Array de microlentes de cristal líquido con control gradual de índice de refracción

Se ha diseñado y construido un array de microlentes cilíndricas de cristal líquido (CL) y se ha llevado a cabo un estudio sobre su comportamiento electroóptico. El array lenticular es novedoso en cuanto a los materiales empleados en su fabricación. Se ha utilizado Níquel como material clave para la implementación de un electrodo de alta resistividad. La combinación del electrodo de alta resistividad junto al CL (cuya impedancia paralelo es elevada) da lugar a un divisor reactivo que proporciona un gradiente de tensión hiperbólico del centro al extremo de cada lente. Este efecto, unido al alineamiento homogéneo de las moléculas de CL, permite la generación de un gradiente de índice de refracción, comportándose el dispositivo como una lente GRIN (GRadient Refraction INdex). Para la caracterización de su funcionamiento se ha analizado su perfil de fase empleando métodos interferométricos y procesamiento de imágenes. Además se han efectuado también diferentes medidas de contraste angular.

Pulse shortening of gain switched single mode semiconductor lasers using a variable delay interferometer

We propose a pulse shaping and shortening technique for pulses generated from gain switched single mode semiconductor lasers, based on a Mach Zehnder interferometer with variable delay. The spectral and temporal characteristics of the pulses obtained with the proposed technique are investigated with numerical simulations. Experiments are performed with a Distributed Feedback laser and a Vertical Cavity Surface Emitting Laser, emitting at 1.5 µm, obtaining pulse duration reduction of 25-30%. The main asset of the proposed technique is that it can be applied to different devices and pulses, taking advantage of the flexibility of the gain switching technique.

Simultaneously measuring two ultrashort laser pulses on a single-shot using double-blind frequency-resolved optical gating

We demonstrate a simple self-referenced single-shot method for simultaneously measuring two different arbitrary pulses, which can potentially be complex and also have very different wavelengths. The method is a variation of cross-correlation frequency-resolved optical gating (XFROG) that we call double-blind (DB) FROG. It involves measuring two spectrograms, both of which are obtained simultaneously in a single apparatus. DB FROG retrieves both pulses robustly by using the standard XFROG algorithm, implemented alternately on each of the traces, taking one pulse to be ?known? and solving for the other. We show both numerically and experimentally that DB FROG using a polarization-gating beam geometry works reliably and appears to have no nontrivial ambiguities.

High-peak-power pulse generation from a monolithic master oscillator power amplifier at 1.5 μm

We present an experimental study on the generation of high-peak-power short optical pulses from a fully integrated master-oscillator power-amplifier emitting at 1.5 μm. High-peak-power (2.7 W) optical pulses with short duration (100 ps) have been generated by gain switching the master oscillator under optimized driving conditions. The static and dynamic characteristics of the device have been studied as a function of the driving conditions. The ripples appearing in the power-current characteristics under cw conditions have been attributed to mode hopping between the master oscillator resonant mode and the Fabry-Perot modes of the entire device cavity. Although compound cavity effects have been evidenced to affect the static and dynamic performance of the device, we have demonstrated that trains of single-mode short optical pulses at gigahertz frequencies can be conveniently generated in these devices.

Lenticular arrays based on liquid crystals

Lenticular array products have experienced a growing interest in the last decade due to the very wide range of applications they can cover. Indeed, this kind of lenses can create different effects on a viewing image such as 3D, flips, zoom, etc. In this sense, lenticular based on liquid crystals (LC) technology is being developed with the aim of tuning the lens profiles simply by controlling the birefringence electrically. In this work, a LC lenticular lens array has been proposed to mimic a GRIN lenticular lens array but adding the capability of tuning their lens profiles. Comb control electrodes have been designed as pattern masks for the ITO on the upper substrate. Suitable high resistivity layers have been chosen to be deposited on the control electrode generating an electric field gradient between teeth of the same electrode. Test measurements have allowed us to demonstrate that values of phase retardations and focal lengths, for an optimal driving waveform, are fairly in agreement. In addition, results of focusing power of tuneable lenses were compared to those of conventional lenses. The behaviour of both kinds of lenses has revealed to be mutually similar for focusing collimated light and for refracting images.

