A fiber bend based humidity sensor with a wide linear range and fast measurement speed

A fast relative humidity sensor is presented which can operate over a wide humidity range, based on a bent single mode optical fiber. We discuss the selection of a suitable hygroscopic coating and also the influence of bend radius on sensor performance. A sensor fabricated using an agarose coating with a bend radius of 16.75 mm shows a linear response in the range 25–90% RH. The measured response time of the sensor is about 50 ms, which is significantly faster than other optical humidity sensors. The humidity sensitivity of the sensor is wavelength dependent and high sensitivity is observed at higher wavelengths. This sensor offers the advantages of simple structure and low cost and its response is reversible in nature.