The locking bandwidth of an actively mode-locked semiconductor laser i
s a measure of its tolerance to variations in the input drive frequenc
y. At frequencies outside the locking bandwidth, the output pulses fro
m the laser exhibit large amplitude fluctuations and timing jitter. Th
is paper investigates the locking bandwidths of fundamentally driven a
nd harmonically driven high-repetition-rate actively mode-locked semic
onductor lasers. We show that the locking bandwidth is maximized when
the cavity length is minimized. The locking bandwidth is related to an
important constant, the ''pull-in time''. Experimental data and numer
ical modeling show that the pull-in time is a function of the optical
bandwidth of the system and the RF drive level.