An adjustable optical power splitter based on MMI-phase shifter cascade and design method

Abstract

The application discloses a tunable optical power splitter based on MMIs-cascaded phase shifters and a design method, the splitter comprising an input waveguide, a tapered transition waveguide, an output waveguide, M MMIs and M-1 phase shifters for cascading the M MMIs; the input waveguide has N input ports coupled with the input end of the first MMI; the tapered transition waveguide is between the input and output waveguides and the MMIs; the MMIs comprise integer MMIs and half-integer MMIs for receiving optical signals and generating multimode interference; each phase shifter is arranged between two adjacent MMIs for independently adjusting the phase of the optical signal of each channel according to the optimal phase configuration; the output waveguide has N output ports coupled with the output end of the last MMI for outputting the optical signal. The application can realize continuous adjustment of the splitting ratio of the output ports, has a more compact structure, is suitable for dynamic power distribution requirements in different optical network scenes and greatly improves flexibility.

Description

Technical Field

[0001] This invention relates to an adjustable optical power beam splitter and its design method based on MMI-phase shifter cascade, belonging to the field of photonic integration technology. Background Technology

[0002] With the rapid development of the Internet and optical communication technologies, communication systems are placing higher demands on signal transmission bandwidth and information exchange rates. Photonic integration technology, with its significant advantages of high integration, large transmission bandwidth, and low power consumption, has become a core development direction in the next generation of high-speed optical interconnects. Optical power beamsplitters, as fundamental components in photonic integrated circuits, have a wide range of applications, covering optical feedback circuits, online optical power monitoring, passive optical networks, and many other fields. Their core function is to distribute the input optical signal to multiple target output ports according to equal or unequal power ratios through waveguide optical field coupling and distribution. Currently, the design methods for optical power beamsplitters are mainly divided into two categories: forward design and reverse design.

[0003] Forward design, also known as traditional design, relies on the designer's experience, physical mechanism analysis, and finite parameter optimization to design device structures. Currently, typical power beam splitter structures fabricated based on forward design mainly include Y-branch and multimode interference (MMI) couplers. The Y-branch structure is simple and easy to fabricate, achieving optical power distribution through a single waveguide bifurcation structure. The MMI structure is currently the most widely used beam splitting structure, offering advantages such as low insertion loss, small device size, and strong robustness. Its beam splitting principle is based on the self-image effect of multimode waveguides—after the optical signal enters the multimode waveguide, it excites different modes. Each mode generates a regular phase difference with the transmission distance and coherently superimposes, periodically reproducing the input optical field along the transmission direction, thereby completing optical power distribution.

[0004] Reverse engineering is a solution method that moves from the performance space to the physical space. Guided by the target performance of the device, it uses various optimization algorithms to simultaneously optimize multiple design parameters, seeking the optimal structural parameters to maximize the match between device performance and design goals. By exploring a broad design space through optimization algorithms, reverse engineering reduces human intervention and helps improve the performance of silicon photonics devices. While reverse-engineered beam splitters can effectively reduce device size, the main waveguide structure cannot be altered after fabrication. Relying on the two-phase switching of phase change materials for control only allows for limited discrete-level splitting ratio adjustment and is extremely sensitive to process errors.

[0005] Meanwhile, traditional multi-port power beam splitters mostly employ a fixed splitting ratio structure, making it impossible to dynamically adjust the output power according to system requirements. With the rapid development of multi-terminal optical interconnect systems, numerous optical signal processing applications have created an urgent need for multi-port, dynamically adjustable beam splitting capabilities.

[0006] Therefore, there is an urgent need for a compact optical power beam splitter that does not require changes to the physical structure and can achieve flexible adjustment of the splitting ratio simply through phase modulation. Summary of the Invention

[0007] The purpose of this invention is to provide an adjustable optical power beam splitter and its design method based on MMI-phase shifter cascade. By utilizing the integer and half-integer characteristics of MMI, the MMI is cascaded with a phase shifter, and a heuristic algorithm is used to solve the phase shifter parameters that satisfy the splitting ratio of the output port in reverse, thereby achieving a highly compact multi-port structure and dynamic adjustability of the splitting ratio.

[0008] To achieve the above objectives, the present invention is implemented using the following technical solution.

[0009] In a first aspect, the present invention provides an adjustable optical power beam splitter based on MMI-phase shifter cascade, comprising: an input waveguide, an output waveguide, a tapered transition waveguide, M MMIs, and M-1 phase shifters for cascading the M MMIs;

[0010] The input waveguide has N input ports, which are coupled to the input of the first MMI, and are used to input optical signals into the first MMI from any one of the input ports;

[0011] One end of the tapered transition waveguide is connected to the input waveguide or the output waveguide, and the other end is connected to the MMI, which is used to achieve mode matching and reduce the coupling loss when the optical signal enters the MMI from the input waveguide;

[0012] The MMI includes integer MMI and half-integer MMI, which are used to receive optical signals and generate multimode interference;

[0013] Each phase shifter is set between two adjacent MMIs to independently adjust the optical signal phase of each channel according to the optimal phase configuration corresponding to the target split ratio.

[0014] The output waveguide has N output ports, which are coupled to the output of the last MMI, and are used to output an optical signal with a power distribution ratio that matches the target splitting ratio after multi-stage phase modulation.

