Polarization-independent lithium niobate cross waveguide based on multimode interference coupling structure

Abstract

The invention provides a polarization-independent lithium niobate crossed waveguide based on a multimode interference coupling structure, which comprises a silicon dioxide substrate, a lithium niobate waveguide and a silicon dioxide cladding, and is characterized in that the lithium niobate waveguide is located on the silicon dioxide substrate, and the silicon dioxide cladding covers the lithium niobate waveguide and the silicon dioxide substrate; the lithium niobate waveguide comprises a rectangular self-imaging area and four width optimization waveguides arranged around the self-imaging area, and the four width optimization waveguides are distributed in a circular array with the center point of the self-imaging area as the circle center and coincide with the four side faces of the self-imaging area respectively and are connected with the four side faces of the self-imaging area. The outer end of the width optimization waveguide is connected with an output waveguide or an input waveguide; the device supports simultaneous propagation of light in TE and TM modes, mutual interference is avoided, and transmission loss and crosstalk are almost consistent, that is, polarization independence is achieved; the LNOI material is adopted, and compared with a silicon material, the lithium niobate material has the advantages of being wide in transparent wave band, low in absorption loss, high in optical damage resistance threshold value and the like.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a polarization-independent lithium niobate cross waveguide based on a multi-mode interference coupling structure. Background Art

[0002] With the rapid development of optical communications, complex photonic systems often need to control and process light with different polarizations, so waveguide crossing is very necessary, which can perfectly transmit light under different polarizations. LNOI (Lithium Niobate on Insulator) is a new type of material. LNOI can well bind light in lithium niobate waveguides by taking advantage of the larger refractive index of lithium niobate than silicon dioxide. And benefiting from the large electro-optic coefficient of lithium niobate, it can be directly modulated, which makes it very convenient to integrate modulators and waveguide devices on the same material platform. This technology uses LNOI materials and realizes the self-focusing of TE and TM light waves at the intersection of waveguides based on the multimode interference (MMI) coupling structure. However, the self-focusing period of traditional MMI is long, which makes the device large in size, complex in structure, poor in versatility, and low in integration. At the same time, its performance in suppressing crosstalk needs to be further improved. Summary of the invention

[0003] In view of the problems in the prior art, the present invention provides a polarization-independent lithium niobate cross waveguide based on a multimode interference coupling structure, the purpose of which is to make the cross waveguide simple in process, small in size, compact in structure, and able to suppress crosstalk and loss, and easy to implement and manufacture.

[0004] A polarization-independent lithium niobate cross waveguide based on a multimode interference coupling structure, comprising a silicon dioxide substrate, a lithium niobate waveguide and a silicon dioxide cladding, wherein the lithium niobate waveguide is located on the silicon dioxide substrate, and the silicon dioxide cladding covers the lithium niobate waveguide and the silicon dioxide substrate; The lithium niobate waveguide includes a rectangular self-imaging area and four width-optimized waveguides arranged around the self-imaging area. The four width-optimized waveguides are distributed in a circular array with the center point of the self-imaging area as the center and overlap and are connected to the four sides of the self-imaging area respectively. An output waveguide or an input waveguide is connected to the outer end of the width-optimized waveguide. The width-optimized waveguide is evenly divided into A tapers along its long direction, and B width change points are evenly selected in the A tapers. The taper width between two adjacent unequal change points changes in a step-like manner, and the amount of change is C. The insertion loss of the lithium niobate waveguide is simulated by the PSO algorithm, and the width of the width change point is continuously optimized. After a set number of iterations, a lithium niobate waveguide with a smaller insertion loss is obtained. Among them, A and B are positive integers, and the value range of C is 10nm~20nm.

[0005] Further, A is 100, B is 13, C is 15nm, and the widths of the 13 width change points are 1.5μm, 1.55μm, 1.76μm, 1.60μm, 1.89μm, 1.89μm, 1.77μm, 1.77μm, 1.8μm, 1.81μm, 1.7μm, 1.8μm1, and 1.93μm from the outer end to the inner end of the width optimized waveguide.

[0006] Furthermore, the material of the lithium niobate waveguide is lithium niobate, which is an anisotropic crystal.

[0007] Furthermore, the thickness of the lithium niobate waveguide is 300 nm, the side wall inclination angle of the width-optimized waveguide is 70°, and the cross-section of the width-optimized waveguide is an isosceles trapezoid.

[0008] Furthermore, the width of the input waveguide and the output waveguide are both 1.2 μm, and the length of the width-optimized waveguide is 4.5 μm.

[0009] Further, the refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.13].

[0010] Furthermore, the self-imaging area is formed by the cross-overlapping parts of the width-optimized waveguides.

