Limits to pulse advance and delay in mode-locked lasers

A fundamental limitation on the net pulse advance or delay induced by the time-varying gain and loss in a mode-locked laser is described as equal to the width of the impulse response of the intracavity filter. This restriction for proper mode locking is due to the limited diffusion distance of the intracavity filter, which acts to expand the laser pulse and permits preferential amplification of the front or the rear wing. Beyond this limit, amplified spontaneous emission continually replaces the pulse, whereas with proper mode locking, the new pulse is produced by a transformation of the old pulse.