Pedestal analysis of jitter in a 20.97 GHz regeneratively mode-locked laser formed with a short polarization-maintaining fiber amplifier.

We demonstrated a 20.97 GHz regeneratively mode-locked Er fiber laser that has a short ring length of 5.84 m. The short active ring enabled large filter-rejection of adjacent supermodes during extraction of its clock signal for regenerative feedback. Very stable mode locking was observed with the generation of ∼5.5 Gaussian pulses. A characterization of the noise pedestals in the photodetected first and second harmonics of the mode-locked pulses indicated there was cross-correlated coupling between the timing and amplitude jitter. From an analysis of the noise pedestals in these harmonics, as well as the noise spectrum at baseband, we estimated an amplitude jitter of ∼0.19, and a timing jitter of ∼60. Using a generalized harmonic analysis approach, we estimated a cross correlation of 0.11% between the timing and amplitude jitter.