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1 environments, affecting the performance and durability of the sensors. Besides, the size of most of the electronic sensors, ranging from few to tens of millimeters, could limit their use. Some of these drawbacks are difficult to overcome and have opened the door to other technologies such as optical sensors. Among them those based on optical fiber technology have demonstrated a good capacity in dealing with these limitations. Other advantages of the fiber-optic sensors over their electronic counterparts are the high sensitivity, lightness, capability for multiplexing and remote sensing [6]. Furthermore, they are biocompatible, allowing them to be used in bio-medical sensing applications within the human body. In recent years, an important number of optical fiber sensors to measure RH have been proposed, in reflection and transmission configurations, based on the optical absorbance, fluorescence, or evanescent field interaction using longperiod gratings [7], fiber Bragg gratings [8], hetero-core fiber structures for multimode interference [9], coreexposed fiber [10], lossy mode resonances [11], fiber Mach-Zehnder interferometers [12], [13] and fiber Fabry-Perot interferometers (FFPI) [14]-[24]. Special attention has been paid to FFPI-based RH sensors since they are simple to fabricate, highly stables, and ultra-compact. An important number of high-performance RH sensors based on single and dual fiber Fabry-Perot interferometers (FFPI and DFFPI) has been proposed using single-mode [15], [16], microstructure [17], photonic crystal [18], four-holes suspended-core [19], or multimode [20] fibers. In these approaches fibers have been coated with RH-sensitive materials such as polymethyl methacrylate (PMMA) [15], polyvinyl alcohol (PVA) [18], optical adhesives [16], [19], [21], Nafion [20], and thin films of SnO2 [17], [24], Al2O3 [22], or TiO2/SiO2/TiO2 [23]. Some of these schemes were also capable to sense temperature simultaneously [17][18][19], however, both parameters were monitored by tracking the phase shift of the interference pattern. The analysis of the changes in the reflectance of the FFPI, due to changes in the RH of the surrounding environment, has been traditionally carried out in

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The dynamic range of optical fiber refractive index sensors is mainly determined by the refractive index (RI) of the sensor surface in contact with the sample under test. In case of the refractive index sensor based on the hybrid fiber Fabry–Perot interferometer (HFFPI), the largest measurable refractive index value is equal to that of the fiber core. In this work, we demonstrate that it is possible to extend the refractive index dynamic range of a HFFPI by simply adjusting the optical path length (OPL) of the air and solid cavity to be equal or differ by just a small amount. Two isometric versions of the HFFPI (i-HFFPI) with a total length of 100 and 172 μm, where the OPL of the air and solid cavity are very similar, were fabricated and tested. The interferometers were immersed in different samples with a refractive index ranging from 1.000 to 1.733. The response of the interferometers was analyzed in the Fourier domain, and it was possible to establish a one-to-one relationship between the refractive index of the liquid sample and the amplitude of one of the peaks in the Fourier spectra. The amplitude of this peak experienced a linear increment when the RI of the surrounding medium was increased. Tracking the amplitude changes of a Fourier spectrum peak is straightforward which simplifies the online monitoring of the sensor. These features make this compact refractive index fiber sensor very appealing for biosensing applications.

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