Phase sensitive detection of light reflected from a Fabry–Pérot interferometer

We present an analysis of the Fabry–Perot response to a phase‐modulated light in the reflection mode, by considering the general problem of the lock‐in detection at the pth harmonics of the rf modulating frequency. Suitable frequency modulation conditions for servo‐locking purposes are obtained and the values of modulation index which maximize the sensitivity for the first, third, and fifth harmonics are found. Moreover, we investigate the effects of the residual amplitude modulation introduced by the electro‐optic frequency modulator, the presence of laser amplitude and frequency noise, and the dependence of the achievable closed‐loop frequency fluctuation spectrum on the modulation index and detection noise.