Showing papers by "Ignacio Moreno published in 2008"

Time fluctuations of the phase modulation in a liquid crystal on silicon display: characterization and effects in diffractive optics

Abstract: In this paper we provide evidence of the temporal fluctuations of the phase modulation property of a liquid crystal on silicon (LCoS) display, and we analyze its effect when the device is used for displaying a diffractive optical element. We use a commercial twisted nematic LCoS display configured to produce a phase-only modulation, and we provide time resolved measurements of the diffraction efficiency that show rapid fluctuations of the phase modulation, in the millisecond order. We analyze how these fluctuations have to be considered in two typical methods for the characterization of the phase modulation: two beam interference and diffraction from a binary grating. We finally provide experimental results on the use of this device for displaying a computer generated hologram. A reduction of the modulation diffraction efficiency results from the phase modulation fluctuation.

Mueller-Stokes characterization and optimization of a liquid crystal on silicon display showing depolarization

Abstract: In this paper we characterize the polarimetric properties of a liquid crystal on silicon display (LCoS), including depolarization and diattenuation which are usually not considered when applying the LCoS in diffractive or adaptive optics. On one hand, we have found that the LCoS generates a certain degree (that can be larger than a 10%) of depolarized light, which depends on the addressed gray level and on the incident state of polarization (SOP), and can not be ignored in the above mentioned applications. The main origin of the depolarized light is related with temporal fluctuations of the SOP of the light reflected by the LCoS. The Mueller matrix of the LCoS is measured as a function of the gray level, which enables for a numerical optimization of the intensity modulation configurations. In particular we look for maximum intensity contrast modulation or for constant intensity modulation. By means of a heuristic approach we show that, using elliptically polarized light, amplitude-mostly or phase-mostly modulation can be obtained at a wavelength of 633 nm.

Wavelength dependence of polarimetric and phase-shift characterization of a liquid crystal on silicon display

Abstract: We present a full characterization of a liquid-crystal-on-silicon (LCoS) display, including a polarimetric analysis based on the Mueller-Stokes formalism, and a phase shift modulation calibration. Results for different wavelengths are compared. The goal of this work is two fold. On one side, previous papers dealing with the illumination wavelength 633 nm have shown that LCoS produce a non negligible amount of depolarized light. This may have a negative impact in certain applications. Here we want to establish how this depolarization varies with the wavelength. On the other side, to use the LCoS as a spatial light modulator (SLM) we need to obtain optimal configurations enabling for phase-only or for amplitude-only modulation. Here we show how phase-only modulation is obtained, and how it evolves with the wavelength. In principle, for shorter wavelengths the phase modulation depth increases and the energy throughput may also be increased. However, these phase-only configurations may be partially degraded by the presence of depolarization at certain gray levels. Thus, the Mueller-Stokes formalism is necessary to get a full picture of the performance exhibited by the LCoS at each wavelength

Time-resolved Mueller matrix analysis of a liquid crystal on silicon display

Abstract: We present a full polarimetric characterization of a liquid crystal on silicon (LCoS) display, with time resolution measurements below the frame period of the device. This time-resolved analysis shows evidence of temporal fluctuations in the millisecond range in the state of polarization of the beam reflected by the display. We demonstrate that light reflected by the display is maintained fully polarized, but these temporal fluctuations result in an effective depolarization effect when detectors with long time integration intervals are used in the characterization of the display.

Combined Mueller and Jones matrix method for the evaluation of the complex modulation in a liquid-crystal-on-silicon display

Abstract: We apply the polar decomposition of the Mueller matrix describing a liquid-crystal-on-silicon display to identify the diattenuator, depolarizer, and retarder contributions as a function of the gray level. The retarder contribution is expressed in terms of the equivalent Jones matrix to apply previous techniques to evaluate the phase modulation. This allows searching for optimized polarization configurations for phase- or amplitude-only modulation responses. We present results for lambda=633 nm showing a phase-only modulation up to 2 pi rad and flat intensity modulation.

