Quasi-continuous-wave (quasi-cw) operation of a laser means that its pump source is switched on only for certain time intervals, which are short enough to reduce thermal effects significantly, but still long enough that the laser process is close to its steady state, i.e. the laser is optically in the state of continuous-wave operation. The duty cycle (percentage of “on” time) may be, e.g., a few percent, thus strongly reducing the heating and all the related thermal effects, such as thermal lensing and damage through overheating. Therefore, quasi-cw operation allows the operation with relatively high output power during limited time intervals.
Pulsed operation with much shorter pumping times, where an optical steady state is not reached, is called gain switching.
Quasi-continuous-wave operation is most often used with diode bars and diode stacks. Such devices are sometimes even designed specifically for quasi-cw operation: their cooling arrangement is designed for a smaller heat load, and the emitters can be more closely packed in order to obtain a higher brightness and beam quality. Compared with ordinary continuous-wave operation, additional lifetime issues can result from quasi-cw operation, related e.g. to higher optical peak intensities or to frequent temperature changes.
Some doped-insulator solid-state lasers are also operated in quasi-cw operation. Such lasers are sometimes called heat capacity lasers.
In a completely different sense, mode-locked operation of a laser could be considered as quasi-continuous operation. Due to the very high pulse repetition rate, where the pulse energy is typically very far below the saturation energy of the laser gain medium, gain saturation is essentially the same as for continuous-wave operation with the same average power. The performance in terms of average power can thus be calculated as for a continuous-wave laser. However, the term quasi-continuous-wave operation is usually not applied to that situation.
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