Jorge García-Márquez

Flicker minimization in an LCoS spatial light modulator

We present a method for reducing the phase flicker originated by the pulsed modulation of a Liquid Crystal on Silicon (LCoS) Spatial Light Modulator (SLM). It consists in reducing the temperature of the LCoS in a controlled way, in order to increase the viscosity of the liquid crystal. By doing this, we increase the time response of the liquid crystal, and thus reduce the amplitude of phase fluctuations. We evaluate the efficacy of this method quantifying the temporal evolution of phase shift using an experiment that is insensitive to optical polarization fluctuations. Additionally, we determine the effect of the temperature reduction on the effective phase modulation capability of the LCoS. We demonstrate that a reduction of up to 80% of the flicker initial value can be achieved when the LCoS is brought to -8 °C.

Three-dimensional displacement fields measured in a deforming granular-media surface by combined fringe projection and speckle photography

D displacement fields on a diffuse surface are measured by a combination of two optical methods, fringe projection and speckle photography. The use of a single camera for recording information from the two methods implies that no calibration procedures are necessary, as is the case in stereoscopy-based techniques. Out-of-plane displacements are measured by fringe projection whereas speckle photography yields the 2D in-plane component. By using this technique, we analyze in detail the morphological spatial-temporal evolution of an analogue model of the Earths crust while subjected to compression forces. We discuss the experimental results and their relevance to the micromechanics of a surface of dry, non-cohesive and dilatant granular media. The results show that the combination of fringe projection and speckle photography is well suited for this type of study and allows the characterization of strain at the grain scale.

Analysis of interferograms with a spatial radial carrier or closed fringes and its holographic analogy.

It is well known that an interferogram can be demodulated to find the wave-front shape if a linear carrier is introduced. We show that it can also be demodulated if it has many closed fringes or a circular carrier appears. A basic assumption is that the carrier fringes are of a bandwidth adequate to contain the wave-front distortion. This phase determination, called here demodulation, is made in the space domain, as opposed to demodulation in Fourier space, but the low-pass filter characteristics must be properly chosen. For academic purposes a holographic analogy of this demodulation process is also presented, which shows that the common technique of multiplying by a sine function and a cosine function is equivalent to holographically reconstructing with a tilted-flat wave front. Alternatively, a defocused (spherical) wave front can be used as a reference to perform the reconstruction or demodulation of some closed-fringe interferograms.

High resolution kilometric range optical telemetry in air by radio frequency phase measurement

We have developed an optical Absolute Distance Meter (ADM) based on the measurement of the phase accumulated by a Radio Frequency wave during its propagation in the air by a laser beam. In this article, the ADM principle will be described and the main results will be presented. In particular, we will emphasize how the choice of an appropriate photodetector can significantly improve the telemeter performances by minimizing the amplitude to phase conversion. Our prototype, tested in the field, has proven its efficiency with a resolution better than 15 μm for a measurement time of 10 ms and distances up to 1.2 km.

Phase-shifting algorithms for a finite number of harmonics: first-order analysis by solving linear systems

From generalized phase-shifting equations, we propose a simple linear system analysis for algorithms with equally and nonequally spaced phase shifts. The presence of a finite number of harmonic components in the fringes of the intensity patterns is taken into account to obtain algorithms insensitive to these harmonics. The insensitivity to detuning for the fundamental frequency is also considered as part of the description of this study. Linear systems are employed to recover the desired insensitivity properties that can compensate linear phase shift errors. The analysis of the wrapped phase equation is carried out in the Fourier frequency domain.

On telescope performance evaluation

Abstract We analyse the imaging performance of synthetic-aperture optical systems (diluted aperture and segmented aperture) using the modulation transfer function. We select a single figure-of-merit, the functional cut-off frequency, over the traditional cut-off frequency, as the most useful one for assessing the optical performance of an instrument for its imaging capacity. A simplified aperture layout is proposed with the performance equivalent to that of the Keck telescope, incorporating a smaller number of segments. The detailed analysis in the spatial frequency plane leads us to observe that the central ring of the hexagons in the Keck telescope does not contribute to its resolution.

Characterization and reduction of the amplitude-to-phase conversion effects in telemetry

We are developing a telemeter based on the measurement of the phase accumulated by an RF sine wave during its propagation in air. This wave is carried by a laser beam by an intensity modulation. The main limitation of this technique lies in amplitude-to-phase conversion occurring in the detection of this modulation. Therefore, we characterize this phenomenon for a given telemetric system and discuss how to reduce its effects on the resolution and the accuracy of the distance measurement. Finally, a solution is implemented and tested outdoors in real conditions of use.

Catadioptric interfaces for designing VLC antennae

This paper presents a model to design bi-aspherical catadioptric lenses with limited image diffraction. A first refractive Cartesian oval surface that does not introduce any spherical aberration is used. When total internal reflection occurs, this surface can also be simultaneously used as a mirror. The reflective characteristics of Cartesian ovals are also well described in this paper. The theoretical work described here can considerably reduce computing time in optical system design. This model is applied to examples of antennae design for visible light communications (VLC).

Flicker reduction in an LCoS spatial light modulator

The spatial light modulators based on liquid crystal (LC-SLMs) have found applications in areas like beam shaping, optical tweezers, and microscopy. The use of these devices for pupil engineering has been an active research field. Many experiments with LC-SLMs involve the use of a digital camera, or even an additional LC modulator. Discrepancies in the refreshing rates of the modulator and the camera, or even between two nominally equal modulators, give rise to beating and intensity fluctuations, known as flicker, in the point spread function generated by the engineered pupil. In this paper we present a method for reducing the flicker caused by a liquid crystal on silicon (LCoS) SLM. It consists in reducing the temperature of the LCoS in a controlled form, in order to increase the viscosity of the liquid crystal, and with this reduce the amplitude of the intensity fluctuations due to the refreshing mechanism. We demonstrate that the flicker has been reduced to only the 25% of its initial value, when the LCoS is brought at 0 °C at a given gray level. Additionally, we found that the proposed reduction of temperature does not affect the dynamical range of phase control.

Basic Fourier properties for generalized phase shifting and some interesting detuning insensitive algorithms

In this manuscript, some interesting properties for generalized or nonuniform phase-shifting algorithms are shown in the Fourier frequency space. A procedure to find algorithms with equal amplitudes for their sampling function transforms is described. We also consider in this procedure the finding of algorithms that are orthogonal for all possible values in the frequency space. This last kind of algorithms should closely satisfy the first order detuning insensitive condition. The procedure consists of the minimization of functionals associated with the desired insensitivity conditions.