It is expected that you don't find a TEM00 each 0.5 MHz frequency shift. As reported in entry #661, since the AOM is put on the green path, the change in the frequency which it induces is compensated by the servo with a change on the IR which is half of the frequency change in the AOM. This means that a shift of 1 MHz in the driving frequency of the AOM corresponds to a shift of 500 kHz in the frequency of the IR light. 

I think that we can now start to tweak the lens position to improve the matching.  It would be also good to do a cavity scan as the one done in entry #776.