Quasi-distributed measurement of surrounding refractive index using photon-counting time domain reflectometry

Using optical fibers as refractometers has attracted considerable attention thanks to their weak intrusion, remote operation and good sensitivity in aqueous media. To measure the surrounding refractive index (SRI) with an optical fiber, the core mode has to be brought into contact with the surrounding medium. This can be done through offset splices or etching, which decreases the mechanical strength. Radiative fiber gratings overcome this issue. However, their manufacturing needs dedicated equipment (e.g. expensive UV laser). We demonstrate here that a single arc-fusion can make the fiber locally sensitive to the SRI, as it breaks the waveguiding conditions and couples light from the core to the cladding. Quasi-distributed measurements are obtained with different arcs along a single fiber interrogated by a photon-counting Optical Time Domain Reflectometer. Its high spatial resolution allows us to detect a modification of the backreflected signal due to a variation of the SRI. Another advantage of this new kind of sensor is its temperature insensitivity.