[0015] Furthermore, the parameters of the tapered transition waveguide are determined through a two-stage collaborative optimization process using particle swarm optimization and the adjoint shape method.

[0016] In the first stage, with mode mismatch loss as the optimization target, the particle swarm optimization algorithm is used for global optimization to obtain the set tapered transition waveguide substrate profile.

[0017] In the second stage, the contour of the tapered transition waveguide substrate is used as the deformation constraint boundary. Based on the adjoint gradient, the contour boundary of the tapered transition waveguide is smoothly fine-tuned to jointly optimize the mode matching loss and the splitting ratio fluctuation.

[0018] Furthermore, the design method of the MMI includes:

[0019] The width of the multimode waveguide in the MMI is determined to ensure that the multimode waveguide supports multiple guided modes;

[0020] Initially obtain the integer interference length and half-integer interference length that meet the total power requirement set at the MMI output port;

[0021] The tapered transition waveguide is incorporated, and its length and width are optimized to reduce mode conversion loss;

[0022] Select the integer or half-integer interference length as the design length of the MMI, and verify its interference performance through simulation; if the performance does not meet the requirements, return to optimize the tapered transition waveguide parameters until the requirements are met; if the performance meets the requirements, extract the S-parameters to obtain the amplitude and phase relationship between each input and output port.

[0023] When the present invention uses a half-integer interference length (MMI) as the design length, the structure of the designed optical power unit is more compact. While keeping the total power of the output port constant, the total insertion loss is low, which can not only significantly reduce the overall device footprint, but also inherit the high tolerance capability of conventional multimode interferometers. When using an integer interference length (MMI) as the design length, uniform power distribution can be achieved at each output port.

[0024] Furthermore, under the condition of the same MMI multimode waveguide width, the half-integer MMI interference length is half of the integer MMI interference length, that is, the position of the first N-fold self-image point at half the length, which is used to improve the compactness of the device.

[0025] The integer MMI is used to distribute the optical signal evenly, making phase modulation more flexible.

[0026] In a second aspect, the present invention provides a design method for a tunable optical power beamsplitter based on MMI-phase shifter cascade, employing the tunable optical power beamsplitter described in any one of the first aspects, and the design method includes:

[0027] Determine the design length of the MMI and extract the S-parameters. Use the S-parameters to obtain the amplitude and phase relationship between each input and output port.

[0028] M MMIs and M-1 phase shifters are cascaded according to a set cascading method. The overall transmission matrix is ​​established using the S-parameters and the phase delay model of the phase shifters.

[0029] A heuristic algorithm is used to solve for the optimal phase value of each phase shifter in reverse, with the goal of minimizing the error between the target splitting ratio and the actual output power.

[0030] By adjusting the refractive index of the waveguide material through a phase modulation mechanism, each phase shifter can achieve its optimal phase value, thereby obtaining an optical power distribution consistent with the target splitting ratio at the N output ports of the output waveguide.

[0031] This invention employs a design method that cascades an MMI (Mechanical Management Interface) with a phase shifter array, resulting in a smaller chip footprint and a more compact structure. The selection of input ports is also more flexible; any one of the N input ports can be used as an optical signal input. While ensuring low insertion loss, the phase shifter's phase adjustment function enables the redistribution of optical power.

[0032] Furthermore, the defined cascading methods include:

[0033] All MMIs are integer MMIs, and each integer MMI is used to distribute the input optical signal evenly, making the phase shifter adjustment more flexible;

[0034] All MMIs are half-integer MMIs, and each half-integer MMI is used to shorten the device size and improve the structural compactness;

[0035] M MMIs are cascaded in an alternating pattern of integer MMIs and half-integer MMIs, with phase shifters between adjacent MMIs; the first is an integer MMI, used to evenly distribute the input optical signal, and the next is a half-integer MMI, used to shorten the device size, balancing the flexibility of phase modulation and the compactness of the structure.

[0036] Furthermore, the phase modulation mechanism employs a thermo-optical effect; the thermo-optical effect changes the waveguide temperature by heating, thereby adjusting the effective refractive index of the waveguide and adjusting the phase of the optical signal to the optimal phase configuration obtained by the algorithm, so that the output power distribution is consistent with the target splitting ratio.

[0037] Furthermore, the total transmission matrix is ​​obtained by multiplying the transmission matrices of M MMIs by the transmission matrices of M-1 phase shifters; wherein, the transmission matrix of each phase shifter is a diagonal matrix, the main diagonal elements of which are the complex exponents of phase modulation introduced by each phase shifter, and the remaining elements are all zero, so as to characterize that each optical path only generates phase delay independently without inter-channel coupling.

[0038] The transmission matrix of the MMI is obtained by using the S-parameters extracted from simulation, or by using the phase relationship expression of the input and output ports of the MMI.

[0039] As can be seen from the above general transmission matrix expression, the distribution ratio of output optical power is determined by the input optical signal, the phase configuration of the phase shifter array, and the transmission characteristics of each MMI. Under the premise that the input port and output port parameters are determined, the output splitting ratio can be made consistent with the target splitting ratio by adjusting the phase value of each phase shifter.

[0040] Furthermore, the heuristic algorithm is based on the device spectral response calculated by the transfer matrix method, and constructs a fitness function with the target splitting ratio error as the core. It iteratively optimizes the optimal phase configuration of the phase shifter to achieve precise control under arbitrary splitting ratios.

