Rapid heat insulation 3dB power beam splitter with lithium niobate film and implementation method of rapid heat insulation 3dB power beam splitter

Abstract

The invention discloses a lithium niobate film rapid heat insulation 3dB power beam splitter, and relates to the technical field of integrated photons. Comprising an upper cladding layer, an x-cut lithium niobate film layer, a buried oxide layer and a silicon substrate layer which are sequentially stacked from top to bottom, the x-cut lithium niobate layer contains a lithium niobate film ridge waveguide formed by an etching technology; the lithium niobate film waveguide comprises an input end conical waveguide, an input end S-bent waveguide, a rapid adiabatic mode evolution waveguide and an output end S-bent waveguide which are connected in sequence; on the basis of a traditional linear adiabatic cone, nonlinear change is achieved in combination with adiabatic parameters so as to accelerate the adiabatic process, and the excellent characteristics of broadband and robustness of the adiabatic power beam splitter are reserved while the length of the adiabatic power beam splitter is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of integrated photons, and in particular to a lithium niobate thin film fast adiabatic 3dB power beam splitter. Background Art

[0002] Integrated photonic technology provides high-performance, miniaturized, low-cost and scalable solutions for the new generation of high-speed optical communications, optical interconnection and optical computing, and has broad application prospects. Among the many integrated photonic platforms, lithium niobate film has become a very influential integrated photonic platform with its wide transparent window, low light absorption loss, high linear electro-optic coefficient and second-order nonlinear coefficient. As a key device in the field of integrated photonic technology, the 3dB power beam splitter can achieve 1:1 uniform light splitting and is widely used in optical switches, modulators, optical communications and sensor networks.

[0003] The technical means of 3dB power splitting include: directional coupler, which couples two phase-matched waveguides to each other. When the coupling length is half the beat length, uniform distribution of light is achieved. The structure is the simplest [Optics Letters 48.5(2023):1264-1267]. However, it is easily affected by the strict phase matching conditions and is easily affected by the processing tolerance, resulting in uneven optical power distribution and limited bandwidth; multimode interference coupler, based on the self-image effect generated by the interference of light in the multimode waveguide, can achieve uniform distribution of optical power at the output end by designing the interference and phase relationship between the modes in the waveguide. It has the characteristics of broadband and compactness [Optics Express 31.17(2023):27266-27273.]. However, multimode interference easily introduces additional optical loss, resulting in reduced overall efficiency; adiabatic coupler, by slowly changing the geometric structure of the optical waveguide, adiabatic mode coupling is achieved, which has the advantages of high efficiency, large bandwidth and high stability [Optics letters 41.13(2016):3041-3044]. However, in order to meet the adiabatic coupling conditions, the device length is very long, on the order of hundreds of microns, and the integration level is low.

[0004] Traditional adiabatic couplers usually use linear transformation parameters, which do not require an accurate definition of the transmission length of the device power, but it must be long enough to meet the adiabatic condition to reduce the energy loss caused by power coupling to other modes, and the integration is low. Based on the traditional linear adiabatic, the fast thermal adiabatic theory uses nonlinear transformation to accelerate the adiabatic process, while reducing the length of the coupler while retaining its excellent characteristics of large bandwidth, low insertion loss, compact structure and strong robustness. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a lithium niobate thin film fast adiabatic 3dB power beam splitter.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A lithium niobate thin film fast adiabatic 3dB power beam splitter comprises, in a top-view direction, an upper cladding layer, an x-cut lithium niobate thin film layer, a buried oxide layer and a silicon substrate layer which are stacked in sequence from top to bottom; the x-cut lithium niobate layer contains a lithium niobate thin film ridge waveguide formed by etching technology; the thin film lithium niobate waveguide comprises an input end tapered waveguide, an input end S-bend waveguide, a fast adiabatic mode evolution waveguide and an output end S-bend waveguide which are connected in sequence.

