I study on modulation frequency for SHG cavity.

Because the cavity is very short (30mm or less),

it has FSR of 4 GHz and has a finesse of 75.

These correspond to resonant width of 53 MHz.

By using FINESSE simulator,

I checked the PDH signals for each modulation frequency.

fmod                  merit                                                    demerit

600MHz            non-resonant sideband                   modulation frequency is too high

15MHz              TAMA used this freq.                        The PDH signal get smaller by a factor of 10.

I think that modulation frequency of 15 MHz is acceptable for us.