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The Photonics Spotlight

The Gain Bandwidth of Laser Crystals and Glasses

Posted on 2007-10-25 as a part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2007_10_25.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Abstract: This article points out that the term gain bandwidth in the context of laser gain media is by far more difficult that it seems at a first glance. There are different definitions, and specifications according to such definitions cannot easily be converted into each other. It requires a decent understanding to deal with that term.

Dr. Rüdiger Paschotta

Ref.: encyclopedia article on gain bandwidth

At a first glance, one might really not expect any difficulties with the term gain bandwidth: isn't it simply the width of the frequency or wavelength region where some laser medium provides gain? Well, basically it is, only I suggest to drop the word “simply”.

At the heart of most of the confusion in the literature is the fact that a gain spectrum is hardly ever shaped like a rectangle, with a constant gain within some bandwidth, and no gain outside. This aspect alone might not look frightening; in optics, we are very much used to such smooth distributions and are familiar with appropriate definitions e.g. for a beam radius or a pulse duration. One might expect that we only have to agree on some percentage of the peak gain and simply measure the gain bandwidth as the difference of the two frequencies or wavelengths where the gain has dropped to that level. However, additional complications arise:

A reader of my article on the term gain bandwidth once wrote in his feedback that apparently it takes a physicist to make things so complicated. Well, my view is somewhat different: these things are rather complicated by their nature, and it takes a physicist to clarify the situation. Quite obviously, it doesn't help to keep things simple by ignoring all these pitfalls and naively dealing with published specifications. This article should have convincingly shown that when dealing with a term like “gain bandwidth”, one needs all the following:

Don't forget the latter – in the end, we should be able to apply that knowledge in practice. As an example, it is a both nontrivial and very important question whether the pulse duration achieved with a particular mode-locked laser is limited by the gain bandwidth or by something else, such as optical nonlinearities. In the latter case, it can be a great waste of time to experiment with more broadband gain media in an attempt to get shorter pulses.

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 Encyclopedia of Laser Physics and Technology.

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