CN116243424A - Beam splitting mode converter, design method, preparation method and optical device thereof - Google Patents
Beam splitting mode converter, design method, preparation method and optical device thereof Download PDFInfo
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Abstract
The application provides a beam splitting mode converter, a design method, a preparation method and an optical device thereof. Wherein, this beam splitting mode converter includes: an input waveguide, a mode conversion structure, and a plurality of output waveguides. One end of the mode conversion structure is connected with the input waveguide, the other end of the mode conversion structure is connected with the plurality of output waveguides, the mode conversion structure is of a two-dimensional structure and is formed by a plurality of unit blocks extending forward and backward along a plane where the two-dimensional direction is located according to a target arrangement and combination mode, and the plane where the two-dimensional direction is located is parallel to the propagation directions of light in the input waveguide and the plurality of output waveguides. The unit blocks comprise a first unit block and a second unit block, the first unit block and the second unit block are respectively formed by a first medium and a second medium, and the refractive index of the first medium is larger than that of the second medium. The device can realize that the beam splitting and mode conversion functions are realized on the limited size, so that the flexibility of device design is improved, and the device is easier to integrate in a miniaturized way and can be applied to a mode division multiplexing system.
Description
Technical Field
The application relates to the technical field of optoelectronic devices, in particular to a beam splitting mode converter, a design method, a preparation method and an optical device thereof.
Background
Fiber optic communications have evolved at a remarkable rate since the advent of the prior art. However, with the continuous development of information technology in modern society, the transmission capacity of a communication system based on a single-mode fiber has come close to the limit due to the influence of optical nonlinearity and fiber fusion effect, and it is increasingly difficult to meet the increasing communication traffic demand. To solve this problem, various multiplexing techniques such as wavelength division multiplexing, mode division multiplexing, partial division multiplexing, and the like have been proposed successively. The technology of the module multiplexing (Mode Division Multiplexing, MDM) is to take mutually independent different modes as information carriers, take few-mode optical fibers as information transmission media, and can provide multiplexing functions of more data in a limited space, thereby meeting the requirement of an optical interconnection system on exponentially increasing bandwidth, and receiving extensive attention of researchers. The MDM system is built without a plurality of basic functional devices, such as a mode division multiplexing/de-multiplexing device, a mode switch, a mode converter and the like, which play different roles in the MDM system, and cooperate with each other to jointly realize the function of increasing the channel capacity. The mode converter is used for converting between different modes at a receiving end and an emitting end in the MDM system, and is one of key devices for improving flexibility of the MDM system.
Mode converters have been proposed today based on various structures such as asymmetric directional couplers, mach-zehnder (MZI) interferometers, bragg gratings, photonic crystals, Y-branches, trench waveguides, long period gratings, etc. Most of the mode converters are single-port input and single-port output structures, namely, m-order modes input from an input port are converted into n-order modes through the mode converters and output from an output port (m and n are non-negative integers, and m is not equal to n), so that a certain limit is placed on the improvement of the flexibility of the mode division multiplexing system. And there are also many limitations to the shape and design of these structures. For example, the existing design method of the mode converter generally sets parameters of a device manually based on the existing analytical theory, then uses optical simulation software to perform simulation calculation or experiment, and adjusts and optimizes various parameters according to data results obtained by the simulation calculation or experiment until satisfactory parameters are determined. However, in such a design method, it is difficult to obtain a globally optimal result by manually adjusting a certain parameter, and when the parameters related to the device are relatively large, optimizing a plurality of parameters at the same time consumes huge labor and time costs, and meanwhile, because of the limitation of the analytical theory, the manually adjusting the parameters of the device is only suitable for designing device structures with some shape rules, so that the performance and the function of the device are limited to a certain extent. In addition, the size of the device designed according to the analysis theory is generally larger, which is not beneficial to improving the integration level. Therefore, the mode converter has yet to be further optimized and improved in device shape structure, performance and design method.
Disclosure of Invention
Aiming at the defects of the related art, the application provides a beam splitting mode converter, a design method, a preparation method and an optical device thereof, wherein the beam splitting mode converter can realize single-input multi-output, flexible position and direction of an output waveguide, smaller size, simple design method and preparation process, and is used for solving the problems that the mode converter in the related art can only realize single-input single-output, oversized size or complex design method.
The application provides a beam splitting mode converter, comprising: an input waveguide, a mode conversion structure, and a plurality of output waveguides. The input waveguide is used for transmitting optical signals of m-order transverse electric modes, wherein m is a non-negative integer. Each output waveguide is used for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer. One end of the mode conversion structure is connected with the input waveguide, and the other end is connected with the plurality of output waveguides, and is used for converting an optical signal of an m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode. Wherein the mode conversion structure is a two-dimensional structure and is formed by a plurality of unit blocks extending forward and backward along a plane where two-dimensional directions are located according to a target arrangement and combination mode, and the two-dimensional structure is formed by combining the unit blocks The plane of the direction is parallel to the propagation direction of the light in the input waveguide and the plurality of output waveguides. The unit blocks comprise a first unit block and a second unit block, the first unit block and the second unit block are respectively formed by a first medium and a second medium, and the refractive index of the first medium is larger than that of the second medium.
As can be seen from the above embodiments, the present application contemplates a beam splitting mode converter including an input waveguide, a mode switching structure, and a plurality of output waveguides. The mode conversion structure is designed into a two-dimensional structure, and the two-dimensional structure is formed by a plurality of unit blocks extending forward and backward along the plane where the two-dimensional direction is located according to a target arrangement and combination mode. Wherein the unit blocks are divided into two types, a first unit block and a second unit block, and the refractive indexes of the first unit block and the second unit block are different. Can realize the splitting and conversion of an optical signal of an m-order transverse electric mode transmitted by an input waveguide into n transmitted by a plurality of output waveguides 1 、n 2 、……、n i Optical signals of the order transverse electric mode. The beam splitting mode converter provided by the application realizes beam splitting and mode conversion functions on the limited size, so that more accommodating spaces are provided for other devices in the mode division multiplexing system, the flexibility of device design is improved, the device is easier to integrate in a miniaturized mode, and the beam splitting mode converter can be used for the mode division multiplexing system to realize beam splitting and mode conversion.
