Sub-Hz Intrinsic Linewidth Power Efficient Microcombs

The synthesis of microcombs with ultralow-phase noise is of critical importance for the generation of low phase noise microwaves [1] and optical clocks relying on optical frequency division. In microcombs, the optical linewidth distribution of the frequency lines follows an elastic tape model [2], with frequency noise increasing quadratically for carriers farther away from the pump. In [3], we demonstrated an approach for self-injection-locking a microcomb to a sideband, resulting in a dramatic modification of the frequency noise distribution. From the perspective of the elastic-tape model, the SIL point “pins down” an otherwise drifting mode, thereby constraining the frequency noise of the comb lines, in a similar manner to the “Kerr optical synchronization scheme [4] but using a single optical laser. In this work, we attain SIL with power-efficient dissipative Kerr soliton microcombs [5] pumped by an ultra-low-noise CW laser. This results into a power-efficient microcomb with sub-Hz intrinsic linewidth over the entire C-band.