A multimode interference coupling structure double-mode cross waveguide based on LNOI material

Abstract

The application provides a multimode interference coupling structure double-mode cross waveguide based on LNOI material, which comprises a silica substrate, a lithium niobate waveguide and a silica cladding, the lithium niobate waveguide is located on the silica substrate, and the silica cladding covers the lithium niobate waveguide and the silica substrate; the lithium niobate waveguide comprises a rectangular self-imaging region and four width-optimized waveguides arranged around the self-imaging region, the four width-optimized waveguides are distributed in a circular array with the center point of the self-imaging region as the center and are respectively coincident with and connected to four side surfaces of the self-imaging region, and an output waveguide or an input waveguide is connected to the outer end of the width-optimized waveguide; the application supports the simultaneous propagation of TE0 and TE1 modes and the influence between the two modes is very small; the LNOI material is adopted, and the lithium niobate material has the characteristics of wide transparent waveband, low absorption loss and high optical damage threshold compared with the silicon material.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication technology, and particularly relates to a multimode interference coupling structure double-mode cross waveguide based on LNOI material. BACKGROUND

[0002] With the rapid development of optical communication, complex photonic systems usually need to transmit different modes of light at the same time, so waveguide crossing is very necessary, which can ensure low loss while transmitting multiple modes of light.

[0003] LNOI (Lithium Niobate on Insulator) is a new type of material, which can well confine light beams in the lithium niobate waveguide by using the characteristic that the refractive index of lithium niobate is larger than that of silicon dioxide. And thanks to the large electro-optic coefficient of lithium niobate, the modulation can be directly integrated on the same material platform.

[0004] The technology uses LNOI material, and realizes self-focusing of TE0 and TE1 modes of light waves at the waveguide crossing based on multimode interference (MMI) coupling structure, but the self-focusing period of the traditional MMI is long, so the device size is large, the structure is complex, the versatility is poor, the integration degree is low, and the performance of suppressing crosstalk needs to be further improved. SUMMARY

[0005] In view of the problems in the prior art, the present application provides a multimode interference coupling structure double-mode cross waveguide based on LNOI material, which aims to suppress crosstalk and loss, has small size, compact structure, is easy to realize and manufacture.

[0006] A multimode interference coupling structure double-mode cross waveguide based on LNOI material, comprising a silicon dioxide substrate, a lithium niobate waveguide and a silicon dioxide cladding, 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;

[0007] The lithium niobate waveguide comprises a rectangular self-imaging region and four width-optimized waveguides arranged around the self-imaging region, the four width-optimized waveguides are distributed in a circular array with the center point of the self-imaging region as the center and respectively coincide with and connect four side surfaces of the self-imaging region, and an output waveguide or an input waveguide is connected to the outer end of the width-optimized waveguide;

[0008] The long direction of the width-optimized waveguide is evenly divided into 100 tapers, 13 width change points are evenly selected from the 100 tapers, the width of the tapers between adjacent two unequal change points changes in a stepped manner, and the change amount is 15nm; the widths of the 13 width change points are 2.38um, 2.13um, 2.17um, 2.23um, 2.40um, 2.43um, 2.33um, 2.26um, 2.26um, 2.22um, 2.7um, 1.8um, 2.7um from the outer end to the inner end of the width-optimized waveguide.

[0009] Further, the material of the lithium niobate waveguide is lithium niobate and is an anisotropic crystal.

[0010] Further, the thickness of the lithium niobate waveguide is 300nm, the side wall inclination angle of the width-optimized waveguide is 70°, and the cross section of the width-optimized waveguide is isosceles trapezoidal.

[0011] Further, the widths of the input waveguide and the output waveguide are both 1.2um, and the length of the width-optimized waveguide is 4.5um.

[0012] Further, the refractive index of the lithium niobate waveguide is [nx, ny, nz]=[2.2111, 2.2111, 2.13].

[0013] Further, the self-imaging region is formed by the part of the width-optimized waveguide that overlaps.

[0014] The beneficial effects of the present application are: 1. Support TE0, TE1 mode light to propagate at the same time and the influence between them is minimal. 2. The structure is simple, stable, and the size is very small, which is convenient for integration. 3. LNOI material is used, and compared with silicon material, lithium niobate material has the characteristics of wide transparent waveband, low absorption loss, high optical damage threshold, etc., especially it has high nonlinear optics, electro-optics, acousto-optics, and thermal-optics coefficients. 4. The use of the width-optimized waveguide is beneficial to reduce loss and crosstalk compared with the traditional rectangular cross waveguide, and the transmission efficiency of the device structure of the multi-mode interference coupling structure double-mode cross waveguide based on LNOI material is not less than 88%, and the crosstalk is less than-30dB. 5. Compared with the tapered 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 even modes, and the transmission efficiency is higher in the micro-nano MMI waveguide. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The schematic diagram of the multi-mode interference coupling structure double-mode cross waveguide structure based on LNOI material proposed by the present application is shown in the figure.

