A low-loss wavelength division multiplexer coupling optimization method

Abstract

The application discloses a low-loss wavelength division multiplexer coupling optimization method, and belongs to the optical coupling technical field of the wavelength division multiplexer. First, rib taper is utilized to match with a large-core optical fiber, and mode size is moderately reduced, so that the mode conversion problem in the large-core optical fiber is solved; in the optimization process, the input size of two-stage taper is determined by using modal overlap analysis, and the optimal height of the double taper and the high-performance length range are determined by using EME optical propagation embedded scanning. Secondly, the reduced mode is coupled into a submicron silicon waveguide by using an inverted taper, and the inverted taper multi-stage series structure is matched, so that high conversion efficiency of the coupler is ensured, and the required space is minimized. In the application, the rib taper structure is adopted, high coupling efficiency and compact size design are combined, high tolerance between the optical fiber and the chip is achieved, and the application is suitable for photonic integrated circuits of different platforms; the multi-stage taper series structure adopted in the application performs well under TM polarized light.

Description

Technical Field

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

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

[0003] To obtain short and efficient tapered couplers suitable for high-density photonic integrated circuits, various configurations have been extensively explored on different platforms, such as nonlinear tapers, stepped cascade tapers, multilayer tapers, metamaterials, as well as double-tip and sinusoidal tapers. Recently, double-stage adiabatically mounted tapers have been widely adopted in device waveguide mode conversion technology, showing significant advantages. This technology proposes the use of designed waveguide structures to achieve high-efficiency conversion between waveguide modes under different conditions by gradually varying the 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. When the total taper length is 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). This approach also combines various Si3N4 thicknesses, but can also lead to additional manufacturing complexity and cost. 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, see the article “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, method for forming the same, and optical coupling system”, its multi-layer structure and multiple connecting posts arranged at the edge of the semiconductor component form an edge connector that 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 coupled mode division multiplexer”, the multiplexer disclosed therein includes an optical waveguide layer and a break layer. By adding multiple coupling structures, the coupled mode power is increased, and the connection efficiency between the optical waveguide modes is improved while the optical waveguide spacing remains 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 device size and cost, and achieve the conversion of more modes remains an important challenge facing this field. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art in wavelength division multiplexer optical fiber mode conversion, namely coupling, the present invention 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 in the present invention is:

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

[0007] The structure of the 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 to the bottom end), 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 faces of the two sub-rib-cone waveguides constitute the rectangular input face of the double-stage tapered coupler, and the end faces of the rib cone tips of the two sub-rib-cone waveguides are rectangular, and the two sub-rib-cone waveguides are The rib-cone tips of the rib-cone waveguides have the same end face length but different end face widths. The rib-cone tip of the upper sub-rib-cone waveguide has a smaller end face width. The inverted-cone waveguide is a series structure formed by multiple stages of tapered 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 passes through the rib-cone waveguide and enters the tip waveguide of the inverted-cone waveguide, then advances 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 of the edge coupler input surface dimensions (H×W) and fiber position through modal overlap analysis.

[0010]

[0011] Among them, overlap represents the modal overlap rate, 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, which are set differently based on actual conditions), which is also the power fraction in mode 2 that can propagate in mode 1 (for both forward and backward propagating fields);

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

[0013] 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, where 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 a rib-tapered structure into the middle waveguide of the edge coupler and optimizing it to obtain a rib-tapered waveguide of a double-stage tapered coupler;

[0015] Step 201: Determine 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 , wherein 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: Set 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 The value range of the sub-rib tapered waveguide is that the rib cavity height h' of the sub-rib tapered waveguide is the same as the inverted cone height of the inverted cone 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 inverted cone length t In the simulation value, the rib cone length is selected based on the highest conversion efficiency. r and the inverted cone length t The optimal value of , and the initial optimal rib cone length is obtained r and the optimal inverted cone length t ;

[0017] Step 203: Optimal ratio of desired inverted cone height to 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 inverted cone length t The rib cone length corresponding to the highest conversion efficiency is selected from the simulation values r and the inverted cone length 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] based on the optimal rib taper length length corresponding to the minimum inverted taper height h r and the optimal inverted taper length length t to obtain the final optimal rib taper length length r and the optimal inverted taper length length t ; or the optimal rib taper length length corresponding to the highest conversion efficiency r and the optimal inverted taper length length t , the value corresponding to the minimum inverted taper height h is filtered out to obtain the final optimal rib taper length length r and the optimal inverted taper length length t ; so as to further obtain the optimal length range of the rib taper and the inverted taper under the high-performance α range based on the expected conversion efficiency threshold (such as 95% conversion efficiency) as the standard;