[0041] This invention uses a heuristic algorithm to reverse-engineer the phase of the phase shifter corresponding to the target splitting ratio. The required parameters of the phase shift module are automatically obtained through algorithm iteration, requiring less involvement from the designer and achieving higher accuracy.

[0042] Furthermore, the heuristic algorithm employs an improved genetic algorithm (GA) to avoid getting trapped in local optima. The process includes:

[0043] An initial population is randomly generated, and each individual consists of the phase values ​​of (M-1)×N phase shifters, representing a set of candidate phase configurations;

[0044] The spectral response of the device under the phase configuration is calculated using the transfer matrix method. Based on the error between the actual output power and the target output power, the fitness values ​​of all individuals are calculated and ranked.

[0045] An elite retention mechanism is adopted, in which high-fit individuals are directly retained to the next generation, while the rest of the ordinary individuals enter the crossover and mutation stage;

[0046] Perform a crossover operation on a regular individual to generate new offspring individuals;

[0047] The mutation rate is dynamically adjusted according to the iteration stage. A high mutation rate is used in the early stage of the iteration to ensure global exploration capability, and the mutation rate is reduced in the later stage of the iteration to enhance local fine search.

[0048] The retained elite individuals are merged with newly generated offspring individuals to form a new generation of population, and the current optimal phase shifter phase configuration is updated.

[0049] Check if the preset stopping condition is met: if it is met, the algorithm ends and outputs the optimal phase configuration; if it is not met, continue iterative optimization.

[0050] This invention employs an improved GA that integrates dynamic adaptive mutation and elite retention strategies to solve for the optimal phase shifter phase configuration that satisfies the target output response. Meanwhile, the MMI used in this invention has a high tolerance for manufacturing errors, further improving the practicality and yield of the device.

[0051] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0052] 1. The adjustable optical power beam splitter based on MMI-phase shifter cascade provided by this invention uses MMI and its self-imaging effect to construct a phase-controlled multimode interference basis. Both integer MMI and half-integer MMI have high tolerance and low insertion loss. Half-integer MMI can further shorten the device length by half. The beam splitter adopts an optical path architecture of MMI and phase shifter array cascade. Without changing the overall structure of the device, the interference result in the MMI can be changed by phase control alone, realizing continuous adjustment of the splitting ratio at the output port. It can flexibly adapt to the dynamic power allocation requirements in different optical network scenarios, greatly improving the flexibility of the device.

[0053] Furthermore, the phase shifter array is integrated on the input and output waveguides of the MMI, and can independently adjust the phase difference of the optical signals in each channel through the thermo-optical effect, thereby controlling the multimode interference results within the MMI; the phase shifter array can realize the continuous adjustment of the splitting ratio at the output port, effectively enhancing the device's adaptability to the power allocation requirements of complex optical links.

[0054] 2. The design method provided by this invention combines the S-parameters of the MMI with the phase delay model of the phase shifter array to construct the total transmission matrix from the input to the output port, enabling rapid calculation of the device's output power distribution under arbitrary phase configurations. Based on this, a heuristic algorithm is used to inversely solve for the optimal phase configuration of the phase shifter, achieving automatic mapping from the target splitting ratio to the phase shifter phase, avoiding repeated manual trial and error. The improved GA fusion dynamic mutation rate adjustment and elite retention strategy ensures global exploration capability through a high mutation rate in the early stage, and strengthens local fine-grained search with a low mutation rate in the later stage, effectively avoiding getting trapped in local optima and significantly improving design efficiency and control accuracy.

[0055] The design method of this invention has a high degree of automation and fast convergence speed, and can meet the design requirements of tunable optical power beam splitters with different numbers of ports and different splitting ratios. Attached Figure Description

[0056] Figure 1 A schematic diagram of the overall structure of a tunable optical power beam splitter based on MMI-phase shifter cascade provided according to an embodiment of the present invention;

[0057] Figure 2A schematic diagram of the structure of a single MMI in a cascaded MMI-phase shifter tunable optical power beam splitter according to an embodiment of the present invention;

[0058] Figure 3 A flowchart illustrating the design of a tunable optical power beam splitter based on MMI-phase shifter cascade according to an embodiment of the present invention;

[0059] Figure 4 This is a schematic diagram of the structure of an integer MMI and half-integer MMI cascaded phase shifter in a tunable optical power beam splitter based on MMI-phase shifter cascaded according to an embodiment of the present invention;

[0060] Figure 5 A schematic diagram illustrating the variation of MMI output port power with multimode region length in a cascaded MMI-phase shifter-based tunable optical power beam splitter according to an embodiment of the present invention;

[0061] Figure 6 The present invention provides a schematic diagram of the mode field distribution from port 8 to the integer MMI and the half-integer MMI in a cascaded MMI-phase shifter based on an embodiment of the present invention, wherein (a) is a schematic diagram of the mode field distribution from port 8 to the integer MMI, and (b) is a schematic diagram of the mode field distribution from port 8 to the half-integer MMI.

[0062] Figure 7 The graphs of insertion loss versus wavelength at the output ports of half-integer MMIs and integer MMIs in a cascaded MMI-phase shifter tunable optical power beam splitter according to an embodiment of the present invention are shown.

