Response of an optical cavity to phase-controlled incomplete power switching of nearly resonant incident light

The response of an optical cavity to incomplete extinction of nearly resonant incident light was experimentally examined. Measurements were performed using a Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer (CRDS) that allowed the laser frequency detuning from the cavity resonance center to be controlled at Hz-level resolution. It is shown that an insufficient laser light extinction ratio combined with a phase shift and frequency detuning may lead to non-exponential cavity pumping and decay signals. The experimental results can be explained with a simple analytical model. The non-exponential decay can lead to a systematic shift as high as 0.5% in the ring-down time constants, dependent on the laser frequency detuning from the cavity mode center and on the extinction ratio. This can lead to appreciable systematic errors in the absorption coefficients determined with the CRDS technique.