Mode-locking is a technique used in lasers to generate ultrashort pulses of light with durations on the order of picoseconds or femtoseconds. This method involves synchronizing the phases of a laser's longitudinal modes to produce a train of short pulses. Mode-locking allows for the generation of high peak power pulses, which can be used in a variety of applications such as microscopy, spectroscopy, telecommunications, and laser machining. The process involves the use of a mode-locking device, such as a saturable absorber or a semiconductor modulator, to selectively enhance certain modes in the laser cavity while suppressing others. This results in the generation of a train of ultra-short, high-intensity pulses with well-defined temporal and spectral characteristics.