A DISTRIBUTED OPTICAL-FIBER SENSING SYSTEM FOR MULTIPOINT HUMIDITY MEASUREMENT

A novel multi-point distributed humidity-sensing system has been constructed based on the principle of using the absorption spectrum of a colorimetric reagent (cobalt chloride) immobilized on the surface of the core of a multimode optical fibre by employing a thin gelatin film. Two-wavelength detection is employed to provide intensity referencing. Light attenuation at the reference wavelength is only 0.2 dB. The basic design, construction and testing of an experimental sensor in the humidity region 20-80% RH and temperature range 25-50 degrees C is described. Resolution can be better than 2% RH and the time constant is about one second. Repeatability is at least as good as the resolution of measurement. A two-wavelength optical time-domain reflectometer (OTDR) has been developed and a four-sensor network has been built. The resolution of measurement on the network can be better than 4% RH, but because of noise in the OTDR the resolution value is dependent on the averaging time in the instrument.