Francisco Javier Salgado-Remacha

Collimation method using a double grating system

We present a collimation technique based on a double grating system to locate with high accuracy an emitter in the focal plane of a lens. Talbot self-images are projected onto the second grating producing moiré interferences. By means of two photodetectors positioned just behind the second grating, it is possible to determine the optimal position of the light source for collimation by measuring the phase shift between the signals over the two photodetectors. We obtain mathematical expressions of the signal in terms of defocus. This allows us to perform an automated technique for collimation. In addition, a simple and accurate visual criterion for collimating a light source using a lens is proposed. Experimental results that corroborate the proposed technique are also presented.

Self-images location of amplitude/phase binary gratings

We analyze the near-field behavior of binary amplitude/phase diffraction gratings, which modulate at the same time the amplitude and phase of the incident light beam. As it is expected, the distance between two consecutive self-images of the grating depends only on the period of the grating and the wavelength of the illumination. However, the location of the self-images depends on the specific properties of the grating. In this work, we analyze the location of the self-images in terms of the Fourier coefficients of the grating, obtaining analytical expressions. This analysis can be useful in applications in which the position of the self-images must be at certain fixed distances from the grating. Finally, an experimental and numerical verification of the proposed theory is performed.

Three-dimensional diffraction of a thin metallic cylinder illuminated in conical incidence: application to diameter estimation.

We present a model to determine the far-field diffraction pattern of a metallic cylinder of infinite length when it is illuminated in oblique incidence. This model is based on the Helmholtz-Kirchhoff integral using the Beckmann conditions for reflection. It considers the three-dimensional nature of the diffracting object as well as the material of which the cylinder is made. This model shows that the diffraction orders are placed in a cone of light. The amplitude at the far field can be divided into three terms: the first term accounts for Babinets principle, that is, the contribution of the cylinder projection; the second term accounts for the three dimensionality of the cylinder; and the third term accounts for the material of which the cylinder is made. This model is applied to the diameter estimation of the cylinder. Since the amplitude of the Babinet contribution is much larger than the light reflected by the surface, the cylinder diameter can be obtained in a simple way. With this approximation, the locations of the diffraction minima do not vary when the cylinder is inclined. On the other hand, when the reflected light is considered the location of the minima and, hence, the estimation of the diameter, varies. Also, a modification of the diffraction minima is produced by the material of which the cylinder is made. Experimental results are also obtained that corroborate the theoretical approach.

Laguerre–Gaussian beam shaping by binary phase plates as illumination sources in micro-optics

The lack of intensity in the central region of Laguerre-Gaussian beams reduces their applicability as illumination sources. For this reason, it is usual to shape a Laguerre-Gaussian beam to a nearly Gaussian beam using a binary phase mask. The behavior of this rectified Laguerre-Gaussian beam is analyzed in this work in the Fresnel regime. A comparison between diverse Laguerre-Gaussian beams with rectified Laguerre-Gaussian beams shows that there appear two different regions along the propagation axis: first is a transition region (for lower distances) with a flat intensity distribution, and next is a second region (for longer distances) where the rectified Laguerre-Gaussian tends to a Gaussian shape. The results of this work are very valuable for the use of this kind of beam in micro-optical applications.

Micromachining of Diffractive Optical Elements Embedded in Bulk Fused Silica by Nanosecond Pulses

Micro-optical devices embedded in transparent materials are usually manufactured focusing a pulsed laser in bulk fused silica. Under this condition, pulsewidth becomes the most important parameter that rules the size of the inscriptions. Ultrafast pulses (pico- and femtosecond pulses) avoid thermal effects and the results present a high efficiency. Nevertheless, nanosecond lasers are more available due the reduced costs. Therefore, a study of the optical behavior of embedded elements micromachined by nanosecond pulses is required. In this study, we show that this regime of pulses can still be used for engraving diffractive optical elements in transparent materials, regardless of the thermal damage. A Fresnel zone plate and a far-field beam shaper have been manufactured as an example of the functionality of these devices.

