Abstract

We performed an experimental demonstration of a wedge-shaped optical 90° hybrid coupler on the thin-film lithium niobate (TFLN) platform, utilizing a paired-interference-based 2 × 4 multimode interference (MMI) coupler and a general-interference-based 2 × 2 MMI coupler. The fabricated optical 90° hybrid coupler has a compact footprint with a width of 18 µm and a length of 134 µm. In a coherent receiving system, the hybrid coupler directly connects to the balanced photodiode array, eliminating the need for waveguide crossings or cascaded phase shifters. The device exhibits a < 1.1 dB excess loss, a > 20 dB common-mode rejection ratio (CMRR), a < 1.3 dB wavelength sensitive loss, and a < ±5° phase deviation over a spectral range of 1530–1560 nm, which is promising to enable a compact heterogeneously integrated coherent receiving system on the thin-film lithium niobate platform.

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