A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have been made in recent years in this field. The present paper reports the current trends of optical fiber humidity sensors. The evolution of optical structures developed towards humidity sensing, as well as the novel materials used for this purpose, will be analyzed. Well-known optical structures, such as long-period fiber gratings or fiber Bragg gratings, are still being studied towards an enhancement of their sensitivity. Sensors based on lossy mode resonances constitute a platform that combines high sensitivity with low complexity, both in terms of their fabrication process and the equipment required. Novel structures, such as resonators, are being studied in order to improve the resolution of humidity sensors. Moreover, recent research on polymer optical fibers suggests that the sensitivity of this kind of sensor has not yet reached its limit. Therefore, there is still room for improvement in terms of sensitivity and resolution.

The measurement of chemical and biomedical parameters can take advantage of the features exclusively offered by optical fibre: passive nature, electromagnetic immunity and chemical stability are some of the most relevant ones. The small dimensions of the fibre generally require that the sensing material be loaded into a supporting matrix whose morphology is adjusted at a nanometric scale. Thanks to the advances in nanotechnology new deposition methods have been developed: they allow reagents from different chemical nature to be embedded into films with a thickness always below a few microns that also show a relevant aspect ratio to ensure a high transduction interface. This review reveals some of the main techniques that are currently been employed to develop this kind of sensors, describing in detail both the resulting supporting matrices as well as the sensing materials used. The main objective is to offer a general view of the state of the art to expose the main challenges and chances that this technology is facing currently.

An optical fiber tip interferometer for the measurement of relative humidity (RH) and temperature is proposed. The optical fiber structure used, a dual-cavity optical fiber Fabry-Perot interferometer (DFFPI), is simply-to-fabricate, compact, and robust. The reflectance (RDFFPI) of the interferometer is sensitive to the refractive index (RI) and temperature of the external medium. Consequently, when the cross-section of the fiber tip was coated with a SnO2 thin film, whose RI changes according to the humidity of the surrounding ambient, the measurement of the RH was possible. An increment of the RH produced a decrement of RI of the SnO2 thin film, then the reflectance of the fiber tip end-face diminished, and this produced a decrement of the visibility of the interference fringes. The analysis of the RDFFPI was carried out in the Fourier domain, using a novel processing method it was possible to establish that the amplitude of two peaks of Fourier spectrum changed at a ratio of 39.49x10-3 %RH-1 in the range of 40 to 90 RH%. On the other hand, the temperature of the humidity chamber was monitored, from 25 to 60 °C at a fixed RH%, by analyzing the phase shift of the interference pattern produced by the changes in the optical path length of the cavities. The good sensitivity, stability, reproducibility, and compactness of the fiber tip RH sensor make this proposal very appealing in a wide range of applications.

In this work an optical fiber tunable filter based on lossy guided-mode resonances (LGMR) is proposed. It consists of a multilayer structure deposited onto the surface of a plastic cladding removed multimode fiber. The first layer is used to generate the LGMR and to work as the first electrode as well; the second one to tune the filter and the outer layer forms the other electrode. The fabricated filter has demonstrated a good sensitivity to the applied voltage showing a change of the LGMR wavelength of 0.4 nm/V. Among other applications, this filter is intended to be used as electro-optic wavelength filter or modulator.