[0020] Step 3, optimizing the inverted taper waveguide of the two-stage taper coupler;

[0021] Step 301, determining the minimum input width of the inverted taper waveguide (tip waveguide) based on the current manufacturing process level, that is, determining the minimum input width of the inverted taper waveguide based on the minimum manufacturing width, and taking the value of the inverted taper height h corresponding to the final optimal rib taper length length r and the optimal inverted taper length length t determined in step 2 as the reference, adjusting the value of the inverted taper height h, simulating to obtain the conversion efficiency under each value, and obtaining the optimal input size of the inverted taper waveguide based on the value corresponding to the maximum conversion efficiency and the minimum input width;

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

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

[0024] Step 303-1, fixing the input width w i and the output width w o of the current stage taper waveguide, obtaining the inverted taper length length t under different taper angles θ, and obtaining the conversion efficiency of the propagation under different taper angles θ through simulation;

[0025] wherein, 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-stage tapered waveguide in series, the input width w i The minimum input width of the inverted tapered waveguide is 0.2 μm. The output width w is the smallest possible. Theoretically, the thinner the better, 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 tapered angle θ that meets the target conversion efficiency, the acceptable maximum tapered 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 under 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 tapered waveguide at the current stage 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 Obtain 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 Obtain the optimized structure of the current-stage tapered waveguide;

[0031] Step 303-4, the output width w of the tapered waveguide at the current stage o As the initial input width of the next tapered waveguide, repeat steps 303-1 to 303-3 until the total length of all tapered waveguides in the current stage 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 the manufacturing process into consideration, 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 tapered coupler structure is built on the simulation platform, and the rib cone length is set to meet the specified light propagation conversion efficiency threshold. r and the inverted cone length t In the simulation value, the rib cone length is selected based on the highest conversion efficiency. r and the inverted cone length t The optimal value of , and the initial optimal rib cone length is obtained r and the optimal inverted cone length t Specifically include:

[0033] (1) The double-stage tapered 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) Using the control variable method to fix the length of the inverted cone and the output waveguide group, perform EME scanning on the length of the rib cone group and filter the EME scanning results with the first target conversion rate to obtain the rib cone length. r and loss r ) relationship;

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

[0036] (4) Set up embedded scanning and filter out the length of the rib coner and the inverted cone length 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 inverted cone length t sum;

[0037] (5) According to the highest conversion efficiency, the initial optimal rib cone length is selected. r and the optimal inverted cone length t , that is, to screen out 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 parabolic 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 is set to a conversion efficiency threshold of 95%; if the width value is higher than or equal to 500 nm, the target conversion efficiency is set to a conversion efficiency threshold of 97%.

[0042] The present invention's low-loss wavelength division multiplexer coupling optimization method is achieved by firstly utilizing a ribbed taper to match a large-core fiber, appropriately reducing the mode size and resolving the mode conversion issue within the large-core fiber. During the optimization process, modal overlap analysis is used to determine the input dimensions of the two-stage taper, and EME light propagation inline scanning is used to determine the optimal height and high-performance length range of the dual tapers. Secondly, an inverted taper is used to couple the reduced mode into a submicron silicon waveguide. Combined with a multi-stage inverted taper structure, this ensures high conversion efficiency while minimizing the required space for the coupler.

[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 combines high coupling efficiency with 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 following briefly introduces the drawings required for use in the description of the embodiments. 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 paying any creative work.

[0047] Figure 1 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 face size of a coupler according to an embodiment of the present invention;

[0049] Figure 3 2 is a schematic diagram of the structure of an edge coupler according to embodiment 1 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 the intermediate straight waveguide of Example 1 of the embodiment of the present invention;

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

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

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

[0055] Figure 9 The inverted cone length and loss of the embodiment of the present invention are r ) relationship diagram;

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

[0057] Figure 11 This is a diagram of the dimensions of a double cone with an insertion loss of less than 0.45 dB according to an embodiment of the present invention;

[0058] Figure 12 This is a flowchart of inverted cone shape optimization according to an embodiment of the present invention.

