Joaquín Otón

Multipoint phase calibration for improved compensation of inherent wavefront distortion in parallel aligned liquid crystal on silicon displays

The inherent distortion of a reflective parallel aligned spatial light modulator (SLM) may need compensation not only for the backplane curvature but also for other possible nonuniformities caused by thickness variations of the liquid crystal layer across the aperture. First, we build a global look-up table (LUT) of phase modulation versus the addressed gray level for the whole device aperture. Second, when a lack of spatial uniformity is observed, we define a grid of cells onto the SLM aperture and develop a multipoint calibration. The relative phase variations between neighboring cells for a uniform gray level lead us to build a multi-LUT for improved compensation. Multipoint calibration can be done using either phase-shift interferometry or Fourier diffraction pattern analysis of binary phase gratings. Experimental results show the compensation progress in diffractive optical elements displayed on two SLMs.

Dynamic compensation of chromatic aberration in a programmable diffractive lens

A proposal to dynamically compensate chromatic aberration of a programmable phase Fresnel lens displayed on a liquid crystal device and working under broadband illumination is presented. It is based on time multiplexing a set of lenses, designed with a common focal length for different wavelengths, and a tunable spectral filter that makes each sublens work almost monochromatically. Both the tunable filter and the sublens displayed by the spatial light modulator are synchronized. The whole set of sublenses are displayed within the integration time of the sensor. As a result the central order focalization has a unique location at the focal plane and it is common for all selected wavelengths. Transversal chromatic aberration of the polychromatic point spread function is reduced by properly adjusting the pupil size of each sublens. Longitudinal chromatic aberration is compensated by making depth of focus curves coincident for the selected wavelengths. Experimental results are in very good agreement with theory.

Chromatic compensation of programmable Fresnel lenses

Two proposals to compensate chromatic aberration of a programmable phase Fresnel lens displayed on a liquid crystal device and working under polychromatic illumination are presented. They are based on multiplexing a set of lenses, designed with a common focal length for different wavelengths, and a multicolor filter that makes each sublens work almost monochromatically. One proposal uses spatial multiplexing with mosaic aperture. The other uses a rotating scheme, a color filter against an array of lens sectors, and hybrid spatial-time integration. The central order focalization has a unique location at the focal plane. We have drastically reduced the transversal chromatic aberration of the polychromatic point spread function by properly adjusting the pupil size of each sublens. Depth of focus curves have been made coincident too for the selected wavelengths.

Dynamic calibration for improving the speed of a parallel-aligned liquid-crystal-on-silicon display

The speed of most parallel-aligned liquid-crystal-on-silicon (LCoS) spatial light modulators (SLMs) is limited to the video rate of their drivers, which is a limitation for high-speed applications. However, the LCoS SLM presented here has a driver allowing a frequency range of up to 1011 Hz. Using the static phase modulation characterization and the static lookup table (LUT), the phase modulation characterization versus frequency shows that the SLM can operate at around 130 Hz or even higher for small phase changes and at 32 Hz for extreme phase changes. A dynamic calibration is carried out, and we propose a method allowing an increase of the frame rate while maintaining a maximum phase modulation of 2pi. Experimental results of dynamic diffractive optical elements displayed on the SLM at a frame rate of 205 Hz show that the dynamic LUT improves the reconstruction quality.

Nonlinear pattern recognition correlators based on color-encoding single-channel systems

In color pattern recognition, color channels are normally processed separately and afterward the correlation outputs are combined. This is the definition of multichannel processing. We combine a single-channel method with nonlinear filtering based on nonlinear correlations. These nonlinear correlations yield better discrimination than common matched filtering. The method codes color information as amplitude and phase distributions and is followed by correlations related to binary decompositions. The technique is based on binary decompositions of the red, green, and blue and the hue, saturation, and intensity monochromatic channels of the reference and of the input scene, after which the binary information on the red, green, and blue channels and that of the hue, saturation, and intensity channels are encoded as different angles of a phase distribution. We have applied the method to images degraded by high levels of substitutive noise. Results show that the sliced orthogonal nonlinear generalized correlation detects the target with a high degree of discrimination when other methods fail.

Spatial Light Modulators For Information Processing: Applications And Overview

We present the characterization of a ferro-electric liquid crystal spatial light modulator and of a nematic liquid crystal spatial light modulator. It is shown that the nematic device can be compensated for its phase distortions and a method is proposed for increasing its frame rate while maintaining a maximum phase modulation of 2π. Several applications in information processing are presented with experimental results.

Advances in LCoS SLM characterization for improved optical performance in image processing

Electrically addressed spatial light modulators (SLM) are widely used in optical image processors to display not only input images but also a huge variety of optical components such as lenses, complex filters and other diffractive elements. These components are fully programmable and dynamically controllable by computer thus bringing flexibility and new degrees of freedom to current optical and digital image processors. A good characterization is the most important step in the SLM initialization. The quality and effectiveness of the optical component addressed to the SLM strongly depends on the knowledge of the device response. This work deals with the spatial and temporal characterizations of reflective zerotwist liquid-crystal on silicon (LCoS) SLM. The signal is spatially modified before addressing it to the LCoS SLM to compensate for the distortions internally introduced by the device. For time varying optical components, the signal is also modified before addressing it to the LCoS SLM to compensate for the distortions internally introduced by the device when phase variations of 2π are required at high rate. Experimental results and applications in image processing are shown.

Imaging Characteristics Of Programmable Lenses Generated By SLM

The imaging characteristics of programmable lenses generated by twist‐nematic liquid crystal pixelated displays are determined from the technical specifications of a commercial modulator. We calculate the ranges of power and aperture of the feasible lenses reproduced by the modulator. Simulated and experimental point spread functions (PSF) are obtained to evaluate the programmed lens behaviour for different technical constraints. Imaging features of programmable multilenses are compared for two different configurations.

Multiplexing schemes for an achromatic programmable diffractive lens

A multiplexed programmable diffractive lens, displayed on a pixelated liquid crystal device under broadband illumination, is proposed to compensate for the severe chromatic aberration that affects diffractive elements. The proposed lens is based on multiplexing a set of sublenses with a common focal length for different wavelengths. We consider different types of integration of the optical information (spatial only, temporal only and hybrid spatial-temporal) combined with a proper selection of the spectral bandwidth. The properties and limits of the achromatic programmable multiplexed lens are described. Experimental results are presented and discussed.

Compensation of inherent wavefront distortion in zero‐twist LCoS spatial light modulators

A good and complete calibration is the most important step in the electrically addressed spatial light modulator (SLM) initialization. It is clear that the quality and effectiveness of the optical component addressed to the SLM strongly depends on the knowledge of the device response. In fact, the signal must be modified before addressing it to the SLM to compensate for the distortions internally introduced by the device and hence, to eventually reproduce the optical component with the desired performance. In this work we deal with the spatial calibration of reflective zero‐twist liquid‐crystal on silicon (LCoS) SLM by testing the aperture of the device to characterize the inherent phase distortion. The inherent distortion of the reflective LCoS SLM may need not only the compensation for the backplane curvature but also for other possible non‐uniformities caused by thickness variations of the liquid crystal layer across the aperture. Firstly, we build a global Look‐Up Table (LUT) of phase modulation versu...