[0011] The beneficial effects of the present invention are as follows: 1. It supports the simultaneous propagation of light in TE and TM modes without mutual interference, and the transmission loss and crosstalk are almost the same, that is, polarization is independent. 2. The structure is simple and stable, and the size is very small, which is convenient for integration. 3. Using LNOI material, lithium niobate material has the characteristics of wide transparent band, low absorption loss, high optical damage threshold and the like compared with silicon material, especially it has very high nonlinear optical, electro-optical, acousto-optical and thermo-optical coefficients. 4. The use of the width optimized waveguide is conducive to reducing loss and crosstalk compared with the traditional rectangular cross waveguide. The transmission efficiency of the device structure of the polarization-independent lithium niobate cross waveguide based on the multi-mode interference coupling structure is not less than 93%, and the crosstalk is less than -30db. 5. Compared with the conical MMI waveguide cross structure, the device structure of the polarization-independent lithium niobate cross waveguide based on the multi-mode interference coupling structure has a symmetrical structure and only has even-order modes, and has higher transmission efficiency in micro-nano MMI waveguides. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Schematic diagram of the polarization-independent lithium niobate cross-waveguide structure based on the multi-mode interference coupling structure proposed in the present invention; Figure 2 A schematic cross-sectional view of the overall structure of the polarization-independent lithium niobate cross-waveguide based on the multi-mode interference coupling structure proposed in the present invention; Figure 3This is a schematic diagram of the simulation results of electric field transmission when TE mode light is input into the cross waveguide proposed in the present invention; Figure 4 This is a schematic diagram of the simulation results of electric field transmission when TM mode light is input into the cross waveguide proposed in the present invention; Figure 5 A schematic diagram of the transmission spectrum when TE mode light is input into the cross waveguide proposed in the present invention; Figure 6 Schematic diagram of the transmission spectrum when TM mode light is input into the cross waveguide proposed in the present invention. DETAILED DESCRIPTION

[0013] The present invention is described in detail below in conjunction with the accompanying drawings. The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention. The directional terms such as left, middle, right, top, and bottom in the embodiments of the present invention are only relative concepts or are based on the normal use state of the product, and should not be considered as restrictive.

[0014] Reference Figures 1 to 6 , a polarization-independent lithium niobate cross waveguide based on a multimode interference coupling structure, which is composed of the same vertical and horizontal parts that cross each other perpendicularly, and the vertical and horizontal parts are composed of an input waveguide 1, a first width optimized waveguide 2, a self-imaging area 3, a second width optimized waveguide 4, and an output waveguide 5. The entire structure is centrally symmetrical, and the center of symmetry is at the center of the self-imaging area. Among them, after the single-mode light enters the first width optimized waveguide 2 through the input waveguide 1, the width will suddenly change, and it will periodically change to single mode, multi-mode, and single mode during the propagation process, that is, a multi-mode interference effect is generated, and imaging points are periodically generated in the waveguide. The width change function of the optimized waveguide part described in the present invention is to control its multi-mode interference effect through the width change, so that the imaging point is formed at the center of the self-imaging area 3, so as to reduce the crosstalk caused by propagation to the waveguide intersection. Due to the central symmetry of the waveguide, after passing through the second width optimized waveguide 4, the second imaging point comes to the output waveguide 5 and outputs as single-mode light, which greatly reduces the loss during the propagation process.

[0015] Reference Figure 1In order to adapt to practical applications, the width of the front input waveguide area is selected as 1.2 microns in the present invention. The lengths of the first width optimized waveguide and the second width optimized waveguide are both 4.5 microns, which are extremely small and easy to integrate. The self-imaging area 3 is formed by the first width optimized waveguide 2, the second width optimized waveguide 4, and the cross-overlapping parts of the two longitudinally centrally symmetrical optimized waveguides, that is, the size of the self-imaging area 3 is formed by the ends of the four width optimized waveguides connected and spliced. In the present invention, the design is carried out by combining theoretical analysis with Finite-Difference Time-Domain (FDTD) simulation and Particle Swarm Optimization (PSO) to obtain the optimal device structure parameters.

[0016] All structural modules are made of LNOI material. At a wavelength of 1550 nanometers, the core refractive index is about 2.2, the cladding refractive index is 1.48, the LNOI waveguide side wall inclination angle θ is 70 degrees, and the waveguide thickness is 300 nanometers. The material of the lithium niobate waveguide is lithium niobate and it is anisotropic crystal. The refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.13]. Due to the anisotropy of the lithium niobate material and the asymmetry of the structure, the cross-waveguide based on the material has polarization dependence. There is no report on the polarization-independent cross-waveguide of the material. Through the design of the waveguide structure and the optimization of the algorithm, the structure described in the present invention can pass light in both TE and TM modes at the same time, and the insertion loss and crosstalk are extremely small, that is, a polarization-independent lithium niobate cross waveguide based on a multi-mode interference coupling structure is realized.

