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Parasitic Lasing

Definition: unwanted laser operation in a laser or amplifier device

German: parasitisches Lasen

Categories: lasers, optical amplifiers

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In lasers and amplifiers with high gain, laser operation may unintentionally take place on some closed beam path. Some examples are briefly described in the following:

The tendency for parasitic lasing is highest when there is a high unsaturated laser gain. This is the case e.g. in Q-switched lasers during the pumping period. In borderline cases, parasitic lasing may occur only at low pulse repetition rates, where the stored energy in the gain medium is higher.

Diagnosing Parasitic Laser Operation

It is not always simple to diagnose parasitic lasing. For example, when observing a laser crystal with an infrared viewer, it can be difficult to distinguish effects of parasitic lasing from those of parasitic reflections of laser light (without a closed beam path), or from pump light which may also be scattered in different directions.

For sufficiently high gain (e.g. several tens of decibels), there may also be amplified spontaneous emission (ASE). The optical spectrum usually exhibits sharp and unstable peaks in the case of parasitic lasing, whereas ASE leads to a spectrum with a smoother shape.

Effects of Parasitic Lasing

The dominating detrimental effect of parasitic lasing is usually the unwanted extraction of energy, which leads to gain clamping and may thus prevent the intended lasing altogether or at least limit its power or the pulse energy. It is also possible that parasitic lasing leads to the emission of coherent light in unexpected directions, which can constitute a laser hazard.

See also: lasers, optical amplifiers, amplified spontaneous emission
and other articles in the categories lasers, optical amplifiers

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