Editing Laser (section)

=== Continuous-wave operation ===
{{^| [[Continuous wave laser]] and [[Continuous-wave laser]] redirect here}}Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as a ''[[continuous-wave]]'' (''CW'') laser. Many types of lasers can be made to operate in continuous-wave mode to satisfy such an application. Many of these lasers lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the [[Free spectral range#Fabry–Pérot interferometer|frequency spacing]] between modes), typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous-wave" as their output power is steady when averaged over longer periods, with the very high-frequency power variations having little or no impact on the intended application. (However, the term is not applied to [[Mode locking|mode-locked]] lasers, where the ''intention'' is to create very short pulses at the rate of the round-trip time.)

For continuous-wave operation, the population inversion of the gain medium needs to be continually replenished by a steady pump source. In some lasing media, this is impossible. In some other lasers, it would require pumping the laser at a very high continuous power level, which would be impractical, or destroying the laser by producing excessive heat. Such lasers cannot be run in CW mode.