Optical frequency combs based on a period-doubling mode-locked Er-doped fiber laser.

The phase-locking mechanism and results of a frequency comb based on a period-doubling mode-locked (PD-ML) fiber laser were investigated. A mode-locked fiber laser was designed to switch from fundamental mode locking (FML) to PD-ML with similar output pulses by simply changing the pump. Experimental results show that the new comb teeth generated in the PD-ML are strongly correlated with the original teeth and have a consistent carrier-envelope offset (CEO) frequency. Controlling the pump and cavity length is also suited for phase-locking the PD-ML laser. With the same f-to-2f heterodyne beat system and locking circuit, phase locking of both PD-ML and FML-based optical combs with two repetition rates, and switching between them, were obtained by changing the pump only.