Alfonso Serrano-Heredia

Computer-generated holograms with optimum bandwidths obtained with twisted-nematic liquid-crystal displays

We discuss a computer-generated hologram for encoding arbitrary complex modulation based on a commercial twisted-nematic liquid-crystal display. This hologram is implemented with the constrained complex modulation provided by the display in a phase-mostly configuration. The hologram structure and transmittance are determined to obtain on-axis signal reconstruction, maximum bandwidth, optimum efficiency, and high signal-to-noise ratio. We employed the proposed holographic code for the experimental synthesis of first-order Bessel beams.

Synthesis of analog apodizers with binary angular sectors.

We describe a new procedure for generating complex amplitude distributions along the optical axis of an optical processor by the use of binary masks in the form of binary angular sectors. This type of binary spatial filter acts as a gray-level apodizer with rotational symmetry. Experimental verifications are included.

Talbot array illuminators with liquid crystal displays

First we show that a binary amplitude grating, altered by a suitable discrete phase modulation is transformed, by Fresnel diffraction, into another binary grating with reduced opening ratio. We employ this process as the basis for discussing segmented versions of Talbot array illuminators that can be displayed with liquid crystal displays.

Recovery of three-dimensional shapes by using a defocus structured light system

We present a new concept for 3D shape recovery using Defocused Structure Light (DSL) images. DSL technique externally extracts the depth information for the scene by using projections of cylindrical wavefronts on the object. These projections show different degrees of defocus as a function of the depth.

Trading dimensionality in signal processing

By trading one spatial dimension, in an optical processor, one can implement simple optical set-ups for signal synthesis and signal processing. We show that this concept, of trading dimensionality, has been applied advantageously in four classical optical processors. We also discuss another four applications of the same concept: the generation and parallel display of certain special functions, the synthesis of complex amplitudes with binary screens, and the generation of radially symmetric apodizers with binary sectors.

Simplified optimum phase-only configuration for a TNLCD

We propose and implement simplifications to the optimum configuration of a twisted-nematic liquid-crystal display (TNLCD) operating as a phase modulator. As previously proposed, such an optimum configuration requires a generator of elliptic polarization, at the input, and a detector of elliptic polarization, at the output. Both the generator and the detector of elliptic polarization are formed by a quarter wave plate and a linear polarizer, appropriately arranged. As a first modification of the optimum phase configuration we removed the quarter-wave plate at the output of the TNLCD. The remaining components, two polarizers and a wave plate are arranged and oriented in such a way that the quality of phase modulation is very similar to that obtained with the arrange using two wave plates. This modification reduces complexity and cost of the mostly-phase setup arranged with the TNLCD. Our experimental implementation of the modified phase configuration employed a laser with a wavelength of 457 nm. As another modification of the setup, instead of a quarter-wave plate, optimized at 457 nm, we employed a half-wave plate optimized at 633 nm. The required elliptic state at the input of the TNLCD was generated by the appropriate arrangement of the linear polarizer and the wave plate.

Recovery of three-dimensional shapes by using defocused structured light

We present a new concept for three-dimensional shape recovery using defocused structured light (DSL) images. The DSL technique externally extracts the depth information from the scene by using projections of cylindrical wavefronts on the object. These projections show different degrees of defocus as a function of the depth. Efficient algorithms for shape recovery are developed taking advantage of the spatial regularity of the projected light and the profile of each fringe. In this way a depth map of the scene is obtained using only one image. Experimental results are shown and we discuss the possibility of a real time implementation.

Numerical evaluation of commercial amplitude spatial light modulator for synthesis of general complex fields

In this work we present the numerical analysis of fully complex fields employing computer generated holograms, implemented with a commercial amplitude liquid-crystal spatial light modulator (SLM). The hologram transmittance is obtained by adding an appropriate bias function to the real cosine computer hologram of the encoded signal. For the purpose of this work, we consider an appropriated bias function for a modulator in which the amplitude transmittance is coupled with a phase modulation. This type of coupled phase appears in a commercial Twisted Nematic Liquid Crystal Display (TNLCD), configured to provide amplitude modulation, with only two polarizers as external components. This modulation curve was experimentally obtained and adjusted to a mathematical function. The numerical evaluation is done by calculating the signal to noise ratio of the reconstructed complex fields for the real modulation curve. Results show that for a good behavior for complex fields a constant bias function should be implemented. As a particular interesting case we generate propagation invariant complex beams with high accuracy and simplicity.

Development and application of Talbot images technique for reconstruction of three dimensional objects

We present a non-contact system for obtaining three-dimensional objects topography. The described system combines the fringe projection technique and the Talbot effect which is knowed like Talbot interferometry. In fringe projection technique, the digitalization is realized when black and white lines are projected over the object and this image is captured by the CCD. In Talbot interferometry, the object is collocated on one of the grating auto-image planes. The deformed grating image is captured by the CCD and superposed with other reference one that can be physical or computer generated (virtual/synthetic) for obtaining a moire pattern which gives information about the object topography. The topography of a coin and phalangeal articulation are obtained by using of this technique. The Spatial Synchronous Detection and Fourier Method were incorporate to retrieve the phase.

Experimental synthesis of general complex fields using an amplitude modulator

In this work we present the experimental synthesis of fully complex fields employing computer generated holograms, implemented with an amplitude liquid-crystal spatial light modulator (SLM). The hologram transmittance is obtained by adding an appropriate bias function to the real cosine computer hologram of the encoded signal. The effect of finite pixel size in the SLM is compensated by digital pre-filtering of the encoded complex signal. For the purpose of this work, we consider an appropriated bias function for a modulator in which the amplitude transmittance is coupled with a linear phase modulator. This type of coupled phase appears in a commercial Twisted Nematic Liquid Crystal Display (TNLCD), configured to provide amplitude modulation, with only two polarizers as external components. We employ a commercial TNLCD to generate experimentally Laguerre-Gauss beams and non-diffracting Bessel beams of several orders.