Detection of high humidity by optical fibre sensing at telecommunications wavelengths

We demonstrate the proof of principle for a novel fibre optic sensor for the detection of high humidity. The device uses single-mode fibre at telecommunications wavelengths, making it particularly suitable for application in passive optical networks. The sensing mechanism relies on a humidity-induced refractive index change in an organic film coated onto the cladding of a bent fibre. The sensor was shown to respond at relative humidities > 62% and > 80% using gelatine and polyethylene oxide films respectively, by the onset of a strong interaction between the LP₀₁ mode and a whispering gallery mode. © 1998 Elsevier Science B.V.