Passive Mode-Locking with Reverse Saturable Absorption

Abstract

In the passive mode-locking of a laser, the leading edge of the mode-locked pulse is shaped by the rapid reduction of loss due to absorption saturation of the absorbing material. There are two mechanisms in general use for shaping the trailing edge of a mode-locked pulse. For gain media which saturate easily, such as organic dyes, the saturation of the gain which follows the peak of the pulse shapes the lagging edge of the pulse [1]. For gain media which cannot be saturated by an individual mode-locked pulse, because the saturation energy is too high, the trailing edge of the pulse continues to see gain and is not suppressed. For solid-state lasers (e.g., alexandrite, ruby, Nd:YAG, and Nd:Glass, etc.) which have high saturation energy, a saturable absorber with a fast relaxation time is used since a pulse with energy greater than the absorber’s saturation energy will experience the least loss in propagating through the cavity when the pulse length is shorter than the absorber’s relaxation time [2]. Therefore, by this mechanism the pulsewidths of broadly tunable, solid-state mode-locked lasers are limited by the relaxation times of saturable absorbers rather than the lasing bandwidth of the media.