When the (optical or electrical) pump power of a laser gain medium is increased, this will usually lead to an increase in the resulting optical gain. However, there are situations where the gain is firmly clamped to some precisely defined value which cannot be exceeded even with strongly increased pump powers. In particular, this is the case when laser action occurs. The gain is then clamped exactly to the value of the optical resonator losses; any increase of gain would lead to an exponentially rising laser power, which is obviously not consistent with steady-state conditions. Note that this mechanism leads to a much more precisely defined gain than e.g. saturation of the gain for high pump powers.
In lasers and amplifiers (e.g. fiber amplifiers), gain clamping limits not only the optical gain but also the (spatially integrated) excitation density (upper-state population). In turn, this also limits the stored energy and the strength of other effects such as quenching processes or excited-state absorption.
Gain clamping is sometimes exploited in fiber amplifiers for stabilization of the optical gain [1, 2]. Here, lasing is deliberately allowed at some wavelength outside the range of signal wavelengths; this can be achieved by incorporating fiber Bragg gratings into the device. Fluctuations of the pump or signal power will then have only a small effect. Note, however, that gain clamping stabilizes only the steady-state gain; transient phenomena can still occur, for example for fast changes of pump or signal input power.
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