Observation of superluminal and negative group velocities in a Mach-Zehnder interferometer

Abstract: We demonstrate superluminal and negative group velocity regimes in a linear passive Mach–Zehnder interferometer. This phenomenon occurs in a narrow frequency region around the interferometer’s transmission minima. Experiments are performed in the radio frequency range by using coaxial cables and 1×2 wave splitters. Group velocities of 2c and tunneling with a maximum fractional advancement of 0.12 were measured for electromagnetic sinusoidal wave packets of 2 μs width. These results agree with theoretical predictions using the interferometer’s transmission phase function. This system is proposed as a simpler alternative to photonic crystals and active or microstructured multiple-beam interferometers for sustaining anomalous group velocities.

Polarization-splitting common-path interferometer based on a zero-twist liquid crystal display.

Abstract: We present a compact optical polarization-splitting common-path interferometer based on a zero-twist liquid crystal display (LCD). The LCD is encoded with a diffraction grating pattern and illuminated with a polarization state with both horizontal and vertical components. The polarization component perpendicular to the director axis of the liquid crystal molecules is not affected by the LCD and forms the reference beam. However, the polarization component parallel to the director axis is diffracted at an angle determined by the period of the grating. By imposing an analyzer polarizer, these two beams create an interferogram that can either display retardance patterns encoded onto the LCD or analyze external birefringent optical elements. The programmability of the system allows new ways of increasing the utility of the interferograms. Experimental results are provided, including the visualization of optical vortices with different and opposite topological charges.

Wavelength-dependent diffraction patterns from a liquid crystal display.

Abstract: Liquid crystal displays (LCDs) are invaluable for a variety of optical applications, including the encoding of programmable diffractive optical elements. The pixel structure in these devices produces a set of diffracted orders of which the central order is the strongest. In most devices that we have examined, the intensity distribution of the diffraction pattern is independent of the wavelength of the illuminating light. Recently we have been examining the performance of LCDs having very small pixel sizes. We compare results for two devices from the same manufacturer. One of them exhibits the normal behavior. For the other, we find surprisingly strong wavelength dependence. The diffraction pattern varies from having most of the energy in the zero order for long wavelengths to having the energy distributed among 50-60 orders as the wavelength decreases. We attribute this behavior to a phase structure over each pixel. We analyze this behavior using a simple two-dimensional model that qualitatively explains the phenomenon. These results can be viewed in two ways--on the positive side this behavior might lead to optical logic or fan-out applications. On the negative side, there is less intensity available in the normally used zero order.

Comparative Analysis of Time and Spatially Multiplexed Diffractive Optical Elements in a Ferroelectric Liquid Crystal Display

Abstract: We present a very simple method of generating time-multiplexed optical diffractive elements. We use a commercially available ferroelectric liquid crystal device originally designed to visualize colour images by sequentially displaying the three red, green, and blue (RGB) colour channels. We substitute the original light emitting diode (LED) light source by monochromatic illumination. Then we generate a three time-multiplexed diffractive element simply by addressing a colour RGB image where each colour component corresponds to a different diffractive screen. We carry out computer simulations in order to compare this simple time multiplexing technique with various spatial multiplexing techniques proposed in the literature. We numerically evaluate the different methods in terms of light efficiency, noise level and the quality of the hologram reconstruction. We provide experimental results that verify the simulations and show the advantage of using the time multiplexing technique.

Hiding binary optical data with orthogonal circular polarizations

Abstract: We discuss a very simple method to encode binary data as circular polarization states with opposite helicity by means of a standard twisted nematic liquid crystal display (TN-LCD). This idea is applied to produce a steganographic imaging system, where binary data are hidden on a background with the same polarization but opposed helicity. A variant is also proposed that maintains some visible information. Experimental results obtained with a commercial TN-LCD are presented.

