Special Issue on Liquid Crystal on Silicon Devices: Modeling and Advanced Spatial Light Modulation Applications

This special issue provides a collection of papers demonstrating the impact of LCoS microdisplays in current and future spatial light modulation applications. State-of-the-art in LCoS device technology, LC materials, modeling and characterization techniques are presented

Misalignment error analysis in polychromatic division of focal plane Stokes polarimeters

Snapshot imaging polarimeters are used in many different areas. Recently, division of focal plane (DoFP) polarimeters have proved useful as snapshot polarimeters for dynamic applications. For an optimal performance of polarimeters, different works dealing with the error analysis of such devices are proposed in literature. In terms of noise amplification from intensity measurements to the final polarization calculations, well-established quality metrics, as in the condition number or the equally weighted variance criteria, are used. Other studies analyze systematic errors due to deviations in the construction parameters. However, something not considered so far is the effect produced by misalignment between the various pixelated masks over the pixelated structure of the camera sensor, always occurring in experimental implementations of DoFPs. In this work, we study the effect of such misalignments in DoFP polarimeters and demonstrate how they lead to polarimetric systems composed of partially depolarized analyzers. We calculate the combined degree of polarization related to different amounts of misalignment and analyze the corresponding system performance. From this study, we show how an imaging polarimeter based on partially polarized analyzers can still lead to a robust and accurate polarimetric performance, and we also provide the misalignment limits in which an acceptable performance is obtained. We evaluate both the monochromatic and the polychromatic cases with special focus on the latter.Ministerio de Economía y Competitividad (MINECO) (FIS2015-66328-C3-1-R, fondos FEDER)

Optical image encryption technique based on deterministic phase masks

The double-random phase encoding (DRPE) scheme, which is based on a 4f optical correlator system, is considered as a reference for the optical encryption field. We propose a modification of the classical DRPE scheme based on the use of a class of structured phase masks, the deterministic phase masks. In particular, we propose to conduct the encryption process by using two deterministic phase masks, which are built from linear combinations of several subkeys. For the decryption step, the input image is retrieved by using the complex conjugate of the deterministic phase masks, which were set in the encryption process. This concept of structured masks gives rise to encryption-decryption keys which are smaller and more compact than those required in the classical DRPE. In addition, we show that our method significantly improves the tolerance of the DRPE method to shifts of the decrypting phase mask-when no shift is applied, it provides similar performance to the DRPE scheme in terms of encryption-decryption results. This enhanced tolerance to the shift, which is proven by providing numerical simulation results for grayscale and binary images, may relax the rigidity of an encryption-decryption experimental implementation setup. To evaluate the effectiveness of the described method, the mean-square-error and the peak signal-to-noise ratio between the input images and the recovered images are calculated. Different studies based on simulated data are also provided to highlight the suitability and robustness of the method when applied to the image encryption-decryption processes

Compact LCOS–SLM Based Polarization Pattern Beam Generator

In this paper, a compact optical system for generating arbitrary spatial light polarization patterns is demonstrated. The system uses a single high-resolution liquid crystal (LC) on silicon (LCOS) spatial light modulator. A specialized optical mount is designed and fabricated using a 3D printer, in order to build a compact dual optical architecture, where two different phase patterns are encoded on two adjacent halves of the LCOS screen, with a polarization transformation in between. The final polarization state is controlled via two rotations of the Poincaré sphere. In addition, a relative phase term is added, which is calculated based on spherical trigonometry on the Poincaré sphere. Experimental results are presented that show the effectiveness of the system to produce polarization patterns

Optical triple random phase encryption

We propose an optical security technique for image encryption using triple random-phase encoding (TRPE). In the encryption process, the original image is first double-random-phase encrypted. The obtained function is then multiplied by a third random-phase key in the output plane, to enhance the security level of the encryption process. This method reduces the vulnerability to certain attacks observed when using the conventional double random-phase encoding (DRPE). To provide the security enhancement of the proposed TRPE method, three attack cases are discussed: chosen-plaintext attacks, known-plaintext attacks, and chosen-ciphertext attacks. Numerical results are presented to demonstrate feasibility and effectiveness of the proposed method. Compared with conventional DRPE, the proposed encryption method can provide an effective alternative and has enhanced security features against the aforementioned attacks

Nou polarímetre basat en la refracció cònica

Els polarímetres són instruments òptics que permeten la determinació de la polarització de la llum i la caracterització d'elements polaritzants a partir de mesures d'intensitat. Aquests dispositius s'utilitzen en un gran nombre d'aplicacions en la indústria, la medicina i l'astronomia, entre altres. Investigadors de la UAB han dissenyat un nou model de polarímetre complet basat en el fenomen de la refracció cònica format per dos cristalls biàxics que suposa una interessant alternativa als ja existents.Los polarímetros son instrumentos ópticos que permiten la determinación de la polarización de la luz y la caracterización de elementos polarizantes a partir de medidas de intensidad. Estos dispositivos se utilizan en un gran número de aplicaciones en la industria, la medicina y la astronomía, entre otros. Investigadores de la UAB han diseñado un nuevo modelo de polarímetro completo basado en el fenómeno de la refracción cónica formado por dos cristales biáxicos que supone una interesante alternativa a los ya existentes

