When the pump power of a laser (particularly a solid-state laser) is suddenly switched on, the laser output power may exhibit several spikes, i.e. energetic pulses, before it approaches its steady-state value via relaxation oscillations, as shown in Figure 1. Similar effects occur when the resonator losses are suddenly reduced, after some time where the gain medium was pumped (→ Q switching).
The duration of the first spike can be of the order of a few times the resonator round-trip time, and is thus often as short as a few tens of nanoseconds. Subsequent spikes then become longer and longer. The repetition frequency of the spikes (sometimes called the spiking frequency) is of the same order of magnitude as the relaxation oscillation frequency.
Pronounced spiking occurs for lasers where the upper-state lifetime is much larger than the cavity damping time. This is the case for, e.g., solid-state lasers based on ion-doped crystals or glasses, particularly when built with a short laser resonator. Spiking may then be reduced, but hardly suppressed altogether, with electronic feedback systems. Gas lasers often operate in an entirely different regime, with the upper-state lifetime being substantially smaller than the cavity damping time, so that spiking phenomena do not occur.
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