Ignacio R. Matias

Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition.

The deposition of an overlay of higher refractive index than the cladding in a Long Period Fiber Grating (LPFG) permits to improve the sensitivity to ambient refractive index changes in a great manner. When the overlay is thick enough, one of the cladding modes is guided by the overlay. This causes important shifts in the effective index values of the cladding modes, and henceforward fast shifts of the resonance wavelength of the attenuations bands in the transmission spectrum. This could be applied for improving the sensitivity of LPFG sensors. The problem is analysed with a numerical method based on LP mode approximation and coupled mode theory, which agrees with so far published experimental results.

Optical fiber humidity sensor based on a tapered fiber coated with agarose gel

Abstract An optical fiber humidity sensor (OFHS) has been fabricated using a hydrophilic gel (agarose) deposited on the thinner zone of a biconically tapered single-mode optical fiber. A variation of up to 6.5 dB of the transmitted optical power was obtained with relative humidity (RH) changes between 30% and 80%. Furthermore, the sensor shows an invariant behavior with time. This inexpensive material showed a good reproducibility with low hysteresis and its use with tapered optical fiber makes it useful for industrial and environmental applications.

OPTICAL FIBER HUMIDITY SENSOR USING A NANO FABRY-PEROT CAVITY FORMED BY THE IONIC SELF-ASSEMBLY METHOD

Abstract The ionic self-assembly monolayer process was utilized to fabricate a novel optical fiber humidity sensor based on a nano interferometric cavity. A wide operation range, from 11.3% to 100% relative humidity with a maximum variation of 4.77 dB, was experimentally demonstrated. Due to the short length of the interferometric cavity, less than 400 nm, an LED was used as the light source instead of a laser. The fast response time of this humidity sensor, less than 1.5 s, makes it possible to monitor human breathing.

Lossy Mode Resonance Generation With Indium-Tin-Oxide-Coated Optical Fibers for Sensing Applications

Surface plasmon resonances and lossy mode resonances (LMRs) can be generated with indium tin oxide (ITO) coated optical fibers. Both phenomena are analyzed and compared. LMRs present important advantages: they do not require a specific polarization of light, it is possible to generate multiple attenuation bands in the transmission spectrum, and the sensitivity of the device to external parameters can be tuned. The key parameter is the thickness of the ITO coating. The study is supported with both theoretical and experimental results. The main purposes are sensing and generation of multiple-wavelength filters.

Design of Humidity Sensors Based on Tapered Optical Fibers

In this paper, the response of a tapered optical fiber humidity sensor is optimized, attending to the thickness of the sensitive coating, the dimensions of the taper, the light source, and the utilization of sensitive materials with different refractive indexes. The main novelty of this study is that the thickness of the sensitive layer is on the order of magnitude of the light wavelength used to excite the sensor. It is shown here that an estimation of the sensor sensitivity can be deducted from the transmitted optical power curve obtained during the layer-by-layer construction process. Theoretical and experimental results are presented

Deposition of overlays by electrostatic self-assembly in long-period fiber gratings.

It was proved that the deposition of an overlay material onto a long-period fiber grating causes important shifts in the wavelengths of the typical attenuation bands that are caused by coupling between cladding and core modes [Opt. Lett. 27, 682 (2002)]. A theoretical model for analyzing a multilayer cylindrical waveguide is presented that permits the phenomenon to be understood and predicted. An overlay of higher refractive index than the cladding starts to guide a mode if a certain thickness value is exceeded. This causes large shifts in the resonance wavelength induced by the grating. One important application of this phenomenon to sensors is enhancement of the sensitivity of a long-period fiber grating to ambient conditions. Theoretical results are corroborated with experimental ones obtained by electrostatic self-assembly.

Optical fiber refractometers based on lossy mode resonances supported by TiO 2 coatings

We obtain lossy mode resonances by the coupling of light from a multimode optical waveguide to a TiO(2)/PSS coating deposited with the layer-by-layer method. The resonances can be generated in a wide wavelength range from the ultraviolet to the infrared region of the optical spectrum. The transmission spectrum is monitored as a function of the number of bilayers deposited, and the experimental results agree with the theoretical predictions. Moreover, each of the resonances owns a particular sensitivity to the external refractive index. This permits us to use the sensor as a refractometer with multiple-wavelength monitorization.

An experimental study about hydrogels for the fabrication of optical fiber humidity sensors

A study about the optical sensitivity of four different hydrogels to the humidity is presented. The investigated hydrogels were poly-hydroxyethyl methacrylate, poly-acrylamide, poly-N-vinyl pyrrolidinone and agarose. These hydrogels were deposited on optical fiber by means of direct polymerization on the optical fiber surface. In addition, these materials were examined with different light sources, temperature and relative humidity conditions. The conclusion deducted from the experiments is that increasing the pore size of the hydrogels, the sensitivity and response time of these materials with the humidity is remarkably improved.

Optical fiber nanometer-scale Fabry-Perot interferometer formed by the ionic self-assembly monolayer process.

The ionic self-assembly monolayer process is a novel technique that has already been used to deposit ultrathin films on glass, polymer, and silicon substrates of different sizes and shapes. This technique is presented as a new tool with which to apply coatings on optical fibers. A nanometer-scale interferometric cavity was built up at the end of an optical fiber with discrete thickness increments of 4.75 nm for a total thickness of 1 mum . Theoretical and experimental aspects of the nanometer-scale Fabry-Perot cavity are described, and both theoretical and experimental results show good agreement.

Design rules for lossy mode resonance based sensors

Lossy mode resonances can be obtained in the transmission spectrum of cladding removed multimode optical fiber coated with a thin-film. The sensitivity of these devices to changes in the properties of the coating or the surrounding medium can be optimized by means of the adequate parameterization of the coating refractive index, the coating thickness, and the surrounding medium refractive index. Some basic rules of design, which enable the selection of the best parameters for each specific sensing application, are indicated in this work.