Rough Fresnel zone plates over metallic surfaces

We analyze the focusing properties of Fresnel zone plates fabricated over steel tapes using laser ablation. Our intention is to implement the use of micro-optical elements when the use of conventional chrome-glass elements is not indicated. Because of the manufacture process, the surface presents a certain anisotropic roughness, which reduces the focusing properties. First, we develop numerical simulations by means of the Rayleigh-Sommerfeld approach, showing how roughness in both levels of the Fresnel zone plate affects the focalization of the lens. We also manufacture Fresnel zone plates over steel tape, and perform experimental verification that corroborates the numerical results.

Optimized square Fresnel zone plates for microoptics applications

Polygonal Fresnel zone plates with a low number of sides have deserved attention in micro and nanoptics, because they can be straightforwardly integrated in photonic devices, and, at the same time, they represent a balance between the high-focusing performance of a circular zone plate and the easiness of fabrication at micro and nano-scales of polygons. Among them, the most representative family are Square Fresnel Zone Plates (SFZP). In this work, we propose two different customized designs of SFZP for optical wavelengths. Both designs are based on the optimization of a SFZP to perform as close as possible as a usual Fresnel Zone Plate. In the first case, the criterion followed to compute it is the minimization of the difference between the area covered by the angular sector of the zone of the corresponding circular plate and the one covered by the polygon traced on the former. Such a requirement leads to a customized polygon-like Fresnel zone. The simplest one is a square zone with a pattern of phases repeating each five zones. On the other hand, an alternative SFZP can be designed guided by the same criterion but with a new restriction. In this case, the distance between the borders of different zones remains unaltered. A comparison between the two lenses is carried out. The irradiance at focus is computed for both and suitable merit figures are defined to account for the difference between them.

Spectral attenuation of brain and retina tissues in the near-infrared range measured using a fiber-based supercontinuum device

A novel setup for the efficient constant optical measurements of biological tissues in the near infrared is presented. The system combines the use of a fiber-based supercontinuum source with a simple optics fiber collimator. This configuration allows a wide spectral range of measurement and, at the same time, can efficiently filter the straightforward transmitted light while avoiding scattered light. As a performance example, the optical characterization of rat brain and retina tissues are shown. The attenuation coefficient for both tissues in the near infrared region is also obtained. This technique could be applied in clinical research as a noninvasive method with several potential practical applications.

Octave-spanning supercontinuum generation in highly nonlinear silica fibres based on cost-effective fibre amplifiers

We report a simple method for supercontinuum generation. The set-up consists of an Er-doped mode-locked fibre laser, used as seed, and a highly nonlinear fibre with zero dispersion at 1550 nm. Thus, all the components are easily attainable. With this novel system the requirements in terms of control of dispersion are reduced. In addition, the spectral width is optimized using fibres with positive and negative dispersion. The supercontinuum emission is characterized by means of an optical spectrum analyser and a PbS photodetector, showing an octave-spanning spectral width, with a flat profile from 1100 nm up to 2100 nm. Compared to other supercontinuum sources, this new proposal results in a very competitive and attainable system, particularly in the 1500–2100 nm region.

Diffraction gratings over cylindrical surfaces by laser ablation

espanolEn este trabajo presentamos un sistema para realizar la grabacion de redes de difraccion sobre sustratos cilindricos. La grabacion de dichas redes se realiza mediante un sistema de laser pulsado de nanosegundos modulado por Q-switching. Disponemos de un sistema mecanico de seis ejes motorizados (cinco de ellos lineales paso a paso, mas un motor rotador de alta precision) con el fin de desplazar verticalmente el cabezal laser y girar angularmente el anillo de forma sincronizada para realizar la grabacion de las redes. Se dispone tambien de diversos dispositivos que monitorizan en tiempo real el proceso de grabacion. Se ha realizado un software de control para sincronizar todo el sistema. Por ultimo mostraremos diversos resultados obtenidos tanto de los pasos previos al grabado como de las redes de difraccion grabadas. EnglishWe present a system for diffraction gratings engraving over cylindrical substrates. The manufacturing of the gratings is performed using a Q-switched nanosecond pulsed laser. We have used a mechanical system with six motorized stages. Five of them are linear stages, and the other one is a high precision rotator. The assembly also comprises various devices that monitorize in real time the manufacturing process. A home-made software synchronizes and controls the entire system.