Frequency locking of a tunable liquid-crystal filter

We describe a technique for frequency locking of a tunable liquid-crystal Fabry–Perot filter. An ac control voltage at frequency ω, used to tune the cavity, also produces a small signal modulation at 2ω. When phase sensitively detected, the 2ω signal results in a first derivative signal. Frequency tracking of the Fabry-Perot resonator is performed by a feedback system that minimizes this derivative signal. We show that this technique compensates for temperature fluctuations that would otherwise cause frequency drifts of the resonator.