Polarization‐dependent gain characterization in x‐cut LNOI erbium‐doped waveguide amplifiers

Abstract Erbium‐doped waveguide amplifiers (EDWAs) are vital for photonic integration, yet most are built on z‐cut lithium niobate, incompatible with the mainstream x‐cut platform. This work presents a combined theoretical and experimental study of polarization‐dependent gain in x‐cut Er:LNOI. Using Judd–Ofelt theory, we analyze how crystal orientation governs TE‐mode coupling to Er 3+ ions, predicting stark differences in transition strengths between α ‐ and π ‐polarizations. Experiments confirm these predictions: at 1,531 nm, the absorption and emission cross sections for α ‐polarization are 1.8 times larger than for π ‐polarization. At 1,550 nm, the α ‐polarization shows a gain coefficient of 3.3 dB/cm versus 2.2 dB/cm for π ‐polarization. In the small‐signal regime, the α ‐polarized amplifier achieves 32.01 dB signal enhancement with 11.18 dB internal net gain. With 9.1 dBm on‐chip input power, it delivers 21.18 mW unsaturated output power under pumping levels exceeding 200 mW. This work demonstrates feasible optical amplification on x‐cut LNOI, providing crucial support for large‐scale photonic and microwave photonic systems.