[0059] Figure 13Schematic diagram of the inverted cone profile formula according to an embodiment of the present invention. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in detail and completely in conjunction with the drawings in the implementation of the present invention. Obviously, the described embodiments are only part of the embodiments of the present application, not 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 Insertion loss: A parameter that describes the attenuation of a signal transmitted from port 1 to port 2 in an RF system. Insertion loss is the attenuation of a slow signal during transmission and is typically expressed in decibels (dB). The square of its absolute value is the conversion efficiency.

[0065] Figure 1This is a schematic diagram of the optimized structure of a two-stage tapered coupler (also known as a two-stage adiabatic evolved tapered coupler) proposed in an embodiment of the present invention for the purpose of reducing wavelength division multiplexer losses. This two-stage tapered coupler is a novel two-stage in-plane coupler that combines a rib-tapered and an inverted-tapered (reverse-tapered) structure. During fabrication, the two-stage tapered coupler includes a substrate (optionally a Si substrate), a cladding (optionally a SiO2 cladding), and a cascaded rib-tapered waveguide, an inverted-tapered waveguide, and an output waveguide. The cladding is located on the substrate and serves as the upper cladding for the rib-tapered waveguide and the inverted-tapered waveguide. The resulting two-stage tapered coupler consists solely of the cascaded rib-tapered waveguide, the inverted-tapered waveguide, and the output waveguide. Among them, the rib-cone waveguide is a combined structure of two stacked sub-rib-cone waveguides, the rib cavities of the two sub-rib-cone 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 side length of the two sub-rib-cone waveguides are the same. The bottom end surfaces of the two sub-rib-cone waveguides constitute a rectangular input surface of the double-stage taper coupler, and the size of the input surface is defined as H×W, where H is twice the rib cavity height of the sub-rib-cone waveguide, and W is the bottom side length of the sub-rib-cone waveguide; and the top surfaces of the two sub-rib-cone waveguides are rectangular, and the bottom end surfaces of the two sub-rib-cone waveguides are rectangular. The angle between the side and bottom of the upper sub-rib-taper waveguide is smaller than that of the lower sub-rib-taper waveguide. That is, the rib-cone tip end facets of the two sub-rib-taper waveguides have the same length but different widths, with the upper rib-cone tip having a smaller width. This allows light from the optical fiber to be coupled to an input facet with matching mode dimensions. After entering the rib-taper waveguide, when the rib cavity becomes sufficiently narrow, the light field is primarily confined to the wider bottom portion of the rib-taper waveguide (i.e., the lower sub-rib-taper waveguide), thereby matching the light field from the larger optical fiber to the smaller intermediate waveguide. The inverted-taper waveguide is a composite structure formed by multiple stages of tapered waveguides connected in series. The top facet of the sub-rib-taper waveguide below the rib-taper waveguide is connected to the tip waveguide of the inverted-taper waveguide. Light entering the rib-taper waveguide begins at the tip waveguide of the inverted-taper waveguide and proceeds step by step to the last stage of the inverted-taper waveguide, ultimately reaching 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 the following Figure 1 The optimized structure of the two-stage tapered coupler shown in the figure utilizes a ribbed tapered structure to match a large-core fiber, moderately reducing the mode size and thus enabling mode conversion in the large-core fiber. The optimal structural parameters are determined using FDE modal overlap analysis and EME light propagation scanning. Multiple stages of tapered structures are connected in series as an inverted cone structure to couple the reduced mode into a submicron silicon waveguide. The optimized design of the multi-stage inverted cone combined with the ribbed cone achieves high conversion efficiency and a small footprint for the coupler.

[0067] As a possible implementation manner, the embodiment of the application provides a specific implementation step of the low-loss wavelength division multiplexer coupling optimization method, which comprises the following steps:

[0068] Step S1, determining the optimal input face size of the two-stage taper coupler, the specific process of which is as follows Figure 2 The method comprises the following steps:

[0069] Step S1.1, design the following simulation structure as example 1, build an edge coupler basic structure, and perform FDE optimization on the input face size HxW and the fiber position through modal overlap analysis; the edge coupler basic structure is designed as shown in Figure 3 , and is built on the Lumerical simulation platform. The structure is composed of a substrate, an input high refractive index waveguide with uniform y span, a tapered part of the high refractive index waveguide with variable y span, and a low refractive index polymer waveguide. The structure parameters are shown in Table 1, the SMF-28 parameters are shown in Table 2, and an FDE solver is added, as shown in Table 3, wherein x, y and z represent the coordinates of the horizontal center point of the structure.