In addition, in the method, the mode converter is formed by arranging and combining a two-dimensional structure formed by dividing the mode converter into a plurality of unit blocks, so that the position and the direction of the output waveguide can be flexibly adjusted according to practical application conditions, and extra loss caused by introducing a bent waveguide to change the transmission direction can be reduced in a photon loop.
In one embodiment, the first medium is silicon and the second medium is silicon dioxide, a polymer or air.
In one embodiment, the maximum dimension of the mode conversion structure along the direction of forward and reverse extension of the plane in which the two-dimensional direction is located is the same as the dimension of the input waveguide and the plurality of output waveguides along the direction of forward and reverse extension of the plane in which the two-dimensional direction is located.
In one embodiment, the plane of the mode conversion structure in the two-dimensional direction is divided into a×b unit planes which are arranged in rows and columns, and A, B are all positive integers greater than or equal to 2. And each unit surface corresponds to each unit block.
In one embodiment, the direction of propagation of light in the output waveguide is parallel to the direction of propagation of light in the input waveguide.
In one embodiment, the direction of propagation of light in the output waveguide is perpendicular to the direction of propagation of light in the input waveguide.
In one embodiment, the beam splitting converter further comprises a substrate. The substrate is positioned on the same side of the input waveguide, the plurality of output waveguides and the mode conversion structure and is used for fixing the positions of the input waveguide, the plurality of output waveguides and the mode conversion structure.
In one embodiment, the beam-splitting converter further comprises a cladding. The cladding layer is positioned at the other side of the input waveguide, the plurality of output waveguides and the mode conversion structure relative to the substrate and is used for cladding the input waveguide, the plurality of output waveguides and the mode conversion structure.
In one embodiment, the target permutation and combination is determined by a reverse design algorithm that includes a direct binary search algorithm.
In one embodiment, the inverse design algorithm determines the target permutation and combination mode by comparing quality factors of the unit blocks under different permutation and combination modes, the quality factors are calculated by substituting the transmittance of the transverse electric mode in each output waveguide under different permutation and combination modes into a preset formula, and the transmittance of the transverse electric mode in each output waveguide is determined by combining optical simulation software with a time domain finite difference algorithm.
The application also provides a design method of the beam splitting mode converter, which comprises the following steps:
an initialized optical three-dimensional simulation model of the beam-splitting mode converter is established based on optical simulation software,the initialized optical three-dimensional simulation model includes an input waveguide, a mode conversion structure, and a plurality of output waveguides. The input waveguide is used for transmitting optical signals of m-order transverse electric modes, wherein m is a non-negative integer. Each output waveguide is used for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer. One end of the mode conversion structure is connected with the input waveguide, and the other end is connected with the plurality of output waveguides, and is used for converting an optical signal of an m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode. The mode conversion structure is a two-dimensional structure and is formed by a plurality of unit blocks extending forward and backward along a plane where the two-dimensional direction is located according to a preset arrangement and combination mode, and the plane where the two-dimensional direction is located is parallel to the propagation directions of light in the input waveguide and the plurality of output waveguides. The unit block is arbitrarily switchable between a first state in which a refractive index of the unit block is greater than a refractive index of the unit block in a second state.
And obtaining the transmittance of a transverse electric mode of each unit block of the mode conversion structure in the initialized optical three-dimensional simulation model in each preset arrangement and combination mode based on a time domain finite difference algorithm, substituting the transmittance into a preset formula, and calculating to obtain the quality factor of each unit block of the mode conversion structure in each preset arrangement and combination mode.
And determining a target arrangement and combination mode by comparing the quality factors of the unit blocks under the preset arrangement and combination modes based on a reverse design algorithm.
And determining a target optical three-dimensional simulation model of the beam splitting mode converter in the target arrangement and combination mode, and preparing the beam splitting mode converter based on the target optical three-dimensional simulation model.
According to the embodiment, the problem of the traditional forward design is solved by adopting the reverse design algorithm, and the mode converter is further optimized.
In one embodiment, the determining the target permutation and combination mode based on the inverse design algorithm by comparing the quality factors of the respective unit blocks under the respective preset permutation and combination modes includes:
and enabling the unit blocks in the mode conversion structure to be in initial states, wherein the initial states are set to be first states.
Any preset unit block is selected, the preset unit block is kept in a first state, the states of the rest unit blocks are unchanged, a first arrangement and combination mode is displayed, and a first quality factor in the first arrangement and combination mode is obtained. And switching the preset unit blocks into a second state, wherein the states of the rest unit blocks are unchanged, so that a second permutation and combination mode is presented, and a second quality factor in the second permutation and combination mode is acquired.
Comparing the first quality factor with the second quality factor, and if the first quality factor is greater than or equal to the second quality factor, keeping the preset unit block in a first state. And if the first quality factor is smaller than the second quality factor, switching the preset unit block into a second state.
Repeating the operation performed on the preset unit blocks for the rest unit blocks until all the unit blocks finish the operation, and displaying a finish state.
And setting the completion state as the initial state, repeating the operation until the preset iteration times are reached, and optimizing the quality factor to determine a target permutation and combination mode.
The application also provides a preparation method of the beam splitting mode converter, which is obtained based on the design method of the beam splitting mode converter and comprises the following steps:
a silicon layer is formed on a substrate.
And performing patterning etching on the silicon layer according to the target arrangement and combination mode of the target optical three-dimensional simulation model to form an input waveguide, a mode conversion structure and a plurality of output waveguides, wherein a unit block area in a second state in the mode conversion structure is in an etching state.
In one embodiment, after the patterning etching of the silicon layer, the method further comprises:
a cladding layer is formed on a side of the silicon layer remote from the substrate to cladding the input waveguide, the mode switching structure, and the plurality of output waveguides.
The present application also provides an optical device comprising a beam splitting mode converter as described above.
Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and together with the description, serve to explain the principles of the application.
