Graphene-Oxide-Coated Long-Period Grating-Based Fiber Optic Sensor for Relative Humidity and External Refractive Index

Graphene oxide is a very attractive material for refractive index and humidity sensing due to its unique 2-D 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 (RH) and external refractive index (as well as temperature to provide a correction for any changes) is reported. In fabricating the probes, an improved Hummer's method was used to synthesize 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 band 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 RH range from 60% RH to 95% RH, at room temperature (25 degrees C). A blue shift of the resonance band wavelength was recorded when the sensor was exposed to vary temperature conditions from 25 degrees C to 70 degrees 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 similar to 17 dB/RIU in the lower refractive index region (1.33-1.38) and similar to 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.