Optical Time-of-Flight Chemical Detection: Absorption-Modulated Fluorescence for Spatially Resolved Analyte Mapping in a Bidirectional Distributed Fiber-Optic Sensor

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Abstract
A continuous chemically sensitive optical fiber is used with optical time-of-flight chemical detection (OTOF-CD) for spatially resolved analyte mapping. To enhance signal levels and to improve their reproducibility, two novel principles for signal generation and processing are introduced. In the first, the fluorescence of an analyte-insensitive fluorophore is monitored as a function of the evanescent wave absorption of an analyte-sensitive indicator. The resulting signal levels are well 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 that can be detected with absorption reagents used in OTOF sensors. The second method raises signal-to-noise ratios by 3−4.5-fold for measurements 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 complementary information, only the first half of each of the collected waveforms is used for analyte quantitation. The introduced concepts were experimentally verified with a distributed sensor constructed from a 40-m-long continuous chemically sensitive optical fiber. This sensing element was produced by immobilization of an ammonia-sensitive absorbing reagent (phenol red) and an analyte-insensitive fluorophore (rhodamine 640) into the original silicone cladding of the plastic-clad silica fiber.