[0008] Furthermore, the input-end tapered waveguide comprises a linear tapered waveguide a and a linear tapered waveguide b arranged in parallel, wherein the linear tapered waveguide a is located at the bottom, has a length of L1, and a gap between the two is gap1; the width of the linear tapered waveguide b linearly changes from w0 to w1, w0>w1, and the width of the linear tapered waveguide a linearly changes from w0 to w2, w0>w1. <w2。

[0009] Furthermore, the input end S-curved waveguide is composed of two symmetrical third-order Bessel curved waveguides, namely, input end S-curved waveguide a and input end S-curved waveguide b from bottom to top, both of which have a length of L2, a width that remains unchanged, and a gap that changes from gap1 to gap2, gap1>gap2.

[0010] Furthermore, the rapid adiabatic mode evolution waveguide is composed of a rapid adiabatic mode evolution waveguide a and a rapid adiabatic mode evolution waveguide b arranged in parallel to form a dual waveguide system, the rapid adiabatic mode evolution waveguide a is located at the bottom, has a length of L3, and a spacing of gap2; the width of the rapid adiabatic mode evolution waveguide a gradually changes from w1 to w1-D(w), and the width of the rapid adiabatic mode evolution waveguide a gradually changes from w2 to w2+D(w), D(w) is the spacing function, and w is the change in waveguide width; the ends of the rapid adiabatic mode evolution waveguide a and the fast adiabatic mode evolution waveguide b have the same width w0.

[0011] Furthermore, the output end S-bend waveguide is composed of two symmetrical third-order Bessel bending waveguides, which are output end S-bend waveguide a and output end S-bend waveguide b from bottom to top, with a length of L4 and a width of w0, and a gap gradually changing from gap2 to gap3, gap2 <gap3。

[0012] Preferably, the material of the upper cladding layer is silicon dioxide or air, and the material of the buried oxide layer is silicon dioxide.

[0013] At the same time, the present invention also discloses a method for realizing a lithium niobate thin film fast adiabatic 3dB power beam splitter, the method is used to prepare the lithium niobate thin film fast adiabatic 3dB power beam splitter as claimed in claim 5, comprising the following steps:

[0014] S1. Determine the waveguide widths w1 and w2 and the waveguide spacing gap2 in the fast adiabatic mode evolution waveguide. w0, gap1, gap3, L1, L2 and L4 are parameters of the non-mode evolution waveguide region, which only need to avoid unnecessary coupling (choose a safe parameter as much as possible), so they do not need to be determined and optimized separately. The selection of these parameters should follow the following principles: L1 should be long enough to ensure adiabatic transmission of the mode, and w0 needs to be consistent with the width of the end of the fast adiabatic mode evolution waveguide; L2 and L4 are obtained by the third-order Bessel function definition, with the criterion of minimizing bending loss; gap1 and gap3 are set large enough to ensure that no coupling occurs between waveguides in this area;

[0015] S2. Calculate the effective refractive index, propagation constant, electric field and magnetic field corresponding to the symmetric mode and antisymmetric mode in the fast adiabatic mode evolution waveguide by using the Lumerical Mode Solutions numerical simulator;

[0016] S3. Calculate the adiabatic parameters of the fast adiabatic mode evolution waveguide, and obtain the optimized spacing function D(w) after length normalization through the fast adiabatic theory;

[0017] S4. The fast adiabatic taper function F(z) is obtained by optimizing the spacing function D(w), and the nonlinear transformation structure of the mode evolution waveguide is obtained. The transmission length L3 required to meet the 3dB power splitting is calculated by the Lumerical Eme Solutions numerical simulator.

