Low-loss wavelength division multiplexer coupling optimization method

Abstract

The invention discloses a coupling optimization method for a low-loss wavelength division multiplexer, and belongs to the technical field of optical coupling of wavelength division multiplexers. According to the method, firstly, the rib taper is matched with the large-core optical fiber, the mode size is properly reduced, the mode conversion problem in the large-core optical fiber is solved, in the optimization process, the input size of the two-stage taper is determined through modal overlapping analysis, and the optimal height and the high-performance length range of the double-taper are determined through EME light propagation embedded scanning. And secondly, the reduced mode is coupled into the submicron silicon waveguide by utilizing an inverted cone, and the required space can be minimized while the high conversion efficiency of the coupler is ensured by matching with an inverted cone multi-stage series structure. A rib cone structure is adopted, high coupling efficiency and compact size design are achieved, high tolerance is achieved for coupling between an optical fiber and a chip, and the optical fiber coupler is suitable for photon integrated circuits of different platforms; the multi-stage conical series structure adopted in the invention is good in performance under TM polarized light.

Description

Technical Field

[0001] The invention belongs to the technical field of optical coupling of wavelength division multiplexers, and more specifically, relates to a coupling optimization method for a low-loss wavelength division multiplexer. Background Art

[0002] Fiber waveguide mode conversion technology is a key technology in the field of optical communications and optical information processing. It involves how to effectively convert optical signals between different fiber modes. This conversion plays an important role in improving the transmission efficiency of optical networks, achieving wavelength management in high-density wavelength division multiplexing (DWDM) systems, and enhancing the flexibility and functionality of optical signal processing. However, traditional mode conversion technology often faces challenges such as low conversion efficiency, large device size, and high manufacturing precision requirements.

[0003] In order to obtain short and efficient tapered couplers suitable for high-density photonic integrated circuits, various configurations have been widely explored on different platforms, such as nonlinear tapers, step-cascade tapers, multilayer tapers, metamaterials, and double-tip and sinusoidal tapers. Recently, double-stage adiabatic mounted tapers have been widely adopted in the device waveguide mode conversion technology, showing significant advantages. This technology proposes to use designed waveguide structures to achieve high-efficiency conversion between waveguide modes in different situations by step-wise varying waveguide width or sparse distribution. K. Shiraishi's team used a two-stage structure, including a horizontal taper with variable width and a vertical taper with variable thickness, and successfully achieved a 0.5dB conversion transmission from the silicon wire waveguide to the 5.1×9.2μm mode. The connector length is about 2mm. For details, please refer to the literature "K. Shiraishi, H. Yoda, A. Ohshima, H. Ikedo, and CSTsai, "A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers," Appl. Phys. Lett. 2007, 91 (14), 141120". In order to further miniaturize the connector, a fiber-to-wave connector can be used, which contains two tapers with different thicknesses. With a total taper length of 151μm, this connector can achieve a 0.58dB connection. Please refer to the literature "J. Fernández, R. D. Doménech, C. Domínguez, and P. "Low-loss invertedtaper edge coupler in silicon nitride," IET Optoelectron, 2019, vol. 13, no. 2, pp. 62–66"). In addition, this method also combines a variety of Si3N4 thicknesses, but it may also lead to additional complexity and cost in the manufacturing process. Multi-level tapers have also been shown to be suitable for applications such as waveguide-to-waveguide and evanescent connectors in different platforms. For details, please refer to the literature "RFDangel et al., "Polymer waveguides enabling scalable low-loss adiabatic optical coupling for silicon photonics," IEEE J. Sel. Topics Quantum Electron., vol. 24, no. 4, Jul. / Aug. 2018, Art. no. 8200211". In the patent application with application number CN202210030543.2 and application name “Semiconductor component and its formation method and optical coupling system”, its multilayer structure and multiple connecting columns arranged at the edge of the semiconductor component form an edge connector, which can effectively couple the optical waveguide to the optical fiber located at the edge of the semiconductor component, with good coupling efficiency, low loss-related loss and compatibility with the complementary metal-oxide-semiconductor (CMOS) platform. In the patent application with application number CN202210210877.8 and application name “An optical waveguide multi-stage coupling mode division multiplexer”, the multiplexer disclosed therein includes an optical waveguide layer and an interruption layer. The patent application increases the power of the coupled mode by adding multiple coupling structures, and improves the connection efficiency between the optical waveguide modes while keeping the optical waveguide spacing unchanged. Although the multiplexer of the patent application has made some progress in improving the connection efficiency, how to further improve the conversion efficiency, reduce the size and cost of the equipment, and realize the conversion of more modes is still an important challenge facing this field. Summary of the invention

[0004] The present invention overcomes the shortcomings of the prior art in the conversion of optical fiber modes of wavelength division multiplexers, namely coupling, and provides a low-loss wavelength division multiplexer coupling optimization method to achieve high conversion efficiency and small footprint of its coupler.

