On the accuracy of decay constant measurement by swept-cavity heterodyne cavity ringdown spectroscopy

Abstract

Cavity ringdown spectroscopy (CRDS) measures the decay time, of a resonant optical cavity containing a measurand, as a function of optical frequency. The measurand is identified and quantified by the cavity decay time, which is modified by the measurand within. As coupling light into a high-finesse optical cavity is difficult, the throughput of the cavity is small. A recent variant, swept-cavity heterodyne CRDS, interferes backward escaping cavity light, with light reflected from the cavity input mirror, providing better signal sensitivity due to the heterodyne advantage. The measured interference signal is demodulated and log-amplified to produce a signal whose slope is representative of the cavity decay time. This paper, for the first time, examines the conditions required for high-fidelity measurements of the cavity decay time using swept-cavity heterodyne CRDS and log-amplification technique. We demonstrate that, due to the very large bandwidth and dynamic range of the log-amplifier, for realistic measurement conditions, the log-amplifier does not impose any significant restrictions on the measurement accuracy. We also demonstrate, however, the measurement accuracy is limited by two factors, the detector bandwidth, and segment of acquired data used to measure the slope.