FIG. 1 is a schematic top plan view of a beam splitting mode converter according to the present application;
FIG. 2 is a diagram showing the simulation result of the field distribution of the beam splitting mode converter in FIG. 1 in simulation software;
FIG. 3 is a schematic top plan view of another beam splitting mode converter provided herein;
FIG. 4 is a diagram showing the simulation result of the field distribution of the beam splitting mode converter in FIG. 3 in simulation software;
fig. 5a to fig. 5g are schematic flow structure diagrams of a preparation method of a beam splitting mode converter according to an embodiment of the present application.
Wherein: 1-a substrate; 11-a silicon substrate; 12-silicon dioxide; a 2-silicon layer; 21-an input waveguide; 22-mode switching structure; 23-an output waveguide; 231-a first output waveguide; 232-a second output waveguide; 3-electron beam photoresist; 4-cladding.
Detailed Description
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, the same numbers in different drawings refer to the same or similar elements, unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. Rather, they are merely examples of apparatus and methods consistent with some aspects of the present application as detailed in the accompanying claims.
The terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used in this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and/or" as used herein refers to and encompasses any or all possible combinations of one or more of the associated listed items.
The research shows that the size of the device designed according to the analysis theory is generally larger, which is unfavorable for improving the integration level. In addition, in practical applications, two different modes may need to be modulated to realize or verify a specific function, and under the existing condition, two laser sources are needed or after the same laser source is split, the basic mode is converted into the high-order mode through two different mode converters, so that the system complexity is further optimized.
The application provides a beam splitting mode converter, a design method, a preparation method and an optical device thereof, and aims to solve the technical problems in the related art.
The technical terms that may be referred to in the present application are explained first to facilitate understanding.
TE mode, transverse ElectricMode, transverse electric mode is a mode in which the electric field is entirely distributed in a cross section perpendicular to the direction of propagation of electromagnetic waves, and the magnetic field has a component of the direction of propagation.
The MDM, mode Division Multiplexing, mode division multiplexing technology, is one of the space division multiplexing technologies, that is, the orthogonality of different modes in multimode optical fibers is utilized, and the different modes are used as different transmission channels to independently transmit information, so that the information can be transmitted in the optical fibers at the same time.
FDTD, finite Difference Time Domain, time domain finite difference method first discretizes the field by some network partitioning method. And secondly, discretizing the partial differential equation and various boundary conditions, and establishing a differential format to obtain a differential equation. Finally, combining the solutions of the selected algebraic equations, and obtaining the numerical solutions of the side value problems through programming. For solving the effective refractive index of the waveguide.
The reverse design is that, from the aspects of design target, spectrum characteristic and the like of the device, firstly, some possible variables affecting the performance of the device, namely the related degrees of freedom, are extracted to be designed into parameterized objective functions, then the parameterized objective functions are set with corresponding ranges, and then the parameter space, namely the maximum value of the objective functions, is solved. In other words, the reverse design method relies on parameter space set in advance, the matching degree of the device performance and the design target is measured by an objective function, and the optimal solution is finally found through a series of algorithm optimization limits. The topology optimization algorithm which is commonly used in reverse design mainly comprises a particle swarm optimization algorithm, a genetic algorithm, a direct binary search algorithm, an accompanying method and the like.
The DBS, direct-binary search algorithm is an iterative optimization algorithm based on pixels, and the minimum unit of optimization is a regular graph (generally square or circular), similar to the binary coding problem. The convergence rate is high, the method is simple and easy to operate, the dense optimization effect is generally a local optimal solution, and other principles can be assisted to search a global optimal value.
The beam splitting mode converter, the design method, the preparation method and the optical device in the embodiments of the present application are described in detail below with reference to the accompanying drawings. The features of the embodiments described below can be supplemented or combined with one another without conflict.
The application provides a beam splitting mode converter, comprising: input waveguide 21, mode conversion structure 22, and plurality ofAnd an output waveguide 23. Wherein the input waveguide 21 is used for transmitting an optical signal of an m-order transverse electric mode, wherein m is a non-negative integer; each output waveguide 23 is used for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer; one end of the mode conversion structure 22 is connected to the input waveguide 21, and the other end is connected to a plurality of output waveguides 23 for converting optical signals of the m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode; the mode conversion structure 22 is a two-dimensional structure, and is formed by a plurality of unit blocks extending forward and backward along a plane in which the two-dimensional direction is located in a target arrangement and combination manner, wherein the plane in which the two-dimensional direction is located is parallel to the propagation directions of light in the input waveguide 21 and the plurality of output waveguides 23; the unit blocks comprise a first unit block and a second unit block, the first unit block and the second unit block are respectively formed by a first medium and a second medium, and the refractive index of the first medium is larger than that of the second medium.
The beam-splitting mode converter provided by the application comprises an input waveguide 21, a mode conversion structure 22 and a plurality of output waveguides 23. The pattern conversion structure 22 is designed into a two-dimensional structure, and the two-dimensional structure is formed by a plurality of unit blocks extending forward and backward along a plane in which the two-dimensional direction is located according to a target arrangement and combination mode. Wherein the unit blocks are divided into two types, a first unit block and a second unit block, and the refractive indexes of the first unit block and the second unit block are different. Can realize the splitting and conversion of the optical signals of the m-order transverse electric mode transmitted by the input waveguide 21 into n transmitted by a plurality of output waveguides 23 1 、n 2 、……、n i Optical signals of the order transverse electric mode. The beam splitting mode converter provided by the application realizes beam splitting and mode conversion functions on the limited size, so that more accommodating spaces are provided for other devices in the mode division multiplexing system, the flexibility of device design is improved, the device is easier to integrate in a miniaturized mode, and the beam splitting mode converter can be used for the mode division multiplexing system to realize beam splitting and mode conversion.
In addition, in the present application, since the mode converter is formed by two-dimensional structure of a plurality of unit blocks, the position and direction of the output waveguide 23 can be flexibly adjusted according to practical situations, and the extra loss caused by introducing the curved waveguide to change the transmission direction in the photonic circuit can be reduced.
