# Frequency modulation in laser locking - what frequency?

In laser locking, e.g. using the PDH technique, the light (say of frequency $f$) is modulated so as to add two symmetric sidebands of frequency $\Omega$.

I know the maths, I understand how this leads to the error signal etc.

Physically and intuitevely, how would I choose the modulation frequency $\Omega$?

I.e. : it should not be the same as the carrier light because... / it should be far or close to it because...

The mixing frequency creates side bands at $f+-\Omega$; these have to be "far from" the resonance of the FP resonator, which sets a minimum frequency; but since the frequency is also needed when you mix down for the detection of the error signal I suspect that smog it too large will give rise to error terms in the error signal. Finally you need to consider that you want the frequency large enough the you can easily filter out the side bands to obtain a pure error signal. The higher the mixing frequency, the easier it is to filter, and the faster the response to the error signal can be.