Connecting the microscopic depolarizing origin of samples with macroscopic measures of the Indices of Polarimetric Purity

In this work we show how a specific set of three depolarizing observables, the Indices of Polarimetric Purity (IPP), P1, P2 and P3, are ideal metrics to study the depolarization characteristic of media. We simulate different depolarizing scenarios, based on different depolarizing origins, and we study the corresponding IPP values. The simulations are based on the incoherent addition of multiple elemental polarizing elements, as ideal polarizers and/or retarders with different specific characteristics (orientation, retardance, transmittance, etc.). Further depolarizing scenarios are also studied by including the effect of ideal depolarizers. We show for the first time how by analyzing depolarizing systems through IPP we unravel two different depolarizing origins: isotropic and anisotropic depolarization, with meaningful physical interpretation. The former, isotropic depolarization is related to pure scattering processes, and mainly connected with P3 observable. The later, anisotropic depolarization is originated by microscopic constituent elements showing polarimetric anisotropy (dichroic and/or birefringent elements with different characteristics) and anisotropic scattering produced by these elements, and mainly described by P1 and P2 observables. Both effects can be simultaneously observed in real samples and give us information of the processes that give rise to depolarization in light-matter interactions. The simulated results are experimentally validated by analyzing the depolarizing behavior, in terms of IPP, of diverse real samples with easy physical interpretation, and direct connection with simulations. The present study could be of interest in multiple scenarios, to further understand the depolarizing response of samples, and it can be of special interest for the study of biological tissues and pathologies, as they present important depolarizing behavior.Monica Canabal-Carbia reports financial support was provided by Spain Ministry of Science and Innovation (PID2021-560 126509OB-C21 and PDC2022-133332-C21). Juan Campos reports financial support was provided by Spain Ministry of Science and Innovation (PID2021-560 126509OB-C21 and PDC2022-133332-C21). Angel Lizana reports financial support was provided by Spain Ministry of Science and Innovation (PID2021-560 126509OB-C21 and PDC2022-133332-C21). Irene Estevez reports financial support was provided by Government of Catalonia (Beatriu de Pinos, 2021-BP-00206). Ignacio Moreno reports financial support was provided by Spain Ministry of Science and Innovation (PID2021-126509OB-C22). Andres Marquez reports financial support was provided by Government of Valencia. Andres Marquez reports financial support was provided by Spain Ministry of Science and Innovation ( PID2021-123124OB-I00). Esther Nabadda reports financial support was provided by Government of Valencia. Mónica Canabal-Carbia, Angel Lizana and Juan Campos reports financial support was provided by the Generalitat de Catalunya (2021SGR00138)

Self-addressed diffractive lens schemes for the characterization of LCoS displays

Ponència presentada a: Emerging Liquid Crystal Technologies (San Francisco, California: 13th, 27 January - 1 February 2018)We proposed a self-calibration method to calibrate both the phase-voltage look-up table and the screen phase distribution of Liquid Crystal on Silicon (LCoS) displays by implementing different lens configurations on the studied device within a same optical scheme. On the one hand, the phase-voltage relation is determined from interferometric measurements, which are obtained by addressing split-lens phase distributions on the LCoS display. On the other hand, the surface profile is retrieved by self-addressing a diffractive micro-lens array to the LCoS display, in a way that we configure a Shack-Hartmann wavefront sensor that self-determines the screen spatial variations. Moreover, both the phase-voltage response and the surface phase inhomogeneity of the LCoS are measured within the same experimental set-up, without the necessity of further adjustments. Experimental results prove the usefulness of the above-mentioned technique for LCoS displays characterization

Dual polarization split lenses

We report the realization of polarization sensitive split lens configurations. While split lenses can be used to easily generate different types of controlled structured light patterns, their realization has been limited so far to scalar beams. Here we propose and experimentally demonstrate their generalization to vectorial split lenses, leading to light patterns with customized intensity and state of polarization. We demonstrate how these polarization split lenses can be experimentally implemented by means of an optical system using two liquid crystal spatial light modulators, each one phase modulating one orthogonal polarization component. As a result, we demonstrate the experimental generation of vectorial beams with different shapes generated with these dual polarization split lenses. Excellent experimental results are provided in each case. The proposed technique is a simple method to generate structured light beams with polarization diversity, with potential applications in polarimetry, customized illuminators or quantum optics

Polarization gating based on Mueller matrices

We present mathematical formulas generalizing polarization gating (PG) techniques. PG refers to a collection of imaging methods based on the combination of different controlled polarization channels. In particular, we show how using the measured Mueller matrix (MM) of a sample, a widespread number of PG configurations can be evaluated just from analytical expressions based on the MM coefficients. We also show the interest of controlling the helicity of the states of polarization used for PG-based metrology, as this parameter has an impact in the image contrast of samples. In addition, we highlight the interest of combining PG techniques with tools of data analysis related to the MM formalism, such as the well-known MM decompositions. The method discussed in this work is illustrated with the results of polarimetric measurements done on artificial phantoms and real ex-vivo tissues