Mercedes Fernández-Alonso

Single-pixel polarimetric imaging.

We present an optical system that performs Stokes polarimetric imaging with a single-pixel detector. This fact is possible by applying the theory of compressive sampling to the data acquired by a commercial polarimeter without spatial resolution. The measurement process is governed by a spatial light modulator, which sequentially generates a set of preprogrammed light intensity patterns. Experimental results are presented and discussed for an object that provides an inhomogeneous polarization distribution.

Fractional derivative Fourier plane filter for phase-change visualization

Fractional derivatives of two-dimensional images have been discussed theoretically in terms of Fourier optics and computer simulated. Filters that realize the half-order differentiation can be either complex or real. We prove, in terms of fractional calculus, that the semiderivative filter is useful for the visualization of phase changes in a phase object in such a way that the output-image intensity is directly proportional to the first derivative of the input object. We give computer-simulated results of one-dimensional semidifferentiating.

Phase-only modulation with a twisted nematic liquid crystal display by means of equi-azimuth polarization states

The capability of a twisted nematic liquid crystal display to generate a set of equi-azimuth polarization states is used to achieve a phase-only modulation regime. For this purpose, a liquid crystal display followed by a quarter-wave plate is launched between two polarizers. Theoretical support is provided by means of the Jones matrix calculus and the Poincaré sphere representation. Laboratory results for a commercial liquid crystal display are presented. A phase modulation deep of 270 masculine is obtained at 514 nm with a residual intensity variation which is lower than 2.5 %.

Dispersion-compensated beam-splitting of femtosecond light pulses: Wave optics analysis.

Recently, using parageometrical optics concepts, a hybrid, diffractive-refractive, lens triplet has been suggested to significantly improve the spatiotemporal resolution of light spots in multifocal processing with femtosecond laser pulses. Here, we carry out a rigorous wave-optics analysis, including the spatiotemporal nature of the wave equation, to elucidate both the spatial extent of the diffractive spots and the temporal duration of the pulse at the output plane. Specifically, we show nearly transform-limited behavior of diffraction maxima. Moreover, the temporal broadening of the pulse is related to the group velocity dispersion, which can be pre-compensated for in practical applications. Finally, some numerical simulations of the spatiotemporal wave field at the output plane in a realistic case are provided.

Spatial-chirp compensation in dynamical holograms reconstructed with ultrafast lasers

A computer generated hologram (CGH) reconstructed with a sub-100-fs laser pulse at the focal plane of a conventional refractive lens experiences a large amount of spatial chirp. We report the shaping of a 12 fs laser pulsed beam by means of a Fourier CGH implemented onto a spatial light modulator, using a hybrid diffractive-refractive lens triplet that provides spatial-chirp compensation. Experimental results demonstrate that parallel, arbitrary, and high-resolution patterning is possible with the proposed device.

Phase-change visualization in two-dimensional phase objects with a semiderivative real filter

A method of visualization of phase changes in two-dimensional pure-phase objects by use of two orthogonal Fourier plane filters that realize the half-order differentiation is presented. Real semiderivative filters used in two dimensions and in sequence yield output-image intensity signals proportional to the first derivatives of the input-object phase that appear on a constant background. This nonlinear filtration of spatial frequencies permits the alleviation of the consequences of square-law detection and makes phase changes visible. Phase changes in gradient-index phosphate glass are calculated experimentally. We discuss the accuracy of the proposed method.

Generation of programmable 3D optical vortex structures through devil’s vortex-lens arrays

Different spatial distributions of optical vortices have been generated and characterized by implementing arrays of devils vortex lenses in a reconfigurable spatial light modulator. A simple design procedure assigns the preferred position and topological charge value to each vortex in the structure, tuning the desired angular momentum. Distributions with charges and momenta of the opposite sign have been experimentally demonstrated. The angular velocity exhibited by the phase distribution around the focal plane has been visualized, showing an excellent agreement with the simulations. The practical limits of the method, with interest for applications involving particle transfer and manipulation, have been evaluated.

Quasi-wavelength-independent broadband optical Fourier transformer

The chromatic behaviour associated with diffractive optical elements is exploited herein to design a hybrid (diffractive-refractive) lens triplet showing very great wavelength-compensation capabilities for the Fraunhofer diffraction pattern of any diffracting screen under broadband point-source illumination. Within the paraxial Fresnel diffraction theory, we show that perfect compensation for the axial position of the Fourier transform of the input can be accomplished if we neglect the secondary spectrum of the refractive objective. Simultaneously, an achromatic correction for the scale of the Fraunhofer pattern is achieved. In this way, even for white light, only a low residual transversal chromatic aberration remains.

On-axis diffraction of an ultrashort light pulse by circularly symmetric hard apertures

The analytical solution of the Rayleigh-Sommerfeld on-axis diffraction integral for an ultrashort light pulse diffracted by circularly symmetric hard apertures is derived. The particular case of a circular aperture is treated in detail. The time changes of the instantaneous intensity along the axial direction are predicted. An analysis of the standard deviation width shows a pulse broadening about the axial positions where the instantaneous intensity reaches a zero value. We show that the temporal shape of the instantaneous intensity depends on the number of oscillation cycles at the central frequency of the real electric field.

Millimeter-wave and microwave signal generation by low-bandwidth electro-optic phase modulation

We propose, analyze and numerically illustrate a photonic-based technique for waveform generation of electrical signals approaching the 50 GHz bandwidth with time apertures as large as a few nanoseconds, by low-frequency, up to 2 GHz, electro-optic phase modulation of time-stretched optical pulses. Synthesis of the electrical waveform relies on phase-to-amplitude conversion of the modulated signal by a group delay dispersion circuit designed to behave as a transversal filter with N taps. Although arbitrary waveform generation capabilities are limited, a wide variety of user-defined signals are numerically demonstrated by appropriately designing the low-frequency signal driving the electro-optical modulator. Frequency upshifting is controlled by the chirp of the stretched pulse which provides an additional degree of freedom. Finally, optical-to-electrical conversion allows for the user-defined electrical waveform. Simulations are given for square waveform generation demonstrating the high resolution and wide-band capabilities of the technique.