Free-running performance and full control of a passively phase-stable Er:fiber frequency comb

Optical frequency combs based on erbium-doped fiber lasers are attractive tools in precision metrology due to their inherent compactness and stability. Here we study a femtosecond Er:fiber comb that passively eliminates the carrier–envelope phase slip by difference frequency generation. Quantum statistics inside the all-fiber soliton oscillator governs its free-running performance. Active stabilization of the repetition rate supports a subhertz optical linewidth and does not necessitate additional intracavity elements. Direct locking to an optical atomic frequency standard enables generation of a 100 MHz microwave signal with a stability of 3.4 mHz maintained over 15 min.