Person: Calvo Padilla, María Luisa
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 28
PublicationCorrections to Raman-Nath Diffraction by Volume Holograms(Taylor and Francis Ltd., 1982) Juncos del Egido, Purificación; Calvo Padilla, María LuisaIn this work we present a general formulation of the corrections to the Raman-Nath approximation for the diffracted field by a volume hologram, based on integral equations. An approximate analytic expression is given for the first correction. Numerical evaluations of this correction are also presented. PublicationSingle-mode Anisotropic Cylindrical Dielectric Waveguides(Taylor and Francis Ltd., 1983) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María LuisaWe study anisotropic cylindrical dielectric waveguides under the following conditions: (1) their cross section Ω has arbitrary shape; (2) the dielectric permittivity tensor ε has discontinuities at the boundaries, as well as a graded spatial variation; (3) Ω is small, and ε minus the unit 3 × 3 matrix (I) is small and smooth, so that only one pair of propagation modes is allowed. A general and exact dispersion relation for the propagation modes is derived, using integral equation techniques and the long wavelength singularity of the free-space Green's function in two dimensions. We present approximate explicit solutions of the dispersion relation to first order in (ε− I), which exhibit a logarithmic dependence and birefringence, in general. Some numerical estimates are also given. Approximate dispersion relations up to and including second order in (ε − I) are also discussed. The scattering problem is treated briefly. PublicationElectromagnetic scattering by an infinite inhomogeneous dielectric cylinder: New Green’s function and integral equations(American Institute of Physics, 1980) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María LuisaThe scattering of electromagnetic waves by an infinite dielectric cylinder with variable dielectric permeability presents, in general, certain mathematical difficulties regarding the construction of rigorous solutions. We give a new divergenceless tensorGreen’s function, specially appropriate for cylindrical symmetry, and, in terms of it, present new scattering integral equations. We prove that the series for med by all successive iterations of those scattering integral equations converge under certain conditions. Suitable transformations lead to new integral equations with Hilbert–Schmidt kernels, which imply further rigorous results. PublicationCoupled wave analysis for a reflection dephased mixed hologram grating(Kluwer Academic Publishers, 1986-05) Guibelalde del Castillo, Eduardo; Calvo Padilla, María LuisaA coupled-wave analysis is given for a mixed hologram grating. The effect of spatial dephasing between the absorption and phase grating which was previously presented in transmission gratings is now developed for reflection ones. The effect is presented analytically and evaluated numerically for several cases. The analysis is motivated by the photorefractive effect in electro-optic crystals. PublicationHolographic gratings in the transition regime(Butterworth - Heinemann Ltd., 1988-06) Guibelalde del Castillo, Eduardo; Calvo Padilla, María LuisaThe transition from the Raman-Nath to a Bragg regime for holographic grating diffracted fields is studied using a new integral equation and Fourier transform techniques. Analytical and numerical studies are presented and evaluated. PublicationInfluence of Small Phase Variations in the Diffraction Pattern of a Biological Cell(SPIE--The International Society for Optical Engineering., 1985) Carreras Béjar, Carmen; Calvo Padilla, María Luisa; Chevalier del Río, Margarita; Dwyer, Samuel J. III; Schneider, Roger H.In preceding works, a theoretical model has been introduced for the transmission function of a biological cell, in which the physical effects of the three transmission functions for the cytoplasm, the nucleus and the cellular membrane have been considered and studied1. Since biological cells are generally objects having small phase variations, we introduce this specific characteristic in the proposed theoretical model, in the present work, Its Fourier transform is numerically analyzed. PublicationInhomogeneous Slab and Cylindrical Dielectric Waveguides with Discontinuous Permittivities: Propagation Modes(Taylor and Francis Ltd., 1982) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María LuisaWe study monomode slab and cylindrical waveguides, in which the dielectric permittivity l has both a graded spatial variation and finite discontinuities at the boundaries. New integral equations for the H-type propagation mode in the slab and for the one in the cylinder are obtained, which display separate contributions due to both the graded variations and the discontinuities, and in which the boundary value problem arising from the latter is solved. Based on the specific long wavelength singularities of the free-space Green's functions in one and two dimensions, exact dispersion relations are presented for the slab and cylinder propagation modes, respectively. We give approximate explicit solutions of those dispersion relations, which are correct up to and including second order (in some parameter measuring the deviation of l from unity), and present applications for parabolic permittivity profiles in both geometries. The discontinuities of l at the boundary give rise to contributions of second and higher orders in the dispersion relations, but never at first order. Electromagnetic scattering by the cylinder in general and, in particular, its long wavelength behaviour, are studied briefly. PublicationInitial field and energy flux in absorbing optical waveguides. I. Theoretical formalism(Optical Society of America, 1987-06) Lakshminarayanan, Vasudevan; Calvo Padilla, María LuisaAn exact formulation of the electromagnetic field striking an optical waveguide is presented and compared with the decomposition relationship used in the literature. The expression for the critical angle is derived as a natural consequence of this formulation. Equations for the total fraction of power confined within a waveguide are derived and analyzed for the special case of absorbing waveguides. An explicit expression is derived for the fraction of energy confined within a waveguide supporting two sets of modes. PublicationNeutron fibres. II. Some improving alternatives and analysis of bending losses(IOP Publishing Ltd, 1986-06-14) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María LuisaFor pt.I see ibid., vol.17, p.475 (1984). The possible confined propagation of slow (thermal) neutrons along cladded waveguides of small cross section (fibres) is studied, in order to improve a previously published analysis. Quite suitable possibilities for the core of the waveguide could be either Al or Si (or vacuum), so as to allow for confined propagation of neutrons along reasonable lengths, without large scattering and absorption losses. Tentative possibilities for the cladding could be either Ni or Fe. The bending losses are analysed for a curved neutron fibre in two spatial dimensions. Such bending losses turn out to be completely negligible, except in the case of a small radius of curvature Rc. To quote some typical values, for any fibre diameter between 0.1 and 1 mu m and reasonable lengths for it (between 5 and 10 cm), bending losses can be neglected other than for Rc PublicationThree-dimensional analysis of bending losses in dielectric optical waveguides with arbitrary refractive-index profile(Optical Society of America, 1987-04) Fernández Álvarez-Estrada, Ramón; Calvo Padilla, María LuisaA three-dimensional analysis of bending losses in dielectric optical waveguides is presented. It constitutes a nontrivial generalization of previous two- and three-dimensional studies by other authors. Our analysis is based on homogeneous integral equations for the total radiation field and suitable asymptotic approximations for Green’s functions. A key role is played by a new three-dimensional approximation for a relevant Bessel function with large order and argument (the former being larger than the latter). A nontrivial check of the consistency of all those approximations is given. General formulas are presented for the radiated field and the energy flow and for a bending-loss coefficient in three dimensions. Numerical results are also given, in order to assess the difference between the results of other authors and ours. Such a difference is rather small for monomode behavior near cutoff, increases as the behavior of the waveguide changes from monomode to multimode, and decreases as the parameter V increases for a given core radius and propagation mode.