Response of a Fabry-Perot cavity to phase modulated light

A solution to the lock-in detection method, which is valid for arbitrary values of the modulation frequency, is described. The theory for determining frequency modulation of a single frequency laser is presented. The theoretical predictions for the optimum modulation index are compared to experimental solutions for the quasi-static regime. Signal line shapes for first, second, and third harmonic generations are derived and examined. It is noted that there is good correlation between experimental and quasi-static theory data for the relation between small modulation frequency and resonance linewidth.