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arXiv · 2608.12902

Inverse-Designed Lithium Niobate Wavelength Demultiplexer via Birefringent Effective Index Approximation

Chihyeon Kim·Minho Choi·Munseong Bae·Hyounghan Kwon·Haejun Chung
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Abstract

Inverse design of thin-film lithium niobate (TFLN) photonic devices is computationally demanding because optical birefringence and fabrication-induced slanted sidewalls generally require three-dimensional electromagnetic models. We introduce a birefringent effective-index (BEI) method to reduce this problem to two dimensions while retaining polarization-dependent slab confinement and a representative cross section of the etched geometry. The method is integrated with adjoint topology optimization and fabrication constraints to design a 30 x 10 um demultiplexer that routes 1550 and 775 nm light to separate output ports. Quantitative comparisons with three-dimensional finite-difference time-domain simulations establish the accuracy and etch-depth dependence of the reduced model. The fabricated device provides mean signal-to-crosstalk ratios of 13.9 dB across 1540-1560 nm and 13.3 dB across 770-780 nm. A two-stage cascaded configuration increases the output extinction ratio to 26.8 dB in the telecom band and 20.3 dB in the near-visible band. This fabrication-aware reduced-dimensional approach enables optimizations of the multifunctional photonic devices for nonlinear optical and quantum applications on the TFLN platform.

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Chihyeon Kim, Minho Choi, Munseong Bae, Hyounghan Kwon, Haejun Chung. 2026-08-13. Inverse-Designed Lithium Niobate Wavelength Demultiplexer via Birefringent Effective Index Approximation. https://arxiv.org/abs/2608.12902

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