Harnessing micro-Fabry–Pérot reference cavities in photonic integrated circuits

Compact photonic systems that offer high frequency stability and low noise are of increasing importance to applications in precision metrology, quantum computing, communication and advanced sensing technologies. However, on-chip resonators comprising dielectrics cannot match the frequency stability and noise characteristics of Fabry–Pérot cavities, whose electromagnetic modes live almost entirely in vacuum. Here we present a novel strategy to interface microfabricated Fabry–Pérot cavities with photonic integrated circuits to realize compact, high-performance integrated systems. Using this new integration approach, we demonstrate the self-injection locking of an on-chip laser to a millimetre-scale vacuum-gap Fabry–Pérot cavity using a circuit interface that transforms the reflected cavity response to enable efficient feedback to the laser. This system achieves a phase noise of –97 dBc Hz–1 at 10-kHz offset frequency, a fractional frequency stability of 5 × 10−13 at 10 ms, a 150-Hz 1/π integral linewidth and a 35-mHz fundamental linewidth. We also present a complementary integration strategy that utilizes a vertical-emission grating coupler and a back-reflection cancellation circuit to realize a fully co-integrated module that effectively redirects the reflected signals and isolates back-reflections with a 10-dB suppression ratio, serving as a key for on-chip Pound–Drever–Hall locking. Together, these results highlight how vacuum-gap Fabry–Pérot reference cavities can be harnessed for ultrastable, low-noise photonic systems. Self-injection locking of an on-chip laser to a milimetre-scale vacuum-gap Fabry–Pérot cavity is demonstrated, with a phase noise of –97 dBc Hz–1 at a 10-kHz offset frequency and a fractional frequency stability of 5 × 10−13 at 10 ms, enabling next-generation high-performance integrated systems.