µm to 2.3µm, and the length is 50µm.

[0055] In the second filter, the input waveguide width is 2.3 μm, and the output waveguide width is 2.7 μm. The width of the wide-gradient waveguide in the adiabatic coupling region transitions from the input waveguide width to the output waveguide width. The widths of the front 180° circular arc waveguide and the rear 90° circular arc waveguide are 0.5 μm and 0.4 μm, respectively. The width of the narrow-gradient waveguide in the adiabatic coupling region gradually changes from the width of the front 180° circular arc waveguide to the width of the rear 90° circular arc waveguide. The wide-gradient and narrow-gradient waveguides in the adiabatic coupling region have the same length, 80 μm. The inner diameter of the circular arc waveguide is 100 μm, and the spacing between the wide-gradient and narrow-gradient waveguides in the adiabatic coupling region is 0.6 μm. The multimode waveguide Bragg grating has a waveguide width of 2.7 μm. Based on FDTD simulation calculations, a grating period of 405 nm, a total number of structures of 500, an apodization intensity of 8, a grating height of 50 nm, and a center wavelength of... The width of the through-through waveguide is 3.3µm, and the width of the tapered through-through waveguide gradually changes from 2.7µm to 3.3µm, with a length of 50µm.

[0056] In the third filter, the input waveguide width is 3.3 μm, and the output waveguide width is 3.6 μm. The width of the wide-gradient waveguide in the adiabatic coupling region transitions from the input waveguide width to the output waveguide width. The widths of the front 180° circular arc waveguide and the rear 90° circular arc waveguide are 0.5 μm and 0.4 μm, respectively. The width of the narrow-gradient waveguide in the adiabatic coupling region gradually changes from the width of the front 180° circular arc waveguide to the width of the rear 90° circular arc waveguide. The wide-gradient and narrow-gradient waveguides in the adiabatic coupling region have the same length, 100 μm. The inner diameter of the circular arc waveguide is 100 μm, and the spacing between the wide-gradient and narrow-gradient waveguides in the adiabatic coupling region is 0.6 μm. The multimode waveguide Bragg grating has a waveguide width of 3.6 μm. Based on FDTD simulation calculations, a grating period of 412 nm, a total number of structures of 500, an apodization intensity of 8, a grating height of 50 nm, and a center wavelength of... The through-through waveguide is a single-mode waveguide with a width of 1µm. The tapered through-through waveguide has a width that gradually changes from 3.6µm to 1µm and a length of 100µm.

[0057] The device was simulated and verified using a three-dimensional finite-difference time-domain algorithm over a broadband range of 1500 to 1600 nm. Figure 8 Simulation results of transmission spectra measured in three channels are presented. Channel 1 is in TE1 mode with a center wavelength of 1525 nm, channel 2 is in TE2 mode with a center wavelength of 1540 nm, and channel 3 is in TE3 mode with a center wavelength of 1560 nm. The channel spacing is <20 nm, the inter-channel crosstalk is <-15 dB, and the loss is <-2 dB. Therefore, this invention utilizes silicon heterostructure integrated lithium niobate waveguides to introduce different higher-order modes, resulting in a three-channel wavelength division multiplexer with a small channel space, low loss, and low inter-channel crosstalk.

Claims

1. A three-channel wavelength division multiplexer based on thin-film lithium niobate, characterized in that, It includes three-stage filters connected in sequence, including a first filter, a second filter and a third filter. The input of the first filter receives multi-wavelength fundamental mode optical signals, and the first to third filters output three single-wavelength fundamental mode optical signals respectively. The first to third filters each include a multimode combined waveguide (1), a curved gradient waveguide (2), a multimode waveguide Bragg grating (3), and a straight waveguide (4). The multimode combined waveguide (1) and the curved gradient waveguide (2) together form an adiabatic coupling region, which is used to broaden the mode spot of the forward fundamental mode optical signal and to convert the reflected single-wavelength optical signal into a fundamental mode optical signal and output it. A multimode waveguide Bragg grating (3) is used to convert the phase-matched wavelength fundamental mode optical signal into the corresponding TE mode and reflect it to the multimode combined waveguide (1) and the curved gradient waveguide (2). The through waveguide (4) is used to transmit the fundamental mode optical signal that is not reflected by the multimode waveguide Bragg grating (3) to the next stage filter or directly output.

2. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 1, characterized in that, The multimode combined waveguide (1) includes a wide gradient waveguide (1-2) in the adiabatic coupling region, and the curved gradient waveguide (2) includes a narrow gradient waveguide (2-2) in the adiabatic coupling region. The positions of the wide gradient waveguide (1-2) in the adiabatic coupling region and the narrow gradient waveguide (2-2) in the adiabatic coupling region correspond to each other and together constitute the adiabatic coupling region.

3. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 2, characterized in that, The curved gradient waveguide (2) further includes a front 180° arc-shaped curved waveguide (2-1) and a rear 90° arc-shaped curved waveguide (2-3). The front 180° arc-shaped curved waveguide (2-1), the thermally adiabatic coupling region narrow gradient waveguide (2-2), and the rear 90° arc-shaped curved waveguide (2-3) are connected in sequence. The fundamental mode optical signal of a single wavelength is output from the lower end of the front 180° arc-shaped curved waveguide (2-1).

4. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 1, characterized in that, The multimode combined waveguide (1), the curved gradient waveguide (2), and the straight waveguide (4) all adopt a lithium niobate ridge waveguide structure.

5. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 1, characterized in that, The multimode waveguide Bragg grating (3) is constructed on a multimode lithium niobate ridge waveguide structure, wherein a silicon Bragg grating is deposited on the multimode lithium niobate ridge waveguide.

6. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 5, characterized in that, The silicon Bragg grating adopts a rectangular silicon Bragg grating structure, with the rectangles arranged in an antisymmetric distribution. By adjusting the number of silicon Bragg grating structures, the apod intensity, and the grating period, the first to third filters are matched to different wavelengths.

7. The three-channel wavelength division multiplexer based on thin-film lithium niobate according to claim 5, characterized in that, The multimode waveguide Bragg grating (3) includes a silicon dioxide substrate (7), a thin-film lithium niobate layer (8), and a silicon load layer (9). The thin-film lithium niobate layer (8) is bonded to the upper surface of the silicon dioxide substrate (7). A lithium niobate ridge waveguide is formed by first etching the thin-film lithium niobate layer (8). The silicon load layer (9) is deposited on the upper surface of the thin-film lithium niobate layer (8). The silicon load layer (9) is then etched a second time to form a subwavelength Bragg grating structure.

8. A three-channel wavelength division multiplexing method based on thin-film lithium niobate, characterized in that, The wavelength division multiplexing (WDM) using the three-channel WDM multiplexer of claim 1 includes the following steps: The multi-wavelength fundamental mode optical signal is input into the first filter. The multi-mode waveguide Bragg grating 3 of the first filter converts the fundamental mode optical signal of the first wavelength into the TE1 mode. The TE1 mode optical signal is reflected to the multi-mode combined waveguide (1) and the curved gradient waveguide (2) of the first filter and then converted into the fundamental mode optical signal of the first wavelength and output. The unreflected fundamental mode optical signal is transmitted to the second filter through the direct waveguide (4) of the first filter. The multimode waveguide Bragg grating (3) of the second filter converts the fundamental mode optical signal of the second wavelength into the TE2 mode. The TE2 mode optical signal is reflected to the multimode combined waveguide (1) and the curved gradient waveguide (2) of the second filter and then converted into the fundamental mode optical signal of the second wavelength and output. The unreflected fundamental mode optical signal is transmitted to the third filter through the direct waveguide (4) of the second filter. The multimode waveguide Bragg grating (3) of the third filter converts the fundamental mode optical signal of the third wavelength into the TE3 mode. The TE3 mode optical signal is reflected to the multimode combined waveguide (1) and the curved gradient waveguide (2) of the third filter and then converted into the fundamental mode optical signal of the third wavelength and output.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the computer program is loaded into the processor, it implements the three-channel wavelength division multiplexing method based on thin-film lithium niobate as described in claim 8.

10. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by the processor, it implements the three-channel wavelength division multiplexing method based on thin-film lithium niobate as described in claim 8.

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