In some embodiments, i=2, the split-mode converter in the present application includes two output waveguides 23, namely a first output waveguide 231 and a second output waveguide 232, for transmitting n 1 Optical signal sum n of order transverse electric mode 2 Optical signals of the order transverse electric mode.
In some embodiments, as shown in fig. 1 or 3, the plane in which the two-dimensional direction is located is a plane formed by the X-direction and the Y-direction together.
Illustratively n 1 =1，n 2 =2, the first output waveguide 231 transmits TE 1 The second output waveguide 232 transmits TE for the mode optical signal 2 Mode optical signal.
In some embodiments, m < n, n is n 1 、n 2 、……、n i Any one of the above.
Illustratively, m=0, the input waveguide 21 transmits TE 0 Mode optical signal.
It should be noted that, the mode converting structure 22 in the present application may also include a plurality of input waveguides 21, for example, a first input waveguide 21, a second input waveguide 21, and the like. Each input waveguide 21 corresponds to a plurality of output waveguides 23, and those skilled in the art can set the same according to the actual situation.
In some embodiments, the first medium is silicon and the second medium is silicon dioxide, a polymer, or air.
Illustratively, the first medium is silicon and the second medium is air. The refractive index of silicon is greater than that of air.
The cell blocks of the present application also include a third cell block, a fourth cell block, and so on in some embodiments. The refractive index of each unit block is different, and can be set by those skilled in the art according to the actual situation.
In some embodiments, the input waveguide 21 and the plurality of output waveguides 23 are made of a light-transmitting material, which allows light signals to pass through.
In one embodiment, as shown in fig. 5f, the maximum dimension of the mode conversion structure 22 in the direction of forward and reverse extension of the plane in which the two-dimensional direction is located is the same as the dimension of the input waveguide 21 and the plurality of output waveguides 23 in the direction of forward and reverse extension of the plane in which the two-dimensional direction is located.
In this embodiment, the dimensions of the directions of forward and reverse extension of the plane in which the two-dimensional directions are located and the maximum dimensions of the directions of forward and reverse extension of the plane in which the two-dimensional directions are located of the mode conversion structure 22 are the same, so that the optical signal transmitted in the input waveguide 21 can be converted into the optical signal propagated in the output waveguide 23 by the mode conversion structure 22.
In some embodiments, the plane in which the mode conversion structure 22 is located in the two-dimensional direction is divided into a×b unit planes that are arranged in rows and columns, and A, B are all positive integers greater than or equal to 2. Each unit surface corresponds to each unit block.
In some embodiments, the plane in which the mode conversion structure 22 is located in the two-dimensional direction is divided into a×b cell blocks of a rows and B columns.
Illustratively, as shown in fig. 1, the plane in which the mode conversion structure 22 is located in the two-dimensional direction is divided into 30×30 cell blocks of 30 rows and 30 columns.
Illustratively, as shown in fig. 3, the plane in which the mode conversion structure 22 is located in the two-dimensional direction is divided into 25×25 unit blocks of 30 rows and 30 columns.
In some embodiments, the cell faces corresponding to each cell block are circular, oval, triangular, rectangular, polygonal, or irregularly shaped. Those skilled in the art can set the setting according to the actual situation.
Illustratively, as shown in fig. 1 or 3, the shape of the unit surface corresponding to the unit block in the present application is rectangular.
In some embodiments, the direction of light propagation in the output waveguide 23 is perpendicular to the direction of light propagation in the input waveguide 21.
Illustratively, as shown in fig. 1, the mode converting structure 22 has a rectangular shape, the input waveguide 21 extends along the Y-direction, and the first output waveguide 231 and the second output waveguide 232 are disposed on opposite sides of the mode converting structure 22, extending along the X-direction and extending in the opposite direction of the X-direction, respectively.
In some embodiments, the direction of propagation of light in the output waveguide 23 is parallel to the direction of propagation of light in the input waveguide 21.
Illustratively, as shown in fig. 3, the mode converting structure 22 is rectangular, the input waveguide 21 extends in the Y direction, the first output waveguide 231 and the second output waveguide 232 are disposed on adjacent sides of the mode converting structure 22, the first output waveguide 231 extends in the X direction, and the second output waveguide extends in the opposite direction to the Y direction.
In some embodiments, as shown in fig. 5 a-5 g, the beam-splitting converter further comprises a substrate 1. The substrate 1 is located on the same side of the input waveguide 21, the plurality of output waveguides 23 and the mode converting structure 22 for fixing the positions of the input waveguide 21, the plurality of output waveguides 23 and the mode converting structure 22.
Illustratively, the base 1 may be an insulating substrate. As shown in fig. 5a to 5g, the insulating substrate has a structure in which a silicon substrate 11 and silicon dioxide 12 are stacked in this order. It should be noted that the substrate 1 in the present application may have a single-layer or multi-layer structure, and those skilled in the art may set the substrate according to practical situations, and is not limited thereto.
In some embodiments, the beam-splitting converter further comprises a cladding layer 4. The cladding layer 4 is located on the other side of the input waveguide 21, the plurality of output waveguides 23 and the mode converting structure 22 with respect to the substrate 1, and is used for cladding the input waveguide 21, the plurality of output waveguides 23 and the mode converting structure 22.
Illustratively, the material of the cladding layer 4 may be silicon dioxide or a polymer. The polymer type may be SU-8 photoresist or polymethyl methacrylate (polymethyl methacrylate, PMMA).
In some embodiments, the target permutation and combination is determined by a reverse design algorithm that includes a direct binary search algorithm.
It should be noted that, the reverse design algorithm adopted in the application may be a genetic algorithm, a direct binary search algorithm or an accompanying method.
In some embodiments, the inverse design algorithm determines the target permutation and combination mode by comparing the quality factors of the unit blocks under different permutation and combination modes, the quality factors are calculated by substituting the transmittance of the transverse electric mode in each output waveguide 23 under different permutation and combination modes into a preset formula, and the transmittance of the transverse electric mode in each output waveguide 23 is determined by combining optical simulation software with a time domain finite difference algorithm.