[0005] The technical solution adopted by the present invention is:

[0006] A low-loss wavelength division multiplexer coupling optimization method, comprising:

[0007] The structure of a double-stage tapered coupler is set: the double-stage tapered coupler includes a cascaded rib-cone waveguide, an inverted-cone waveguide and an output waveguide; wherein the rib-cone waveguide is a stacked structure composed of two sub-rib-cone waveguides, the length (the length from the tip surface to the bottom surface), the rib cavity height (i.e., the thickness of the sub-rib-cone waveguide) and the bottom side length of the two sub-rib-cone waveguides are all the same, the bottom end surfaces of the two sub-rib-cone waveguides constitute a rectangular input surface of the double-stage tapered coupler, and the end surfaces of the rib cone tips of the two sub-rib-cone waveguides are rectangular, and the two sub-rib-cone waveguides are The end face lengths of the rib cone tips of the rib cone waveguides are the same, but the end face widths are different, and the end face width of the rib cone tip of the sub-rib cone waveguide located above is smaller; the inverted cone waveguide is a series structure formed by connecting multiple stages of cone waveguides in series, and the rib cone tip of the sub-rib cone waveguide below the rib cone waveguide is connected to the tip waveguide of the inverted cone waveguide, and the light entering from the rectangular input surface of the double-stage taper coupler enters the tip waveguide of the inverted cone waveguide through the rib cone waveguide, and then moves step by step to the last stage of the inverted cone waveguide, and finally reaches the output waveguide at the end;

[0008] The structural parameters of the double-stage tapered coupler are optimized through the following optimization steps:

[0009] Step 1, design the basic structure of the edge coupler, and perform finite difference eigenmode FDE optimization on the input surface size H×W and the fiber position of the edge coupler through modal overlap analysis;

[0010]

[0011] Wherein, overlap represents the modal overlap ratio, Re[] represents a mathematical symbol, which represents the real part of a complex number. A complex number consists of a real part and an imaginary part. The real part represents the projection of the complex number on the real axis or the real part. and Represent the electric field strength of mode 1 and mode 2 respectively, and Represent the magnetic field strength of mode 1 and mode 2 respectively, and S represents the area of ​​the input surface; overlap measures the proportion of the electromagnetic field that overlaps between two field distributions (i.e., two modes, two different modes are set based on actual conditions), which is also the power fraction in mode 2 that can propagate in mode 1 (for forward and backward propagating fields);

[0012] That is, the modal overlap rate overlap corresponding to different input surface sizes H×W is calculated, and the optimization result of the input surface size is obtained based on the input surface size H×W corresponding to the highest overlap;

[0013] Then, the input surface size H×W corresponding to the optimization result is used as the rectangular input surface size of the double-stage tapered coupler, wherein H is twice the rib cavity height h′ of the sub-rib tapered waveguide of the rib tapered waveguide of the double-stage tapered coupler, and W is the bottom side length of the sub-rib tapered waveguide;

[0014] Step 2, introducing and optimizing a rib-cone structure in the middle waveguide of the edge coupler to obtain a rib-cone waveguide of a double-stage tapered coupler;

[0015] Step 201, determining the narrowest rib cone tip width W of the sub-rib cone waveguide above the rib cone waveguide based on the minimum manufacturing width rib , where the width of the rib-cone tip of the sub-rib-cone waveguide below the rib-cone waveguide is greater than W rib big;

[0016] Step 202, setting the rib cone length of the double-stage taper coupler rib cone waveguide r and the inverted cone length of the inverted cone waveguide t In the range of values, under the premise that the rib cavity height h′ of the sub-rib tapered waveguide is the same as the inverted cone height of the inverted tapered waveguide, a double-stage tapered coupler structure is built on the (MODE) simulation platform. The rib cone length length that meets the specified light propagation conversion efficiency threshold is r and the length of the inverted cone t In the simulation value selection, the rib cone length is selected based on the highest conversion efficiency. r and the length of the inverted cone t The optimal value of , and the initial optimal rib cone length length r and the optimal inverted cone length t ;

[0017] Step 203, based on the optimal ratio of the desired inverted cone height and the rib cone height and the determined height H of the rectangular input surface of the double-stage tapered coupler, and obtain the initial optimal value of the inverted cone height h;

[0018] Taking the initial optimal value of the inverted cone height h as the reference value, adjust the value of the inverted cone height h, and build a double-stage taper coupler structure on the simulation platform for each value. r and the length of the inverted cone t The rib cone length corresponding to the highest conversion efficiency is selected from the simulation values r and the length of the inverted cone t , get the optimal rib cone length length for the current value of the inverted cone height h r and the optimal inverted cone length t ;

[0019] The optimal rib cone length length corresponding to the minimum inverted cone height h r and the optimal inverted cone length t Get the final optimal rib cone length r and the optimal inverted cone length t ; or the optimal rib cone length corresponding to the highest conversion efficiency r and the optimal inverted cone length t The value corresponding to the minimum inverted cone height h is selected to obtain the final optimal rib cone length length r and the optimal inverted cone length t ; Thus, taking the desired conversion efficiency threshold (such as 95% conversion efficiency) as the standard, the optimal length range of the rib cone and the inverted cone in the high-performance α range is further obtained;

