Sánchez Brea, Luis Miguel

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First Name
Luis Miguel
Last Name
Sánchez Brea
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
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Search Results

Now showing 1 - 10 of 36
  • Publication
    Double grating systems with one steel tape grating
    (Elsevier Science BV, 2008-12-01) Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel; Bernabeu Martínez, Eusebio
    Steel tape gratings are used in different metrology applications. As the period of these gratings was large (around 100μm,), its analytical study has been performed, up to date, using a geometrical approach. Nowadays, steel tape gratings can be manufactured with lower periods, around 20–40 μm, and diffractive effects must be taken into account. Also, due to the roughness of the surface, statistical techniques need to be considered to analyze their behavior. In this work, an analysis of the pseudo-imaging formation in a double grating system including one steel tape grating is performed. In particular Moiré and Lau configurations are analyzed. We have found that roughness significantly affects to Moiré configuration. However, its effect is negligible in Lau configuration. Generalized grating imaging configuration is also studied in depth. It is shown that roughness does not affect to the contrast of pseudoimages, but it modifies their depth of focus.
  • Publication
    Variogram-based method for contrast measurement
    (The Optical Society Of America, 2007-08-01) Sánchez Brea, Luis Miguel; Torcal Milla, Francisco José; Bernabeu Martínez, Eusebio
    We present a technique for determining the contrast of an intensity distribution in the presence of additive noise and other effects, such as undesired local amplitude or offset variations. The method is based on the variogram function. It just requires the measurement of the variogram at only four points and, as a consequence, it is very fast. The proposed technique is compared with other standard techniques, showing a reduction in the error of the contrast measurement.
  • Publication
    Optimal achromatic wave retarders using two birefringent wave plates
    (The Optical Society Of America, 2013-03-20) Vilas Prieto, José Luis; Sánchez Brea, Luis Miguel; Bernabeu Martínez, Eusebio
    Two plates of different birefringence material can be combined to obtain an achromatic wave retarder. In this work, we achieve a correction for the overall retardation of the system that extends the relation to any azimuth. Current techniques for the design of achromatic wave retarders do not present a parameter that characterizes its achromatism on a range of wavelengths. Thus, an achromatic degree has been introduced, in order to determine the optimal achromatic design composed with retarder plates for a spectrum of incident light. In particular, we have optimized a quarter retarder using two wave plates for the visible spectrum. Our technique has been compared to previous results, showing significant improvement.
  • Publication
    Optical technique for the automatic detection and measurement of surface defects on thin metallic wires
    (The Optical Society Of America, 2000-02-01) Sánchez Brea, Luis Miguel; Siegmann, Philip; Rebollo, María Aurora; Bernabeu Martínez, Eusebio
    In industrial applications of thin metallic wires it is important to characterize the surface defects of the wires. We present an optical technique for the automatic detection of surface defects on thin metallic wires (diameters, 50–2000 µm) that can be used in on-line systems for surface quality control. This technique is based on the intensity variations on the scattered cone generated when the wire is illuminated with a beam at oblique incidence. Our results are compared with those obtained by atomic-force microscopy and scanning-electron microscopy.
  • Publication
    Detection and measurement of waviness on thin metallic wires
    (The Optical Society Of America, 2004-03-01) Tejeda, César; Sánchez Brea, Luis Miguel; Bernabeu Martínez, Eusebio
    We propose a model for determining the far-field diffraction pattern of wires with waviness. Analytical solutions are obtained by means of the stationary phase method, which allows us to determine dimensional parameters such as wire diameter and waviness factor. Experimental results are presented, which are in accordance with our theoretical description.
  • Publication
    Uncertainty Estimation by Convolution Using Spatial Statistics
    (IEEE Institute of Electrical and Electronics Engineers, 2006-10) Sánchez Brea, Luis Miguel; Bernabeu Martínez, Eusebio
    Kriging has proven to be a useful tool in image processing since it behaves, under regular sampling, as a convolution. Convolution kernels obtained with kriging allow noise filtering and include the effects of the random fluctuations of the experimental data and the resolution of the measuring devices. The uncertainty at each location of the image can also be determined using kriging. However, this procedure is slow since, currently, only matrix methods are available. In this work, we compare the way kriging performs the uncertainty estimation with the standard statistical technique for magnitudes without spatial dependence. As a result, we propose a much faster technique, based on the variogram, to determine the uncertainty using a convolutional procedure. We check the validity of this approach by applying it to one-dimensional images obtained in diffractometry and two-dimensional images obtained by shadow moire.
  • 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, Eusebio
    A 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, Valerio
    A 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, Eusebio
    We 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, Eusebio
    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.