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

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 Er³⁺ 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.