Quasi-distributed pH sensor

A Quasi-distributed pH sensing system based on Optical Time Domain Reflectometry (OTDR) was developed to determine the spatial and intensity information from fluorescence signals coupled back into a single fibre. The evanescent wave due to a 488 nm light pulse from a N₂ pumped dye laser was used to excite a pH sensitive fluorophore and the emission around 515 nm was detected. A 3dB Y-coupler was used to convey excitation light to the sensing sites and to deliver emission signals from these sites back to a filtered photomultiplier tube. Data collection was performed using a HP 54540A 500MHz digital scope and analysis was carried out using a dedicated Pentium 166MHz PC. A reproducible laboratory fabrication process was developed to produce sensing sites at discrete intervals along the length of the fibre. A polishing process carefully removed the cladding at each sensing site and photo-polymerisation was then used to covalently bind the fluorophore fluorescein with a co-polymer directly onto the site. The results show the sensors performance over a range of pH4-pH10 with a pKa value of 6.3. The present system was chosen to have sites 10 m apart, however, based on the propagation rate of 5 ns m^-1 for light in the fibre and 10 ns for the fluorescence lifetime, a resolution of approximately 1 m is possible.