OPTICAL TIME-OF-FLIGHT CHEMICAL-DETECTION - ABSORPTION-MODULATED FLUORESCENCE FOR SPATIALLY-RESOLVED ANALYTE MAPPING IN A BIDIRECTIONAL DISTRIBUTED FIBEROPTIC SENSOR

A continuous chemically sensitive optical fiber is used with optical t ime-of-flight chemical detection (OTOF-CD) for spatially resolved anal yte mapping. To enhance signal levels and to improve their reproducibi lity, two novel principles for signal generation and processing are in troduced. In the first, the fluorescence of an analyte-insensitive flu orophore is monitored as a function of the evanescent wave absorption of an analyte-sensitive indicator. The resulting signal levels are wel l above those encountered in optical time domain reflectometry methods that rely upon backscattering for spatially resolved detection. As a result, the method could significantly expand the range of species tha t can be detected with absorption reagents used in OTOF sensors. The s econd method raises signal-to-noise ratios by 3-4.5-fold for measureme nts made at the far ends of the sensing fiber. It functions by sending probe laser-pulses into and monitoring their return sequentially from both ends of the sensing fiber. Because the two pulses provide comple mentary information, only the first half of each of the collected wave forms is used for analyte quantitation. The introduced concepts were e xperimentally verified with a distributed sensor constructed from a 40 -m-long continuous chemically sensitive optical fiber. This sensing el ement was produced by immobilization of an ammonia-sensitive absorbing reagent (phenol red) and an analyte-insensitive fluorophore (rhodamin e 640) into the original silicone cladding of the plastic-clad silica fiber.