Fiber-optic sensors

Optical Communications Group has a broad experience on fiber-optic sensors. From 1992 we have developed fiber sensors and sensors networks. Nowadays we develop both single-point and distributed sensors using our own fabrication technology. We also develop new multiplexing networks for our sensors or commercial ones using optical amplification when needed.

This research field has five main topics:

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Resonance-based fiber-optic sensors

Resonance phenomena related to coated optical waveguides have become a widely explored technique in the field of sensors. Most of the applications developed belong to the well-known phenomenon of Surface Plasmon Resonance (SPR), which occurs when the real part of the thin-film permittivity is negative and higher in magnitude than both its own imaginary part and the real part of the permittivity of the material surrounding the thin film (i.e., the optical waveguide and the surrounding medium in contact with the thin film). Very recently, another less explored resonance base phenomenon, lossy mode resonance (LMR), has been studied theoretically and experimentally by our group for sensing applications. LMR occurs when the real part of the thin-film permittivity is positive and higher in magnitude than both its own imaginary part and the real part of the permittivity of material surrounding the thin film. Due to the previous premises, SPRs are mainly constrained to semiconductor or metallic materials, whereas LMRs can be produced by using oxides or polymeric materials among others. Furthermore, LMRs constitute a promising research field that could compete with the largely established SPRs since they do not depend on the polarization of light and permit the generation of multiple resonances without modifying the optical fiber geometry.

The generation of an LMR can be understood with the experimental setup of Fig. 1. A white light source launches light into one end of an optical fiber. At the other end, light is collected with a spectrometer. The optical fiber has a region where its core is coated with a nanofilm. This is called the sensitive region because the nanofilm can be a sensitive material. At this region, there is a coupling of light from modes guided in the optical fiber to modes guided in the nanofilm. Two conditions must be satisfied for a maximum coupling: considerable overlap between the mode fields, and phase-matching condition (i.e. the equality of real parts of propagation constants). If the absorbance is monitorized as a function of number of bilayers and the incidence light wavelength the resonances can be observed. In Fig. 2 theoretical and experimental results are presented.

Fig2. Absorbance as a function of the number of bilayers of the nanofilma and the wavelength: a) theory, b) experiments

Recently, we have demonstrated theoretically and experimentally the utilization of LMR for sensing purposes using indium tin oxide (ITO) coatings fabricated onto the optical fiber core. The characteristics of ITO satisfy the conditions for SPR at high wavelengths and also satisfy the conditions for LMR at low wavelengths. Additionally, we have proven the generation of LMRs with other materials, such as indium oxide and titanium oxide. Additionally, some polymers fulfill the specifications to produce LMRs and, at the same time, can act as transducers without the need of an extra overlay, which can simplify the fabrication of the devices. This enables the utilization of easy, low-cost and highly effective fabrication methods for the development of LMR sensing devices. Some examples of this are presented in the figure below with the utilization of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), PAH/PAA, polymeric coatings for the fabrication of LMR-based pH and relative humidity sensors.


The geometry of the optical fiber is also important in terms of improving the depth of the resonance as it has been proved recently with nanocoated tapered optical fiber. The LMR shape can be controlled by using adequate parameters in the taper geometry (see Fig. 4)

To sum up, LMR-based devices enable the utilization of an wide range of materials and are easy to implement without the need of highly sophisticated fabrication methods or equipment. Furthermore, the utilization of these devices is the first step towards the fabrication of a multipurpose sensing platform with a vast field of applications in chemistry, or biology by the only addition of an adequate second coating or sensitive overlay such as enzymes or antibodies capable of varying their refractive index in the presence of the selected analytes.

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Contact: Francisco J. Arregui, Ignacio R. Matías, I. Del Villar, Carlos R. Zamarreño, M. Hernaez.