[0020] Step 3, optimizing the inverted tapered waveguide of the double-stage tapered coupler;

[0021] Step 301, determine the minimum input width of the inverted tapered waveguide (tip waveguide) based on the current manufacturing process level, that is, determine the minimum input width of the inverted tapered waveguide based on the minimum manufacturing width, and use the final optimal rib cone length determined in step 2 as the final optimal rib cone length. r and the optimal inverted cone length t The corresponding value of the inverted cone height h is used as a reference, the value of the inverted cone height h is adjusted, the conversion efficiency under each value is obtained by simulation, and the optimal input size of the inverted cone waveguide is obtained based on the value corresponding to the maximum conversion efficiency and the minimum input width;

[0022] Step 302, determining the inverted cone shape of the inverted cone waveguide, fixing the input width and output width of each simulated inverted cone waveguide, obtaining the corresponding conversion efficiency and inverted cone length relationship curve, and taking the contour shape with high conversion efficiency and small inverted cone length as the inverted cone shape;

[0023] Step 303, optimizing the series structure of the inverted tapered waveguide:

[0024] Step 303-1, fix the input width w of the current tapered waveguide i and output width w o , get the inverted cone length length under different cone angles θ t And obtain the conversion efficiency of propagation under different cone angles θ through simulation;

[0025] Among them, θ and length t , w i , wo The relationship between them is: x represents the abscissa with the input end of the inverted tapered waveguide as the starting point of the horizontal axis x;

[0026] For the first-order tapered waveguide in the series structure, the input width w i The minimum input width of the inverted tapered waveguide is 0.2 μm in this invention. The output width w is the thinner the better in theory, but the manufacturing process is limited. o is the desired output width of the inverted tapered waveguide (i.e., w o The value of depends on the output width requirement of the device connected to the double-stage tapered coupler); starting from the second-stage tapered waveguide of the series structure, the input width w i is the output width of the previous tapered waveguide, the output width w o is the desired output width of the inverted tapered waveguide;

[0027] Based on the taper angle θ that meets the target conversion efficiency, the acceptable maximum taper angle θ of the current tapered waveguide is obtained: b , and based on the maximum cone angle θ b The corresponding inverted cone length t Get the initial length L of the current tapered waveguide b ;

[0028] Step 303-2, fix the input width w of the current tapered waveguide i and the maximum cone angle θ b , with an initial length L b The upper limit of the tapered length of the tapered waveguide at the current level is used to obtain the output width w at different length values. o And obtain the conversion efficiency of light propagation corresponding to different length values ​​through simulation;

[0029] The optimal length L of the current tapered waveguide is obtained based on the length that meets the target conversion efficiency. a , and based on the maximum cone angle θ b and the current input width w i , the optimal length L a and its corresponding output width w o Obtaining the optimized structure of the current-stage tapered waveguide;

[0030] Step 303-3, based on the input width w obtained in step 303-2 i , the optimal length L a and output width w o According to the formula Calculate the cone width W(x) at each value of x, based on the cone width W(x), input width w i , the optimal length La , output width w o and the maximum cone angle θ b Obtaining the optimized structure of the current-stage tapered waveguide;

[0031] Step 303-4, using the output width w of the current tapered waveguide o As the initial input width of the next-stage tapered waveguide, repeat steps 303-1 to 303-3 until the total length of all current tapered waveguides approaches the final optimal inverted cone length obtained in step 2. t , that is, the total length and length t The deviation is within the allowable range, thereby obtaining the final tapered waveguide formed by the series connection of multiple tapered waveguides. The present invention not only takes into account the manufacturing process, but also selects a relatively small ratio of the inverted cone height to the rib cone height and the total length of the inverted cone after series connection while ensuring high conversion efficiency.

[0032] Furthermore, in step 2, a double-stage taper coupler structure is constructed on the simulation platform, and the rib taper length length meets the specified light propagation conversion efficiency threshold. r and the length of the inverted cone t In the simulation value selection, the rib cone length is selected based on the highest conversion efficiency. r and the length of the inverted cone t The optimal value of , and the initial optimal rib cone length length r and the optimal inverted cone length t Specifically include:

[0033] (1) The double-stage taper coupler is divided into three cell groups according to the rib cone, inverted cone, and output waveguide structures. A grid is added to the inverted cone part, and a bidirectional eigenmode expansion (EME) solver is used to simulate light propagation.

