For a laser, continuous-wave operation implies that it is continuously pumped and continuously emits light. The emission can occur in a single resonator mode (→ single-frequency operation) or on multiple modes.
The first continuous-wave laser was a helium–neon laser operating at 1153 nm . A version working with the now common emission wavelength of 632.8 nm was demonstrated soon after that. Later on, many other types of lasers were developed which can also be operated continuously: other gas lasers, many types of solid-state lasers (including semiconductor lasers), and dye lasers.
For many lasers with low-gain laser transitions, continuous-wave operation is difficult to achieve, while operation with pulsed pumping is easy to obtain. In some cases, continuous-wave operation is only possible with fiber lasers, but not with bulk lasers, as the fiber geometry greatly increases the gain efficiency. Some so-called self-terminating laser transitions are not suitable at all for continuous-wave operation, or only with additional measures.
The output power of a continuous-wave laser is more or less constant on longer time scales, but it can exhibit substantial power variations e.g. due to mode beating (if single-frequency operation is not achieved) and other kinds of laser noise. Various techniques for the stabilization of lasers concerning output power and/or optical frequency can be applied, often involving additional optical elements in or around the laser resonator.
In continuous-wave operation, some lasers exhibit too strong heating of the gain medium. The heating can then be reduced by quasi-continuous-wave operation, where the pump power is only switched on for limited time intervals, but long enough to reach steady-state optical conditions.
Continuous-wave Mode Locking
Some lasers are called continuous-wave mode-locked. This means that the laser is mode-locked, and the pulse energy (or average power) is constant, i.e. there is no Q-switched mode locking. Despite the emission of pulses, mode locking can be considered as a kind of continuous-wave operation in the sense that the resonator modes are excited with constant powers.
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