Based on the same inventive concept, the present application further provides a method for designing a beam splitting mode converter, including the following steps:
s100: an initialized optical three-dimensional simulation model of the beam-splitting mode converter is established based on optical simulation software, and the initialized optical three-dimensional simulation model comprises an input waveguide 21, a mode conversion structure 22 and a plurality of output waveguides 23. Wherein the input waveguide 21 is for transmitting an optical signal of an m-order transverse electric mode, where m is a non-negative integer. Each output waveguide 23 is used for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer. One end of the mode conversion structure 22 is connected to the input waveguide 21, and the other end is connected to a plurality of output waveguides 23 for converting optical signals of the m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode. The mode conversion structure 22 is a two-dimensional structure formed by a plurality of unit blocks extending in the forward and reverse directions of a plane in which the two-dimensional direction is located, which is parallel to the propagation direction of light in the input waveguide 21 and the plurality of output waveguides 23, in a preset arrangement and combination manner. The cell block is arbitrarily switchable between a first state and a second state, the refractive index of the cell block in the first state being greater than the refractive index of the cell block in the second state.
S200: and acquiring the transmittance of the transverse electric mode in each output waveguide 23 under each preset arrangement and combination mode of each unit block of the mode conversion structure 22 in the initialized optical three-dimensional simulation model based on a time domain finite difference algorithm, and substituting the transmittance into a preset formula to calculate and obtain the quality factor of each unit block of the mode conversion structure 22 under each preset arrangement and combination mode.
S300: and determining a target arrangement and combination mode by comparing quality factors of each unit block under each preset arrangement and combination mode based on a reverse design algorithm.
S400: and determining a target optical three-dimensional simulation model of the beam splitting mode converter in a target arrangement and combination mode, and preparing the beam splitting mode converter based on the target optical three-dimensional simulation model.
The beam splitting mode converter obtained according to the design method can synchronously realize beam splitting and mode conversion, reduces the complexity of the mode converter in the existing mode division multiplexing system, further improves the flexibility of the mode division multiplexing system and improves the integration level of devices.
In some embodiments, the predetermined formula for obtaining the quality Factor (FOM) is
Where i is the number of output waveguides 23, i is an integer equal to or greater than 2. t is t pq Is the nth output from the p-th output port q Transmittance of the order mode, t when p=q pq Is the nth q The conversion rate of the order mode output at the p-th output port; when p.noteq, t pq Is the nth q The order mode outputs crosstalk at the p-th output port. X is x p For n output by the first output waveguide 231 1 Transmittance t of the order mode 11 When the output port is the reference, the nth output port outputs p The spectral ratio of the order mode.
It should be noted that the number of the substrates,
representing the mode conversion efficiency of the beam-splitting mode converter, i.e. when the m-order mode input from the input waveguide 21 is converted into the p-th output waveguide 23 output n q The more the order modes (p=q), the n output from the p-th output waveguide 23 q The smaller the order pattern (p+.q), i.e. the smaller the loss and crosstalk of the beam-splitting mode converter, the larger the calculation result value, the larger the FOM value. />
For evaluating n output from the p-th output waveguide 23 q Whether the spectral ratio of the order mode (p=q) meets the expectations, when t pq Deviation->
The larger (p=q), the larger the calculation result, the smaller the FOM value; when t pq The closer->
When (p=q), the smaller the calculation result, the larger the FOM value.
In some embodiments, step S300 includes the steps of:
s310: making the unit blocks in the mode conversion structure 22 be in an initial state, and setting the initial state as a first state;
s320: selecting any preset unit block, keeping the preset unit block in a first state, keeping the states of the rest unit blocks unchanged, and obtaining a first quality factor in a first arrangement and combination mode by presenting the first arrangement and combination mode; switching the preset unit blocks into a second state, wherein the states of the rest unit blocks are unchanged, so that a second permutation and combination mode is presented, and a second quality factor in the second permutation and combination mode is obtained;
S330: comparing the first quality factor with the second quality factor, and if the first quality factor is greater than or equal to the second quality factor, keeping the preset unit block in a first state; if the first quality factor is smaller than the second quality factor, switching the preset unit block into a second state;
s340: repeating the operations S320-S330 executed on the preset unit blocks for the rest unit blocks until all the unit blocks finish the operations, and presenting a finish state;
s350: and setting the completion state as an initial state, repeating the operations S310-S340 until the preset iteration times are reached, and optimizing the quality factor to determine the target permutation and combination mode.
Based on the same inventive concept, the present application further provides a method for preparing a beam splitting mode converter, as shown in fig. 5a to 5g, which is obtained based on the method for designing a beam splitting mode converter, and includes:
as shown in fig. 5a, a silicon layer 2 is formed on a substrate 1.
As shown in fig. 5b to 5f, the silicon layer 2 is patterned and etched according to the target arrangement and combination mode of the target optical three-dimensional simulation model to form an input waveguide 21, a mode conversion structure 22 and a plurality of output waveguides 23, and a unit block area in the second state in the mode conversion structure 22 is in an etched state.
In some embodiments, as shown in fig. 5g, after the patterned etching of the silicon layer 2, the method further includes:
a cladding layer 4 is formed on the side of the silicon layer 2 remote from the substrate 1 to cladding the input waveguide 21, the mode switching structure 22 and the plurality of output waveguides 23.
For ease of understanding, the following specific examples are provided to further illustrate the beam splitting mode converter provided herein.
