ASE recirculation effects in pulsed frequency shifted feedback lasers at large frequency shifts

An analysis of the different emission regimes (continuous wave, Q-switched, and different forms of modelocking) of a C-band Er:fiber frequency shifted feedback laser at large frequency shifts is presented. We clarify the role of amplified spontaneous emission (ASE) recirculation in the origin of various spectral and dynamical properties of this type of laser. Specifically, we show that Q-switched pulses are supported by a noisy, quasiperiodic ASE recirculation pattern that univocally identifies the pulses within the sequence, and that these Q-switched pulses are chirped as a consequence of the frequency shift. A specific pattern of ASE recirculation, in the form of a periodic stream of pulses, is identified in resonant cavities, namely, those where the free spectral range and the shifting frequency are commensurable. The phenomenology associated with this pattern is explained through the moving comb model of ASE recirculation. Modelocked emission is induced from both integer and fractional resonant conditions. It is shown that ASE recirculation coexists with modelocked pulses, originates a secondary peak in the optical spectrum, and also drives Q-switched modelocking near resonant conditions. Harmonic modelocking with variable harmonic index is also observed in non-resonant cavities.