Modulation instability-induced fading in phase-sensitive optical time-domain reflectometry

Phase-sensitive optical time-domain reflectometry (?OTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. We show in this Letter that modulation instability (MI) can induce a position-dependent signal fading in long-range ?OTDR over conventional optical fibers. This fading leads to a complete masking of the interference signal recorded at certain positions and therefore to a sensitivity loss at these positions. We illustrate this effect both theoretically and experimentally. While this effect is detrimental in the context of distributed vibration analysis using ?OTDR, we also believe that the technique provides a clear and insightful way to evidence the Fermi?Pasta?Ulam recurrence associated with the MI process.

Método de calibración para interrogadores de redes de Bragg en fibra óptica

Se ha desarrollado un método de calibración de interrogadores de redes de Bragg en fibra óptica que compara la respuesta del equipo de prueba con un simulador de red de Bragg compuesto por un filtro sintonizable unido a un espejo de fibra óptica. La longitud de onda simulada obtenida se mide con un interferómetro de referencia calibrado en el Instituto de Óptica del Consejo Superior de Investigaciones Científicas (IO?CSIC). Este método de calibración ha sido aplicado para calibrar varios tipos de interrogadores de redes de Bragg: estáticos y dinámicos. La incertidumbre en la calibración absoluta de los interrogadores es de ±88 pm, y es debida principalmente a la anchura espectral del filtro que simula la red de Bragg. Sin embrago, en calibraciones relativas la incertidumbre baja a ±3 pm, dominado por la incertidumbre en la calibración del interferómetro de referencia.

Simulation of the transmission performance for equalized holographic ROADM designs

A complete simulation of the transmission performance for Equalized Holographic ROADM (Reconfigurable Optical Add-Drop Multiplexer) designs is presented in this paper. These devices can address several wavelengths from the input to different output fibres, according to the holograms stored in a SLM (Spatial Light Modulator), where all the outputs are equalized in power. All combinations of the input wavelengths are possible at the different output fibres. To simulate the transmission performance of the EH-ROADM, a software program, from Optiwave, has been used. The correspondence between physical blocks of the device (grating, SLM, lens...) and those simulated in the program (filters, losses, splitters...) has been defined in order to obtain a close agreement between the theoretical transmission performance and the simulated one. To complete the review about Equalized Holographic ROADMs some guidelines about its design have been done.

Magneto-optic current sensor with Faraday mirror for linear birefringence compensation

Fiber optic sensors have some advantages in subjects related with electrical current and magnetic field measurement. In spite of the optical fiber utilization advantages we have to take into account undesirable effects, which are present in real non-ideal optical fibers. In telecommunication and sensor application fields the presence of inherent and induced birefringence is crucial. The presence of birefringence may cause an undesirable change in the polarization state. In order to compensate the linear birefringence a promising method has been chosen. This method employs orthogonal polarization conjugation in the back propagation direction of the light wave in the fiber. A study and a simulation of an experimental setup are realized with the advantage of a significant sensitivity improvement.

Efficient design and optimization of bio-photonic sensing cells (BICELLs) for label free biosensing

In previous works we demonstrated the benefits of using micro–nano patterning materials to be used as bio-photonic sensing cells (BICELLs), referred as micro–nano photonic structures having immobilized bioreceptors on its surface with the capability of recognizing the molecular binding by optical transduction. Gestrinone/anti-gestrinone and BSA/anti-BSA pairs were proven under different optical configurations to experimentally validate the biosensing capability of these bio-sensitive photonic architectures. Moreover, Three-Dimensional Finite Difference Time Domain (FDTD) models were employed for simulating the optical response of these structures. For this article, we have developed an effective analytical simulation methodology capable of simulating complex biophotonic sensing architectures. This simulation method has been tested and compared with previous experimental results and FDTD models. Moreover, this effective simulation methodology can be used for efficiently design and optimize any structure as BICELL. In particular for this article, six different BICELL's types have been optimized. To carry out this optimization we have considered three figures of merit: optical sensitivity, Q-factor and signal amplitude. The final objective of this paper is not only validating a suitable and efficient optical simulation methodology but also demonstrating the capability of this method for analyzing the performance of a given number of BICELLs for label-free biosensing.

