Evanescent Wave Cavity Ring-down Spectroscopy: a Future Technology for Process Sensing

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is described. The total internal reflection (TIR)-ring and monolithic folded cavity designs are briefly discussed. The performance of EW-CRDS for trace water vapor detection is evaluated by accounting for surface chemical and physical properties as well as intrinsic resonator properties. INTRODUCTION Cavity ring-down spectroscopy (CRDS) uses the photon intensity decay time or ring-down time in a lowloss optical cavity to measure optical absorption. In the conventional implementation, the cavity typically consists of a pair of high reflectivity dielectric mirrors that are arranged to form a stable optical resonator. When a laser beam is injected into the cavity, the intensity decays exponentially with a ring-down time constant given by, 0 ) ( L + ) ( L t = ) (