[0016] Figure 2A cross-sectional schematic view of a whole structure of a multimode interference coupling structure double-mode cross waveguide based on LNOI material according to the present application;

[0017] Figure 3 A schematic view of electric field transmission simulation results when a cross waveguide according to the present application is input with TE0 mode light;

[0018] Figure 4 A schematic view of electric field transmission simulation results when a cross waveguide according to the present application is input with TE1 mode light;

[0019] Figure 5 A schematic view of transmission spectrum when a cross waveguide according to the present application is input with TE0 mode light;

[0020] Figure 6 A schematic view of transmission spectrum when a cross waveguide according to the present application is input with TE1 mode light. DETAILED DESCRIPTION

[0021] The present application will be described in detail below with reference to the accompanying drawings. The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the drawings, in which the same or similar reference numerals represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the drawings are exemplary and are for the purpose of explaining the present application only, and should not be interpreted as limiting the present application. The terms left, middle, right, top, bottom, and the like in the embodiments of the present application are merely relative concepts or are with reference to the normal use state of the product, and should not be considered as limiting.

[0022] A double-mode cross waveguide based on a multimode interference coupling structure of LNOI material, which is composed of the same mutually perpendicular intersecting vertical and horizontal parts, and each of the vertical and horizontal parts is composed of an input waveguide 1, a first width-optimized waveguide 2, a self-imaging region 3, a second width-optimized waveguide 4, and an output waveguide 5. The whole structure is center-symmetrical, and the center of symmetry is at the center of the self-imaging region. After single-mode light enters the first width-optimized waveguide 2 through the input waveguide 1, the width will change, and the light will periodically change into single mode, multimode, and single mode during propagation, that is, a multimode interference effect is generated, and an imaging point is periodically generated in the waveguide. The width change of the optimized waveguide part in the present application controls the multimode interference effect, so that the imaging point is formed at the center of the self-imaging region 3, thereby reducing the crosstalk caused by propagation to the waveguide intersection. Due to the center symmetry of the waveguide, the second imaging point comes to the output waveguide 5 as single-mode light after passing through the second width-optimized waveguide 4, thereby greatly reducing the loss during propagation.

[0023] REFERENCE Figure 1, the width of the front end input waveguide region is 1.2 microns in the application to adapt to practical applications, the length of the first width-optimized waveguide and the second width-optimized waveguide is 4.5 microns, which is extremely small and facilitates integration; the self-imaging region 3 is formed by the first width-optimized waveguide 2, the second width-optimized waveguide 4 and the part of the two longitudinally central symmetrical width-optimized waveguides that cross and overlap, that is, the size of the self-imaging region 3 is formed by the end connection and splicing of the four width-optimized waveguides. In the application, the method of combining theoretical analysis, finite-difference time-domain (FDTD) simulation and particle swarm optimization (PSO) is used to design the optimal device structure parameters.

[0024] All structural modules use LNOI material, the core refractive index of which is about 2.2 at a wavelength of 1550 nm, and the cladding refractive index is 1.48, the LNOI waveguide side wall angle is 70 degrees, and the waveguide thickness is 300 nanometers. The lithium niobate waveguide is made of lithium niobate and is an anisotropic crystal, and the refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.13]. Due to the limitation of current processing conditions, the processing difficulty of lithium niobate material is higher, and because the high-order mode light is more sensitive to small changes in waveguide structure, when the waveguide side wall has an angle, the high-order mode transmission may encounter more scattering points, resulting in greater loss. Moreover, because the TE1 mode light is often more scattered and has greater loss during propagation than the TE0 mode light and requires a wider waveguide for propagation, the TE0 mode light often has higher loss in a wider waveguide. The optimized waveguide described in the application is to optimize the structure of the MMI region to ensure that the TE0 can propagate in a wider waveguide with low insertion loss and low crosstalk while reducing the insertion loss and crosstalk of the TE1 mode light caused by the waveguide structure during propagation.

