High-performance fibre-optic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film

Abstract A high-performance humidity sensor based on a side-polished single-mode fibre (SPF) coated with graphene oxide film was fabricated with a wheel side-polishing technique and spontaneous evaporation. The wavelength-selective coupling gave rise to a loss dip in the transmitted spectrum when a phase match condition was satisfied at a given resonant wavelength. The swelling effect of GO under high RH, which was confirmed by XRD and a spectroscopic ellipsometer, could increase the GO film thickness and led to a redshift at resonant wavelength. Experimental results show that the GO-film-coated SPF (GFC-SPF) is capable of working in two modes: tracing-wavelength mode and intensity variation mode. For the two working modes, the GFC-SPF has high sensitivity, good reversibility, good repeatability and excellent linearity. For the tracing-wavelength mode, a very high sensitivity of 0.145 nm/RH% and a high linear correlation of 99.6% were achieved in a low RH range of 32%–85%, while an ultrahigh sensitivity of 0.915 nm/RH% and a linear correlation of 98.7% were achieved in a high RH range of 85%–97.6%. For the intensity variation mode, a high sensitivity of 0.427 dB/RH% and an excellent linearity correlation of 99.8% were achieved in a wide RH range of 58.2%–92.5%. Such a high performance allows GFC-SPF to have widespread potential applications, such as in the fields of biology, chemical processing and food processing.