Real time modulable multifocality through annular optical elements

Abstract: We present and analyze new multifocal optical elements based on an annular distribution of the transmittance. These elements provide selectable number of foci and can be designed to work between two fixed positions or even to provide extended focal depth. The energy of the foci can be modulated through a single parameter that controls the area of each ring. In our study we analyze the quality of the peaks and also the limit number of foci that can be obtained. The properties shown by these elements make them usable in instrumental optics or in ophthalmic optics, as new intraocular implants, where multifocal elements are required. The implementation has been done on a twisted nematic spatial light modulator, thus allowing real time reconfiguration of the element.

Influence of the incident angle in the performance of LCoS displays

Abstract: Liquid Crystal on Silicon (LCoS) displays can be very useful in numerous optical applications due to some technical features of these devices, like the large fill factor and resolution. Here, we have developed a study related to the performance of the LCoS displays. It has been demonstrated that the LCoS display produces certain amount of depolarized light and the Mueller-Stokes formalism has been required for a full polarimetric characterization. In a previous paper we obtained the experimental Mueller matrix of the LCoS display as a function of the addressed gray level, at the very small angle of 2 degrees between the incident beam and the LCoS display normal, and for the wavelength of 633 nm. In the present paper we extend this study to different angles of incidence in order to analyze the influence of this parameter on the performance of the LCoS display. We also analyze the influence of the angle of incidence on the degree of polarization of the reflected beam. A comparison between the obtained results is presented in this paper. Keywords: Mueller matrix, liquid crystal display, spatial light mo dulator, diattenuation, pol arizance, retardance, degree of polarization.

Chromatic dispersion compensation of a ferroelectric liquid crystal modulator for optical switching applications

Abstract: In this work we present a characterization of the phase shift chromatic dispersion introduced by a ferroelectric liquid crystal modulator. Maximum optical contrast is obtained when the proper design wavelength illuminates the device. In this situation the modulator acts as a perfect half-wave plate. However, for many applications the illuminating sources available are different from the design one and the optical contrast is reduced. In a previous work we proposed an optimization method to increase the optical contrast ratio by illuminating the device with an optimal elliptically polarized light. Here, we explore further the optimization of the ferroelectric modulator performance as an optical switching device for different monochromatic and polychromatic sources. For that purpose we use an achromatic quarter wave-plate placed in front of the modulator. For the optimal situations, the two output states of the modulator will be almost linear and orthogonal providing a highly contrasted modulated optical signal. Experimental results are obtained using a commercially available single pixel ferroelectric liquid crystal modulator from CRL-Opto, model LCS2N-G, with an active area of 25.5×25.5 mm 2 .

Spectral Characterization of a Ferroelectric Liquid Crystal Modulator and Performance Optimization

Abstract: In this work we present a characterization of a ferroelectric liquid crystal modulator and its performance optimization as an optical switch. We present a procedure to characterize the orientation of the liquid crystal layer and its change with the applied voltage. Then we illuminate the modulator with white light and analyze the transmitted light with a portable spectro‐photometer. The chromatic dispersion of the phase shift introduced by the modulator is obtained from the spectral intensity of the transmitted light. Then we apply this result to evaluate the response as an optical switch both for white light and monochromatic laser light with different wavelengths. We show that it is possible to optimize the response of the modulator for wavelengths that are far from the design wavelength. For that purpose a quarter wave‐plate is placed in front of the modulator. In this configuration the modulator is illuminated with elliptically polarized light, being the polarization ellipse selected to produce at the...

Analysis of polarization vortices generated from a polarization diffractive mask

Abstract: In this work we analyze the diffraction pattern generated by a four quadrant polarization mask. Different quadrants transmit different linear polarizations. We show that the diffraction pattern, in the absence of analyzer, shows the intensity distribution characteristic from a square aperture, but the state of polarization is not uniform. In particular, we demonstrate that an array of singular points (polarization vortices) is being generated. When an analyzer is placed behind the polarization diffractive mask, the intensity of the diffracted field is changed as the analyzer rotates. Experimental verification is performed with a twisted nematic liquid crystal display.