Modes of Laser Operation
Lasers can be used in distinct modes of operation, the most important of which are:
- Continuous-wave operation (cw operation): the laser is continuously pumped and emits light continuously, either on a single resonator mode (→ single-frequency operation) or on multiple modes (see also: single-mode operation).
- Quasi-continuous-wave operation (quasi-cw operation): the pump source is switched on only for short time intervals to prevent excessive heating.
- Gain-switched operation: gain switching means that the pump source is turned out only for very short time intervals (below the upper-state lifetime) in order to obtain short pulses.
- Q-switched operation: the intracavity losses are modulated, so that the laser emits energetic pulses. Pumping may be continuous or pulsed.
- Mode-locked operation: initiated and stabilized by an optical modulator or a saturable absorber, one or several ultrashort pulses are circulating in the laser resonator, so that a regular train of pulses is generated. Mode-locked lasers are usually continuously pumped, but mode locking with quasi-continuous pumps is also possible.
- Q-switched mode-locked operation: simultaneous mode locking and Q switching occurs.
In more exotic cases, lasers exhibit chaotic oscillations or fluctuations, or exhibit only amplified spontaneous emission.