A bandwidth response humidity sensor with micro-nano fibre Bragg grating

A polyimide-coated fibre Bragg grating (FBG) relative humidity sensor with bandwidth response is proposed and demonstrated. A 12-mu m-diameter micro-nano fibre is fabricated and the FBG was written by a 244-nm-UV laser, followed by the coating of polyimide on the FBG to form the sensor. The principle of the bandwidth response sensing based on the relationship between the humidity and the reflection spectra bandwidth is described. The sensor is examined under different relative humidity conditions. The results show that, with an increase in the relative humidity, the bandwidth changes at a rate of 0.00297 nm/%RH, and the linearity value is 99.3%. However, when the relative humidity is decrease, the linearity values are 97% and 98%, and the sensitivity values are 0.0035 nm/%RH and 0.0008 nm/%RH at ranges of 90-65 %RH and 65-30 %RH, respectively. In addition, the sensor has been tested the bandwidth change with temperature change under different humidity, the bandwidth-temperature sensitivity is about 0.35 pm/degrees C.