[0034] (2) The length of the inverted cone and the output waveguide group is fixed by the control variable method, and the length of the rib cone group is scanned by EME. The EME scanning results are screened with the first target conversion rate to obtain the rib cone length. r and loss r )

[0035] (3) Using the control variable method to keep the length of the rib cone and the output waveguide group constant, perform EME scanning on the length of the inverted cone group and screen the EME scanning results with the first target conversion rate to obtain the inverted cone length t and loss t )

[0036] (4) Set up embedded scanning to filter out the length of the rib coner and the length of the inverted cone t Perform staggered scanning one by one to obtain different rib cone lengths r , inverted cone length t The relationship between the total length of the device and the loss, and the relationship between the total length of the device and the loss, where the total length of the device is the cone length r and the length of the inverted cone t sum;

[0037] (5) According to the highest conversion efficiency, select the initial optimal rib cone length r and the optimal inverted cone length t , that is, to select the rib cone length with the smallest total length and the smallest loss r and the optimal inverted cone length t .

[0038] Furthermore, the first target conversion rate is a conversion efficiency threshold of 95%.

[0039] Furthermore, in step 302, the shape of the outer contour includes a linear shape, a parabola shape, and an exponential shape.

[0040] Furthermore, in steps 303-1 and 303-2, the target conversion efficiency used is specifically:

[0041] If the width value (input width or output width) is lower than 500 nm, the target conversion efficiency selects a conversion efficiency threshold of 95%; if the width value is higher than or equal to 500 nm, the target conversion efficiency selects a conversion efficiency threshold of 97%.

[0042] The low-loss wavelength division multiplexer coupling optimization method of the present invention is implemented as follows: first, the rib taper is matched with the large core fiber to appropriately reduce the mode size, thereby solving the mode conversion problem in the large core fiber. In the optimization process, the input size of the two-stage taper is determined by modal overlap analysis, and the optimal height and high-performance length range of the double taper are determined by EME light propagation embedded scanning. Secondly, the reduced mode is coupled to the submicron silicon waveguide by using an inverted taper, and the inverted taper multi-stage series structure is used to ensure the high conversion efficiency of the coupler while minimizing the required space.

[0043] The technical solution provided by the present invention brings at least the following beneficial effects:

[0044] (1) The present invention adopts a rib cone structure, which has both high coupling efficiency and compact size design, has high tolerance for coupling between optical fiber and chip, and is suitable for photonic integrated circuits on different platforms;

[0045] (2) The multi-stage tapered series structure used in the present invention performs well under TM polarized light. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0047] Figure 1 is a schematic structural diagram of a double-stage tapered coupler according to an embodiment of the present invention;

[0048] Figure 2 is a flow chart of optimizing the input surface size of a coupler according to an embodiment of the present invention;

[0049] Figure 3 is a schematic diagram of the structure of an edge coupler of Example 1 of an embodiment of the present invention;

[0050] Figure 4 is a schematic cross-sectional view of SMF-28 in an example of the present invention;

[0051] Figure 5 is a mode cross-sectional view of an intermediate straight waveguide of Example 1 of an embodiment of the present invention;

[0052] Figure 6 is a double-cone length optimization flow chart of an embodiment of the present invention;

[0053] Figure 7 is a schematic diagram of the structure of a coupler according to embodiment 2 of the embodiment of the present invention;

[0054] Figure 8 is the rib cone length and loss (loss) of the embodiment of the present invention r ) relationship diagram;

[0055] Fig. 9 is the inverted cone length and loss (loss) of the embodiment of the present invention r ) relationship diagram;

[0056] Fig.10 is a height ratio optimization flow chart of an embodiment of the present invention;

[0057] Fig.11 It is a double cone dimension diagram of an embodiment of the present invention with an insertion loss less than 0.45dB;

[0058] Fig.12 It is a flow chart of inverted cone shape optimization according to an embodiment of the present invention.

[0059] Fig.13It is a schematic diagram of the inverted cone shape contour formula of an embodiment of the present invention. DETAILED DESCRIPTION

[0060] 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 described in detail and completely in conjunction with the drawings in the implementation of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings can be arranged and designed using 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 present application for protection, but merely represents selected embodiments of the present invention.

[0061] For ease of understanding, the relevant professional terms appearing in the embodiments of the present invention are explained as follows:

[0062] FDE (Finite Difference Eigenmode): finite difference eigenmode;

[0063] EME (Eigenmode Expansion): Eigenmode expansion;

[0064] S 21 : Refers to the attenuation parameter that describes the signal transmission from port 1 to port 2 in the RF system. Insertion loss is used to slow the signal during transmission, usually expressed in decibels (dB). The square of its absolute value is the conversion efficiency.

