Highly tunable birefringent phase-matched second-harmonic generation in an angle-cut lithium niobate-on-insulator ridge waveguide.

Phase-matched nonlinear wave mixing, e.g., second-harmonic generation (SHG), is crucial for frequency conversion for integrated photonics and applications, where phase matching wavelength tunability in a wide manner is important. Here, we propose and demonstrate a novel design of angle-cut ridge waveguides for SHG on the lithium niobate-on-insulator (LNOI) platform via type-I birefringent phase matching (BPM). The unique strong birefringence of LN is used to achieve flexible temperature tuning. We experimentally demonstrate a normalized BPM conversion efficiency of 2.7%W-1cm-2 in an angle-cut LN ridge waveguide with a thermo tuning slope of 1.06 nm/K at the telecommunication C band. The approach effectively overcomes the spatial walk-off effect and avoids the need for periodic domain engineering. Furthermore, the angle-cut ridge waveguide scheme can be universally extended to other on-chip birefringent platforms where domain engineering is difficult or immature. The approach may open up an avenue for tunable nonlinear frequency conversion on integrated photonics for broad applications.