[0018] The specific implementation method of S3 is as follows:

[0019] S31. The interval function D(w) adopts the linear taper function F(z)=(D f / L)z, F(z) means that w changes linearly along the propagation distance z, and the boundary conditions satisfy F i =F(0)=0, the waveguide width starts to change at the initial end of the rapid adiabatic mode evolution waveguide, and the boundary condition satisfies F f =F(L)=(w1-w2) / 2, at the end of the fast adiabatic mode evolution waveguide, the two waveguide widths are the same;

[0020] S32. The adiabatic parameter is defined as

[0021]

[0022] Among them, β m , β n is the propagation constant of eigenmodes m and n, E mt and H ntare the transverse components of the electric and magnetic fields of the mth and nth eigenmodes, respectively. The integration region is the entire waveguide cross section, and z is the propagation distance. is a unit vector, s is the integration plane, ε is a minimum parameter, is the conjugate of the transverse component of the magnetic field;

[0023] The fast adiabatic theory requires that at least one parameter in the optical waveguide system changes with the propagation distance z. According to the fast adiabatic theory and the chain rule, D(w) is selected as the only parameter, and the adiabatic parameter is

[0024]

[0025] in, The overlap integral of the electromagnetic fields of mode m and mode n varies with D and is independent of z; A(D) varies linearly adiabatically with the adiabatic parameter c lin (z) Export

[0026]

[0027] D f , D i For the boundary conditions mentioned above, the adiabatic parameters are simplified to

[0028]

[0029] The relationship between z and D is

[0030]

[0031] S33. Integrate both sides of equation (5) and substitute the boundary condition z(D i )=0,z(D f )=L, and the adiabatic parameter is obtained

[0032]

[0033] S34. Substituting equation (6) into equation (5), we can get the variation of the interval function D with the propagation distance z:

[0034]

[0035] S35. Substituting equation (7) into the taper function formula, we get the fast adiabatic taper function F(z) = (D f / L)z FAQUAD .

[0036] The lithium niobate thin film fast adiabatic 3dB power beam splitter described in the present invention adopts the above technical solution and has the following technical effects compared with the prior art:

[0037] The lithium niobate thin film fast adiabatic 3dB power splitter proposed in the present invention can achieve the goal of exciting only one of the eigenmodes in the dual-waveguide system at the input, while ensuring that the energy is basically not coupled to the other eigenmode after passing through the coupling region, and the excited eigenmode is dominant. At the same time, the length of the coupling region that meets the 3dB power splitting can be quickly determined, which is conducive to the efficient completion of device design.

[0038] The present invention proposes an optimization formula for the spacing function of a fast adiabatic 3dB power beam splitter. The optimization formula for the spacing function is formula (7). By using the spacing function, the corresponding taper function F(z)=(D f / L)z FAQUAD , the optimal variation trend of the waveguide width in the coupling region can be quickly obtained through the taper function, and the length of the device can be further reduced while satisfying the adiabatic evolution condition. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the structure of the top view of the lithium niobate thin film waveguide layer of the present invention, the two illustrations on the top are schematic diagrams of the evolution of the mode field in the mode evolution region when the narrow waveguide inputs the light, and the two illustrations on the bottom are schematic diagrams of the evolution of the mode field in the mode evolution region when the wide waveguide inputs the light;

[0040] Figure 2 A comparison diagram of the optimized spacing function of the fast adiabatic mode evolution waveguide and the optimized spacing function of the linear adiabatic power beam splitter mode evolution region waveguide in an embodiment of the present invention;

[0041] Figure 3 A comparison diagram of adiabatic parameters of a fast adiabatic power beam splitter and adiabatic parameters of a linear adiabatic power beam splitter in an embodiment of the present invention;

[0042] Figure 4 is a relationship diagram between the waveguide length in the mode evolution region and the transmittance of the output port of the linear adiabatic power beam splitter in an embodiment of the present invention;

[0043] Figure 5 is a graph showing the relationship between the waveguide length in the mode evolution region and the transmittance of the output port of the fast adiabatic power beam splitter in an embodiment of the present invention;

[0044] Figure 6 It is a simulation diagram of the transmission efficiency of the eigenmode excited by the fast adiabatic power beam splitter in the embodiment of the present invention;

[0045] Figure 7 is the transmission spectrum of the fast adiabatic power beam splitter in the embodiment of the present invention;

[0046] Figure 8 This is a simulated electric field intensity propagation diagram of the fast adiabatic power beam splitter in an embodiment of the present invention. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0048] See also Figure 1-8 , the specific implementation modes of the present invention are as follows.

