Million-Q integrated Fabry-Perot cavity using ultralow-loss multimode retroreflectors

Abstract

The monolithic integration of Fabry-Perot cavities has many applications, such as label-free sensing, high-finesse filters, semiconductor lasers, and frequency comb generation. However, the excess loss of integrated reflectors makes it challenging to realize integrated Fabry-Perot cavities working in the ultrahigh- Q regime ( > 10 6 ). Here, we propose and experimentally demonstrate what we believe is the first silicon integrated million- Q Fabry-Perot cavity. Inspired by free-space optics, a novel monolithically integrated retroreflector is utilized to obtain near-unity reflectance and negligible reflection losses. The corner scattering in the retroreflector is prevented by the use of the TE 1 mode, taking advantage of its zero central field intensity. Losses incurred by other mechanisms are also meticulously engineered. The measurement results show resonances with an ultrahigh intrinsic Q factor of ≈ 3.4 × 10 6 spanning an 80-nm bandwidth. The measured loaded Q factor is ≈ 2.1 × 10 6 . Ultralow reflection losses ( ≈ 0.05    dB ) and propagation losses ( ≈ 0.18    dB/ cm ) are experimentally realized.