Graphene oxide-coated Long Period Grating-based fibre optic sensor for relative humidity and external refractive index

IEEE Graphene oxide is a very attractive material for refractive index and humidity sensing due to its unique two-dimensional structure, which results in faster response times and improved sensitivity over alternative materials. In this paper, response of a graphene oxide-coated long period grating-based sensor to changes in relative humidity and external refractive index (as well as temperature to provide a correction for any changes) is reported. In fabricating the probes, an improved Hummer & #x0027;s method was used to synthesis the graphene oxide dispersion used as its basis, allowing coating of a functionalized long period grating by using a dip-coating technique. A consistent and stable response of the resonance dip intensity of the graphene oxide-coated long period grating was observed to the change in humidity, achieving a sensitivity of 0.15 dB / %RH with a linear correlation coefficient of 0.980 over the relative humidity range from 60%RH to 95%RH, at room temperature (25 & #x00B0;C). A blue shift of the resonance band wavelength was recorded when the sensor was exposed to varying temperature conditions from 25 & #x00B0;C to 70 & #x00B0;C and the response was found to be linear, with a correlation coefficient of 0.997. When evaluating its performance as an external refractive index sensor, sensitivities of ~17dB/RIU in the lower refractive index region (1.33-1.38) and ~55 dB/RIU over the higher refractive index region (1.40-1.45) were achieved. The graphene oxide-coated long period grating sensor probe performed well, showing a good stability and repeatability over a number of test cycles in the performance evaluation carried out