Distributed hydrogen determination with fiber-optic sensor

In order to develop a distributed hydrogen sensor, a fiber-optic evanescent-wave sensor was characterized using optical time domain reflectometry (OTDR). A silica core (Ø = 100 μm) fiber was coated with platinum-supported tungsten trioxide (Pt/WO3) thin film as hydrogen sensitive cladding. The fiber-optic sensors of 15 cm length were spliced into a transmitting fiber cable. Light pulse of 20 ns emitted from laser diode at 1.3 μm wavelength was applied in OTDR measurement. The power loss of about 5 dB in backscattering light at spatial position of the sensor was observed with the exposure to 1 vol.% H2/99 vol.% N2. This indicates that the sensor has the potential for distributed measurement to detect the location of hydrogen leakage points along a fiber. The response to hydrogen increased as the fiber sensor length increases (62 dB/m). On the other hand, the OTDR measurement revealed 14 dB/m of propagation loss associated with the sensor device in air. Then, step index (SI) and quasi step index (QSI) multimode optical fiber sensors, which have different refractive index profile were prepared and optical responses were compared. Although the QSI type sensor showed lower sensitivity, light level was higher than that of SI type sensor. Finally, multipoint sensing using three sensors spliced in series was demonstrated.