Multi-mode interference coupler based on thin-film lithium niobate

Abstract

The invention discloses a multimode interference coupler based on thin-film lithium niobate. The multimode interference coupler comprises a silicon-based substrate, a protective layer and an optical waveguide. The optical waveguide is composed of an input waveguide, a multimode interference area and two identical output waveguides. The multi-mode interference area is composed of a multi-mode interference area A and a multi-mode interference area B; the output waveguide is composed of an output gradual change area C, an output gradual change area D and an output straight waveguide. The input gradual change waveguide, the multimode interference area A, the multimode interference area B, the output gradual change area C and the output gradual change area D are all of isosceles trapezoid structures. According to the invention, the additional loss is 0.009 dB under the wavelength of 1.55 [mu] m. Under the additional loss of 1dB, the bandwidth of the device is greater than 0.8 [mu] m. Compared with other MMI structures, the MMI structure has the advantages of being large in bandwidth, low in loss and high in process tolerance.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a multimode interference coupler based on thin-film lithium niobate. Background Art

[0002] Multimode interference (MMI) couplers have the advantages of excellent device performance, simpler manufacturing process, and more compact device structure. A multimode interference coupler with high bandwidth, low loss, and high process tolerance occupies a dominant position in photonic circuits such as electro-optic modulators, optical switches, and optical multiplexers. Summary of the invention

[0003] The purpose of the present invention is to provide a high process tolerance and low loss MMI structure design based on thin film lithium niobate, which solves the problems of high insertion loss, small working bandwidth and low process tolerance of the splitter while ensuring compact structure and good integration, and can be applied to related fields such as optical transceiver modules and optical quantum computing.

[0004] The multimode interference coupler provided by the present invention comprises a silicon-based substrate, a protective layer and an optical waveguide. The optical waveguide is made of a thin film lithium niobate material with a thickness of 0.3 um. The protective layer is located on the silicon-based substrate and the optical waveguide to completely wrap the optical waveguide.

[0005] The optical waveguide is a tapered structure that gradually expands in the transmission direction. The first part is the transformation of the optical waveguide transmission to a larger waveguide width; the second part is the multi-mode interference region, and the multi-mode interference region is divided into two parts; the third part is to transform the waveguide with a wide multi-mode interference region into two single-mode transmission waveguides.

[0006] The optical waveguide is composed of an input waveguide, a multimode interference region and two identical output waveguides; the input waveguide is composed of an input straight waveguide and an input gradient waveguide; the multimode interference region is composed of a multimode interference region A and a multimode interference region B; the output waveguide is composed of an output gradient region C, an output gradient region D and an output straight waveguide. The input gradient waveguide, the multimode interference region A, the multimode interference region B, the output gradient region C and the output gradient region D are all isosceles trapezoidal structures.

[0007] The optical waveguide is made of thin film lithium niobate material with a thickness of 0.3um. The input straight waveguide is butted with one end of the input gradient waveguide, and the width of the butt joint is W1=1.5um; the other end of the input gradient waveguide is butted with one end of the multimode interference region A, and the width of the butt joint is W2=2.5um; the other end of the multimode interference region A is butted with one end of the multimode interference region B, and the width of the butt joint is W3=5um; the other end of the multimode interference region B is butted with one end of the two output gradient regions C, and the width of the butt joint is W4=6um; the other ends of the two output gradient regions C are butted with the two output gradient regions D respectively, and the butt joint is W4=6um. The width at the joint is W5=2.5um; the other ends of the two output gradient areas D are respectively connected to the two output straight waveguides, and the width of the joint is W6=1.5um; the length of the input straight waveguide is L1=10um, the length of the input gradient waveguide is L2=10um, the length of the multimode interference area A is L3=5um, the length of the multimode interference area B is L4=22um, the length of the output gradient area C is L5=0.5um, the length of the output gradient area D is L6=5um, and the length of the output straight waveguide is L7=10um; the error of the above dimensions is ±10%.

