Photonic crystal fiber interferometer coated with a PAH/PAA nanolayer as humidity sensor

[Display omitted] •The proposed humidity sensor combines LbL method and photonic crystal fiber.•LbL allows the thickness of the sensing coating to be optimized at nanometer scale.•The coating thickness is kept below the evanescent field penetration depth.•Fast Fourier Transform is applied to improve the linearity of the sensor.•The sensor is reproducible with long-term stability and a resolution of 0.074%/RH. In this paper, an optical fiber interferometric humidity sensor is presented. The device consists of 1cm-long segment of photonic crystal fiber (PCF) spliced to standard single mode fibers (SMFs), forming an interferometer: the two collapsed interfaces between PCF and SMF segments produce the excitation and recombination of core and cladding modes. The latter interact with a poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) polymeric nanocoating deposited on the PCF by the well-established layer-by-layer nano assembly (LbL) technique. Humidity modifies the index of the polymeric nanolayer which in turns alters the cladding modes along the PCF segment and causes a detectable shift to the interference pattern. A study of different nanocoting thicknesses is presented in order to obtain the best possible sensibility for the sensor. Furthermore, the interrogation of the humidity sensor is presented not only by the conventional study of the spectrum shift amplitude, but also making use of the Fast Fourier Transform (FFT), which yields a linearization of the device response. The sensor here presented is reproducible, can resolve 0.074% of relative humidity (RH) and operates in the 20–95% RH range. Moreover, it exhibits response time of 0.3s, a negligible cross sensitivity to temperature as well as long term stability.