The transition from uni-directional to bi-directional behaviour in amplified spontaneous emission lasers

Amplified spontaneous emission lasers normally operate in one of two modes. Either the travelling wave mode whereby the short duration pump and laser fields propagate simultaneously along the lasant towards the output; there is thus a strong forward beam, and negligible backward - uni-directional ou tput. Or the quasi-steady mode in which the gain is uniform in space and ti me and identical beams propagate forward and backward - bi-directional outp ut. If the pump is applied over times comparable to the beam transit time, the mode of operation will change from one of these modes to the other. We examine this transition concentrating particularly on the case of strongly saturated emission. It is found that if the laser is started in the uni-dir ectional mode, the transition to bi-directional is an exponential decay, an d may be slow if the gain-length product is large. (C) 2001 Elsevier Scienc e B.V. All rights reserved.