quasi-continuous-wave operation (original) (raw)

Author: the photonics expert (RP)

Definition: an operation mode of lasers (e.g. diode bars) where the pump power is switched on for short time intervals to limit thermal effects

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category light pulses light pulses

Related: continuous-wave operationmodes of laser operationgain switchingdiode barsdiode stacks

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DOI: 10.61835/kpr Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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What is Quasi-continuous Operation of Lasers?

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 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.

Mode-locked Lasers

In an entirely 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.

Bibliography

[1] T. Dascalu et al., “100-W quasi-continuous-wave diode radially pumped microchip composite Yb:YAG laser”, Opt. Lett. 27 (20), 1791 (2002); doi:10.1364/OL.27.001791
[2] C. Y. Li et al., “QCW diode-side-pumped Nd:YAG ceramic laser with 247 W output power at 1123 nm”, Appl. Phys. B 103 (2), 285 (2010); doi:10.1007/s00340-010-4228-y
[3] M. Hübner et al., “A 1.4 kW 780 nm pulsed diode laser, high duty cycle, passively side-cooled pump module”, Opt. Express 29 (7), 9749 (2021); doi:10.1364/OE.416527

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