All-Optical Noise Quenching of An Integrated Frequency Comb

Integrated frequency combs promise transformation of lab-based metrology into disruptive real-world applications. These microcombs are, however, sensitive to stochastic thermal fluctuations of the integrated cavity refractive index, with its impact becoming more significant as the cavity size becomes smaller. This tradeoff between microcomb noise performance and footprint stands as a prominent obstacle to realizing applications beyond a controlled lab environment. Here, we demonstrate that small footprint and low noise become compatible through the all-optical Kerr-induced synchronization (KIS) method. Our study unveils that the phase-locking nature of the synchronization between the cavity soliton and the injected reference pump laser enables the microcomb to no longer be limited by internal noise sources. Instead, the microcomb noise is mostly limited by external sources, namely, the frequency noise of the two pumps that doubly pin the microcomb. First, we theoretically and experimentally show that the individual comb tooth linewidths of an octave-spanning microcomb remain within the same order-of-magnitude as the pump lasers, contrary to the single-pumped case that exhibits a more than two order-of-magnitude increase from the pump to the comb edge. Second, we theoretically show that intrinsic noise sources such as thermorefractive noise in KIS are quenched at the cavity decay rate, greatly decreasing its impact. Experimentally, we show that even with free-running lasers, the KIS microcomb can exhibit better repetition rate noise performance than the predicted thermorefractive noise limitation in absence of KIS.