Searched around over various axial modes in order to find a beat.
I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.
On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.
For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).
We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.
With this setup, I find that a sub-100-MHz beat occurs for north slow at 5.015 V and south slow at 0.725 V. This south slow voltage corresponds to a south cavity TEM00 mode, but the nearest north slow voltage is at 5.298 V.