Editing Mode locking (section)

===Active mode locking===

The most common active mode-locking technique places a standing wave [[electro-optic modulator]] into the laser cavity. When driven with an electrical signal, this produces a sinusoidal [[amplitude modulation]] of the light in the cavity. Considering this in the frequency domain, if a mode has optical frequency {{Mvar|&nu;}} and is amplitude-modulated at a frequency {{Mvar|f}}, then the resulting signal has [[sideband]]s at optical frequencies {{Math|''&nu;'' &minus; ''f''}} and {{Math|''&nu;'' + ''f''}}. If the modulator is driven at the same frequency as the cavity mode spacing {{Math|&Delta;''&nu;''}}, then these sidebands correspond to the two cavity modes adjacent to the original mode. Since the sidebands are driven in-phase, the central mode and the adjacent modes will be phase-locked together. Further operation of the modulator on the sidebands produces phase locking of the {{Math|''&nu;'' &minus; 2''f''}} and {{Math|''&nu;'' + 2''f''}} modes, and so on until all modes in the gain bandwidth are locked. As said above, typical lasers are multi-mode and not seeded by a root mode, so multiple modes need to work out which phase to use. In a passive cavity with this locking applied, there is no way to dump the [[entropy]] given by the original independent phases. This locking is better described as a coupling, leading to a complicated behavior and not clean pulses. The coupling is only dissipative because of the dissipative nature of the amplitude modulation; otherwise, the phase modulation would not work.

This process can also be considered in the time domain. The amplitude modulator acts as a weak "shutter" to the light bouncing between the mirrors of the cavity, attenuating the light when it is "closed" and letting it through when it is "open". If the modulation rate {{Mvar|f}} is synchronised to the cavity round-trip time {{Mvar|&tau;}}, then a single pulse of light will bounce back and forth in the cavity. The actual strength of the modulation does not have to be large; a modulator that attenuates 1% of the light when "closed" will mode-lock a laser, since the same part of the light is repeatedly attenuated as it traverses the cavity.

Related to this amplitude modulation (AM), active mode locking is [[frequency-modulation]] (FM) mode locking, which uses a modulator device based on the [[acousto-optic effect]]. This device, when placed in a laser cavity and driven with an electrical signal, induces a small, sinusoidally varying frequency shift in the light passing through it. If the frequency of modulation is matched to the round-trip time of the cavity, then some light in the cavity sees repeated upshifts in frequency, and some repeated downshifts. After many repetitions, the upshifted and downshifted light is swept out of the gain bandwidth of the laser. The only light unaffected is that which passes through the modulator when the induced frequency shift is zero, which forms a narrow pulse of light.

The third method of active mode locking is synchronous mode locking, or synchronous pumping. In this, the pump source (energy source) for the laser is itself modulated, effectively turning the laser on and off to produce pulses. Typically, the pump source is itself another mode-locked laser. This technique requires accurately matching the cavity lengths of the pump laser and the driven laser.