Distributed humidity measurements using fibre-segment interferometry

A novel approach to distributed humidity sensing using polyimide-coated optical fibres and fibre segment interferometry (FSI) is demonstrated. FSI interrogates the cumulative strain along fibre segments formed between pairs of ultrashort FBG reflectors, with measurement sensitivity scaling with segment length. Polyimide, a hygroscopic coating material, induces strain on the optical fibre in response to humidity changes. Experiments were conducted using a serial array of fibre segment interferometers, comprising three 10 mm long fibre segments, with polyimide coatings on two segments and the third uncoated for control. The polyimide-coated segments exhibit strong and consistent phase responses to changes in relative humidity (RH), with responsivities of 0.049±0.05 rad/mm/%RH and 0.043±0.05 rad/mm/%RH, respectively over the 30% RH range tested. The uncoated segment displayed no measurable response to humidity.