[0065] Figure 1The present invention is a schematic diagram of an optimized structure of a double-stage tapered coupler (also called a double-stage adiabatic evolved tapered coupler) proposed for the purpose of reducing the loss of a wavelength division multiplexer in an embodiment of the present invention. The double-stage tapered coupler is a new type of two-stage in-plane coupler, which combines a rib-cone and an anti-cone (inverted cone) structure. When preparing the double-stage tapered coupler, it includes a substrate (optionally a Si substrate), a cladding (optionally a SiO2 cladding), and cascaded rib-cone waveguides, inverted cone waveguides, and output waveguides, wherein the cladding is located on the substrate, and the cladding is the upper cladding of the rib-cone waveguide and the inverted cone waveguide, and the double-stage tapered coupler obtained after preparation only includes cascaded rib-cone waveguides, inverted cone waveguides, and output waveguides. The rib-conical waveguide is a combined structure of two stacked sub-rib-conical waveguides, the rib cavities of the two sub-rib-conical waveguides gradually become thinner along the light propagation direction of the coupler, and the length (length from the tip surface to the bottom surface), height (rib cavity height) and bottom length of the two sub-rib-conical waveguides are the same, and the bottom end surfaces of the two sub-rib-conical waveguides constitute a rectangular input surface of the double-stage tapered coupler, and the size of the input surface is defined as H×W, wherein H is twice the rib cavity height of the sub-rib-conical waveguide, and W is the bottom length of the sub-rib-conical waveguide; and the top end surfaces of the two sub-rib-conical waveguides are rectangular, and the position The angle between the side and bottom of the upper sub-rib cone waveguide is smaller than the angle between the side and bottom of the lower sub-rib cone waveguide, that is, the rib cone tip end face lengths of the two sub-rib cone waveguides are the same, but the end face widths are different, and the width of the upper rib cone tip is smaller, so that the light from the optical fiber is coupled to the input surface with matching mode size. After entering the rib cone waveguide, when the rib cavity becomes narrow enough, the light field is mainly confined to the wider bottom of the rib cone waveguide (that is, the sub-rib cone waveguide located below), thereby matching the light field from the large optical fiber to the smaller intermediate waveguide. The inverted cone waveguide is a combined structure formed by multiple levels of cone waveguides in series. The top end face of the sub-rib cone waveguide below the rib cone waveguide is connected to the tip waveguide of the inverted cone waveguide. The light entering the rib cone waveguide starts from the tip waveguide of the inverted cone waveguide, and enters the last level of the inverted cone waveguide step by step, and finally reaches the output waveguide at the end.

[0066] The embodiment of the present invention also provides a low-loss wavelength division multiplexer coupling optimization method to obtain Figure 1 The optimized structure of the double-stage tapered coupler shown in the figure uses the rib tapered shape to match the large core fiber to moderately reduce the mode size, thereby realizing mode conversion in the large core fiber. The optimal structural parameters are determined using FDE modal overlap analysis and EME light propagation scanning. The multi-stage tapered series is used as an inverted cone structure to couple the reduced mode into the submicron silicon waveguide. The optimized design of the multi-stage series inverted cone combined with the rib cone achieves high conversion efficiency and small footprint of the coupler.

[0067] As a possible implementation manner, the specific implementation steps of the low-loss wavelength division multiplexer coupling optimization method provided in the embodiment of the present invention include:

[0068] Step S1, determine the optimal input surface size of the double-stage taper coupler, the specific process is as follows Figure 2 Said, including;

[0069] Step S1.1, design the following simulation structure as Example 1, build the edge coupler basic structure, and perform FDE optimization on the input surface size H×W and the fiber position through modal overlap analysis; design the edge coupler basic structure as follows Figure 3 , and built on the Lumerical simulation platform. The structure will consist of a substrate, an input high-refractive index waveguide with a uniform y span, a tapered section of a high-refractive index waveguide with a varying y span, and a low-refractive index polymer waveguide. Its structural parameters are shown in Table 1, the SMF-28 parameters are shown in Table 2, and the FDE solver is added as shown in Table 3, where x, y, and z represent the coordinates of the horizontal center point of the structure;

[0070] Table 1 Example 1 - Edge coupler structure parameter table

[0071]

[0072] Table 2 Example 1-SMF-28 structural parameters

[0073]

[0074]

[0075] Table 3 Example 1 - FDE solver parameter table

[0076] name property value General background index 1 Geometry x, y, z (μm) 0、0、0 y, z span (μm) 30、14

[0077] S1.2. Enable the "SMF-28" object and disable "taper". Run FDE and calculate the mode. Right-click the basic TE mode to add to the global deck. The mode field profile of SMF-28 is as follows: Figure 4 Switch to layout. Disable "SMF-28" and enable "Taper". Run FDE to calculate the modes. The mode field profile of the straight waveguide is as follows: Figure 5As shown. Select the fundamental TE mode of the waveguide and click the Overlap Analysis tap under the Eigenmode Analysis window. Select the fiber mode saved to the global panel and calculate the overlap between the two modes. Click "Optimize Position" to calculate the optimized fiber position for maximum modal overlap. The length and width of the input waveguide are discretely scanned in the range of 9 to 15μm. In this case, the fiber is offset by 7.30336e-12μm in the y direction and 0.0503861μm in the z direction to achieve an optimal overlap of 97.6642% and a power coupling of 0.976636. The input surface size is determined to be 12.3*11.3μm;

[0078] S2. Optimize the parameters related to the rib-cone waveguide. The process is as follows: Figure 6 As shown. A tapered coupler is built on the simulation platform, and a rib-cone structure of a rib-cone waveguide is introduced into the middle waveguide as Example 2, and the parameters are shown in Table 4;

