Environmentally induced noises in an actively mode-locked erbium fibre laser operating in the second-order rational harmonic mode locking regime

摘要

We investigate the influence of environmentally induced cavity length fluctuations on the energy and phase noises of an optical pulse train generated by an actively mode-locked erbium-doped fibre laser that is operated in the second-order rational harmonic mode-locking regime. Using a simple model that was recently proposed to describe this particular regime, we identify the specific nature of these energy and phase noises. and we demonstrate that their values averaged over two consecutive pulses in the train can be measured directly by implementing the time-domain demodulation technique. For a known cavity length variation, we compare the values of the average energy and phase fluctuations that are obtained both theoretically, using the model, and experimentally, through time-domain demodulation. This study brings out a mechanism of coupling between the energy and phase fluctuations that appears when cavity length fluctuations are at play. This coupling is caused by the periodic intracavity amplitude modulation and strongly depends on the average detuning of the cavity length. Through the measurement of the cross-correlation between energy and phase noises. we identify environmental perturbations as a major source of the noises that affect a pulse train generated through second-order rational harmonic mode locking. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 33]