Mode locked Laser

Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example in modern refractive surgery. The basis of the technique is to induce a fixed-phase relationship between the longitudinal modes of the laser’s resonant cavity. Constructive interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be ‘phase-locked’ or ‘mode-locked’. Check wikipedia for Mode-locking .

Mode locking is the most important technique for generating pulses with picosecond and femtosecond durations. When a laser is mode locked, one or sometimes several pulses are circulating in the laser resonator. The case of a single pulse is most common. Each time the pulse hits the output coupler mirror, a part of its energy is emitted, so the laser output is a regular pulse train. The gain medium replenishes the pulse energy in each roundtrip.

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