Showing papers by "Ignacio Moreno published in 2021"

Encoding complex amplitude information onto phase-only diffractive optical elements using binary phase Nyquist gratings

Abstract: We reexamine a simple technique for encoding complex amplitude information onto a phase-only spatial light modulator (SLM) The basis for the approach is to spatially vary the diffraction efficiency of a two-dimensional checkerboard binary phase diffraction grating where the period for the Nyquist grating is two pixels As the phase depth of this 2D grating changes spatially, the amount of light diffracted into the zero order can be controlled Unwanted information is encoded onto the first diffraction orders and is directed away from the center This process uses a very simple coding algorithm to generate a complex beam reconstruction on-axis and allows exploiting the full spatial resolution for encoding amplitude However, its experimental realization with the current liquid-crystal on silicon (LCOS) technology is strongly affected by the limitations imposed by the fringing effect in these devices We provide experimental evidence of how this effect impacts the efficiency of diffraction gratings displayed on the SLM We then show how it affects the encoding technique, both in the near field and in the Fourier transform domain, where the limitations imposed by the fringing effect are clearly visible in the form of a focused peak These results provide evidence of the usefulness of the technique but also about the limitations imposed by the current LCOS technology, which do not allow fully exploiting their high resolution Finally, we discuss the performance of these newer LCOS devices compared to other SLMs

Customized depolarization spatial patterns with dynamic retardance functions.

Abstract: In this work we demonstrate customized depolarization spatial patterns by imaging a dynamical time-dependent pixelated retarder. A proof-of-concept of the proposed method is presented, where a liquid-crystal spatial light modulator is used as a spatial retarder that emulates a controlled spatially variant depolarizing sample by addressing a time-dependent phase pattern. We apply an imaging Mueller polarimetric system based on a polarization camera to verify the effective depolarization effect. Experimental validation is provided by temporal integration on the detection system. The effective depolarizance results are fully described within a simple graphical approach which agrees with standard Mueller matrix decomposition methods. The potential of the method is discussed by means of three practical cases, which include non-reported depolarization spatial patterns, including exotic structures as a spirally shaped depolarization pattern.

Indices of polarimetric purity: application in biological tissues

Abstract: Complete characterization of biological samples is of potential interest in different industrial and research areas, as for instance, in biomedical applications, for the recognition of organic structures or for the early detection of some diseases. During the last decades, polarimetric methods are experiencing an increase of attention in the study of biomedical tissues, and they are nowadays used in such framework to provide qualitative (polarimetric imaging) and quantitative (data processing) information for the studied samples. Polarimetric methods are based on the analysis of polarization modifications produced by light-matter interactions which can be triggered by a number of complex internal processes but can be roughly understood as the result of the combination of three pure polarimetric features of the sample: its diattenuation, retardance and depolarization. For the analysis of the depolarization content, we propose the use of the Indices of Polarimetric Purity (IPP) to describe the sample behavior. Related with the randomness of the scattering processes, IPPs provide more information of depolarizing systems than the widely used depolarization index (pu), which further synthetize the depolarization content of samples. Moreover, certain combinations of IPP parameters leads to pu. As a result, IPPs allow the revelation of some structures from tissue samples hidden in regular intensity images of even in the pu channel, leading to better tissue classification results. In this work, we present different applications of IPPs in biomedical tissue that show its potential, which are not restricted to the biomedical framework as relevant results in plants characterization are also presented.

Retrieving physical information of depolarizing systems

Abstract: Light interaction with material systems may introduce depolarization to the incident light. This phenomenon comes from multiple scattering processes that take place inside the media and strongly depends on the particle characteristics. In the case of botany, plant leaves can be understood as depolarizing systems. A non-contact method to analyze these samples consist of illuminating them with well-known polarized light and study the scattered light to retrieve the physical characteristics of the sample. This physical study can be done by measuring the Mueller matrix of samples, in which the physical information of samples is encoded in their 16 elements and further mathematical treatment is required to extract the information. In the case of scattering systems, the depolarization content carries very valuable information but it is usually not inspected in the botanic field. A way to study depolarized content is by determining the so-called depolarization index PΔ, which gives an overall measure of the degree of depolarization of a system but it does not measure possible anisotropic dependence of the depolarization. For instance, a depolarizer equally depolarizing any fully polarized input polarization or a depolarizer that depolarizes them in a strongly heterogenous way, may lead to the same PΔ value. In contrast, the Indices of Polarimetric Purity (IPP) are a group of metrics that further synthesize the depolarizing content, taking into account the anisotropic depolarization. In this work, we describe the main physical characteristics of samples achieved by using these IPP through plant samples. Moreover, we show how IPP highlights some structures hidden in regular intensity measurements, highlighting the potential of these metrics for botanical applications.

Programmable supercontinuum laser spectrum generator based on a liquid-crystal on silicon spatial light modulator

Abstract: Supercontinuum (SC) lasers combine a broadband light spectrum with the unique properties of single-mode lasers In this work we present an optical system to spectrally filter a SC laser source using liquid-crystal on silicon (LCoS) spatial light modulators (SLM) The proposed optical system disperses the input laser and the spectrally separated components are projected onto the LCoS-SLM, where the state of polarization of each wavelength is separately modulated Finally, recombining the modulated spectral components results in an output laser source where the spectrum can be controlled dynamically from a computer The system incorporates two branches to independently control the visible (VIS) and the near infrared (NIR) spectral content, thus providing a SC laser source from 450 nm to 1600 nm with programmable spectrum This new ability for controlling at will the wide spectra of the SC laser sources can be extremely useful for biological imaging applications

Unraveling the physical information of depolarizers

Abstract: The link between depolarization measures and physical nature and structure of material media inducing depolarization is nowadays an open question. This article shows how the joint use of two complementary sets of depolarizing metrics, namely the Indices of polarimetric purity and the Components of purity, are sufficient to completely describe the integral depolarizing properties of a sample. Based on a collection of illustrative and representative polarimetric configurations, a clear and meaningful physical interpretation of such metrics is provided, thus extending the current tools and comprehension for the study and analysis of the depolarizing properties of material media. This study could be of interest to those users dealing with depolarization or depolarizing samples.