J. Ascorbe

Optical Fiber Current Transducer Using Lossy Mode Resonances for High Voltage Networks

This paper presents the development and characterization of a fiber optic transducer based on lossy mode resonances (LMRs) to monitor current transformers in high voltage networks. A multilayer structure was created in order to generate and to shift the LMR peak by an electro-optic material deposited between two electrodes. Three devices of different sensitive layer thickness have been compared and resonance wavelength displacements greater than 200 nm have been achieved, what makes this device suitable for current monitoring.

Tunable electro-optic wavelength filter based on lossy-guided mode resonances

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.

Micro and Nanostructured Materials for the Development of Optical Fibre Sensors

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.

Recent developments in fiber optics humidity sensors

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.

Humidity Sensor Based on Bragg Gratings Developed on the End Facet of an Optical Fiber by Sputtering of One Single Material

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development of wavelength-modulated optical fiber humidity sensors. A theoretical analysis has also been performed in order to study the different parameters involved in the fabrication of this optical structure and how they would affect the sensitivity of these devices. Experimental and theoretical results are in good agreement. A sensitivity of 150 pm/%RH was obtained for relative humidity changes from 20% to 60%. This kind of humidity sensors shows a maximum hysteresis of 1.3% relative humidity.

Optical fiber humidity sensor based on a tapered fiber asymmetrically coated with indium tin oxide

In this work a new humidity sensor based on lossy mode resonances (LMR) generated on a biconically tapered single mode optical fiber is presented. The tapered fiber was coated only on one side using indium tin oxide (ITO) by means of sputtering in order to generate the LMR. This asymmetrical structure (180 degrees of coating) makes the device sensitive to the polarization of the light so that the LMR can be separated in its TE and TM modes. The polarization dependence produces the effect of a smaller bandwidth in the LMR resonance with respect to the symmetrical coating, allowing a better resolution in the peak detection procedure. The sputtering process was carefully monitored in order to have the LMR placed in the range of measurement. Then the sensor was characterized using a climatic chamber with humidity in the range 20-80%RH. A sensitivity of 200 pm/%RH was obtained using a tapered fiber of 35 μm of waist diameter.

Magnetic field sensor based on a single mode-multimode-single mode optical fiber structure

A single mode-multimode-single mode structure (SMS) has been used for developing a magnetic field sensor without using magnetic fluids. The SMS optical fiber was coated with a magneto-optical material by means of sputtering. The sputtering process was monitored in order to achieve the optimum thickness of the coating and the best sensitivity. Sensitivities of 1.45 dB/T and 6 nm/T have been achieved. Repeatability in the fabrication and in the behaviour of the device has been checked.

Fabrication of Bragg Gratings on the End Facet of Standard Optical Fibers by Sputtering the Same Material

A sputtering process has been applied to deposit quarter-wavelength stacks on the end facet of cleaved optical fibers by using only one sputtering target. Standard multimode optical fibers were used as substrates to fabricate broadband filters, and the experimentally measured spectral responses of these devices are shown. Periodical changes in the refractive index of the coating have been achieved by changing the vacuum chamber pressure. A reflected peak with a full-width at half-maximum of 20 nm centered at 440 nm has been obtained, which provides a good structure for the development of optical fiber sensors working with the wavelength detection technique. This optical structure can be used for several purposes: as tunable wavelength filters or optical fiber sensors or to improve the performance of fluorescence sensors. A theoretical analysis of these structures corroborates the experimental results and allows some rules to be obtained.

Nanocoated optical fibre for lossy mode resonance (LMR) sensors and filters

Nanometer scale coatings with a complex refractive index deposited on optical fibre permit to obtain attenuation bands in the transmission spectrum, whose central wavelength coincides with the moment when a mode guided in the optical fibre cladding starts to be guided in the coating. Due to the complex refractive index of the coating, the guided mode is a lossy mode. Consequently, these attenuation bands receive the name of lossy mode resonances. This phenomenon can be used for development of ultra-high sensitivity photonic devices (for detection, among others, of volatile organic compounds, pH and refractive index) or for optical filtering. In this work, rules for adequate design are indicated based on numerical results obtained with FIMMWAVE and on experimental results that corroborate the theoretical predictions.

Low voltage transducer based on the changes in the wavelength of the attenuation band

In this work a fiber optic voltage transducer using oriented poly(vinylidene fluoride) (PVDF) films prepared by dip coating is presented. The device consists of an inner nanofilm of silver, an intermediate layer of PVdF and an external layer of Indium Tin Oxide (ITO). The metal layers were coated by sputtering, and they generate an electromagnetic resonance that produces an attenuation band, besides acting as electrodes. The fabricated transducer has demonstrated high sensitivity to the applied voltage showing wavelength displacements of up to 200 nm.