The Ideal Pump Intensity Distribution in an End-Pumped Solid State Laser

Posted on 2007-04-01. Permanent link: http://www.rp-photonics.com/spotlight_2007_04_01.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on end pumping, side pumping, thermal lensing, beam quality, cavity modes, mode coupling, and alignment sensitivity; R. Paschotta, "Beam quality deterioration of lasers caused by intracavity beam distortions", Opt. Express 14 (13), 6069 (2006)

In the scientific and technical literature, one often comes across the question what shape and size of pump intensity profile is ideal for an end-pumped laser. The given answers vary quite substantially and sometimes even appear to be in outright contradiction with each other. I suppose that many people would like to know what the right answer is.

Here it is: it depends! I am sorry, but that's true, even if we consider only lasers with diffraction-limited (TEM₀₀) output:

• As long as thermal lensing is weak, and the pump beam quality is sufficiently high, one can use a pump beam radius somewhat below the laser mode radius and easily obtain a diffraction-limited output. The precise pump beam shape doesn't matter very much.
• For high-power lasers with a stronger thermal lens, one might even have to do the opposite: make the pumped area a bit larger than the laser mode. This way, the laser mode doesn't so strongly feel the aberrations of the thermal lens. (Spectra-Physics actually has a patent essentially on this method.)
• A close to rectangular pump profile is sometimes said to be better than a Gaussian profile. This is because aberrations of the thermal lens then occur only at the edge, which the smaller laser mode may not probe. However, the aberrations are then fairly strong at this point, and the laser mode intensity then really has to be small there. If it is too small, however, higher-order modes may start to oscillate. A somewhat smoother pump profile can be beneficial.

In this context, it can be important to understand how exactly aberrations influence the laser's beam quality: via mode coupling – see the paper cited above. Interestingly, this crucially depends not only on mode sizes in the laser crystal, but also on other properties of the cavity design.

So we see that a universal answer cannot be appropriate. To find out what is best in a concrete case will often require either a carefully worked out approach, based on the detailed understanding of the relevant physical issues, or an experimental trial-and-error approach which might be tedious and result in non-ideal performance or stability. There are, however, cases e.g. where only a low output power is required, and the design is not critical.

In any case, however, it is highly recommended to carefully check the pump intensity profile e.g. with a beam profiler even before building the laser cavity. By trying to save a little time at this point one may later on get into trouble e.g. with instabilities, and it takes much longer then to identify and cure the cause.