Línea de retardo sintonizable eléctricamente para trenes de pulsos ópticos basada en el efecto de autoimagen temporal

En este artículo se presenta una nueva técnica para la realización de una línea de retardo para trenes de pulsos ópticos ultracortos basada en dispositivos habituales en enlaces de fibra óptica. El sistema está basado en la introducción, mediante un modulador electro?óptico, de una fase lineal a un tren de pulsos de entrada para su posterior dispersión consiguiéndose el efecto de autoimagen temporal en fibra. La sintonía eléctrica del retardo introducido se consigue mediante la variación de la pendiente de la señal eléctrica aplicada en el modulador, obteniéndose una sintonía continua a lo largo de todo el periodo de la señal. Dada la imposibilidad práctica de realizar un generador lineal cuya salida no esté limitada en el tiempo, se propone el uso de una señal rampa, reduciéndose por tanto el rango de sintonía a un conjunto de valores discretos para evitar la deformación del tren de pulsos a la salida del sistema. Resultados obtenidos por simulación prueban la validez del esquema propuesto.

Inorganic hydrophobic coatings: Surfaces mimicking the nature

Added value products are being developed in ceramic industry. Different optical effects as bright metallic shine or new functionalities as hydrophobicity or bactericide characteristics are the new properties searched on the tiles. In this study, we prepare glassy coatings for tiles based on copper pigment by a conventional industrial process. The obtained coatings present different aesthetical aspects, including bright metallic aspect which confers a high decorative value to the tile. Furthermore, these metallic coatings present hydrophobic properties with contact angles with water as high as 115 degrees and also bactericide characteristics. Superficial microstructure and nanoparticles were found in the bactericide-hydrophobic samples, resembling the surface of hydrophobic leaf surfaces. This structure was formed by the crystallization of CuO nanoparticles as Tenorite due to the copper saturation of the glassy matrix at the surface of the coatings.

Space-time Measurements of Oceanic Sea States

Stereo video techniques are effective for estimating the space–time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. We present an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea and near the southern seashore of the Crimean peninsula, in the Black Sea. We use classical epipolar techniques to reconstruct the sea surface from the stereo pairs sequentially in time, viz. a sequence of spatial snapshots. We also present a variational approach that exploits the entire data image set providing a global space–time imaging of the sea surface, viz. simultaneous reconstruction of several spatial snapshots of the surface in order to guarantee continuity of the sea surface both in space and time. Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics at a point in time that agrees well with probabilistic models. In particular, WASS stereo imaging is able to capture typical features of the wave surface, especially the crest-to-trough asymmetry due to second order nonlinearities, and the observed shape of large waves are fairly described by theoretical models based on the theory of quasi-determinism (Boccotti, 2000). Further, we investigate space–time extremes of the observed stationary sea states, viz. the largest surface wave heights expected over a given area during the sea state duration. The WASS analysis provides the first experimental proof that a space–time extreme is generally larger than that observed in time via point measurements, in agreement with the predictions based on stochastic theories for global maxima of Gaussian fields.

Optics detailed analysis of an improved collimation system for LED light sources

Optics detailed analysis of an improved collimation system for LED light sources

Electrically tunable delay for trains of optical pulses

A technique to implement an electrically tunable delay line with high bandwidth for trains of ultrashort optical pulses is presented. The system is based on the temporal self-imaging effect in fiber gratings and electrooptic modulation.