## Person: Sánchez Brea, Luis Miguel

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##### First Name

Luis Miguel

##### Last Name

Sánchez Brea

##### Affiliation

Universidad Complutense de Madrid

##### Faculty / Institute

Ciencias Físicas

##### Department

Óptica

##### Area

Optica

##### Identifiers

17 results

## Search Results

Now showing 1 - 10 of 17

Publication Near-field diffraction-based focal length determination technique(Elsevier, 2017-05) Torcal Milla, Francisco José; Sánchez Brea, Luis MiguelAn accurate and simple technique for determining the focal length of a lens is presented. It consists of measuring the period of the fringes produced by a diffraction grating at the near field when it is illuminated with a beam focused by the unknown lens. In paraxial approximation, the period of the fringes varies linearly with the distance. After some calculations, a simple extrapolation of data is performed to obtain the locations of the principal plane and the focal plane of the lens. Thus, the focal length is obtained as the distance between the two mentioned planes. The accuracy of the method is limited by the collimation degree of the incident beam and by the algorithm used to obtain the period of the fringes. We have checked the technique with two commercial lenses, one convergent and one divergent, with nominal focal lengths (+100±1) mm and (−100±1) mm respectively. We have experimentally obtained the focal lengths resulting into the interval given by the manufacturer but with an uncertainty of 0.1%, one order of magnitude lesser than the uncertainty given by the manufacturer.Publication Self-imaging technique for beam collimation(Optical Society of America, 2014) Sánchez Brea, Luis Miguel; Torcal Milla, Francisco José; Herrera Fernández, José María; Morlanes, Tomás; Bernabeu Martínez, EusebioA simple collimation technique based on measuring the period of one self-image produced by a diffraction grating is proposed. Transversal displacement of the grating is not required, and then automatic single-frame processing can be performed. The self-image is acquired with a CMOS camera, and the period is computed using the variogram function. Analytical and experimental results are obtained, which show the simplicity and accuracy of the proposed technique.Publication Fast optical source for quantum key distribution based on semiconductor optical amplifiers(The Optical Society Of America, 2011-02-28) Jofre, Marc; Gardelein, Arnaud; Anzolin, Gabriele; Amaya, Waldimar; Capmany Francoy, José; Ursin, Rupert; Peñate Quesada, Laura; López Molina, Demetrio; San Juan, J. L.; Carrasco, José Antonio; García de Quirós, Francisco; Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Bernabeu Martínez, Eusebio; Perdigues Armengol, Josep Maria; Jennewein, Thomas; Pérez Torres, Juan; Mitchell, Morgan W.; Pruneri, ValerioA novel integrated optical source capable of emitting faint pulses with different polarization states and with different intensity levels at 100 MHz has been developed. The source relies on a single laser diode followed by four semiconductor optical amplifiers and thin film polarizers, connected through a fiber network. The use of a single laser ensures high level of indistinguishability in time and spectrum of the pulses for the four different polarizations and three different levels of intensity. The applicability of the source is demonstrated in the lab through a free space quantum key distribution experiment which makes use of the decoy state BB84 protocol. We achieved a lower bound secure key rate of the order of 3.64 Mbps and a quantum bit error ratio as low as 1.14 × 10−2 while the lower bound secure key rate became 187 bps for an equivalent attenuation of 35 dB. To our knowledge, this is the fastest polarization encoded QKD system which has been reported so far. The performance, reduced size, low power consumption and the fact that the components used can be space qualified make the source particularly suitable for secure satellite communication.Publication Self-imaging with curved gratings(Elsevier Science BV, 2010-10-15) Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Salgado Remacha, Francisco Javier; Bernabeu Martínez, EusebioWe analyze the near field behavior of binary amplitude gratings which present a curved profile. This configuration has an important application in rotary optical encoders. This kind of encoders is used to measure the angular displacement between two different parts of the devices. To our knowledge, its behavior in the near field has not been analyzed yet. We have found that Talbot effect is produced but the period of the self-images and Talbot distances change as we separate from the grating. We have fabricated a curved grating and have performed the experiment to corroborate the behavior theoretically found. This fact could be useful in systems that use Talbot effect to measure displacements, since the mechanical tolerances grow.Publication Collimation method using a double grating system(The Optical Society Of America, 2010-06-10) Sánchez Brea, Luis Miguel; Torcal Milla, Francisco