Hybrid mode-locked Er-fiber oscillator with a wide repetition rate stabilization range

We present a hybrid mode-locked Er-fiber oscillator whose repetition rate can be tuned from 101.3 to 103.0 MHz. By incorporating saturable absorption and nonlinear polarization evolution in a ring cavity, mode-locked states can be easily accessed with a 3 dB spectral range wider than 50 nm. In addition, by the combination of a computer-controlled optical delay line and a piezo actuator to change the length of the cavity, the repetition rate can be stabilized at any frequency in the tunable range for the long term. When phase locking to an Rb clock for 14 h continuously, the standard deviation of the fundamental repetition rate is 0.2 mHz with a gate time of 1 s. In addition, the feedback bandwidth can be extended to ∼400 Hz by stabilizing the 10th harmonic instead of the fundamental repetition rate. Furthermore, the linewidth of the comb modes is investigated by beating with a wavelength-tunable diode laser.