Distributed fiber optical sensing of molecular oxygen with OTDR

Spatially resolved sensing of molecular oxygen is important for many biological and environmental applications. For this purpose, time-resolved fluorescence measurements were combined with optical time domain reflectometry (OTDR), a technique which was primarily developed for inspections of optical fiber lines. To achieve spatial resolution of some meters, which are typical for commercial OTDR instruments, the lifetimes of the sensor dyes must be within the range of some nanoseconds, which is much shorter than the decay times of common phosphorescent oxygen probes. Therefore, the measurements were performed with a novel fluorescent triangular-[4]phenylene sensor dye. The fluorescence decay times are around 80 ns in absence of oxygen and around 20 ns in the presence of air. The[4] phenylene sensor dye was applied in toluene solution as well as immobilized in a polymer film. Using a branched model fiber line, oxygen measurements were carried out in a micro-to millimolar concentration range. Oxygen-dependent fluorescence decay times measured with OTDR in toluene were verified by use of time-correlated single photon counting (TCSPC). The Stern-Volmer plots obtained for fluorescence quenching of sensor dyes dissolved in toluene solution and polymer-based sensor spots show good linearity.