Physics Asked on January 6, 2022

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):

For a symmetric cavity (mode waist is largest for a confocal cavity, $R=L$. Mode volume then increases with length. But my guess is that you want the photons to spend more time in the gain medium so, unless you extend the gain medium, my guess is you don’t win for increasing length.

So that is the gain medium should entirely fill the cavity. You probably do better with more uniform mode weight so confocal is the best there again.

Id guess you get more power with a bigger gain medium but I’m sure something g limits that at some point. I’m not sure what.

I’ll post references for how to calculate cavity properties from mirror geometries and properties later.

Disclaimer: as you can see this is a lot of guesses. I’m not a laser engineer, but I do have experience with optical cavities.

*I think if you get the laser up to high enough power that you can saturate the atomic transitions even with a fraction of the circulating cavity light then second constraint will stop being relevant. This condition would be reached if the losses with the cavity round trip become small compared to the gain from the gain medium

edit: With input from the OP I can give some more info now. The OP tells us that the geometry of the gas tube is fixed with radius $R_0$ and length $L_0$.

My hypothesis is that you will get the most power (as a function of cavity geometry) when the geometry of the cavity mode closely matches that of the gas tube. That is, 1) you want the light to be hitting as many He and Ne atoms as possible but 2) you don't want the light spending time where there aren't He and Ne atoms because then you won't be saturating the atomic transitions as strongly as possible.*

The formula for the waist of the cavity mode, $w_0$, and the waists on the mirror surfaces, $w_{text{mir}}$ in terms of mirror radius $R_1=R_2=R$ and $L$, are given by

begin{align} w_0^2 =& frac{lambda}{2pi} sqrt{2R-L}sqrt{L}\ w_{text{mir}} =& w_0 sqrt{frac{2R}{2R-L}} end{align}

We want a roughly cylindrical beam since the gas tube is cylindrical. This means, from the second equation, that we want $Rgg L$ so that $w_{text{mir}} approx w_0$. In this case the first equation can be approximated as

$$ w_0^2 approx frac{lambda}{2pi}sqrt{2RL} $$

From this you could estimate an optimarl $R$ such that $w_0 approx R_0$ to make sure you use the whole gas cylinder but don't waste power outside the cylinder.

The equations above are either given directly or can be derived from equations in Siegmann's *Lasers* textbook.

Answered by Jagerber48 on January 6, 2022

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.

Answered by flippiefanus on January 6, 2022

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