[0063] Figure 8 A flowchart illustrating the improved genetic algorithm in the design method of a tunable optical power beamsplitter based on MMI-phase shifter cascade provided by an embodiment of the present invention;

[0064] Figure 9 The graph shows the convergence performance comparison between the improved genetic algorithm and the traditional genetic algorithm in the design method of tunable optical power beam splitter based on MMI-phase shifter cascade provided by the present invention. (a) is a comparison of the convergence performance of the algorithm when solving for the phase corresponding to the optical switch, and (b) is a comparison of the convergence performance of the algorithm when solving for non-uniform beam splitting.

[0065] Figure 10 This is a diagram illustrating the optical switching function of a device implemented in an tunable optical power beam splitter based on an MMI-phase shifter cascade, according to an embodiment of the present invention.

[0066] Figure 11 This is a diagram illustrating the working state of an 8-channel uniform beam splitter implemented under a cascaded MMI-phase shifter based on an embodiment of the present invention.

[0067] Figure 12 The present invention provides two non-uniform beam splitting operation diagrams for a cascaded MMI-phase shifter-based adjustable optical power beam splitter, wherein (a) is a non-uniform beam splitting operation diagram with a splitting ratio of 3:3:3:3:1:1:1:1; and (b) is a non-uniform beam splitting operation diagram with a splitting ratio of 4:3:2:1:0:0:0:0. Detailed Implementation

[0068] It should be noted that:

[0069] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations thereof. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0070] The term "and / or" simply describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. Additionally, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0071] Example 1

[0072] like Figure 1 As shown, this embodiment introduces a tunable optical power beam splitter based on MMI-phase shifter cascade, including: an input waveguide, an output waveguide, a tapered transition waveguide, M MMIs, and M-1 phase shifters for cascading the M MMIs;

[0073] The input waveguide has N input ports, which are coupled to the input of the first MMI, and are used to input optical signals into the first MMI from any one of the input ports;

[0074] One end of the tapered transition waveguide is connected to the input waveguide or the output waveguide, and the other end is connected to the MMI, which is used to achieve mode matching and reduce the coupling loss when the optical signal enters the MMI from the input waveguide;

[0075] The MMI includes integer MMI and half-integer MMI, which are used to receive optical signals and generate multimode interference;

[0076] Each phase shifter is set between two adjacent MMIs to independently adjust the optical signal phase of each channel according to the optimal phase configuration corresponding to the target split ratio.

[0077] The output waveguide has N output ports, which are coupled to the output of the last MMI, and are used to output an optical signal with a power distribution ratio that matches the target splitting ratio after multi-stage phase modulation.

[0078] In summary, the optical power beam splitter designed in this embodiment adopts an integer / half-integer MMI. Both integer and half-integer MMIs have the advantages of low insertion loss and high process tolerance. The half-integer MMI also shortens the device length to half of the integer interference length, significantly reducing the footprint of the silicon photonics chip. By setting the phase shifter array between the output waveguide of each MMI and the next-stage input waveguide, the phase of the optical signal can be adjusted independently step by step, thereby changing the multimode interference result within the MMI. This allows the splitting ratio at the output port to be continuously adjustable without changing the overall structure of the device, flexibly adapting to the dynamic power allocation requirements of different optical network scenarios, and greatly improving the integration and control flexibility of the device.

[0079] Preferably, the number of input ports can be arbitrarily selected between ≤N.

[0080] Preferably, the parameters of the tapered transition waveguide are determined through a two-stage collaborative optimization process using particle swarm optimization and the adjoint shape method.

[0081] In the first stage, with mode mismatch loss as the optimization target, the particle swarm optimization algorithm is used for global optimization to obtain the set tapered transition waveguide substrate profile.

[0082] In the second stage, the contour of the tapered transition waveguide substrate is used as the deformation constraint boundary. Based on the adjoint gradient, the contour boundary of the tapered transition waveguide is smoothly fine-tuned to jointly optimize the mode matching loss and the splitting ratio fluctuation.

[0083] Preferred, such as Figure 2 As shown, the design method of the MMI includes:

[0084] The multimode waveguide width of the MMI is determined by combining theoretical analysis and design experience to ensure that the multimode waveguide supports multiple guided modes.

[0085] Through formula calculation or parameter scanning simulation, the integer interference length and half-integer interference length that meet the total power requirement set at the output port are initially obtained.

[0086] A tapered transition waveguide is added between the input waveguide and the multimode waveguide, and its length and width are optimized to reduce mode conversion loss.

[0087] Select the integer or half-integer interference length as the design length of the MMI, and verify its interference performance through simulation; if the performance does not meet the requirements, return to optimize the parameters of the tapered transition waveguide until the requirements are met; if the performance meets the requirements, extract the S-parameters to obtain the amplitude and phase relationship between each input and output port.

