A humidity sensor based on a hetero-core optical fiber

A new type of fiber optic sensor has been developed for sensing relative humidity (RH). This RH sensor was fabricated by using a hetero-core optical fiber structure that was coated with hygroscopic polymer layers by a layer-by-layer technique, producing a [poly-glutamic acid/poly-lysine] nanostructured overlay. The refractive index of the polymer layer varies with RH around the sensor, so that the light intensity transmitted in the fiber is found to be affected by the ambient RH because the layer condition influences the light leakage from the fiber. It is found that the intensity change of 0.26 dB is observed at a wavelength of 1310 nm in the range of 50–92.9%RH, which is appreciably large to conventional photo diode detection. Additionally, discussion is made in detail on the operational mechanism of RH sensor from the view point of layers hygroscopic property. The time response of the developed sensor is also investigated by experiments using the flow meter bulb action of a humid air flow system and by a preliminary blow experiment due to human breathing, resulting in a time response of approximately 400 ms.