[0008] Technical features and effects of the present invention:

[0009] The input and output waveguides of the present invention are both composed of gradient waveguides, and the multimode interference region uses a conical structure (or isosceles trapezoidal structure), and the multimode interference region changes from small to large. An imaging law of MMI is a process in which a single mode excites multiple modes, and the imaging position is also a process of continuously widening. The use of a gradient structure can increase the effective width of the multimode interference region, thereby reducing the influence of the wavelength on the imaging position, increasing the bandwidth of the device, and at the same time improving the process tolerance and bandwidth, and reducing the additional loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0011] Figure 2 A top view of the optical waveguide portion of the present invention;

[0012] Figure 3 The process tolerance comparison is for a conventional structure, a multi-mode interference region of a rectangle, and a structure of the present invention;

[0013] Figure 4 The bandwidth comparison is for a conventional structure, a multi-mode interference region of a rectangle, and a structure of the present invention.

[0014] Figure 1 Middle: 1. Input straight waveguide; 2. Input gradient waveguide; 3. Multimode interference region A; 4. Multimode interference region B; 5. Output gradient region C; 6. Output gradient region D; 7. Output straight waveguide. DETAILED DESCRIPTION

[0015] In this embodiment, the waveguide part is made of thin-film lithium niobate material, whose anisotropic refractive index is {2.21, 2.13, 2.21}, and the protective layer wrapping the waveguide part is made of silicon dioxide material, whose refractive index is 1.44. The substrate is made of silicon material, whose refractive index is 3.9.

[0016] In this embodiment, the thickness of the silicon substrate is 300 μm, the height of the silicon dioxide protection layer is 4.70 μm, and the height of the lithium niobate film is 0.6 μm.

[0017] The straight waveguide is composed of a ridge waveguide. In order to meet the conditions of single-mode transmission, the ridge width W1 of the ridge waveguide is designed to be 1.5μm, the ridge height is 0.3μm, and according to the process requirements, the etching angle is 70°, and the straight waveguide length L1 is 10μm.

[0018] The input and output waveguides are composed of gradient waveguides. The gradient waveguides at each position are continuous, and the width and length are optimized to obtain a low-loss MMI result. The wide waveguide input can effectively reduce the number of input modes, thereby reducing phase loss.

[0019] Because of the existing process accuracy problems, there are errors in etching accuracy. For MMI, a small change in size will have a very large impact on imaging, so process tolerance analysis is very necessary. Compared with the y-beam splitter, the process tolerance of the multi-mode interferometer has been improved, and the gradient MMI is proposed to improve the tolerance capability of the process.

[0020] The following is the design and simulation of the entire device model based on the lumerical eigenmode expansion method (EME). And optimize the various parameters of the model. The specific steps of modeling are as follows:

[0021] Modeling: Parametric modeling of the three-dimensional model of MMI, the width of the multi-mode interference is preset to 6μm, and the interference length of the one-to-two splitting is 22μm according to theoretical calculations. The height of the multi-mode interference is 0.3μm, and the height of the lithium niobate at the bottom is 0.3μm. The lithium niobate film is wrapped with silicon dioxide, and the material priority of the lithium niobate is set to 2; the priority of silicon dioxide is set to 3.

[0022] Material setting: The material used for thin film lithium niobate is anisotropic dispersion data. Since there is no lithium niobate data in Lumerical, it needs to be imported. For silicon dioxide, you can directly use the material provided in the software.

[0023] Boundary condition setting: Set the four sides of the model as PML boundary conditions.

[0024] Set the excitation port: There are three ports to set, each port is set to mode field. For each port mode analysis, set it to TE mode transmission.

[0025] Solution settings: In EME, the entire MMI structure is divided into six sections, and each section has a different division unit and excitation mode. The first section is the input straight waveguide, which is divided into 5 cells; the second section is the input gradient waveguide, which is divided into 10 cells; the third section is the multimode interference area 1, which is divided into 1 cell, and the mode is set to 50; the fourth section is the multimode interference area 2, which is set to 50 and divided into 1 cell; the fifth section is the output gradient area, which is divided into 10 cells; the sixth section is the output straight waveguide area, which is divided into 10 cells.

