Light Diffusing Optical Fiber Sensor for Distributed Optical Absorption Spectroscopy and Chemical Sensing

An optical fiber sensor system for distributed optical absorption spectroscopy based on light diffusing optical fiber is proposed and evaluated. The sensor is composed by two light-diffusing glass fibers radiatively coupled. The light from a pulsed laser diode (PLD) propagates along the first fiber and is locally diffused out into the medium between the fibers. The light transmitted by the medium is partially collected by the second fiber and detected at the end of the fiber by an optical time-domain reflectometry (OTDR) detection scheme. The system permits distributed measurements of the optical absorption properties of the sample medium between the fibers. The experimental results confirm the possibility of distributed measurements with a spatial resolution of about 17 cm over 4 m of measurement range. Distributed chemical sensing has also been evaluated by measuring the absorption of water solutions containing copper ions. A limit of detection (LOD) of 710 ppm has been achieved.