When a laser (particularly a solid-state laser) is passively mode locked with a saturable absorber, it can exhibit so-called Q-switching instabilities. In some cases, Q-switched mode locking can lead to the regular emission of bunches of pulses with fairly stable parameters, but in other cases there are substantial fluctuations of the pulse parameters. Typically, rather unstable behavior is obtained when the parameters are chosen such that the bunch period becomes long and the pulse energy can become extremely low between the pulse bunches. In that case, the pulses in each bunch are essentially generated starting from spontaneous emission, and the pulse parameters cannot reach a steady state, because a bunch corresponds only to a few resonator round trips.
The article on Q-switched mode locking discusses possible measures for suppressing Q-switching instabilities, i.e., for obtaining stable continuous-wave mode locking with a constant pulse energy. In certain parameter regions, e.g. ultrahigh pulse repetition rates or high output power, this can be difficult or at least require compromises e.g. concerning the pulse duration.
Note that there are other types of instabilities of mode-locked lasers, which are not easily distinguished from Q-switching instabilities in experiments, but have a definitely different origin (e.g. a too long recovery time of the saturable absorber) and accordingly need different methods to be suppressed.
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