Silica glass fiber photorefractometer.

We report a fiber-optic refractive-index sensor that is applicable to a long-distance measurement. The sensor consists of a silica glass fiber bent into a U shape with a bending radius of typically several hundred micrometers. The cladding at the tip of the sensor is stripped off. The sensing mechanism is based on the variation of the output intensity that is induced by radiation loss at the bend, which enables us to measure the refractive index of the outer medium. A fabrication method of fusing the sensor with a CO(2) laser and etching with HF is described. Multipoint measurements of optical-time-domain reflectometry are also described.