[0079] Table 4 Example 2 - Coupler structure parameter table

[0080]

[0081]

[0082] S2.1. Build a new coupler structure and introduce a rib cone structure in the middle waveguide, such as Figure 7 , the relevant parameters are shown in Table 3. In this example, the rib cone tip width W rib 0.5μm;

[0083] S2.2, EME solver and adding mesh;

[0084] S2.2.1. Add an EME solver, where the size should cover the entire coupler, the simulation wavelength is 1.31 μm, and the background refractive index is 1. Divide the coupler into three cell groups according to the rib cone, inverted cone, and output waveguide structure;

[0085] S2.2.2Add a grid to the inverted cone, set the grid multiplier to 1*5*5, and the size should also cover the entire coupler;

[0086] S2.3 Use the control variable method to fix the length of the inverted cone and the output waveguide group unchanged, and perform EME scanning on the length of the rib cone group. Take 0.176dB (95% conversion efficiency) as the minimum insertion loss standard, screen the EME scanning results, and obtain the relationship between the rib cone length and loss (loss r )

[0087] S2.3.1 first performs EME propagate, then makes 201 scanning points in the range of 400-600μm with a gap of 1 for group span 2 (rib cone length), and performs EME scanning. The results are visualized, and the square of the absolute value of S21 is extracted to obtain the rib cone length and loss (loss r ) relationship, such as Figure 8 As shown;

[0088] S2.3.2 Return to layout mode and perform EME propagate again. Then, make 201 scanning points in the range of 200-600μm with a gap of 1 for group span 3 (cone length) and perform EME scanning. Visualize the results and extract the square of the absolute value of S21 to obtain the inverted cone length and loss (loss r ) relationship, such as Fig. 9 As shown;

[0089] S2.3.3 takes 0.176dB (95% conversion efficiency) insertion loss as the minimum standard, and screens the EME scanning results of S2.3.1 and 2.3.2, and finds that the rib cone has good performance in the range of 300-500μm and the inverted cone in the range of 300-400μm;

[0090] S3 optimizes the height ratio of the rib cone (rib cone waveguide) and the inverted cone (inverted cone waveguide). The optimization process is as follows: Fig.10 shown.

[0091] S3.2.1 Since the input surface size is fixed, the height ratio can be changed by changing the height of the rib cone. For different height ratios (i.e., different rib cone heights), embedded scanning is set, and high-performance rib cones (300-500μm) and inverted cone length ranges (300-400μm) are interlaced scanned one by one to obtain the relationship between different rib cones, inverted cone lengths and losses (LOSS) that can meet the minimum loss of 0.176dB and the relationship between the total length of the device, i.e., the sum of the rib cone and inverted cone lengths, and the loss. Fig.11 The relationship between conversion efficiency and double cone length is shown when α is 0.5. The red part represents the optimal performance range with efficiency higher than 0.95.

[0092] S4 optimizes the input height of the inverted cone;

[0093] S4.1 From S3, it can be seen that when the rib cone length is 400μm, the length of the inverted cone can reach more than 95% in the range of 200-600μm, so when optimizing the inverted cone, the rib cone length is taken as 400μm. The inverted cone width is as sharp as possible according to the manufacturing process, and is taken as 0.05μm in this example. The output waveguide width determines the output width of the inverted cone, and is taken as 6μm in this example. Build the coupler structure of Example 3 according to the above dimensions;

[0094] S4.2 changes the input height of the inverted cone, and takes the optimal inverted cone input height when the square of the absolute value of S21 is the largest;

[0095] S5 optimizes the inverted cone shape structure, and its process is as follows Fig.12 As shown;

[0096] S5.1 Three types of couplers were built on the MODE simulation platform, where the contours of the inverted cone structure were set to linear, parabolic, exponential and other research shapes, and their functions were shown below, with the lengths all set to 320 μm;

[0097]

[0098] In the above formulas (1) to (3), the x-axis is defined with the leftmost end of the graph as the origin, and the meanings of the other variables are as follows: Fig.13 As shown in the figure, x represents the coordinate value of the x-axis, W(x) represents the width of the inverted cone at x, and the subscripts 1 to 3 of W(x) are used to distinguish different shapes, namely, linear, parabolic, and exponential, respectively. i Indicates the input width of the inverted cone, W0 indicates the input width of the inverted cone, L t Indicates the length of the inverted cone.

[0099] S5.2 uses EME solver to simulate light propagation and scans the length of the inverted cone to obtain the relationship curve between the conversion efficiency and length under various shapes. The comparative analysis method shows that the linear shape not only has good performance but also simple manufacturing process, so the inverted cone shape is determined to be linear;

[0100] S5.3 First level cone iterative optimization process;

[0101] S5.3.1 Fixed the inverted cone width to 0.05 μm, the inverted cone output width to 6 μm, and selected the 0.2 dB insertion loss threshold level to determine the maximum acceptable θ for each taper level b , then the initial length L is obtained b ;

[0102] S3.5.2 Fixed W i and θ b , explore the relationship between the taper length and insertion loss. When the conversion efficiency reaches the same level as step S5.3.1, take the current taper length as L a , at this time W i , W0, θ b , L a This is the optimized cone.

