On-chip polarization management for stable nonlinear signal generation in thin-film lithium niobate

Nonlinear signal generation requires precise control of the input polarization to satisfy phase-matching conditions. Conventional polarization management using external fiber polarization controllers or bulk wave plates increases coupling complexity and can degrade polarization fidelity and conversion efficiency in nonlinear photonic systems. Here, we demonstrate on-chip polarization control in thin-film lithium niobate nonlinear photonic circuits. Integrated polarization modulators enable real-time tuning of arbitrary input polarization states and thus provide on-demand control of nonlinear conversion in a periodically poled lithium niobate waveguide. A closed-loop feedback system, which integrates auto-compensation and automatic fiber-chip alignment routines, automatically optimizes the second-harmonic generation intensity and maintains performance over extended periods despite polarization scrambling and environmental perturbations. This integrated approach reduces coupling complexity and offers a scalable route toward fully reconfigurable nonlinear photonic systems.