I should mention that I just found a bug in how it treats oddmirrornumber cavities. For such cavities, HG modes with odd horizontal indices should receive an extra roundtrip phase of pi/2 (due to the rotation by the cavity). Because of a numbering convention issue, arbcav actually used to apply this phase shift to evenorder modes. Essentially, the only difference is that the fundamental mode was shifted to antiresonance. Everywhere else, there are modes at both corresponding locations in frequency space, and so it does not back a big difference in terms of cavity design.
Thanks to this IMC modeling we are doing at the workshop, I caught it! It has been fixed in the SVN.
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I have calculated (using Zach's sweet software) the expected mode content for the various possible PRCs that we can make.
Also, Zach was right about the factor of 2. I see now that I was calculating the mode spacing between a plane wave and a HOM, so the guoy phase had a factor of (n+m+1). The right thing to do is to get the spacing between the 00 mode and HOMs, so the guoy phase just has (n+m). Switching from n+m+1=2 to n+m=1, that fixes the factor of 2 problem.
I attach my results as a pdf, since I'm listing out 5 configurations. Each config has a cartoon, with a small (hard to read) HOM plot, and then at the end, each HOM plot is shown again, but larger. Also, "TM" is the "test mirror", the flat G&H that we're using as the cavity end mirror.