As shown in fig. 1, which is a schematic top plan view of a beam splitting mode converter in embodiment 1, the beam splitting mode converter provided in embodiment 1 includes an input waveguide 21, a mode converting structure 22, a first output waveguide 231 and a second output waveguide 232. Wherein the wavelength of the input light in the input waveguide 21 is 1550nm, the waveguide width of the input waveguide 21 is 500nm, for transmitting TE 0 Mode optical signal. The top plan of the mode switching structure 22 is a square with a side length of 3 μm and a thickness of 220nm. The mode conversion structure 22 is divided into 30×30 rectangular unit pieces, and the top plane of each unit piece is a square with a side length of 100 nm. The unit block includes a first unit block and a second unit block, wherein the first unit block is formed of a first medium, and the first medium is silicon. The second unit block is formed of a second medium, which is air. The arrangement and combination of the unit blocks in this embodiment are determined by combining the quality factors through a reverse design algorithm. The first output waveguide 231 has a waveguide width of 800nm for transmission TE transfusion 1 Mode optical signal. The second output waveguide 232 has a waveguide width of 1300nm for TE transmission 2 Mode optical signal. TE (TE) 1 Mode optical signal and TE 2 The ratio between the optical signals of the modes is 1:1. the directions of the first output waveguide 231 and the second output waveguide 232 are perpendicular to the direction of the input waveguide 21. It should be noted that the beam splitting mode converter in this embodiment further includes an insulating substrate formed by stacking a silicon substrate and a silicon dioxide layer 2, wherein the thickness of the silicon dioxide layer is 3 μm. An input waveguide 21, a mode conversion structure 22, a first output waveguide 231, and a second output waveguide 232 are fixed on the insulating substrate. The input waveguide 21, the mode conversion structure 22, the first output waveguide 231, and the second output waveguide 232 are covered with a cladding layer 4 in a direction away from the insulating substrate, and the material of the cladding layer 4 is silicon dioxide. The thickness of the cladding layer 4 was 1 μm. As shown in fig. 2, which is a diagram of a simulation result of the field distribution of the beam splitting mode converter in the simulation software, the beam splitting mode converter in the embodiment successfully realizes the beam splitting and mode conversion functions.
The present application provides yet another specific example (example 2) for further illustration. As shown in fig. 3, which is a schematic top plan view of another beam splitting mode converter provided in embodiment 2, other parameter conditions in this embodiment are the same as those in embodiment 1, except that the top plan view of the mode converting structure 22 in this embodiment is a square with a side length of 2.5 μm, the mode converting structure 22 is divided into 25×25 rectangular unit blocks, and the top plan view of each unit block is a square with a side length of 100 nm. The first output waveguide 231 is perpendicular to the direction of the input waveguide 21, and the second output waveguide 232 is parallel to the direction of the input waveguide 21. As shown in fig. 4, which is a diagram of a simulation result of the field distribution of the beam splitting mode converter in the simulation software, the beam splitting mode converter in the embodiment successfully realizes the beam splitting and mode conversion functions.
It should be noted that, the quality factors in the embodiments 1-2 are all calculated according to the following preset formulas. Quality factor
Wherein TE is input from the input waveguide 21 at a certain wavelength 0 In mode, TE is monitored at the first output waveguide 231 0 The transmittance of the mode is t 12 ，TE 1 The transmittance of the mode is t 11 Respectively represent TE at the first output waveguide 231 0 Crosstalk and TE of modes 1 A conversion rate of the mode; TE monitored at the second output waveguide 232 0 The transmittance of the mode is t 21 ，TE 2 The transmittance of the mode is t 22 Respectively represent TE at the second output waveguide 232 0 Crosstalk and TE of modes 2 Conversion rate of mode. The calculation of FOM consists of two parts, +.>
Is the mode conversion efficiency of the mode converter, i.e. when TE is input from the input waveguide 21 0 The more the mode is converted into TE output by the first output waveguide 231 1 Mode and TE output by the second output waveguide 232 2 In the mode, the larger the calculation result value is, the larger the FOM value is.
To evaluate TE output by the first output waveguide 231 1 Mode and TE output by the second output waveguide 232 2 Whether the energy ratio of the mode is close to 1:1.
it should be noted that, in embodiments 1-2, based on the reverse design algorithm, the quality factors of each unit block under each preset permutation and combination mode are compared, and the method for determining the target permutation and combination mode specifically includes the following steps:
Step 1: all the unit blocks are made of silicon.
Step 2: and selecting a first rectangular unit block, and simultaneously keeping the states of all the unit blocks unchanged to present a first arrangement and combination mode. Calculating a first quality factor FOM of the beam splitting mode converter in a first arrangement and combination mode by adopting a three-dimensional finite time domain difference (3D-FDTD) function in optical simulation software Lumerical max 。
Step 3: changing the state of the first rectangular unit block to be air, and leaving a singleThe metablock state is unchanged to present a second permutation and combination mode. The second quality factor temp of the beam-splitting mode converter in the second permutation and combination mode is calculated by the same method. The first quality factor FOM max Comparing with the second quality factor temp, if FOM max Not less than temp, the state of the first rectangular unit block is kept to be silicon, if FOM max And (c) changing the state of the first rectangular unit pieces to convert the first rectangular unit pieces into air.
Step 4: repeating the steps 2-3 for the rest unit blocks until all the unit blocks finish the operation, FOM max No longer continues to increase, presenting a completed state;
step 5: and (3) setting the completion state as the initial state in the step (1), repeating the steps (1-4) for a plurality of iterations until the preset iteration times are reached, and optimizing the quality factor to determine the target arrangement and combination mode.
It should be noted that the operations in the steps 3 to 5 are completed by means of a programming language Python. The number of times the operation was performed in step 4 in example 1 was 30×30-1, and the number of times the operation was performed in step 4 in example 2 was 25×25-1. And the iteration times are 3-4 times, so that the optimized quality factor can be obtained.
