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Active Mode Locking
Active mode locking involves the modulation of energy losses incurred by the resonator itself. By synchronizing the modulation with the round-trip time for the resonator, ̶0;ultrashort̶1; pulses of laser energy can be generated. Active mode locking modulation hardware can take the form of electro-optic modulators, acousto-optic modulators, or a semiconductor electroabsorption modulator.
Passive Mode Locking
Passive mode locking can be achieved through the use of a saturable absorber. A property of a saturable absorber is recovery time, which is the time taken for a pulse to decay. Some saturable absorbers have a very short recovery time, allowing them to be used to produce ̶0;ultrashort̶1; pulses of laser energy.
Pumping
Mode locked lasers can be either continuously pumped or synchronously pumped. A pump source, such as a laser diode, supplies laser energy to the resonator either continuously or in bursts. These bursts are synchronized with the timing of the pulses already inside the resonator, hence the term synchronous pumping. Synchronous pumping is used in optical parametric oscillators, because the device is incapable of storing the energy required for continuous pumping. Most mode-locked lasers are continuously pumped, where energy is constantly supplied to the gain medium and removed at regular intervals.
Harmonic Mode Locking
Multiple pulses can be introduced into the resonator at fixed time intervals, allowing for the laser to become harmonically mode locked. These pulses can reach the multi-gigahertz range, even in lasers that are typically only capable of producing pulses in the multi-megahertz range.