For various reasons, I have a HeNe laser setup with two external spherical mirrors that I can position and align at will. I've looked around at various sources, which all make mention of which optical resonator configurations give "decent" or "improved" mode volume and the relative amounts of diffraction losses, but am having trouble nailing down which would ideally give the best output power (I know, it's related to mode volume). I'm pretty confident, based on what I've read, that the spherical cavity configuration would produce lower power outputs, but I can't figure out which of the confocal or the long-radius configurations is better for output power.

So let's keep everything else fixed. Can't change the mirror properties, input power, tube length, etc. Let's also say I don't care about which TEM mode is output nor how hard it is to align the laser. Given this, which optical resonator configuration using two spherical mirrors is better for maximizing output power? Which gives the most bang for your buck? And what are the limitations of this configuration?

For reference, Sam's Laser FAQ - an amazing reference for all things laser - gives a great illustration of many of the common types of configurations ($r$ refers to the radius of curvature of the mirror):

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  • $\begingroup$ I don't know if this is possible: can a HeNe laser be used as q-switched? (if yes it could help to get higher power pulses). CO2 lasers are used in in q-switching, they are inhomogeneously broaden gas lasers, so at least the broadening does not seem a fundamental limitation of HeNe. But I cannot find any info about this, so it's probably unpractical for some reason. $\endgroup$ – scrx2 Sep 4 '17 at 20:09
  • $\begingroup$ @scrx2 I'm just using this as a CW laser. No pulsing $\endgroup$ – Jim Sep 8 '17 at 12:35

The design of optical resonator cavities is a fairly comprehensive topic that is not easily covered in such an answer. Books have been written on the topic [For instance: A. E. Siegman, Lasers, University Science Books, Mill Valley, CA (1986); and N. Hodgson and H. Weber, Laser Resonators and Beam Propagation, 2nd edn., Springer, Berlin (2005).]

However, perhaps one principle to keep in mind, if you want to maximize the power output, given certain constraints, is that you want to maximize the region inside the active medium that is occupied by the beam/mode.

  • $\begingroup$ it's true that I want to maximize the mode volume, which might naively make one assume that, all other things constant, the long-radius configuration would produce the maximal power output compared to the confocal configuration. However, the long-radius has greater diffraction losses than confocal, which might tip the balance. None of the many sources I have read have weighed in on output power in ideal conditions, which is why I want to try to fix as many variables as I can and ask the experts here. $\endgroup$ – Jim Sep 8 '17 at 12:17
  • $\begingroup$ Of course, if the naive answer is the correct answer, I'm also interested in the extent of that. That is, if long radius is the most power-friendly (even if it is harder to align and has high diffraction losses), then is it increasingly effective as the mirror separation becomes ever shorter than the ROC of the mirrors (obviously to the limit of the tube length), or is there some sort of ideal ratio of ROC to separation? $\endgroup$ – Jim Sep 8 '17 at 12:20
  • $\begingroup$ It completely depends on the situation, the most important factor will depend on the materials, the input and output wavelengths required. Generally it is an optimisation problem between thermal lensing and gain overlap, but that is just generally (i.e. smaller spot =/= more power). $\endgroup$ – MJC Mar 11 at 14:21

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