Polarization-insensitive 90° optical hybrid on the thin-film lithium niobate platform

We propose a simple polarization-insensitive 90° optical hybrid on the thin-film lithium niobate (TFLN) platform. The design comprises one 1 × 2 multimode interference (MMI) coupler, three 2 × 2 MMI couplers, and crossing waveguides, effectively reducing the excitation of higher-order modes. By optimizing the length of the 2 × 2 MMI to balance the beat length difference between transverse electric (TE) and transverse magnetic (TM) modes, polarization-insensitive transmission across the entire C-band was achieved. The device exhibits a <5° phase error for TE/TM modes, a <1 dB excess loss, and a >34 dB common-mode rejection ratio (CMRR) over the 1530-1565 nm range. Capable of direct integration with on-chip photodetectors (PDs), this design holds significant potential for applications in coherent receivers integrated with polarization-division multiplexing (PDM).