Laser Operation Far Above Threshold
It is known not to be a good idea to operate a laser only slightly above its threshold: it would sensitively react to changes in pump power or to internal disturbances, and the power efficiency would be low because most of the pump power is translated into fluorescence of the gain medium. On the other hand, operation far above threshold is not a problem, is it?
To begin with efficiency, operation far above threshold may not be good, since it requires a low output coupler transmission, which can reduce the slope efficiency. But this may not always be the case, e.g. if there is plenty of laser gain or the cavity losses are very small.
Thermal effects tend to become stronger when the laser gain is increased by more strongly focusing the pump and laser radiation in the gain medium. Thermal lensing, however, won't necessarily become a more pronounced effect, since a smaller laser mode is less sensitive to lensing. This is often overlooked.
Another aspect is that the intracavity laser power will be high. In particular, the laser intensity within the gain medium will be far above the saturation intensity. In principle this doesn't have to disturb the laser process; in most cases, the laser-active ions or atoms can be cycled around very quickly. However, such high intensities can cause nonlinear effects. Particularly in fiber lasers, there may be plenty of Raman gain, causing the generation of significant power at some longer wavelength. Excessive amounts of self-phase modulation may also not be beneficial.
Special aspects come into play in optical parametric oscillators, where back-conversion can occur and lead to a very complicated kind of interaction. Particularly for synchronously pumped oscillators, this can seriously degrade the performance e.g. in terms of pulse duration, bandwidth, beam quality and stability. Such devices are often not pumped far above threshold – not because the threshold pump power would be hard to exceed, but to avoid the mentioned detrimental effects.
While the particular reasons differ very much between different cases, the bottom line is very frequently that operation moderately above threshold, say with a pump power which 3 or 5 times above threshold (or perhaps somewhat less for OPOs), presents the best compromise: the power efficiency couldn't be much larger for higher pump parameters, and various detrimental effects are safely avoided.
However, operation far above threshold is unavoidable in some cases. This can happen for fiber lasers since their mode area can not be as easily increased as for solid-state bulk lasers. Also, passively mode-locked lasers may have to be operated in this regime to avoid problems with Q-switching instabilities. In any case, the question how far above threshold to operate is one of the essential issues of laser design, and it is obviously helpful to have a detailed understanding of all involved physical effects.
This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.
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