The application also provides a preparation method of the beam splitting mode converter in embodiments 1-2, as shown in fig. 5 a-5 g, comprising the following steps:
step 1: a silicon layer 2 is formed on a substrate 1. Specifically, the chip is dissociated, cleaned and preprocessed. After the eight-inch SOI wafer is separated into small pieces by a dicing blade or dicing machine, NH is used first 4 F and HF solution to remove the oxide layer on the surface of the silicon wafer to improve the adhesion of the photoresist on the surface of the silicon wafer in the subsequent step. Then sequentially passing through acetone, methanol, isopropanol (alcohol) and deionized water for organic removal treatment to reduce scraps and organic impurities on the silicon wafer. And carrying out deep cleaning and surface activation treatment on the silicon wafer subjected to primary cleaning by using a concentrated sulfuric acid hydrogen peroxide mixture and a hydrofluoric acid solution. The deeply cleaned silicon wafer is placed on a hot plate of 120 ℃ or in an oven for 10 minutes and sufficiently baked and dried to obtain a substrate 1 with a silicon layer 2 formed thereon as shown in FIG. 5a Shown. A photoresist (SU-8, AZ5214, etc.) or an electron beam photoresist (PMMA, ma-N2403, etc.) is then spin coated on the silicon wafer. According to the minimum feature size (100 nm) of the devices of example 1 and example 2, electron beam resist 3 is selected, as shown in fig. 5 b. PMMA and Ma-N2403 are positive photoresist and negative photoresist respectively, the ultraviolet exposure part of the positive photoresist is removed when developing, and conversely, the ultraviolet exposure area of the negative photoresist is reserved after developing. The spin coating thickness is determined by the spin coating rotating speed and time of the spin coating machine, and the glue layers with different thicknesses can be obtained through different settings. Before the photoresist is uniformly exposed, the sample needs to be pre-baked to harden the photoresist and dehydrate, and the specific temperature and time length need to be determined according to different photoresist characteristics. The substrate 1 in the present application is specifically a silicon substrate 11 and silicon dioxide 12 in an SOI wafer, and the silicon layer 2 is the top silicon in the SOI wafer. The SOI wafer is used as a preparation base of the beam splitting converter in the present application, which is only one possible implementation, and a person skilled in the art may reasonably change according to the examples provided in the present application, which is not limited in this application.
Step 2: and (3) carrying out patterned etching on the silicon layer 2 in the step (1) according to a target arrangement and combination mode of the target optical three-dimensional simulation model to form an input waveguide 21, a mode conversion structure 22 and a plurality of output waveguides 23, wherein a unit block area in a second state in the mode conversion structure 22 is in an etching state. Specifically, first, ultraviolet lithography pattern transfer is performed. For different precision requirements, the exposure process can adopt electron beam lithography, laser direct writing or ultraviolet lithography, and the two processes only need an electron mask pattern. For the solutions provided in embodiments 1-2, the minimum feature size of the device is 100 nm, so electron beam lithography can be selected. As shown in fig. 5c, the silicon wafer is put into an electron beam exposure machine, and the running can be set by leading in a layout which is drawn and generated in advance, and the exposure is determined according to different glue characteristics. The layout drawn and generated in advance is the target arrangement and combination mode of the target optical three-dimensional simulation model, and the specific positions and forms of the input waveguide 21, the mode conversion structure 22 and the plurality of output waveguides 23. The portion of the mode switching structure 22 to be patterned and etched is the unit block area in the second state. After exposure is completed, the steps of developing, removing residues and the like are carried out according to different photoresists by using corresponding developing solutions to obtain a graph as shown in fig. 5d, and then the silicon wafer is placed on a hot plate for post-baking, so that the etching resistance of the photoresist is improved. And uniformly coating heat-conducting glue on the back surface of the post-baked sample, placing the sample into ICP equipment for etching, setting etching time according to the etching rate of the formula, and transferring the pattern of the photoresist onto the top silicon (namely the silicon layer 2), as shown in figure 5 e. After the etching is completed, as shown in fig. 5f, the residual photoresist is then removed by acetone or oxygen plasma, and the sample is baked.
The cladding layer 4 in examples 1 to 2 was air. As shown in fig. 5f, i.e. a cross-sectional view corresponding to the dashed line I-I' in fig. 1.
In some embodiments, the cladding 4 of the beam-splitting mode converter may also be silica or a polymer. As shown in fig. 5g, the preparation method may further include the following steps:
a cladding layer 4 is formed on the side of the silicon layer 2 remote from the substrate 1 to cladding the input waveguide 21, the mode switching structure 22 and the plurality of output waveguides 23. The material of the cladding 4 is silicon dioxide or polymer.
Based on the same inventive concept, the application also provides an optical device comprising the beam splitting mode converter. The principle and technical effects of the present invention are described in the foregoing embodiments, and are not repeated herein.
The above embodiments of the present application may be complementary to each other without conflict.
Those of skill in the art will appreciate that the various operations, methods, steps in the flow, actions, schemes, and alternatives discussed in the present application may be alternated, altered, combined, or eliminated. Further, other steps, means, or steps in a process having various operations, methods, or procedures discussed in this application may be alternated, altered, rearranged, split, combined, or eliminated. Further, steps, measures, schemes in the related art having various operations, methods, flows disclosed in the present application may also be alternated, altered, rearranged, decomposed, combined, or deleted.
In the description of the present application, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like indicate orientations or positional relationships based on the orientation or positional relationships shown in the drawings, merely to facilitate description of the present application and simplify the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and therefore should not be construed as limiting the present application.
The terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include one or more such feature. In the description of the present application, unless otherwise indicated, the meaning of "a plurality" is two or more.
In the description of the present application, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the terms in this application will be understood by those of ordinary skill in the art in a specific context.
In the description of the present specification, a particular feature, structure, material, or characteristic may be combined in any suitable manner in one or more embodiments or examples.
The foregoing is only a partial embodiment of the present application and it should be noted that, for a person skilled in the art, several improvements and modifications can be made without departing from the principle of the present application, and these improvements and modifications should also be considered as the protection scope of the present application.
Claims (14)
1. A beam-splitting mode converter, comprising:
an input waveguide for transmitting an optical signal of an m-order transverse electric mode, where m is a non-negative integer;
a plurality of output waveguides, each for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer;
a mode conversion structure, one end of which is connected with the input waveguide and the other end of which is connected with the output waveguides and is used for converting the optical signal of the m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode; the mode conversion structure is a two-dimensional structure and is formed by a plurality of unit blocks extending forward and backward along a plane where two-dimensional directions are located according to a target arrangement and combination mode, and the plane where the two-dimensional directions are located is parallel to the propagation directions of light in the input waveguide and the plurality of output waveguides; the unit blocks comprise a first unit block and a second unit block, the first unit block and the second unit block are respectively formed by a first medium and a second medium, and the refractive index of the first medium is larger than that of the second medium.
