Óscar Esteban

In situ salinity measurements in seawater with a fibre-optic probe

We have successfully proved the feasibility of an optical salinity meter for marine applications in a two-week measurement campaign, carried out for the realization of in situ salinity measurements in seawater. An optical instrument (optode), in which the main element is a fibre-optic refractive-index sensor based on surface plasmon resonance (SPR), has been developed for that purpose, and has been especially designed to be able to operate in realistic conditions. The performance of the optode has been evaluated on an oceanographic ship in the Baltic Sea, close to the Vistula estuarine area. The obtained results (in different tests, such as depth-profiling, towing and stationary measurements) show good correlation with the data provided by a commercial probe. Although the device is currently a part of a more complex measuring platform and uses an axial spectrograph as detector, the output power measurement used and the simplicity of its conception allow us to conceive a closed, extremely compact set-up which can be in principle commercially competitive with existing sensors.

Surface plasmon resonance sensors based on uniform-waist tapered fibers in a reflective configuration

We present a configuration for surface plasmon resonance sensors based on uniform-waist tapered optical fibers and reflective elements. Once the fiber is tapered fulfilling the adiabatic criterion, a multilayer including a metallic medium is asymmetrically deposited on the uniform waist of the fiber. This feature provides the resonant excitation of multiple surface plasma waves. In addition, a mirror is produced at the fiber tip by a chemical Tollens reaction. In this way, the sensor operates in a reflective mode, more convenient for dip probes. When these sensors are spectrally interrogated, a high sensitivity of 10-4 refractive index units per nanometer is attained. These devices can be advantageously used for any kind of chemical sensing and biosensing.

Multiple surface-plasmon resonance in uniform-waist tapered optical fibers with an asymmetric double-layer deposition

Novel devices consisting of uniform-waist tapered optical fibers with asymmetric double-layer (metal plus dielectric) depositions have been recently proposed as refractive-index sensors. We study the properties of light transmission by use of this kind of devices, and we specifically perform a detailed study of the generation of surface-plasma waves in the structures. We show that multiple surface plasmons are excited for specific combinations of the constructive parameters of the devices and for specific ranges of the refractive index of the surrounding medium. The behavior also depends on the wavelength and the state of polarization of the incident light. The use of uniform-waist tapers allows for control of constructive parameters and an increase in the interaction length with the outer medium. We show how the plasmons are excited in the region of the taper waist by a coupling with the cladding modes guided in that area. This characterization shows the importance of the presence of a dielectric layer for selection of the operating range of the device. The results are useful for the design of new sensors.

A fibre-optic temperature sensor based on the deposition of a thermochromic material on an adiabatic taper

A fibre-optic sensor has been developed for the measurement of temperature, especially of liquids. The device is conceived as part of an all-optical CTD probe for the control of the physical parameters of a marine medium. The dependence on temperature of the optical properties (specifically, absorbance) of a thermochromic material, namely lophine (2,4,5-triphenylimidazole), is the basis of the sensor. The sensor presents some significant differences with respect to other similar sensors proposed in the literature: the use of adiabatic, long, tapered optical fibres with adjustable geometric parameters; the use of LED illumination in the 800?nm range; improvements in the deposition technique, etc. The sensors show a linear behaviour over the desired temperature range, and their sensitivity is high. Also, the dependence of the response of the sensor with variations of the geometry of the tapers is discussed. Specifically, we have performed measurements with different diameters of the taper waist, and we show the dependence of the slope of the response curve with that parameter.

Plasmonic Sensors Based on Doubly-Deposited Tapered Optical Fibers

A review of the surface plasmon resonance (SPR) transducers based on tapered fibers that have been developed in the last years is presented. The devices have proved their good performance (specifically, in terms of sensitivity) and their versatility and they are a very good option to be considered as basis for any kind of chemical and biological sensor. The technology has now reached its maturity and here we summarize some of the characteristics of the devices produced.

