Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.
Keywords: 42.60.Da; 42.62.Fi; 42.81.Pa; 78.20.Ci; 82.80.Dx; PACS 07.60.Vg; absorption; capillary electrophoresis; cavity ring-down (CRD); fiber Bragg grating (FBG); fiber cavity; fiber-loop; long-period grating (LPG); microfluidics; phase-shift; refractive index.