[0088] Preferably, the MMI has three interferometry modes: general interferometry, paired interferometry, and symmetrical interferometry. Choosing different interferometry modes results in different positions of the input and output waveguides and different self-image lengths for the same width. In this embodiment, the integer MMI uses general interferometry, corresponding to the length of the multimode waveguide. The expression is:

[0089] ;

[0090] In this process, light can excite different modes at different locations within the multimode waveguide after entering the waveguide core layer. The beat length between two low-order modes is expressed as:

[0091] ;

[0092] in, and Let be the propagation constants of the fundamental mode and the first-order mode in the waveguide, respectively. The effective refractive index of the waveguide core layer, The width of the multimode waveguide, The wavelength of the input light;

[0093] The input / output port phase relationship expression of the integer MMI is:

[0094] ;

[0095] in, and These are the bottom-up input and output port numbers of the MMI, representing integers. From The light input at the port is Phase at port output Total number of ports The phase is constant.

[0096] Preferably, while conventional N×N MMIs offer stable performance, their relatively long device length is unfavorable for applications requiring high integration in photonic circuits. To achieve a more compact design, this embodiment also optimizes the N×N MMI using its half-integer characteristics. The interference length of the half-integer MMI is half that of the integer MMI, corresponding to the length of the multimode waveguide. The expression is:

[0097] ;

[0098] The relationship between the input and output port phases of the half-integer MMI is expressed as follows:

[0099] ;

[0100] in, The imaginary unit is used; therefore, the transmission matrix corresponding to the half-integer N×N MMI can be obtained. .

[0101] This embodiment employs an integer / half-integer MMI design. The half-integer MMI retains the high tolerance and low insertion loss advantages of conventional multimode interferometers while reducing the device length to half that of the integer MMI, significantly decreasing the device footprint. The integer MMI, on the other hand, enables uniform power distribution at the output port, simplifying the initial phase configuration of the back-end phase shifter. The two designs complement each other: the integer MMI provides a uniform beam splitting basis, making phase shifter adjustment more flexible; and the half-integer MMI significantly reduces device size while maintaining low insertion loss, improving structural compactness. Furthermore, like the integer MMI, the half-integer MMI ensures that the normalized total output power is close to 1 when input is made at any input port.

[0102] Example 2

[0103] like Figure 3 As shown, based on the cascaded MMI-phase shifter tunable optical power beam splitter of Embodiment 1, this embodiment introduces a design method for a cascaded MMI-phase shifter tunable optical power beam splitter. The design method includes:

[0104] Determine the design length of the MMI and extract the S-parameters. Use the S-parameters to obtain the amplitude and phase relationship between each input and output port.

[0105] M MMIs and M-1 phase shifters are cascaded according to a set cascading method. The overall transmission matrix is ​​established using the S-parameters and the phase delay model of the phase shifters.

[0106] A heuristic algorithm is used to solve for the optimal phase value of each phase shifter in reverse, with the goal of minimizing the error between the target splitting ratio and the actual output power.

[0107] By adjusting the refractive index of the waveguide material through a phase modulation mechanism, each phase shifter can achieve its optimal phase value, thereby obtaining an optical power distribution consistent with the target splitting ratio at the N output ports of the output waveguide.

[0108] Preferably, the cascading method includes:

[0109] All MMIs are integer MMIs, and each integer MMI is used to distribute the input optical signal evenly, making the phase shifter adjustment more flexible;

[0110] All MMIs are half-integer MMIs, and each half-integer MMI is used to shorten the device size and improve the structural compactness;

[0111] M MMIs are cascaded in an alternating pattern of integer MMIs and half-integer MMIs, with phase shifters between adjacent MMIs; the first is an integer MMI, used to evenly distribute the input optical signal, and the next is a half-integer MMI, used to shorten the device size, balancing the flexibility of phase modulation and the compactness of the structure.

[0112] Preferably, in this embodiment, the transmission matrix of the phase shifter array... The expression is:

[0113] ;

[0114] Among them, diagonal elements For the first The amount of phase modulation introduced by each phase shifter;

[0115] The overall transmission matrix The expression is obtained by multiplying each MMI by the transfer matrix of the phase shifter array:

[0116] ;

[0117] in, The MMI transmission matrix in this embodiment can be theoretically obtained from the relationship expression of the input and output port phases of the MMI, or it can be obtained from the S-parameters extracted during simulation. For the input transfer matrix; For the transfer matrix of the second phase shifter, This is the transfer matrix for the first phase shifter.

[0118] The number of MMIs and phase shifters cascaded varies depending on the number of ports. Based on matrix theory, we can initially determine whether the current number of cascades meets the tunability condition: for a 2-port tunable splitter, two MMIs cascaded with one phase shifter can be used; for 4 to 8-port tunable splitters, three MMIs cascaded with two phase shifters can be used.

[0119] Preferably, as can be seen from the above transmission matrix expression, the output optical power is jointly determined by the input signal, the phase of the phase shifter, and the transmission characteristics of the MMI. Therefore, when the input and output parameters are determined, the target splitting ratio can be achieved by adjusting the phase of the phase shifter.

[0120] In this embodiment, a heuristic algorithm is used to solve the optimal phase configuration of each phase shifter in reverse: by constructing a fitness function with the target splitting ratio error as the core, iterative optimization is performed in the phase search space to finally obtain the phase shifter phase set that meets the power allocation requirements of the output port.

[0121] Preferably, the phase modulation function of the phase shifter is achieved by adjusting the refractive index of the waveguide material. The phase modulation mechanism adopts the thermo-optic effect: the thermo-optic effect changes the waveguide temperature by heating, adjusts the effective refractive index of the waveguide, and adjusts the phase of the optical signal to the optimal phase configuration solved by the algorithm, so that the output power distribution is consistent with the target splitting ratio.

