The equation used to find relation between frequency and phase should be restricted inside cavity because it comes from the term phi = 2*pi*(f*L)/c. Since we assume the cavity is kept on resonance, we have relation between f and L. So we don't compare the phase of laser inside and outside cavity.

We measured the phase noise introduced by AOM. According to Fourier transform, the frequency noise is phi/f.

Taking phi/f and cavity pole, we get the frequency noise introduced by AOM as Fig.1. We can see AOM introduce negligible frequency noise of only 25 uHz.