[0049] [Example 1]

[0050] In order to better illustrate this embodiment, the components in the drawings may be enlarged or reduced, and do not represent the actual device size. The waveguide spacing and waveguide width mentioned above refer to the size of the upper surface of the waveguide.

[0051] like Figure 1 The lithium niobate thin film fast adiabatic 3dB power beam splitter comprises, in the top view direction, an upper cladding layer, an x-cut lithium niobate thin film layer, a buried oxide layer and a silicon substrate layer stacked in sequence from top to bottom; the x-cut lithium niobate layer contains a lithium niobate thin film ridge waveguide formed by etching technology; from left to right, the thin film lithium niobate waveguide comprises an input end tapered waveguide, an input end S-bend waveguide, a fast adiabatic mode evolution waveguide and an output end S-bend waveguide connected in sequence. The material of the upper cladding layer is silicon dioxide, and the material of the buried oxide layer is silicon dioxide.

[0052] When the transverse electric fundamental mode TE0 is input at the input end of waveguide a, it is mainly converted into the symmetric mode in the dual waveguide system in the mode evolution region; when the transverse electric fundamental mode TE0 is input at the input end of waveguide b, it is mainly converted into the antisymmetric mode in the dual waveguide system in the mode evolution region. Since the phase matching is satisfied at the end of the mode evolution region, when the adiabatic change condition is met, the power is equally distributed in the two waveguides, realizing 3dB power splitting.

[0053] The input end tapered waveguide comprises a linear tapered waveguide a and a linear tapered waveguide b which are arranged in parallel. The linear tapered waveguide a is located at the bottom, and the length of the linear tapered waveguide a is L1. The distance between the linear tapered waveguide a and the linear tapered waveguide b is gap1. The width of the linear tapered waveguide b linearly changes from w0 to w1, w0>w1. The width of the linear tapered waveguide a linearly changes from w0 to w2, w0>w1. <w2。

[0054] The input-end S-curved waveguide is composed of two 180° symmetrical third-order Bessel-curved waveguides, which are respectively the input-end S-curved waveguide a and the input-end S-curved waveguide b from bottom to top, both of which have a length of L2, a width that remains unchanged, and a spacing that changes from gap1 to gap2, gap1>gap2; in the input-end S-curved waveguide region, the width of waveguide a is w1, and the width of waveguide b is w2.

[0055] The fast adiabatic mode evolution waveguide is composed of a fast adiabatic mode evolution waveguide a and a fast adiabatic mode evolution waveguide b arranged in parallel, and the two waveguides satisfy the field coupling condition to form a dual waveguide system; the fast adiabatic mode evolution waveguide a is located at the bottom, with a length of L3 and a spacing of gap2; the width of the fast adiabatic mode evolution waveguide a gradually changes from w1 to w1-D(w), and the width of the fast adiabatic mode evolution waveguide a gradually changes from w2 to w2+D(w), D(w) is the spacing function, and w is the change in the waveguide width; the ends of the fast adiabatic mode evolution waveguide a and the fast adiabatic mode evolution waveguide b have the same width w0.

[0056] The output end S-bend waveguide is composed of two symmetrical 180° third-order Bessel bending waveguides, which are output end S-bend waveguide a and output end S-bend waveguide b from bottom to top, with a length of L4 and a width of w0. The spacing gradually changes from gap2 to gap3, gap2 <gap3。

[0057] like Figure 1 As shown in the figure, when TE0 is input from waveguide a, the eigenmode excited by the dual waveguide system is an even / symmetric mode. Initially, the mode energy is mainly concentrated in waveguide a. Since the phase matching condition is met at the end of the mode evolution region, the power is finally equally distributed in the two waveguides and finally output through the output waveguide.

