In different occasions, Benoit suggested to use a modulation frequency of 18 MHz to improve the SNR of the locking and alignment signals.
One of the objections raised was that the 18 MHz sideband may be as sensitive to aberrations as the 6 MHz.
For this reason, simulations have been performed, scanning the PR RoC and looking at the gain for sidebands at different modulation frequencies.
The results are plotted in Fig. 1 (All modes) and Fig. 2 (TEM00 mode). As it can be seen, indeed the 18 MHz shows a large sensitivity to aberrations and it seems there would be no gain in using such modulation frequency.