[0025] Particle swarm algorithm has the advantages of fast convergence speed, less parameters, simple algorithm and easy implementation. It converges to the optimal solution faster than genetic algorithm for high-dimensional optimization problems, but it is prone to fall into local optimal solution, so it depends on good initialization. According to the PSO algorithm, first we create a 4.5 micron long taper in the simulation software, in order to make the change uniform and reduce the spikes caused by the width mutation, the taper is uniformly divided into 100 small tapers, except for the small tapers in the middle of the two ends, then 13 width change points are evenly taken in the 4.5 micron taper. When the width between the points changes, the width of each small taper between the two points changes in a stepwise manner, with a change of 15 nanometers. Through the PSO algorithm, the initial value and the width change interval are given, and after the parameter setting is completed, the optimization starts. The width of each linear taper changes every time the simulation runs. If the insertion loss decreases, the change is accepted; otherwise, the running result will be discarded. We set the maximum number of iterations to 10 and the iteration specification to 50. After iteration optimization, the center width of each linear taper is 2.38 μm, 2.13 μm, 2.17 μm, 2.23 μm, 2.40 μm, 2.43 μm, 2.33 μm, 2.26 μm, 2.26 μm, 2.22 μm, 2.7 μm, 1.8 μm1, 2.7 μm.

[0026] Referring to Figure 2 Silicon dioxide is selected as the base material, and a layer of lithium niobate sheet with a thickness of 0.3 microns is placed on the base. The refractive indexes of the silicon dioxide base and the lithium niobate waveguide are 1.48 and 2.2, respectively. Photolithography is performed on the lithium niobate sheet. Since lithium niobate material is harder than traditional silicon wafer, it is difficult to form a right angle by direct etching. Under the current etching conditions, the sidewall inclination angle of the lithium niobate waveguide is generally 70 degrees, and the cross section of the width-optimized waveguide is isosceles trapezoidal. The lithium niobate sheet is buried by a 2-micron-thick silicon dioxide cladding layer.

[0027] Referring to Figure 3 and Figure 4 By analyzing the optical field of TE0 mode and TE1 mode in the multi-mode interference coupling structure double-mode cross waveguide based on LNOI material, it can be seen that the color change of TE0 and TE1 modes is regular, and the process of the spot density changing from dense to sparse and then to dense is the process of single-mode light producing multi-mode interference, and then periodically becoming single-mode light output due to the self-imaging effect.

[0028] Referring to Figure 5 and Figure 6The FDTD simulation is carried out on the TE0 mode and the TE1 mode in the multi-mode interference coupling structure double-mode cross waveguide based on the LNOI material respectively, the insertion loss of the TE0 mode light is about-0.25 dB, the crosstalk is below-31 dB; the insertion loss of the TE1 mode light is about-0.7 dB, the crosstalk is below-28 dB.

[0029] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A dual-mode cross waveguide with a multi-mode interference coupling structure based on LNOI material, characterized by: The invention comprises 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 region and four width-optimized waveguides arranged around the self-imaging region. The four width-optimized waveguides intersect each other perpendicularly at the center of the self-imaging region and overlap and connect with the four side surfaces of the self-imaging region respectively. The output waveguide or the input waveguide is connected to the outer end of the width-optimized waveguide. The width optimized waveguide is evenly divided into 100 tapers along its long direction, and 13 width change points are evenly selected in the 100 tapers. The taper width between two adjacent unequal change points changes in a step-like manner, and the change amount is 15nm; the widths of the 13 width change points are 2.38μm, 2.13μm, 2.17μm, 2.23μm, 2.40μm, 2.43μm, 2.33μm, 2.26μm, 2.26μm, 2.22μm, 2.7μm, 1.8μm1, and 2.7μm from the outer end to the inner end of the width optimized waveguide.

2. The LNOI material-based multimode interference coupling structure dual-mode cross waveguide according to claim 1, characterized in that: The material of the lithium niobate waveguide is lithium niobate, which is an anisotropic crystal.

3. The LNOI material-based multimode interference coupling structure dual-mode cross waveguide according to claim 1, 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.

4. The LNOI material-based multimode interference coupling structure dual-mode cross waveguide according to claim 1, characterized in that: The width of the input and output waveguides is 1.2 μm, and the length of the width-optimized waveguide is 4.5 μm.

5. The LNOI material-based multimode interference coupling structure dual-mode cross waveguide according to claim 1, characterized in that: The refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.13].

6. The LNOI material-based multimode interference coupling structure dual-mode cross waveguide according to claim 1, characterized in that: The self-imaging region is formed by the cross-overlapping sections of the width-optimized waveguides.

Images