[0122] In this embodiment, a total of N×(M-1) phase shifters are included. If the phase is changed by manually adjusting the voltage, not only is the workload large, but it is also affected by process errors and thermal crosstalk during the heating process, resulting in a deviation between the optimal phase obtained by the algorithm and the phase obtained after actual loading. Therefore, in the actual calibration and testing phase, an automatic voltage calibration scheme based on an improved genetic algorithm is adopted. This scheme communicates with the programmable DC power supply through a power monitor and connects to the same host computer. The host computer executes the genetic algorithm calibration process: in each iteration of the genetic algorithm, the host computer outputs a set of voltage population data to the programmable DC power supply and simultaneously receives the corresponding output power signal collected by the power monitor; the host computer completes a quantitative evaluation of the current spectral performance according to a preset fitness function, and updates and generates the next generation of voltage population through selection, crossover, and mutation operations, iterating until the output spectral ratio converges to the target index.

[0123] The automatic calibration scheme adopted in this invention effectively solves the problems of low efficiency and difficulty in eliminating deviations in manual adjustment, thereby improving the accuracy of the device.

[0124] Preferably, the heuristic algorithm employs an improved GA to avoid getting trapped in local optima, and the process includes:

[0125] An initial population is randomly generated, and each individual consists of the phase values ​​of (M-1)×N phase shifters, representing a set of candidate phase configurations;

[0126] The spectral response of the device under the phase configuration is calculated using the transfer matrix method. Based on the error between the actual output power and the target output power, the fitness values ​​of all individuals are calculated and ranked.

[0127] An elite retention mechanism is adopted, in which high-fit individuals are directly retained to the next generation, while the rest of the ordinary individuals enter the crossover and mutation stage;

[0128] Perform a crossover operation on a regular individual to generate new offspring individuals;

[0129] The mutation rate is dynamically adjusted according to the iteration stage. A high mutation rate is used in the early stage of the iteration to ensure global exploration capability, and the mutation rate is reduced in the later stage of the iteration to enhance local fine search.

[0130] The retained elite individuals are merged with newly generated offspring individuals to form a new generation of population, and the current optimal phase shifter phase configuration is updated.

[0131] Check if the preset stopping condition is met: if it is met, the algorithm ends and outputs the optimal phase configuration; if it is not met, continue iterative optimization.

[0132] Example 3

[0133] like Figure 4 As shown, based on Embodiments 1 and 2, this embodiment takes an 8-port tunable optical power beamsplitter based on cascaded phase shifters with intervals of integer MMI and half-integer MMI as an example. The structure is illustrated by cascading three MMIs (i.e., the first uses an integer MMI, the second uses a half-integer MMI, and the third uses an integer MMI) with two phase shifters at intervals. The tunable optical beamsplitter is fabricated on a standard silicon-on-insulator (SOI) platform, with a top silicon layer thickness of 220 nm, a buried oxide layer thickness of 2 μm, and a top cover of silicon dioxide cladding.

[0134] The tunable beam splitter is designed with a working wavelength centered at 1550nm. The width of the input and output waveguides is set to 0.5μm and the length is set to 3μm. The initial width of the tapered transition waveguide is set to 0.5μm, the width of the connection end with the MMI is set to 1.6μm, and the length is set to 13.5μm. The width of the multimode waveguide for the integer MMI / half-integer MMI is set to 12.8μm.

[0135] like Figure 5 As shown, when the multimode waveguide width is fixed at 12.8 μm, the power at the MMI output port varies with the length of the multimode region as follows: When the multimode waveguide length is 149 μm, the total output power of the MMI is close to 1, and the power of the eight output ports is approximately equal, achieving uniform beam splitting; however, when the multimode waveguide length is halved to 74.5 μm, although the total output power is still close to 1, the power distribution of each port is no longer uniform; this non-uniform beam splitting characteristic provides a new possibility for the design of tunable beam splitters, enabling adjustable power distribution to be achieved by further adjusting the phase shifter parameters while reducing the device area by half; and the uniform beam splitting characteristic of integer MMIs helps to simplify the phase configuration of the back-end phase shifter and reduce the control power consumption.

[0136] like Figure 6 As shown, Figure 6 Figure (a) shows the modulus field distribution of the integer MMI when input from port 8. Figure 6Figure (b) shows the mode field distribution of the half-integer MMI, and its results are consistent with... Figure 5 The scan results shown are consistent.

[0137] Preferred, such as Figure 7 As shown in the simulation curves, the relationship between the insertion loss of the output port of the half-integer MMI and the integer MMI as a function of wavelength shows that, compared with the integer MMI, the half-integer MMI is not only smaller and more compact, but also has a wider bandwidth while ensuring lower insertion loss.

[0138] Preferably, the tunable beam splitter modulates the phase of the optical signal through a phase shifter, causing it to interfere in the MMI, thereby outputting different beam splitting ratios. However, in the actual manufacturing process, silicon-based photonic devices with high refractive index contrast are extremely sensitive to nanoscale deviations in physical geometry, while multimode interference couplers have a higher tolerance for manufacturing errors.

