346 Ultrashort Sources II: Examples
• when attempting to achieve the shortest pulse durations, the pulse frequen-
cies are randomly distributed [4].
One solution to these problems is to combine the techniques of passive and
active mode-locking in a hybrid system [14,15]. Depending on the optical thick-
ness of the absorber, the hybrid mode-locked laser is either a synchronously
mode-locked laser perturbed by the addition of saturable absorption or a pas-
sively mode-locked laser pumped synchronously. The distinction is obvious to
the user. The laser with little saturable absorption modulation will have the noise
characteristics and cavity length sensitivity typical of synchronously pumped
lasers, but a shorter pulse duration. The laser with a deep