Analysis of whispering-gallery microcavity-enhanced chemical absorption sensors

A theoretical analysis of the operation of a chemical sensor based on cavity-enhanced optical absorption is given for a system in which the cavity is a dielectric whispering-gallery microresonator. Continuous-wave input is assumed, and the detection sensitivity is characterized in terms of an effective absorption path length. In the case of tunable single-frequency input, it is shown that monitoring analyte-induced changes in the throughput dip depth enables detection with relative sensitivity greater than that of frequency-shift and cavity-ringdown methods. In addition, for the case of broadband input and drop-port output, an analysis applicable to microcavity-enhanced absorbance spectroscopy experiments is given.