Stability Zones of Laser Resonators

Posted on 2006-10-01 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2006_10_01.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: V. Magni, “Multielement stable resonators containing a variable lens”, J. Opt. Soc. Am. A 4 (10), 1962 (1987); encyclopedia articles on stability zones, resonator design, laser design, alignment sensitivity, and others

It is well known that laser resonators exhibit stability zones, i.e. well-defined regions for parameters like a cavity arm length or the focusing power of an intracavity element, in which stable cavity modes exist. A linear optical resonator normally exhibits two such stability zones, even though one may observe only a single one under certain circumstances. While both stability regions have the same width e.g. in terms of focusing power of the thermal lens in the gain medium, they otherwise have very different properties. In particular, they can be very different concerning the alignment sensitivity of a laser. I have myself encountered an extreme case where a laser was very hard to align due to strong thermal effects in the gain medium. When I completely redesigned the laser resonator to operate in the other stability zone, it generated more than twice the output power and was much less critical to align. Note that the mode size was nearly unchanged compared to the original design, so the mode size is clearly not all what matters.

The mentioned fundamental resonator properties have been analyzed in detail in the paper cited above. As many people are not familiar with these details, they get one of the stability zones of their resonators just at random, and may therefore easily end up with a non-ideal design. With refined software, it is possible to systematically design laser resonators operating at the stationary point of the wanted stability zone while simultaneously meeting several other requirements.