Mode-locked lasers are a type of laser that produces ultrashort pulses of light by actively modulating the laser cavity to produce a series of pulses that are temporally and spatially confined. This results in a high peak power and narrow spectral bandwidth, making mode-locked lasers ideal for a wide range of applications such as optical communications, ultrafast spectroscopy, and micromachining. Mode-locked lasers can operate in different modes, including passive mode-locking, where the modulation is achieved through the introduction of a saturable absorber or semiconductor, or active mode-locking, where an external modulator or fast electronic devices are used. Mode-locked lasers have significant potential for advancing various fields of science and technology due to their unique capabilities for producing intense, short pulses of light.