A tapered single-mode optical fibre as an intrinsic pH sensor

Sensors for monitoring pH have important industrial and biomedical applications. Intrinsic fibre optic sensors, ones that use the fibre itself as the sensor, are of particular interest due to their simplicity in addition to immunity to electrical interference. The sensor presented relies on the evanescent field excitation of a pH sensitive dye in the external region surrounding a tapered optical fibre. The authors have made use of a biconically-tapered single mode optical fibre which allows the lowest order fibre mode to interact directly with the external medium at the taper waist. A pH sensitive dye is placed in the surrounding flow cell, and the level of fluorescence corresponding to the pH of the sample is measured as light is coupled back into the taper