Coverage-Mapping Method Based on a Hardware Model for Mobile-Robot Positioning in Intelligent Spaces

In this paper, we describe a method for relating the precision in phase shift measurements to the navigation areas in which that precision is reached so that a coverage map is built, setting a signal-to-noise ratio (SNR) threshold level that depends on that phase precision. The method brings a novel approach to linking different areas in robotics and is applied to a mobile-robot (MR) local positioning system (LPS) in an intelligent space, where distances are computed from differential phase shift measurements with intensity modulation and direct detection (IMDD) infrared signals. A global model including the parameters of all the parts involved, e.g., optoelectronics, wireless channel, and instrumentation hardware, is developed. Furthermore, based on the model, an analytical expression deduced for the phase shift measurement is used to find the necessary SNR for a desired precision. A complete set of results, applying the coverage cells to a real building covering a path followed by an MR, is shown. The position of the MR can be known, with an accuracy value below 5 cm and tested in a basic rectangular locating cell with dimensions 3.0 m × 2.5 m.

Absorption as a selective mechanism in surface plasmon resonance fiber optic sensors.

A new concept of surface plasmon resonance fiber optic sensor is presented. By tuning the plasmon resonance to a wavelength for which the outer medium is absorptive, a significant variation of the spectral transmittance of the device is produced as a function of the concentration of the analyte. With this mechanism, selectivity can be achieved without the need of any functionalization of the surfaces or the use of recognizing elements, which is a very interesting feature for any kind of chemical sensor or biosensor. Doubly deposited uniform-waist tapered fibers are well suited for the development of these new sensors. Multiple surface plasmon resonance, obtainable in those structures, can be used for the development of microspectrometers based on this principle.

Non-homogenous illumination correction of calibrated incoherent optical-fiber-bundles for image transmission purposes

We present here a new non-homogenous illumination correction method of a calibrated incoherent optical fiber bundle (IOFB) for image transmission purpose. The use of IOFB for image transmission in hazardous areas, where conventional camera cannot be employed because of their incorporated electronic, is fully justified. The high fabrication cost of coherent optical fiber bundles makes IOFB useful for large distances images capture applications. Since the bundle is spatially incoherent, the image coming out at the bundle end is a random set of illuminated points, without any spatial correlation, but once the IOFB calibrated, it can be used to transmit optical images without any electronics. To calibrate such cables, a flat LCD screen is used to emit input images. To solve a chromatic aberration problem, generated by the optical system composed by few conventional lens, diaphragms are used, which also reduce the emitted light, and so solve the saturation camera sensor problem too. It unfortunately generates a non-homogenous output picture illumination, which creates noise in the reconstructed images. In this paper we present and compare three methods to correct the cable output image illumination, once the spatial calibration is achieved, presenting some of the results obtained using this method

Location of Optical Fibers for the Calibration of Incoherent Optical Fiber Bundles for Image Transmission

Image transmission by incoherent optical fiber bundles (IOFBs) requires prior calibration to obtain the spatial in-out fiber correspondence to reconstruct the image captured by the pseudocamera. This information is recorded in a lookup table (LUT), which is later used for reordering the fiber positions and reconstructing the original image. This paper shows how to apply a fiber detection process to minimize the calibration time and improve the quality of the recovered image. Two different fiber detection methods were developed. The former uses the circular Hough transform algorithm based on the image gradient. The second algorithm combines a number of morphological transformations with distance transform. The results demonstrate that this technique provides a remarkable reduction in the processing time while improving fiber detection accuracy.

Calibration of Incoherent Optical Fiber Bundles for Image Transmission. Fibers Detection Process

Image transmission by incoherent optical fibers bundles requires a previous calibration to obtain the fibers spatial in-out correspondence in order to reconstruct at the output side an image presented on the input of the system. This information is recorded in a look-up table for later reordering of the fibers positions and reconstructs the original image. This paper shows how to minimize calibration time and improve the quality of the recovered image, by using a fibers detection process reducing the number of points to take into account in the calibration process. There is, also, an improvement in the quality of the recovered image compared with others techniques. In this paper two detection methods of circular patterns have been applied to a fiber bundle image captured by a CMOS sensor. The former utilizes the algorithm of circular Hough transform based on the gradient field of the image. A second one is introduced as a new hybrid algorithm that mixes some morphological transformations and distance transform. The results demonstrate that this technique offers a remarkable reduction of processing time and good detection.