[0139] like Figure 8 As shown, this embodiment employs an improved GA that integrates dynamic adaptive mutation and elite retention strategies to solve for the optimal set of phase shifters that satisfy the target output response. The genetic algorithm parameters for the tunable beam splitter are set as follows: population size is set to 100, search space dimension is set to 16, corresponding to the phases of 16 phase shifters; search range is set to [0, 2π], and minimum precision of optimization parameters is set to 0.01 rad to ensure the physical realizability of the phase shifters.

[0140] In this embodiment, the overall iterative process of the algorithm includes:

[0141] The initial phase values ​​of 16 phase shifters are randomly generated to form the initial population;

[0142] The spectral response of the device under the current phase configuration is calculated using the transfer matrix method, and the fitness value of each individual is calculated.

[0143] Individuals with fitness values ​​in the top 30% are classified as elite individuals and directly retained to the next generation; the remaining ordinary individuals undergo crossover to generate new offspring individuals.

[0144] In the early stages of the iteration, a high mutation rate is used and random individuals are injected to ensure global exploration capabilities; in the later stages of the iteration, the mutation rate is reduced to enhance local fine-grained search.

[0145] The retained elite individuals are merged with the generated offspring individuals to form a new generation of population. The fitness values ​​are recalculated and sorted, and the current optimal phase shifter configuration is updated.

[0146] Check if the preset stopping condition is met: if it is, the algorithm ends and outputs the optimal phase configuration; otherwise, return to the above to continue the iteration.

[0147] The iterative process adopted in this embodiment effectively balances the exploration and development process, avoids the algorithm from converging to a local optimum too early, and improves the optimization accuracy and convergence stability of the phase shifter phase.

[0148] Preferred, such as Figure 9 As shown, the curves compare the convergence performance of the improved GA and the traditional GA under different encoding states. The horizontal axis represents the number of iterations, and the vertical axis represents the mean squared error (MSE) between the actual output power and the target output power at each port. A lower MSE value indicates a higher degree of matching between the output power distribution and the target state. Figure 9 (a) shows a comparison of the convergence performance of the algorithms when solving for the phase corresponding to the optical switch. Figure 9 Figure (b) shows a comparison of the convergence performance of the algorithms for solving non-uniform beam splitting. As can be seen from the comparison, the improved genetic algorithm proposed in this embodiment exhibits faster convergence speed and higher optimization accuracy under target power distributions of different complexities. It effectively solves the defects of traditional genetic algorithms that are prone to getting trapped in local optima and stagnating in the later stages of convergence, and can provide a better configuration scheme for the phase shifter of multi-port optical power dividers.

[0149] Preferred, such as Figure 10 As shown, the optical switching function of the device is demonstrated: at a wavelength of 1550nm, by adjusting the phase configuration of each MMI and phase shifter, all input optical power is output from a single output port, while the power transmitted to the other ports approaches zero; as shown... Figure 11 As shown, the working state of 8-channel uniform beam splitting is demonstrated: by optimizing the phase configuration, the transmission power of the 8 output ports is made to be consistent, achieving an equal power distribution of 1:1:1:1:1:1:1:1.

[0150] Preferred, such as Figure 12 As shown, in Figure 12 Figure (a) illustrates a non-uniform beam splitting ratio of 3:3:3:3:1:1:1:1. By adjusting the phase of the phase shifter, four ports achieve higher power output, while the remaining four ports achieve lower power output. Figure 12 Figure (b) shows a non-uniform splitting ratio of 4:3:2:1:0:0:0:0, with power output at only 4 ports and the transmission power at the other 4 ports suppressed to near zero, thus achieving partial gating of the output ports.

[0151] The results show that by adjusting the phase configuration of the cascaded integer / half-integer MMI structure, this invention can achieve multiple operating modes, including full-port uniform splitting, single-port gating, and non-uniform splitting with different ratios, verifying the device's excellent tunability and reconfigurability. It should be noted that the above are only some exemplary splitting ratios in typical application scenarios. The adjustable splitting capability achieved by the invention is not limited to the configurations shown in the figures. By further optimizing the phase shifter phase combinations, more non-uniform and differentiated power allocation ratios can be supported to meet the flexible needs of different systems for multi-port optical power allocation.

[0152] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A tunable optical power beam splitter based on MMI-phase shifter cascade, characterized in that, include: Input waveguide, output waveguide, tapered transition waveguide, M MMIs, and M-1 phase shifters for cascading the M MMIs; The input waveguide has N input ports, which are coupled to the input of the first MMI, and are used to input optical signals into the first MMI from any one of the input ports; One end of the tapered transition waveguide is connected to the input waveguide or the output waveguide, and the other end is connected to the MMI, which is used to achieve mode matching and reduce the coupling loss when the optical signal enters the MMI from the input waveguide; The MMI includes integer MMI and half-integer MMI, which are used to receive optical signals and generate multimode interference; Each phase shifter is set between two adjacent MMIs to independently adjust the optical signal phase of each channel according to the optimal phase configuration corresponding to the target split ratio. The output waveguide has N output ports, which are coupled to the output of the last MMI, and are used to output an optical signal with a power distribution ratio that is consistent with the target beam splitting ratio after multi-stage phase modulation. The parameters of the tapered transition waveguide were determined through a two-stage collaborative optimization using particle swarm optimization and the adjoint shape method. In the first stage, with mode mismatch loss as the optimization target, the particle swarm optimization algorithm is used for global optimization to obtain the set tapered transition waveguide substrate profile. In the second stage, the contour of the tapered transition waveguide substrate is used as the deformation constraint boundary. Based on the adjoint gradient, the contour boundary of the tapered transition waveguide is smoothly fine-tuned to jointly optimize the mode matching loss and the splitting ratio fluctuation. The design methodology of the MMI includes: The width of the multimode waveguide in the MMI is determined to ensure that the multimode waveguide supports multiple guided modes; Initially obtain the integer interference length and half-integer interference length that meet the total power requirement set at the MMI output port; The tapered transition waveguide is incorporated, and its length and width are optimized to reduce mode conversion loss; Select the integer or half-integer interference length as the design length of the MMI, and verify its interference performance through simulation; if the performance does not meet the requirements, return to optimize the tapered transition waveguide parameters until the requirements are met; if the performance meets the requirements, extract the S-parameters to obtain the amplitude and phase relationship between each input and output port.