[0103] S3.6 Repeat step S3.5 until the total length is close to Fig.11The optimized length range, taking into account the manufacturing process, while ensuring high conversion efficiency, chooses as small α and the total length of the inverted cones in series as possible, so as to obtain Figure 1 The final optimized structure is shown.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0105] The above are only some embodiments of the present invention. For those skilled in the art, several modifications and improvements can be made without departing from the creative concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A low-loss wavelength division multiplexer coupling optimization method, characterized in that: include: The structure of the double-stage tapered coupler is set: the double-stage tapered coupler includes a cascaded rib-conical waveguide, an inverted-conical waveguide and an output waveguide; wherein the rib-conical waveguide is a stacked structure composed of two sub-rib-conical waveguides, the lengths, rib cavity heights and bottom edge lengths of the two sub-rib-conical waveguides are the same, the bottom end surfaces of the two sub-rib-conical waveguides constitute a rectangular input surface of the double-stage tapered coupler, and the end surfaces of the rib cone tips of the two sub-rib-conical waveguides are rectangular, and the end surfaces of the rib cone tips of the two sub-rib-conical waveguides are The surface lengths are the same, but the end surface widths are different. The end surface width of the rib cone tip of the sub-rib cone waveguide located above is smaller. The inverted cone waveguide is a series structure formed by connecting multiple stages of cone waveguides in series. The rib cone tip of the sub-rib cone waveguide below the rib cone waveguide is connected to the tip waveguide of the inverted cone waveguide. Light entering from the rectangular input surface of the double-stage taper coupler enters the tip waveguide of the inverted cone waveguide through the rib cone waveguide, and then moves step by step to the last stage of the inverted cone waveguide, and finally reaches the output waveguide at the end. The structural parameters of the double-stage tapered coupler are optimized through the following optimization steps: Step 1, design the basic structure of the edge coupler, and perform finite difference eigenmode FDE optimization on the input surface size H×W and the optical fiber position of the edge coupler through modal overlap analysis; calculate the modal overlap rate overlap corresponding to different input surface sizes H×W, and obtain the optimization result of the input surface size based on the input surface size H×W corresponding to the highest overlap; Then, the input surface size H×W corresponding to the optimization result is used as the rectangular input surface size of the double-stage tapered coupler, wherein H is twice the rib cavity height h′ of the sub-rib tapered waveguide of the rib tapered waveguide of the double-stage tapered coupler, and W is the bottom side length of the sub-rib tapered waveguide; Step 2, introducing and optimizing a rib-cone structure in the middle waveguide of the edge coupler to obtain a rib-cone waveguide of a double-stage tapered coupler; Step 201, determining the narrowest rib cone tip width W of the sub-rib cone waveguide above the rib cone waveguide based on the minimum manufacturing width rib , where the width of the rib-cone tip of the sub-rib-cone waveguide below the rib-cone waveguide is greater than W rib big; Step 202, setting the rib cone length of the double-stage taper coupler rib cone waveguide r and the inverted cone length of the inverted cone waveguide t In the range of values, under the premise that the rib cavity height h′ of the sub-rib tapered waveguide is the same as the inverted cone height of the inverted tapered waveguide, a double-stage tapered coupler structure is built on the simulation platform. The rib cone length length that meets the specified light propagation conversion efficiency threshold is r and the length of the inverted cone t In the simulation value selection, the rib cone length is selected based on the highest conversion efficiency. r and the length of the inverted cone t The optimal value of , and the initial optimal rib cone length length r and the optimal inverted cone length t ; Step 203, based on the optimal ratio of the desired inverted cone height and the rib cone height and the determined height H of the rectangular input surface of the double-stage tapered coupler, and obtain the initial optimal value of the inverted cone height h; Taking the initial optimal value of the inverted cone height h as the reference value, adjust the value of the inverted cone height h, and build a double-stage taper coupler structure on the simulation platform for each value. r and the length of the inverted cone t The rib cone length corresponding to the highest conversion efficiency is selected from the simulation values r and the length of the inverted cone t , get the optimal rib cone length length for the current value of the inverted cone height h r and the optimal inverted cone length t ; The optimal rib cone length length corresponding to the minimum inverted cone height h r and the optimal inverted cone length t Get the final optimal rib cone length r and the optimal inverted cone length t ; or the optimal rib cone length corresponding to the highest conversion efficiency r and the optimal inverted cone length t The value corresponding to the minimum inverted cone height h is selected to obtain the final optimal rib cone length length r and the optimal inverted cone length t ; Step 3, optimizing the inverted tapered waveguide of the double-stage tapered coupler; Step 301, determining the minimum input width of the inverted tapered waveguide based on the minimum manufacturing width, and using the final optimal rib cone length determined in step 2 as the final optimal rib cone length. r and the optimal inverted cone length t The corresponding value of the inverted cone height h is used as a reference, the value of the inverted cone height h is adjusted, the conversion efficiency under each value is obtained by simulation, and the optimal input size of the inverted cone waveguide is obtained based on the value corresponding to the maximum conversion efficiency and the minimum input width; Step 302, determining the inverted cone shape of the inverted cone waveguide, fixing the input width and output width of each simulated inverted cone waveguide, obtaining the corresponding conversion efficiency and inverted cone length relationship curve, and taking the contour shape with high conversion efficiency and small inverted cone length as the inverted cone shape; Step 303, optimizing the series structure of the inverted tapered waveguide: Step 303-1, fix the input width w of the current tapered waveguide i and output width w o , get the inverted cone length length under different cone angles θ t And the conversion efficiency of propagation under different cone angles θ is obtained through simulation; Among them, θ and length t , w i , w o The relationship between them is: x represents the abscissa with the input end of the inverted tapered waveguide as the starting point of the horizontal axis x; For the first-order tapered waveguide in the series structure, the input width w i The minimum input width of the inverted tapered waveguide is 0.2 μm in this invention. The output width w is the thinner the better in theory, but the manufacturing process is limited. o is the desired output width of the inverted tapered waveguide; starting from the second-stage tapered waveguide in the series structure, the input width w i is the output width of the previous tapered waveguide, the output width w o is the desired output width of the inverted tapered waveguide; Based on the taper angle θ that meets the target conversion efficiency, the acceptable maximum taper angle θ of the current tapered waveguide is obtained: b , and based on the maximum cone angle θ b The corresponding inverted cone length t Get the initial length L of the current tapered waveguide b ; Step 303-2, fix the input width w of the current tapered waveguide i and the maximum cone angle θ b , with an initial length L b The upper limit of the tapered length of the tapered waveguide at the current level is used to obtain the output width w at different length values. o And the conversion efficiency of light propagation corresponding to different length values ​​is obtained through simulation; The optimal length L of the current tapered waveguide is obtained based on the length that meets the target conversion efficiency. a , and based on the maximum cone angle θ b and the current input width w i , the optimal length L a and its corresponding output width w o Obtaining the optimized structure of the current-stage tapered waveguide; Step 303-3, based on the input width w obtained in step 303-2 i , the optimal length L a and output width w o According to the formula Calculate the cone width W(x) at each value of x, based on the cone width W(x), input width w i , the optimal length L a , output width w o and the maximum cone angle θ b Obtaining the optimized structure of the current-stage tapered waveguide; Step 303-4, using the output width w of the current tapered waveguide o As the initial input width of the next-stage tapered waveguide, repeat steps 303-1 to 303-3 until the total length of all current tapered waveguides approaches the final optimal inverted cone length obtained in step 2. t .