[0058] The method for realizing the above-mentioned lithium niobate thin film fast adiabatic 3dB power beam splitter includes:

[0059] S1. Determine the waveguide widths w1 and w2 and the waveguide spacing gap2 in the rapid adiabatic mode evolution waveguide;

[0060] S2. Calculate the effective refractive index, propagation constant, electric field and magnetic field corresponding to the symmetric mode and antisymmetric mode in the fast adiabatic mode evolution waveguide by using the Lumerical Mode Solutions numerical simulator;

[0061] S3. Calculate the adiabatic parameters of the fast adiabatic mode evolution waveguide, and obtain the length-normalized optimized spacing function D(w) through the fast adiabatic theory; for the light evolution following a single eigenmode, the adiabatic theorem states that the adiabatic parameters must satisfy:

[0062]

[0063] where β m , β n is the propagation constant of eigenmodes m and n, E mt and H nt are the transverse components of the electric and magnetic fields of the mth and nth eigenmodes, respectively, and the integration region is the entire waveguide cross section. The spacing function D uses a linear taper function F(z) = (D f / L)z, the boundary conditions meet F i =F(0)=0,F f =F(L)=(w1-w2) / 2. The fast adiabatic theory requires that at least one parameter in the optical waveguide system changes with the propagation distance z, so D is selected as the only parameter. According to the chain rule, formula (1) becomes

[0064]

[0065] in, The overlap integral of the electromagnetic fields of mode m and mode n varies with D and is independent of z. A(D) can be obtained by linearly adiabatically varying the adiabatic parameter c lin (z) Export

[0066]

[0067] Then the adiabatic condition simplifies to,

[0068]

[0069] The relationship between z and D is

[0070]

[0071] Integrate both sides of the equation and substitute the boundary conditions z(D i )=0,z(D f )=L, and the adiabatic parameter is obtained

[0072]

[0073] Substituting equation (6) into equation (5), we can obtain the variation of the fast adiabatic optimization interval function D with the propagation distance z:

[0074]

[0075] Substituting formula (7) into the taper function formula, we get the fast adiabatic taper function F(z) = (D f / L)z FAQUAD In all the above formulas, L represents the output waveguide length L3 in the text, and L is used for the convenience of description.

[0076] S4. The fast adiabatic taper function F(z) is obtained by optimizing the spacing function D(w), and the nonlinear transformation structure of the mode evolution waveguide is obtained. The transmission length L3 required to meet the 3dB power splitting is calculated by the Lumerical Eme Solutions numerical simulator.

[0077] [Example 2]

[0078] In order to better illustrate this embodiment, the components in the attached drawings may be enlarged or reduced, and do not represent the actual device size. In order to minimize the input and output waveguide losses, the input waveguide spacing gap1 and the output waveguide spacing gap3 are 5.2 μm, and the input waveguide length L1 and the output waveguide length L3 are 38 μm. The selection of the waveguide spacing gap2 has a significant impact on the length of the mode evolution region. When gap2 is smaller, the shortest coupling region length that meets the adiabatic evolution condition is shorter. At the same time, considering that the waveguide spacing will also be affected by the process processing limit, based on the above considerations, gap2 is set to 0.5 μm in the present invention.

[0079] Through the Lumerical Mode Solutions numerical simulator, the symmetric mode and antisymmetric mode excited by the incident light input mode evolution region are found, and the effective refractive index n of the two modes is calculated as the parameter D changes. eff , electric field, magnetic field and propagation constant. According to the fast adiabatic theory, the optimized spacing function and adiabatic coefficient of the fast adiabatic power splitter are calculated. The results are as follows Figure 2 and Figure 3 As shown. Figure 2 It can be seen that the adiabatic parameter c(z) of the linear adiabatic power beam splitter is not evenly distributed over the length L. It is very small at the beginning of the cone and then gradually increases until the end of the cone. Therefore, in order to meet the adiabatic condition, the length L needs to be long enough to ensure that the overall adiabatic parameter is below an acceptable value. Unlike the linear adiabatic power beam splitter, which has a constant rate of change over the entire cone area, the fast adiabatic power beam splitter has the characteristics of fast changes at the starting point of the cone where c(z) is small and slow changes at the end point where c(z) is large. Strategically, dD / dz is allocated on demand, making c(z) a stable value overall. After the same length L, more energy is retained in the target mode, such as Figure 2 and Figure 3 shown.

