
328 Ultrashort Sources I: Fundamentals
In the case of passive mode-locking, the ratio of beam waists in the
absorber–amplifier is important, because this is the parameter that deter-
mines the relative saturation of the gain and absorber.
• a small spot size in the amplifier may be desirable for efficient pumping and
heat removal.
• a round spot is desirable in the passive mode-locking element (for instance,
the amplifier rod in the case of a Ti:sapphire laser, or the saturable absorber
jet in a dye laser) to have the most uniform possible wavefront across the
beam, because it is a region of the cavity which contributes to the phase
modulation.
Mirrors or ...