Exploring fiber optic approach to sense humid environment over nano-crystalline zinc oxide film

A novel humidity sensor made up of nano-crystalline zinc oxide (ZnO) film, coated onto the U-shaped typical glass substrate as a wave guide, conjugated with an optical fiber and He–Ne un-polarized laser source. The nano-crystalline zinc oxide (ZnO) was synthesized using single molecular precursor method. The resulting material was characterized with Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), thermogravimetric-differential thermal analysis (TG-DTA) and scanning electron microscopy (SEM). In the thin film, ZnO particles exhibited the wurtzite phase structure with the particle size in a diameter range of 70–80 nm. The humidity sensing characteristic has been estimated by measuring the optical permeability (OP) as a function of percentage relative humidity (%RH) in the ranging from 5 to 90 inside a closed chamber. The OP decreases linearly with increase in %RH with a respond time of about 30 and recovery time of 35 s. The sensor exhibits the sensitivity of 0.45 in the %RH range 5–50 and 0.30 from 50 to 90.