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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...

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I am no expert on this but after reading about the technique I would offer a couple of suggestions.

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.

All of this seems to argue for a "relatively large" frequency shift.

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  • $\begingroup$ The key point about the PDH technique is that the 2 sideband frequencies are 180 degrees out of phase with each other. So when the laser frequency exactly matches that of the FP cavity the beat signal is zero, and as the laser drifts off in frequency, you'll start to see a modulation signal (this is your error signal.) So, you need a frequency difference that you can detect in a phase sensitive manner. $\endgroup$
    – JQK
    Jan 12 '16 at 21:05

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