[0017] The particle swarm algorithm has the advantages of fast convergence speed, few parameters, simple and easy-to-implement algorithm. For high-dimensional optimization problems, it converges to the optimal solution faster than the genetic algorithm, but it also has the problem of falling into the local optimal solution, so it depends on good initialization. According to the PSO algorithm, we first create a 4.5-micron-long taper in the simulation software. In order to make its change uniform and reduce the spikes caused by width mutation, this taper is evenly divided into 100 small tapers. The small tapers in the middle parts except the two ends are connected end to end, and then 13 width change points are evenly obtained in this 4.5-micron taper. When the width between points changes, the width of each small taper between the two points changes in a step-like manner, with a change of 15 nanometers. The initial value and width change range are given by the PSO algorithm. After the parameter setting is completed, the optimization begins. The width of each linear taper will change in each simulation run. If the resulting insertion loss is reduced, the change is accepted; otherwise, the result of this run will be discarded. We set the maximum number of iterations to 10 and the iteration specification to 50. After iterative optimization, the center widths of each linear taper are 1.5μm, 1.55μm, 1.76μm, 1.60μm, 1.89μm, 1.89μm, 1.77μm, 1.77μm, 1.8μm, 1.81μm, 1.7μm, 1.8μm1, and 1.93μm respectively.

[0018] Reference Figure 2 , silicon dioxide is selected as the substrate material, and a lithium niobate sheet with a thickness of 0.3 microns is placed on the substrate. The refractive indexes of the silicon dioxide substrate and the lithium niobate waveguide are 1.48 and 2.2 respectively. Photolithography is performed on the lithium niobate sheet. Since the lithium niobate material is harder than the traditional silicon wafer, it is difficult to form a right angle by direct etching. Under the current etching conditions, the side wall inclination angle of the lithium niobate waveguide is generally 70 degrees, and the cross-section of the width-optimized waveguide is an isosceles trapezoid. The lithium niobate sheet is buried by a 2-micron silicon dioxide cladding.

[0019] Reference Figure 3 as well as Figure 4 , the light field analysis of the TE mode and the TM mode in the polarization-independent lithium niobate cross waveguide based on the multi-mode interference coupling structure is performed respectively. It can be seen that the changes in the light field colors of the TE and TM modes are regular, and the process of the light spot density changing from dense to sparse and then to dense is the multi-mode interference generated by single-mode light, and then periodically changing back to single-mode light output due to its self-imaging effect.

[0020] Reference Figure 5 as well as Figure 6FDTD simulations were performed on the TE mode and TM mode in the polarization-independent lithium niobate cross-waveguide based on the multimode interference coupling structure. The insertion loss of the TE mode light was about -0.15dB, and the crosstalk was below -32dB; the insertion loss of the TM mode light was lower than -0.4dB, and the crosstalk was below -31dB.

[0021] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A polarization-independent lithium niobate cross waveguide based on a multimode interference coupling structure, characterized in that: It includes a silicon dioxide substrate, a lithium niobate waveguide and a silicon dioxide cladding, wherein the lithium niobate waveguide is located on the silicon dioxide substrate, and the silicon dioxide cladding covers the lithium niobate waveguide and the silicon dioxide substrate; The lithium niobate waveguide includes a rectangular self-imaging area and four width-optimized waveguides arranged around the self-imaging area. The four width-optimized waveguides are distributed in a circular array with the center point of the self-imaging area as the center and overlap and are connected to the four sides of the self-imaging area respectively. An output waveguide or an input waveguide is connected to the outer end of the width-optimized waveguide. The width-optimized waveguide is evenly divided into A tapers along its long direction, and B width change points are evenly selected in the A tapers. The taper width between two adjacent unequal change points changes in a step-like manner, and the amount of change is C. The insertion loss of the lithium niobate waveguide is simulated by the PSO algorithm, and the width of the width change point is continuously optimized. After a set number of iterations, a lithium niobate waveguide with a smaller insertion loss is obtained. Among them, A and B are positive integers, and the value range of C is 10nm~20nm.

2. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 1, characterized in that: A is 100, B is 13, and C is 15nm; the widths of the 13 width change points are 1.5μm, 1.55μm, 1.76μm, 1.60μm, 1.89μm, 1.89μm, 1.77μm, 1.77μm, 1.8μm, 1.81μm, 1.7μm, 1.8μm1, and 1.93μm from the outer end to the inner end of the width optimized waveguide.

3. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 2, characterized in that: The material of the lithium niobate waveguide is lithium niobate, which is an anisotropic crystal.

4. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 2, characterized in that: The thickness of the lithium niobate waveguide is 300 nm, the sidewall inclination angle of the width-optimized waveguide is 70°, and the cross-section of the width-optimized waveguide is an isosceles trapezoid.

5. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 2, characterized in that: The width of the input and output waveguides are both 1.2 μm, and the length of the width-optimized waveguide is 4.5 μm.

6. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 2, characterized in that: The refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.13].

7. The polarization-independent lithium niobate cross-waveguide based on a multi-mode interference coupling structure according to claim 1, characterized in that: The self-imaging region is formed by the cross-overlapping sections of the width-optimized waveguides.

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