[0070] Table 1 shows the edge coupler structure parameter table of example 1

[0071]

[0072] Table 2 shows the SMF-28 structure parameter table of example 1

[0073]

[0074]

[0075] Table 3 shows the FDE solver parameter table of example 1

[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 the “taper”. Run FDE and calculate the mode. Right-click the basic TE mode to add to the global deck, and the mode field profile of the SMF-28 is as shown in Figure 4 . Switch to the layout. Disable “SMF-28” and enable “taper”. Run FDE to calculate the mode, and at this time the mode field profile of the straight waveguide is as shown in Figure 5The basic TE mode of the waveguide is selected, and then the overlap analysis faucet under the eigenmode analysis window is clicked. The fiber mode saved to the global panel is selected and the overlap between the two modes is calculated. Clicking "Optimize Position" can calculate the optimized fiber position of the maximum modal overlap. In the range of 9-15 μm, the length and width of the input waveguide are discretely scanned, and in this case, the fiber is offset by 7.30336e-12 μm in the y direction and 0.0503861 μm in the z direction to achieve an optimal overlap of 97.6642% and a power coupling of 0.976636, and then the input face size is determined to be 12.3*11.3 μm;

[0078] S2, optimize the rib-tapered waveguide related parameters, the process is as shown in Figure 6 The taper coupler is built on the simulation platform, and the rib-tapered structure of the rib-tapered waveguide is introduced in the middle waveguide as an 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, introduce a rib-tapered structure in the middle waveguide, as shown in Figure 7 , and the related parameters are shown in Table 3. In this example, the rib-tapered tip width W rib is 0.5 μm;

[0083] S2.2, EME solver and add grid;

[0084] S2.2.1, add EME solver, where the size should cover the entire coupler, the simulation wavelength is 1.31 μm, and the background refractive index is 1. The coupler is divided into three cells according to the rib-tapered, inverted taper, and output waveguide structure;

[0085] S2.2.2, add grid to the inverted taper part, set the grid multiplier to 1*5*5, and the size also covers the entire coupler;

[0086] S2.3, use the control variable method to fix the length of the inverted taper and output waveguide group unchanged, and perform EME scanning on the length of the rib-tapered group to obtain the relationship between the rib-tapered length and the loss r ) as the lowest standard. The EME scanning results are screened to obtain the relationship between the rib-tapered length and the loss;

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

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

[0089] S2.3.3 uses 0.176dB (95% conversion efficiency) insertion loss as the minimum standard. The EME scan results of S2.3.1 and 2.3.2 were screened and found to have good performance in the range of 300-500μm for ribbed cones and 300-400μm for inverted cones.

[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: Figure 10 shown.

[0091] S3.2.1 Since the input surface dimensions are fixed, the height ratio can be changed by varying the rib cone height. For different height ratios (i.e., different rib cone heights), an inline scan is set up, and interleaved scans are performed on high-performance rib cones (300-500μm) and inverted cone lengths (300-400μm). The relationship between the loss (LOSS) of different rib cones and inverted cone lengths that meet the minimum loss of 0.176dB is obtained, as well as the relationship between the total device length (i.e., the sum of the rib cone and inverted cone lengths) and the loss. Figure 11 The graph shows the relationship between conversion efficiency and biconical length when α is 0.5. The red part represents the optimal performance range with efficiency above 0.95.

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

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

[0094] S4.2 changes the input height of the inverted cone, and the optimal inverted cone input height is obtained 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 Figure 12 As shown;

[0096] S5.1 Build three types of couplers on the MODE simulation platform. The contours of the inverted cone structure are set to linear, parabolic, and exponential shapes, respectively. The functions are shown below, and the length is 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: Figure 13 As shown, 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 inverted cone length.

[0099] S5.2 uses an 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 tapered iterative optimization process;

[0101] S5.3.1 Fixed the inverted taper width to 0.05 μm, the inverted taper 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 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 shape.

