Reaching fiber-laser coherence in integrated photonics

We self-injection-lock a diode laser to a 1.41 m long, ultra-high Q integrated resonator. The hybrid integrated laser reaches a frequency noise floor of 0.006Hz^2/Hz at 4 MHz offset, corresponding to a Lorentzian linewidth below 40 mHz—a record among semiconductor lasers. It also exhibits exceptional stability at low-offset frequencies, with frequency noise of 200Hz^2/Hz at 100 Hz offset. Such performance, realized in a system comprised entirely of integrated photonic chips, marks a milestone in the development of integrated photonics; and, for the first time, to the best of our knowledge, exceeds the frequency noise performance of commercially available, high-performance fiber lasers.