José; Salgado Remacha, Francisco Javier; Morlanes Calvo, Tomás; Jiménez Castillo, Isidoro; Bernabeu Martínez, EusebioWe 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.Publication Near field diffraction of cylindrical convex gratings(IOP Publishing, 2015) Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Bernabeu Martínez, EusebioWe analyze the field produced by a cylindrical convex diffraction grating at the Fresnel regime for several kinds of light sources, including a monochromatic quasipunctual source, finite size, and polychromatic sources. These results can help one understand the functioning of rotary optical encoder technology. A decrease in the self-image contrast is produced for finite nonpunctual sources. In addition, the polychromaticity of the source affects the smoothness of the self-images, making them quasicontinuous from a certain distance from the grating forward. Finally, we experimentally validate the obtained analytical predictions.Publication Far field diffraction of gratings with two roughness levels(IOP Publishing, Ltd., 2019-08) Torcal Milla, Francisco José; Sánchez Brea, Luis MiguelWe investigate the far field diffraction process produced by diffraction gratings with two roughness levels, such as steel tape gratings, which are used in environments where diffraction gratings made of glass are not possible. We obtain the analytical formulation describing the formation of diffraction orders in terms of the roughness parameters. Since roughness is stochastic, we use the mutual coherence function to propagate the field to the Fraunhofer regime. We conclude that diffraction orders exists when at least one correlation length of both roughness is larger than the period of the grating and the standard deviation on heights is around or larger than the illumination wavelength. Finally, we corroborate the analytical results with numerical simulations based on the fast Fourier transform, showing a high agreement.Publication Numerical model of the inhomogeneous scattering by the human lens(Optical Society of America, 2019-05-01) Cuadrado, Alexander; Sánchez Brea, Luis Miguel; Torcal Milla, Francisco José; Quiroga, Juan Antonio; Gómez Pedrero, Jose AntonioWe present in this work a numerical model for characterizing the scattering properties of the human lens. After analyzing the scattering properties of two main scattering particles actually described in the literature through FEM (finite element method) simulations, we have modified a Monte Carlo's bulk scattering algorithm for computing ray scattering in non-sequential ray tracing. We have implemented this ray scattering algorithm in a layered model of the human lens in order to calculate the scattering properties of the whole lens. We have tested our algorithm by simulating the classic experiment carried out by Van der Berg et al for measuring "in vitro" the angular distribution of forward scattered light by the human lens. The results show the ability of our model to simulate accurately the scattering properties of the human lens.Publication Near field of stacked diffraction gratings(Gustav Fischer Verlag, 2013) Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Bernabeu Martínez, EusebioWe obtain a general analytical formulation for determining the near field produced by N diffraction gratings disposed in stack using a scalar approximation. Parameters of the gratings such as type-amplitude/phase-, fill factors, periods and relative positions between gratings along the x and y axes are considered. The obtained formulation is useful for analyzing problems which involve several diffraction gratings, such as optical encoders since it is computationally faster than integral formulations. Finally, analytical results are compared with numerical simulations based on the Rayleigh–Sommerfeld equation.Publication Use of steel substrates in diffractive optics: Near field of high surface quality steel tape gratings(Elsevier Sci. Ltd., 2011-03) Salgado Remacha, Francisco Javier; Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Bernabeu Martínez, EusebioSteel tape is used in optical applications due to its mechanical properties. However, roughness of steel surfaces affects to wave propagation. The effect can be observed using diffraction gratings with low periods. In such a case, self-images of the grating appear at certain periodic distances from the grating (Talbot planes). When standard steel is used, the contrast of Talbot self-images strongly decreases with the distance from the grating. In this work, we prove that controlling the surface quality of steel, it is possible to improve its optical behavior and, as a consequence, high quality surface steel represents a good choice for diffractive optics when the use of chrome-on-glass masks is not indicated. As an example, we have manufactured a diffraction grating over a high quality steel surface by means of an ablation process with a nanosecond pulsed laser. The contrast of the self-images for these gratings decreases very slowly with the distance, in comparison with the self-images obtained with standard steel tape gratings.