[0080] Scan the waveguide length of the linear adiabatic power splitter and the fast adiabatic power splitter to obtain the transmission curve of the power splitter. The results are as follows Figure 4 and Figure 5As shown, the length L of the 3dB coupler can be selected at the intersection of the two output curves. It can be seen from the figure that in order to make the mode evolution area adiabatic transmission, the length of the linear adiabatic power coupling is longer. For example, at wavelength λ = 1550nm, under the condition of input mode TE0, the first 3dB intersection of the linear adiabatic power splitter appears at L = 85.9μm, while the fast adiabatic power splitter is only 28.6μm, and the length of the coupling area is reduced by 66.7%, indicating that under the same adiabatic conditions, the fast adiabatic theory can effectively reduce the length of the device.

[0081] Select the first 3dB intersection length of the fast adiabatic power splitter, build a complete fast adiabatic power splitter, and obtain the transmission spectrum of the entire structure. The results are as follows: Figure 6 As shown; it can be seen from the figure that within the wavelength range of 1500-1600nm, the conversion efficiency between the input light mode coupled to the output intrinsic mode shows high flatness and high conversion efficiency, and the conversion efficiency is above 98%.

[0082] Figure 7 The simulated transmission curves of the two output ports corresponding to the fast adiabatic power splitter at wavelength λ = 1550nm, L3 = 28.6μm. It can be seen that the coupling ratio is within the range of 3±0.55dB in the entire simulated wavelength range. Figure 8 This is a simulated electric field intensity propagation diagram under the same simulation conditions as above. It can be clearly seen that the output light is equally divided into two output ports.

[0083] The present invention is not limited to the above-mentioned optional implementation modes. Anyone can derive other various forms of products under the inspiration of the present invention. However, no matter what changes are made in the parameters, all technical solutions that fall within the scope defined by the claims of the present invention fall within the protection scope of the present invention.

Claims

1. A lithium niobate thin film fast adiabatic 3dB power beam splitter, characterized in that: In the top-down direction, it includes an upper cladding layer, an x-cut lithium niobate thin film layer, a buried oxide layer and a silicon substrate layer stacked in sequence from top to bottom; the x-cut lithium niobate layer contains a lithium niobate thin film ridge waveguide formed by etching technology; the thin film lithium niobate waveguide includes an input end tapered waveguide, an input end S-bend waveguide, a fast adiabatic mode evolution waveguide and an output end S-bend waveguide connected in sequence.

2. The lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 1, characterized in that: The input end tapered waveguide comprises a linear tapered waveguide a and a linear tapered waveguide b which are arranged in parallel. The linear tapered waveguide a is located at the bottom, and the length of the linear tapered waveguide a is L1. The distance between the linear tapered waveguide a and the linear tapered waveguide b is gap1. The width of the linear tapered waveguide b linearly changes from w0 to w1, w0>w1. The width of the linear tapered waveguide a linearly changes from w0 to w2, w0>w1. <w2。 3. A lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 2, characterized in that: The input end S-bent waveguide is composed of two symmetrical third-order Bessel bent waveguides, namely, input end S-bent waveguide a and input end S-bent waveguide b from bottom to top, both of which have a length of L2, a width that remains unchanged, and a gap that changes from gap1 to gap2, gap1>gap2.

