Abstract
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.
Highlights
Cavity ring-down (CRD) spectroscopy and related cavity-enhanced absorption spectroscopic methods have garnered a large following in the past decade [1,2,3]
Cavity ring-down spectroscopy is based on determining the optical losses of a high-finesse cavity by monitoring the lifetime of a photon trapped in the cavity
For the purpose of this review we distinguish between two types of fiber cavities: (1) the linear fiber cavity where light is confined by fiber optic “mirrors” such as reflective coatings or fiber Bragg gratings (FBGs) and (2) a loop of waveguide material
Summary
Cavity ring-down (CRD) spectroscopy and related cavity-enhanced absorption spectroscopic methods have garnered a large following in the past decade [1,2,3]. Fiber loops are inherently broadband cavities and have a low roundtrip loss over the entire transmission range of the fiber material - for silica fibers from approximately 1.7 μm to 250 nm Such a fiber loop cavity can be designed to hold a variety of sensor heads, such as very small absorption cells [20], long-period gratings for refractive index sensing [21], evanescent field blocks [17], tapers for absorption [22,23] or strain [24] sensing, and others. In this review we will first present a brief overview of the theoretical foundations and discuss different designs of sensor elements for fiber-based CRD devices and their applications
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