2. The method according to claim 1, characterized in that, in step 2, a double-stage tapered coupler structure is built on a simulation platform, and the rib cone length length that meets the specified light propagation conversion efficiency threshold is r and the length of the inverted cone t In the simulation value selection, the rib cone length is selected based on the highest conversion efficiency. r and the length of the inverted cone t The optimal value of , and the initial optimal rib cone length length r and the optimal inverted cone length t Specifically include: (1) The two-stage taper coupler is divided into three cell groups according to the rib cone, inverted cone, and output waveguide structures, and a grid is added to the inverted cone part. The bidirectional eigenmode expansion EME solver is used to simulate light propagation; (2) The length of the inverted cone and the output waveguide group is fixed by the control variable method, and the length of the rib cone group is scanned by EME. The EME scanning results are screened with the first target conversion rate to obtain the rib cone length. r Relationship with loss; (3) Using the control variable method to keep the length of the rib cone and the output waveguide group constant, perform EME scanning on the length of the inverted cone group and screen the EME scanning results with the first target conversion rate to obtain the inverted cone length t Relationship with loss; (4) Set up embedded scanning to filter out the length of the rib cone r and the length of the inverted cone t Perform staggered scanning one by one to obtain different rib cone lengths r , inverted cone length t The relationship between the total length of the device and the loss, and the relationship between the total length of the device and the loss, where the total length of the device is the cone length r and the length of the inverted cone t sum; (5) According to the highest conversion efficiency, select the initial optimal rib cone length r and the optimal inverted cone length t , that is, to select the rib cone length with the smallest total length and the smallest loss r and the optimal inverted cone length t .

3. The method of claim 2, wherein the first target conversion rate is a conversion efficiency threshold of 95%.

4. The method according to claim 2, characterized in that, in step 302, the shape of the outer contour includes a linear type, a parabolic type and an exponential type.

5. The method according to claim 2, characterized in that, in steps 303-1 and 303-2, the target conversion efficiency used is specifically: If the width value is lower than 500 nm, the target conversion efficiency selects a conversion efficiency threshold of 95%; if the width value is higher than or equal to 500 nm, the target conversion efficiency selects a conversion efficiency threshold of 97%.

Images