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Definition: the tendency of some lasers to emit spikes of output power, particularly after being switched on

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 light 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 temporal spacing of the spikes is of the order of the inverse relaxation oscillation frequency.

Figure 1: Turn-on dynamics of a laser, simulated with the software RP Q-switch. It is assumed that the pump power is suddenly switched on. Before the steady state is reached, the laser emits a number of spikes and undergoes damped relaxation oscillations.

Figure 2: Phase space representation of the same dynamics as in Figure 1. After turning on, the operation point starts a revolution along the outer curve in counter-clockwise direction, in order then gradually to approach the steady state with smaller and smaller excursions of gain and output power.

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.