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
UCM identifierORCIDScopus Author IDWeb of Science ResearcherIDDialnet IDGoogle Scholar ID

Search Results

Now showing 1 - 10 of 60
  • 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
    Far field of binary phase gratings with errors in the height of the strips
    (SPIE, 2009-06-17) Rico-García, José María; Sánchez Brea, Luis Miguel
    Diffraction gratings are not always ideal but, due to the fabrication process, several errors can be produced. In this work we show that when the strips of a binary phase diffraction grating present certain randomness in their height, the intensity of the diffraction orders varies with respect to that obtained with a perfect grating. To show this, we perform an analysis of the mutual coherence function and then, the intensity distribution at the far field is obtained. In addition to the far field diffraction orders, a "halo" that surrounds the diffraction order is found, which is due to the randomness of the strips height.
  • 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
    Near-field diffraction-based focal length determination technique
    (Elsevier, 2017-05) Torcal Milla, Francisco José; Sánchez Brea, Luis Miguel
    An 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
    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.