[0026] Mesh refinement: In order to increase the accuracy of the simulation and keep the simulation speed not too slow, the simulation mesh accuracy needs to be set reasonably. In the simulation part, mesh analysis is introduced to refine the mesh. The refinement accuracy is selected as one tenth of the height of the ridge waveguide, and the refinement part is selected as the input and output parts.

[0027] An additional loss for an MMI device is calculated as the sum of each output divided by the input:

[0028] EL(dB)=-10lg(∑P OUT / P IN );

[0029] According to the formula, the law of single shot length is:

[0030] Table 1 shows the output positions of single and multiple images under different input excitations.

[0031] This design selects symmetrical interference as the interference mode, and wants to obtain a binary image output position. Substitute the multi-mode interference width into the formula in the table to determine the interference length. In other words, each multi-mode interference width will correspond to an optimal interference length. In the next part, for the multi-mode interference width optimization part, the width and length are changed simultaneously, and the corresponding optimal interference length is obtained.

[0032] In order to satisfy the symmetric interference condition, the input waveguide is selected at the center of the device, while the output position is symmetrical to the center.

[0033] Table 1

[0034]

[0035] In order to better reflect the advantages of the structure of the present invention, the bandwidth and process tolerance of the designed structure will be analyzed. The process tolerance is mainly reflected in the error when etching the waveguide. During simulation, the error caused by the length of the entire device is mainly selected. When the deviation is 10%, the performance change of the device is reflected as follows Figure 3 As shown in the figure, the structure of the present invention has a significant improvement in resistance to process tolerance. As for bandwidth analysis, the wavelength is analyzed from 1.2μm to 2μm. Under the condition of 1dB loss, the bandwidth of structure II and structure III is greater than 0.8μm, which is significantly better than structure I. Figure 4 The structure of the present invention has excellent advantages in bandwidth, additional loss and process tolerance. At a light wavelength of 1.55, the additional loss of the present invention is 0.009 dB.

Claims

1. A multimode interference coupler, comprising a silicon-based substrate, a protective layer and an optical waveguide, characterized in that: The optical waveguide is composed of an input waveguide, a multimode interference region and two identical output waveguides; the input waveguide is composed of an input straight waveguide and an input gradient waveguide; the multimode interference region is composed of a multimode interference region A and a multimode interference region B; the output waveguide is composed of an output gradient region C, an output gradient region D and an output straight waveguide; the input straight waveguide is butted with one end of the input gradient waveguide, and the width of the butt joint is W1=1.5um; the other end of the input gradient waveguide is butted with one end of the multimode interference region A, and the width of the butt joint is W2=2.5um; the other end of the multimode interference region A is butted with one end of the multimode interference region B, and the width of the butt joint is W3=5um; the other end of the multimode interference region B is butted with one end of the two output gradient regions C, and the width of the butt joint is W4=6um; the other ends of the two output gradient regions C are butted with the two output gradient regions D respectively, and the width of the butt joint is W5=6um. The width of the two output gradient regions D is W5=2.5um; the other ends of the two output gradient regions D are respectively connected to the two output straight waveguides, and the width of the connection is W6=1.5um; the length of the input straight waveguide is L1=10um, the length of the input gradient waveguide is L2=10um, the length of the multimode interference region A is L3=5um, the length of the multimode interference region B is L4=22um, the length of the output gradient region C is L5=0.5um, the length of the output gradient region D is L6=5um, and the length of the output straight waveguide is L7=10um; the input gradient waveguide, the multimode interference region A, the multimode interference region B, the output gradient region C and the output gradient region D are all isosceles trapezoidal structures, and the optical waveguide is made of thin film lithium niobate material with a film thickness of 0.3um; the size errors of W1, W2, W3, W4, W5, W6, L1, L2, L3, L4, L5, L6, and L7 are ±10%.

2. The multimode interference coupler according to claim 1, characterized in that: The protective layer is located on the silicon-based substrate and the optical waveguide, and completely wraps the optical waveguide.

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