Person: Sánchez Brea, Luis Miguel
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 28
PublicationNear-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. PublicationOptimal 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, EusebioTwo 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. PublicationSelf-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. PublicationFast 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. PublicationSelf-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. PublicationCollimation 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. PublicationNear-field shaping with two binary diffractive optical elements in tandem(Elsevier Science BV, 2013-06-15) Herrera Fernández, José María; Sánchez Brea, Luis Miguel; Bernabeu Martínez, EusebioDiffractive optical elements (DOEs) are commonly used for beam shaping in the far field. In the present work, we develop an algorithm for near-field beam shaping using a double DOE system with binary modulation, so that it can be easily implemented on a glass substrate. In a first stage, the first amplitude DOE is equal to the target intensity distribution and the second phase DOE is obtained using a simple and fast iterative technique. In order to improve the optical efficiency of the system, we also analyze the behavior of the algorithm when the first DOE is not equal to the target image, but a widened version of it. Several numerical simulations are obtained, and an analysis of tolerances is performed. PublicationDiffractive optical elements with square concentric rings of equal width(John Wiley & Sons INC, 2010-04) Alda, Javier; Sánchez Brea, Luis Miguel; Salgado Remacha, Francisco Javier; Rico-García, José MaríaA diffractive optical element having equal-width concentric square rings is analyzed in this article. This constant width makes possible its realization using spatial light modulators or square pixels phase screens. It allows a simple analytical treatment, and the element is also simulated using the Rayleigh-Sommerfeld approach. An experimental verification of its performance has been compared with the simulated results. PublicationPolarization conversion on nanostructured metallic surfaces fabricated by LIPSS(SPIE. International Society Optical Enginnering, 2019) San Blas, A.; Casquero, N.; Pérez, N.; Martínez Calderón, M.; Sánchez Brea, Luis Miguel; Buencuerpo, J.; Olaizola, S.M.; Rodríguez, A.Waveplates modify polarization by generating a phase change. Laser Induced Periodic Surface Structures (LIPSS) have recently started to be studied as waveplates due to the birefringence in-duced by the nanoripples, easily fabricated in a one-step process by laser, where LIPSS morphology is defined by the characteristics of the laser process parameters and the substrate material. The optical properties of these waveplates are defined by LIPSS parameters such as period, depth or width of the ripples. In this work we have deposited thin film coatings on stainless steel samples containing LIPSS for different coating thickness and composition. Results show that thin film coatings are a good candidate for the tunability of LIPSS birefringence since the coating modifies the induced polarization change and reflectivity of the sample depending on coating thickness and composition, as expected from numerical simulations. PublicationNear 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.