2. The tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 1, characterized in that, Under the condition of multimode waveguide width with the same MMI, the interference length of half-integer MMI is half the interference length of integer MMI, that is, the position of the first N-fold self-image point at half the length, which is used to improve the compactness of the device. The integer MMI is used to distribute the optical signal evenly, making phase modulation more flexible.

3. A design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in any one of claims 1 to 2, characterized in that, The design method includes: Determine the design length of the MMI and extract the S-parameters. Use the S-parameters to obtain the amplitude and phase relationship between each input and output port. M MMIs and M-1 phase shifters are cascaded according to a set cascading method. The overall transmission matrix is ​​established using the S-parameters and the phase delay model of the phase shifters. A heuristic algorithm is used to solve for the optimal phase value of each phase shifter in reverse, with the goal of minimizing the error between the target splitting ratio and the actual output power. By adjusting the refractive index of the waveguide material through a phase modulation mechanism, each phase shifter can achieve its optimal phase value, thereby obtaining an optical power distribution consistent with the target splitting ratio at the N output ports of the output waveguide.

4. The design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 3, characterized in that, The cascading methods specified include: All MMIs are integer MMIs, and each integer MMI is used to distribute the input optical signal evenly, making the phase shifter adjustment more flexible; All MMIs are half-integer MMIs, and each half-integer MMI is used to shorten the device size and improve the structural compactness; M MMIs are cascaded in an alternating pattern of integer MMIs and half-integer MMIs, with phase shifters between adjacent MMIs; the first is an integer MMI, used to evenly distribute the input optical signal, and the next is a half-integer MMI, used to shorten the device size, balancing the flexibility of phase modulation and the compactness of the structure.

5. The design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 3, characterized in that, The phase modulation mechanism employs the thermo-optic effect; the thermo-optic effect changes the waveguide temperature by heating, thereby adjusting the effective refractive index of the waveguide and adjusting the phase of the optical signal to the optimal phase configuration obtained by the algorithm, so that the output power distribution is consistent with the target splitting ratio. The phase modulation of the thermo-optic effect employs an automatic voltage calibration scheme based on an improved genetic algorithm to compensate for initial phase deviation and reduce the impact of thermal crosstalk.

6. The design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 3, characterized in that, The total transmission matrix is ​​obtained by multiplying the transmission matrices of M MMIs with the transmission matrices of M-1 phase shifters; wherein, the transmission matrix of each phase shifter is a diagonal matrix, the main diagonal elements of which are the complex exponents of phase modulation introduced by each phase shifter, and the remaining elements are all zero, which is used to characterize that each optical path generates phase delay independently without inter-channel coupling. The transmission matrix of the MMI is obtained by using the S-parameters extracted from simulation, or by using the phase relationship expression of the input and output ports of the MMI.

7. The design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 3, characterized in that, The heuristic algorithm is based on the device spectral response calculated by the transfer matrix method. It constructs a fitness function with the target splitting ratio error as the core and iteratively optimizes the optimal phase configuration of the phase shifter to achieve precise control under arbitrary splitting ratios.

8. The design method for a tunable optical power beam splitter based on MMI-phase shifter cascade as described in claim 7, characterized in that, The heuristic algorithm employs an improved genetic algorithm to avoid getting trapped in local optima. The process includes: An initial population is randomly generated, and each individual consists of the phase values ​​of (M-1)×N phase shifters, representing a set of candidate phase configurations; The spectral response of the device under the phase configuration is calculated using the transfer matrix method. Based on the error between the actual output power and the target output power, the fitness values ​​of all individuals are calculated and ranked. An elite retention mechanism is adopted, in which high-fit individuals are directly retained to the next generation, while the rest of the ordinary individuals enter the crossover and mutation stage; Perform a crossover operation on a regular individual to generate new offspring individuals; The mutation rate is dynamically adjusted according to the iteration stage. A high mutation rate is used in the early stage of the iteration to ensure global exploration capability, and the mutation rate is reduced in the later stage of the iteration to enhance local fine search. The retained elite individuals are merged with newly generated offspring individuals to form a new generation of population, and the current optimal phase shifter phase configuration is updated. Check if the preset stopping condition is met: if it is met, the algorithm ends and outputs the optimal phase configuration; if it is not met, continue iterative optimization.

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