Inverse Design of a Silicon-Based Compact Polarization Splitter-Rotator

HUI Zhanqiang; ZHANG Xinglong; HAN Dongdong; LI Tiantian; GONG Jiamin

doi:10.11999/JEIT250858

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HUI Zhanqiang, ZHANG Xinglong, HAN Dongdong, LI Tiantian, GONG Jiamin. Inverse Design of a Silicon-Based Compact Polarization Splitter-Rotator[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT250858

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HUI Zhanqiang, ZHANG Xinglong, HAN Dongdong, LI Tiantian, GONG Jiamin. Inverse Design of a Silicon-Based Compact Polarization Splitter-Rotator[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT250858

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Inverse Design of a Silicon-Based Compact Polarization Splitter-Rotator

doi: 10.11999/JEIT250858 cstr: 32379.14.JEIT250858

HUI Zhanqiang^{1, 2
,
,},
ZHANG Xinglong^{1, 2},
HAN Dongdong^{1, 2},
LI Tiantian^{1, 2},
GONG Jiamin^{1, 2}

1.
The School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an 710121, China
2.
The Shaanxi Province International Science and Technology Cooperation Base for Microwave Photon and Next Generation Broadband Communication Technology, Xi’an 710121, China

Funds: The National Key Research and Development Program Project (2022YFB2903201), Shaanxi Provincial Innovation Capacity Support Program Project (2022PT15)

Received Date: 2025-09-02
Accepted Date: 2025-11-13
Rev Recd Date: 2025-11-13

Available Online: 2025-11-18

Abstract

Abstract

Objective The Polarization Splitter-Rotator (PSR) is a key device used to control the polarization state of light in Photonic Integrated Circuits (PICs). Device size has become a major constraint on integration density in PICs. Traditional design methods are time-consuming and tend to yield larger device footprints. Inverse design, by contrast, determines structural parameters through optimization algorithms according to target performance and enables compact devices to be obtained while maintaining functionality. This strategy is now applied to wavelength and mode division multiplexers, all-optical logic gates, power splitters, and other integrated photonic components. The objective of this work is to use inverse design to address size limitations in silicon-based PSRs by combining the Momentum Optimization algorithm with the Adjoint Method. This combined approach improves the integration level of PICs and provides a feasible pathway for the miniaturization of other photonic devices. Methods The design region is defined on a 220 nm Silicon-on-Insulator (SOI) wafer and is discretized into 25×50 cylindrical elements. Each element has a 50 nm radius, a 150 nm height, and an initial relative permittivity of 6.55. The adjoint method is used to obtain gradient information across the design region, and this gradient is processed with the Momentum Optimization algorithm. The relative permittivity of each element is then updated according to the processed gradient. During optimization, the momentum factor is dynamically adjusted with the iteration number to accelerate convergence, and a linear bias is applied to guide the permittivity toward the values of silicon and air as the iterations progress. After optimization, the elements are binarized based on their final permittivity: values below 6.55 are assigned to air, whereas values above 6.55 are assigned to silicon. This results in a structure containing irregularly distributed air holes. To compensate for performance loss introduced during binarization, the etching depth of air holes with pre-binarization permittivity between 3 and 6.55 is optimized. Adjacent air holes are merged to reduce fabrication errors. The final device consists of air holes with five radii, among which three larger-radius types are selected for further refinement. Their etching radii and depths are optimized to recover remaining performance loss. Device performance is evaluated through numerical analysis. Calculated parameters include Insertion Loss (IL), Crosstalk (CT), Polarization Extinction Ratio (PER), and bandwidth. Tolerance analysis is also conducted to assess robustness under fabrication variations. Results and Discussions A compact PSR is designed on a 220 nm SOI wafer with dimensions of 5 μm in length and 2.5 μm in width. During optimization, the momentum factor in the Momentum Optimization algorithm is dynamically adjusted. A larger momentum factor is applied in the early stage to accelerate escape from local maxima or plateau regions, whereas a smaller momentum factor is used in later iterations to increase the weight of the current gradient. Compared with other optimization strategies, this algorithm requires only 20%～33% of the iteration count needed by alternative methods to reach a Figure of Merit (FOM) of 1.7, which improves optimization efficiency. Numerical analysis shows that the device achieves stable performance across the 1 520～1 575 nm wavelength range. The IL remains low (TM0 < 1 dB, TE0 < 0.68 dB), and the CT is effectively suppressed (TM0 < –23 dB, TE0 < –25.2 dB). The PER is high (TM0 > 17 dB, TE0 > 28.5 dB). Tolerance analysis indicates strong robustness to fabrication variations. Within the 1 520～1 540 nm range, performance remains stable under etching depth offsets of ±9 nm and etching radius offsets of ±5 nm, demonstrating reliable manufacturability. Conclusions Numerical analysis demonstrates that combining the adjoint method with the Momentum Optimization algorithm is a feasible strategy for designing an integrated PSR. The design principle relies on controlling light propagation through adjustments to the relative permittivity, which determine the distribution and placement of air holes to achieve polarization splitting and rotation. Compared with traditional design approaches, inverse design uses the design region more efficiently and enables a more compact device structure. The proposed PSR is markedly smaller and shows enhanced fabrication tolerance. It is suitable for future large-scale PICs and provides useful guidance for the miniaturization of other photonic devices.
- Polarization Splitter-Rotator (PSR),
- Inverse design,
- Adjoint method,
- Momentum optimization algorithm,
- Air hole merging

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References(46)

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