4. The lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 3, characterized in that: The fast adiabatic mode evolution waveguide is composed of a fast adiabatic mode evolution waveguide a and a fast adiabatic mode evolution waveguide b arranged in parallel, and the two waveguides satisfy the field coupling condition to form a dual waveguide system; the fast adiabatic mode evolution waveguide a is located at the bottom, with a length of L3 and a gap of gap2; The width of the fast adiabatic mode evolution waveguide a gradually changes from w1 to w1-D(w), and the width of the fast adiabatic mode evolution waveguide a gradually changes from w2 to w2+D(w), where D(w) is the interval function and w is the change in waveguide width; the ends of the fast adiabatic mode evolution waveguide a and the fast adiabatic mode evolution waveguide b have the same width w0.

5. The lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 4, characterized in that: The output end S-bend waveguide is composed of two symmetrical third-order Bessel bending waveguides, which are output end S-bend waveguide a and output end S-bend waveguide b from bottom to top, with a length of L4 and a width of w0. The spacing gradually changes from gap2 to gap3, gap2 <gap3。 6. The lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 1, characterized in that: The material of the upper cladding layer is silicon dioxide or air, and the material of the buried oxide layer is silicon dioxide.

7. A method for realizing a lithium niobate thin film fast adiabatic 3dB power beam splitter, the method being used to prepare the lithium niobate thin film fast adiabatic 3dB power beam splitter as claimed in claim 5, characterized in that: The following steps are involved: S1. Determine the waveguide widths w1 and w2 and the waveguide spacing gap2 in the rapid adiabatic mode evolution waveguide; S2. Calculate the effective refractive index, propagation constant, electric field and magnetic field corresponding to the symmetric mode and antisymmetric mode in the fast adiabatic mode evolution waveguide by using the Lumerical Mode Solutions numerical simulator; S3. Calculate the adiabatic parameters of the fast adiabatic mode evolution waveguide, and obtain the optimized spacing function D(w) after length normalization through the fast adiabatic theory; S4. The fast adiabatic taper function F(z) is obtained by optimizing the spacing function D(w), and the nonlinear transformation structure of the mode evolution waveguide is obtained. The transmission length L3 required to meet the 3dB power splitting is calculated by the Lumerical Eme Solutions numerical simulator.

8. The lithium niobate thin film fast adiabatic 3dB power beam splitter according to claim 5, characterized in that: The specific implementation method of S3 is as follows: S31. The interval function D(w) adopts the linear taper function F(z)=(D f / L)z, F(z) means that w changes linearly along the propagation distance z, and the boundary conditions satisfy F i =F(0)=0, the waveguide width starts to change at the initial end of the rapid adiabatic mode evolution waveguide, and the boundary condition satisfies F f =F(L)=(w1-w2) / 2, at the end of the fast adiabatic mode evolution waveguide, the two waveguide widths are the same; S32. According to the rapid adiabatic theory and the chain rule, D(w) is selected as the only parameter, and the adiabatic parameter is in, Indicates that the overlap integral of the electromagnetic fields of mode m and mode n varies with D and has nothing to do with z; β m , β n is the propagation constant of the eigenmodes m and n, E mt and H nt are the transverse components of the electric and magnetic fields of the mth and nth eigenmodes, respectively. The integration region is the entire waveguide cross section, and z is the propagation distance. is a unit vector, s is the integration plane, ε is a minimum parameter, is the conjugate of the transverse component of the magnetic field; A(D) is the adiabatic parameter c of the linear adiabatic change lin (z) Export D f , D i For the boundary conditions mentioned above, the adiabatic parameters are simplified to The relationship between z and D is S33. Integrate both sides of equation (5) and substitute the boundary condition z(D i )=0,z(D f )=L, and the adiabatic parameter is obtained S34. Substituting equation (6) into equation (5), we can get the variation of the interval function D with the propagation distance z: S35. Substituting equation (7) into the taper function formula, we get the fast adiabatic taper function F(z) = (D f / L)z FAQUAD .

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