[0103] S3.6 Repeat step S3.5 until the total length is close to Figure 11The optimized length range, taking into account the manufacturing process, while ensuring high conversion efficiency, is to choose as small a value as possible and the total length of the inverted cone after series connection, so as to obtain the following 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 various 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 inventive concept of the present invention, which all fall within the scope of protection 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-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, rib cavity height and bottom side length of the two sub-rib-cone waveguides are the same, the bottom end faces of the two sub-rib-cone waveguides constitute the rectangular input face of the double-stage tapered coupler, and the end faces of the rib cone tips of the two sub-rib-cone waveguides are rectangular, and the end faces of the rib cone tips of the two sub-rib-cone waveguides are rectangular. The surface lengths are the same, but the end face widths are different. 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 multiple-stage 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 advances 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 fiber position of the edge coupler through modal overlap analysis. Calculate the modal overlap ratio (overlap) corresponding to different input surface sizes (H×W). Optimize the input surface size based on the input surface size (H×W) with the highest overlap. 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, where 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 a rib-tapered structure into the middle waveguide of the edge coupler and optimizing it to obtain a rib-tapered waveguide of a double-stage tapered coupler; Step 201: Determine 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 , wherein 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: Set the rib-cone length of the double-stage taper coupler rib-cone waveguide. r and the inverted tapered length of the inverted tapered waveguide t The value range of the sub-rib tapered waveguide is that the rib cavity height h' of the sub-rib tapered waveguide is the same as the inverted cone height of the inverted cone 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 inverted cone length t In the simulation value, the rib cone length is selected based on the highest conversion efficiency. r and the inverted cone length t The optimal value of , and the initial optimal rib cone length is obtained r and the optimal inverted cone length t ; Step 203: Optimal ratio of desired inverted cone height to 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 inverted cone length t The rib cone length corresponding to the highest conversion efficiency is selected from the simulation values r and the inverted cone length 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 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. r and the optimal inverted cone length t ; Step 3, optimizing the inverted tapered waveguide of the double-stage tapered coupler; Step 301: 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 inverted cone height h is used as a reference, the value of the inverted cone height h is adjusted, and the conversion efficiency under each value is obtained by simulation. 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: Determine the inverted tapered shape of the inverted tapered waveguide. For each simulated inverted tapered waveguide, fix the input width and output width of the inverted tapered waveguide, obtain a curve showing the relationship between the corresponding conversion efficiency and the inverted tapered length, and select the inverted tapered shape with the highest conversion efficiency and the shortest inverted tapered length. 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 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-stage tapered waveguide in series, the input width w i The minimum input width of the inverted tapered waveguide is 0.2 μm. The output width w is the smallest possible. Theoretically, the thinner the better, 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 series, 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 tapered angle θ that meets the target conversion efficiency, the acceptable maximum tapered 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 under different length values. o And obtain the conversion efficiency of light propagation corresponding to different length values ​​through simulation; The optimal length L of the tapered waveguide at the current stage 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 Obtain 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 Obtain the optimized structure of the current-stage tapered waveguide; Step 303-4, the output width w of the tapered waveguide at the current stage o As the initial input width of the next tapered waveguide, repeat steps 303-1 to 303-3 until the total length of all tapered waveguides in the current stage 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 constructed on a simulation platform, and the rib cone length is length that meets the specified light propagation conversion efficiency threshold. r and the inverted cone length t In the simulation value, the rib cone length is selected based on the highest conversion efficiency. r and the inverted cone length t The optimal value of , and the initial optimal rib cone length is obtained r and the optimal inverted cone length t Specifically include: (1) The double-stage tapered 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. (2) Using the control variable method to fix the length of the inverted cone and the output waveguide group, perform EME scanning on the length of the rib cone group and filter the EME scanning results with the first target conversion rate to obtain the rib cone length. r Relationship with loss; (3) Using the control variable method to fix the length of the rib cone and the output waveguide group, perform EME scanning on the length of the inverted cone group and filter 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 and filter out the length of the rib cone r and the inverted cone length 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 inverted cone length t sum; (5) According to the highest conversion efficiency, the initial optimal rib cone length is selected. r and the optimal inverted cone length t , that is, to screen out the rib cone length with the smallest total length and the smallest loss r and the optimal inverted cone length t . 3 . The method according to 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 is selected as a conversion efficiency threshold of 95%; if the width value is higher than or equal to 500 nm, the target conversion efficiency is selected as a conversion efficiency threshold of 97%.

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