In a Q-switched laser, pumping too hard or holding the switch open too long lets the inversion rebuild and fire a second pulse after the first. In a mode-locked laser, excess intracavity power can split the circulating pulse into two, which shows as modulation fringes on the spectrum and side peaks on the autocorrelation. Either way pulse energy and peak power drop and downstream optics see an unexpected pulse.
Spectrum's got fringes on it, it's double pulsing, back the pump off half a watt and it'll go back to one.
Related terms
Q-switching
Producing short, intense laser pulses by holding cavity losses high while energy builds up, then suddenly dropping them.
Mode locking
Fixing the phases of many longitudinal modes so they add up into a train of ultrashort pulses.
Autocorrelator
An instrument that measures ultrashort pulse duration by overlapping a pulse with a delayed copy of itself.
CW breakthrough
A narrow continuous-wave spike in a mode-locked laser's spectrum, showing part of the power is not pulsing.