| The improved sensitivity of Cavity Ring-down Spectroscopy (CRDS) over traditional molecular absorption spectroscopy techniques has resulted in its widespread adoption. Similarly, the versatility of optical fiber sensors has increased the range of applications for such devices. Traditional CRDS, based on a cavity formed by two highly reflective mirrors, is limited to localized detection in gaseous matrices, but its functionality can be expanded by implementation in optical fiber. In this thesis, we show that the high characteristic sensitivity of the CRDS technique, which is derived from the measurement of small changes in a resonator's decay rate, is also an effective method for determination of the loss in an optical fiber system.; Built from common telecommunications components, a single mode fiber optic CRDS resonator can be adapted for a variety of new applications by the incorporation of a biconically tapered fiber segment, which improves access to the evanescent field in the optical fiber. We demonstrate molecular absorption sensing in such a device by resolving the near-infrared spectrum of 1-octyne with a sensitivity better than common evanescent wave spectroscopy techniques. By chemically modifying the taper's surface, we show detection in the near-field of the taper of biological species at single-cell levels. This instrument, based on a 2.2 km resonator, can also be distributed over an large perimeter, allowing detection of both chemical and biological agents at discrete sensors along its length.; We also extend the CRD technique beyond typical spectroscopic applications with only minor modification to the tapered sensing structure. By configuring the resonator to measure mechanical strain and displacement, a CRD strain gauge shows sensitivity comparable to that of other common optical fiber strain gauges without the expensive acquisition and processing components that they require. This extension of the applicability of CRDS to the measurement of physical parameters exploits the immunity of CRDS from laser intensity fluctuations and external loss contributions that are common sources of noise in optical fiber systems. This demonstration of a simple, inexpensive combination of CRDS and optical fiber technology represents a significant expansion of the versatility of highly sensitive spectroscopic sensing for chemical, biological, and physical properties. |
