Beam Quality Measurements Can Easily Go Wrong

Posted on 2007-06-11. Permanent link: http://www.rp-photonics.com/spotlight_2007_06_11.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on beam quality, M² factor, beam radius

It is well known that the beam quality of a laser beam can be measured by recording the caustic, i.e., the beam radius as a function of position along the beam, and then fitting a relatively simple function to these data. Unfortunately, the obtained results are very often quite wrong, because many details have to be observed. Some examples of possible errors are:

• For beams with non-Gaussian intensity profiles, as often occur e.g. with high power laser diodes, one has to use a proper definition of the beam radius, based on an integral of the intensity distribution.
• It is necessary to choose a suitable set of measurement points along the beam propagation direction. You need sufficiently many points close to the beam focus, and also a similar number of points far enough from the focus – more precisely, in a distance of at least two Rayleigh lengths from the focus.
• Further aspects concern the details of the fitting procedure. For example, you have to minimize the sum of the squared relative deviations.

Such requirements are defined in great detail in ISO Standard 11146. "Why should I care about that standard?", some people may ask. The answer is simple: because you want correct values, whether you use the data yourself, or publish them. And that isn't simply a matter of definition; the details of ISO 11146 have not been determined just arbitrarily, but with good reasons. In particular, they are made so that the calculation of beam propagation e.g. through some focusing optics can bring reliable results. Only with a correctly determined M² factor (beam quality factor), you can describe the beam propagation as for Gaussian beams, but inserting M² times the wavelength for the actual optical wavelength.

It is easy to imagine what can happen otherwise. As an example, you characterize the output beam of a laser diode and use the result to select and place the optics for focusing the pump beam of a diode-pumped laser. As a sanity check, you characterize the obtained beam and find that the focus is not as expected – and not suitable. So you somewhat modify your optics until you obtain the right beam parameters. This iteration is time-consuming. If you skip this, however, and just go with building you laser, it can be even worse – see the previous Spotlight article "Characterize Your Pump Beam"! So you better start with properly characterizing the pump beam, observing all the details in order to avoid the more troublesome extra loops.