2. The beam-splitting mode converter of claim 1, wherein the first medium is silicon and the second medium is silicon dioxide, a polymer, or air.
3. The beam-splitting mode converter according to claim 1, wherein a maximum dimension of the mode conversion structure in a direction of forward and reverse extension of a plane in which the two-dimensional direction is located is the same as a dimension of the input waveguide and the plurality of output waveguides in a direction of forward and reverse extension of a plane in which the two-dimensional direction is located.
4. The beam-splitting mode converter according to claim 1, wherein the plane in which the mode conversion structure is located in the two-dimensional direction is divided into a×b unit planes arranged in rows and columns, and A, B are positive integers of 2 or more; and each unit surface corresponds to each unit block.
5. The split-mode converter of claim 1, wherein the direction of light propagation in the output waveguide is parallel to the direction of light propagation in the input waveguide;
and/or the propagation direction of the light in the output waveguide is perpendicular to the propagation direction of the light in the input waveguide.
6. The beam-splitting mode converter of claim 1, further comprising: a substrate;
The substrate is positioned on the same side of the input waveguide, the plurality of output waveguides and the mode conversion structure and is used for fixing the positions of the input waveguide, the plurality of output waveguides and the mode conversion structure.
7. The beam-splitting mode converter of claim 6, further comprising: a cladding layer;
the cladding layer is positioned at the other side of the input waveguide, the plurality of output waveguides and the mode conversion structure relative to the substrate and is used for cladding the input waveguide, the plurality of output waveguides and the mode conversion structure.
8. The beam-splitting mode converter of claim 1, wherein the target arrangement pattern is determined by a reverse design algorithm comprising a direct binary search algorithm.
9. The beam-splitting mode converter according to claim 8, wherein the inverse design algorithm determines the target permutation and combination by comparing quality factors of the unit blocks in different permutations and combinations, the quality factors being calculated by substituting the transmittance of the transverse electric mode in each of the output waveguides in different permutations and combinations into a predetermined formula, and the transmittance of the transverse electric mode in each of the output waveguides being determined by optical simulation software in combination with a time domain finite difference algorithm.
10. A method for designing a beam-splitting mode converter, comprising:
establishing an initialized optical three-dimensional simulation model of the beam splitting mode converter based on optical simulation software, wherein the initialized optical three-dimensional simulation model comprises an input waveguide, a mode conversion structure and a plurality of output waveguides; the input waveguide is used for transmitting optical signals of m-order transverse electric modes, wherein m is a non-negative integer; each output waveguide is used for transmitting n 1 、n 2 、……、n i Optical signal of order transverse electric mode, wherein n i Is a non-negative integer, i is a positive integer; one end of the mode conversion structure is connected with the input waveguide, and the other end is connected with the plurality of output waveguides, and is used for converting an optical signal of an m-order transverse electric mode into n 1 、n 2 、……、n i Optical signals of the order transverse electric mode; the mode conversion structure is a two-dimensional structure and is formed by a plurality of unit blocks extending forward and backward along a plane where a two-dimensional direction is located in a preset arrangement and combination mode, and the plane where the two-dimensional direction is located is parallel to the propagation directions of light in the input waveguide and the plurality of output waveguides; the unit block can be switched between a first state and a second state at will, and the refractive index of the unit block in the first state is larger than that of the unit block in the second state;
Acquiring the transmittance of a transverse electric mode of each unit block of the mode conversion structure in the initialized optical three-dimensional simulation model in each preset arrangement and combination mode based on a time domain finite difference algorithm, substituting the transmittance into a preset formula to calculate and obtain quality factors of each unit block of the mode conversion structure in each preset arrangement and combination mode;
determining a target arrangement and combination mode by comparing the quality factors of each unit block in each preset arrangement and combination mode based on a reverse design algorithm;
and determining a target optical three-dimensional simulation model of the beam splitting mode converter in the target arrangement and combination mode, and preparing the beam splitting mode converter based on the target optical three-dimensional simulation model.
11. The method of designing a beam-splitting mode converter according to claim 10, wherein the determining a target permutation and combination pattern by comparing the quality factors of the respective unit blocks in the respective preset permutation and combination patterns based on a reverse design algorithm includes:
the unit blocks in the mode conversion structure are all in an initial state, and the initial state is set to be a first state;
Selecting any preset unit block, keeping the preset unit block in a first state, keeping the states of the rest unit blocks unchanged, and obtaining a first quality factor in a first arrangement and combination mode by presenting the first arrangement and combination mode; switching the preset unit blocks into a second state, wherein the states of the rest unit blocks are unchanged, so that a second arrangement and combination mode is presented, and a second quality factor in the second arrangement and combination mode is obtained;
comparing the magnitude between the first quality factor and the second quality factor, and if the first quality factor is greater than or equal to the second quality factor, keeping the preset unit block in a first state; if the first quality factor is smaller than the second quality factor, switching the preset unit block into a second state;
repeating the operation performed on the preset unit blocks for the rest unit blocks until all the unit blocks complete the operation, and presenting a completion state;
and setting the completion state as the initial state, repeating the operation until the preset iteration times are reached, and optimizing the quality factor to determine a target permutation and combination mode.
12. A method for manufacturing a beam-splitting mode converter, based on the target optical three-dimensional simulation model obtained by the method for designing a beam-splitting mode converter according to claim 10 or 11, comprising:
Forming a silicon layer on a substrate;
and performing patterning etching on the silicon layer according to the target arrangement and combination mode of the target optical three-dimensional simulation model to form an input waveguide, a mode conversion structure and a plurality of output waveguides, wherein a unit block area in a second state in the mode conversion structure is in an etching state.
13. The method of claim 12, further comprising, after the patterning etching of the silicon layer:
a cladding layer is formed on a side of the silicon layer remote from the substrate to cladding the input waveguide, the mode switching structure, and the plurality of output waveguides.
14. An optical device comprising a beam splitting mode converter according to any of claims 1 to 9.
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