David Pereda Cubián

Characterization of depolarizing optical media by means of the entropy factor: application to biological tissues

Polarized light imaging is a potential tool to obtain an adequate description of the properties of depolarizing media such as biological tissues. In many biomedical applications, for instance, dermatology, ophthalmology, or urology, imaging polarimetry provides a noninvasive diagnosis of a wide range of disease states, and, likewise, it could be applied to the study of internal tissues though the use of endoscopes that use optical fibers. We introduce an algebraic method, based on the Mueller-coherence matrix, for a clearer analysis of the polarization characteristics of depolarizing media via the entropy factor. First-order errors introduced by the measurement system are corrected. Entropy defines three kinds of media according to their depolarizing behavior, and several examples corresponding to each region are shown. The calculation of this factor provides clearer information than that provided by the traditional Mueller matrix in the analysis of biological tissue properties by polarization measurement techniques.

Variation of the Pauli matrices coefficients in a PCA system under non-desired effects for ellipsometric applications

The theory of Pauli matrices decomposition is described and its application to the analysis of non-ideal devices, which display first-order effects like optical activity or dichroism, has been developed. The analysis is extended to a Polarizer-Compensator-Analyser (PCA) system for ellipsometric applications. This analysis shows that the presence of non-desired effects into the system produces a high influence on the birefringent and dichroical behaviour, represented by the coefficients of the decomposition.

Burn depth determination using high-speed polarization-sensitive Mueller optical coherence tomography with continuous polarization modulation

National Health Interview Survey (NHIS) estimates more than 1.1 million burn injuries per year in the United States, with nearly 15,000 fatalities from wounds and related complications. An imaging modality capable of evaluating burn depths non-invasively is the polarization-sensitive optical coherence tomography. We report on the use of a high-speed, fiber-based Mueller-matrix OCT system with continuous source-polarization modulation for burn depth evaluation. The new system is capable of imaging at near video-quality frame rates (8 frames per second) with resolution of 10 μm in biological tissue (index of refraction: 1.4) and sensitivity of 78 dB. The sample arm optics is integrated in a hand-held probe simplifying the in vivo experiments. The applicability of the system for burn depth determination is demonstrated using biological samples of porcine tendon and porcine skin. The results show an improved imaging depth (1 mm in tendon) and a clear localization of the thermally damaged region. The burnt area determined from OCT images compares well with the histology, thus proving the systems potential for burn depth determination.

Polarization recording of dynamic holograms in saturable absorbers

The potentialities of diffraction methods for wavefront transformations may be considerably widened due to the use of nonlinear recording of dynamic holograms enabling multiwave mixing in media with the fifth and higher order nonlinearities. There is a great variety of such media including the resonant ones for which the presence of higher order nonlinearities is conditioned by the absorption saturation effect and transitions between different excited states of the molecules. However, in the majority of previous studies of multiwave mixing the resonant medium approximation has been used disregarding the induced anisotropy effect. This work presents a theoretical model and experimental studies of the energy efficiency of multiwave mixing in complex molecular media exhibiting higher order nonlinearities, in two cases: when the nonlinear cavity is introduced in the Fabry-Perot interferometer (IFP), and without it. Owing to the use of different combinations of mutually aligning polarizations for interacting waves, one is enabled to determine the contribution into the interaction efficiency by various dynamic gratings and nonlinearity mechanisms. The role of polarization gratings resultant from spatial modulation of the light field polarization state at the orthogonal polarization of the hologram recording waves has been established. Comparisons between the contributions of the above gratings and “normal” gratings recorded by identically polarized light beams into the process of multiwave mixing have demonstrated that the relation of these contributions is dependent on the intensity of interacting waves, and also it has been found that polarization of the diffracted wave is dependent on the diffraction order.

Application of the Quaternions theory to the analysis of the semimagnetic semiconductor microcavity

The Quaternion theory is applied to a Semimagnetic Semiconductor Microcavity, a Fabry-Perot cavity in which a Quantum Well has been placed in its center. The Optical Scattering Matrix calculus and the Mason’s rule are applied to characterize the device, and with the employment of the Pauli decomposition, the different phenomena, like linear and circular birefringence and dichroism, are measured. This analysis is developed for different configurations of the microcavity under the influence of an external magnetic field, which produces very important changes in the polarization properties of the light propagating into the microcavity.

Four-wave mixing in nonlinear interferometer Fabry-Perot with saturable absorbers

In this work the different schemes of propagation and interaction of the light beams in nonlinear Fabry-Perot interferometer have been studied theoretically and experimentally. Degenerate and non-degenerate four-wave mixing have been realized in the cavity of Fabry-Perot type using Rhodamine-6G dye and polymethine dye 3274U solution as saturable absorber. The diffraction efficiency of intracavity dynamic grating has been studied in dependence on intensity of interacting beams and parameters of resonator. The theoretical model of the processes of intracavity degenerate and non-degenerate four-wave mixing has been developed and applied to the analysis of the efficiency of light beams conversion by mean of Bragg diffraction from intracavity dynamic gratings. For theoretical description of typical experimental situations we used the round-trip model of nonlinear interferometer adapted for the geometry of degenerate four-wave mixing, which can be realized in the scheme of symmetrical oblique incidence of pump, signal and probe beams to the front and back mirrors of cavity.

Four-wave mixing in microresonators with resonance and thermal nonlinearities

The results of theoretical and experimental investigations into four-wave mixing in microresonators with resonant and thermal nonlinearities are presented. Theoretical models of dynamically-induced optical gratings have been derived in two cases of the light beams interaction: when a nonlinear medium is incorporated into Fabry-Perot interferometer and for the off-cavity configuration. Experimental realization of intracavity four-wave mixing performed using ethanol solution of Rodamine 6G as a nonlinear medium. It is shown that the use of nonlinear interferometer increases significantly an efficiency of dynamic optical gratings. The potentialities for obtaining of high efficiency conversion of optical information owing the spectroscopic properties of multilevel resonant media are discussed.

Formation of holographic gratings and dynamics of four-wave mixing in nonlinear microresonators

The results of theoretical and experimental investigations into degenerate and non-degenerate four-wave mixing in Fabry-Perot microresonator with dye solution as a nonlinear medium are presented. It is shown that the efficiency of holographic methods for wave-front and frequency conversion may be considerably increased due to the use of recording of dynamic holograms in nonlinear interferometers. Spatial-temporal transformation of light beams for different configurations of interaction has been performed and the methods for control of energy efficiency of intracavity dynamics gratings are discussed.

Frequency conversion of coherent images by thin and volume dynamic holograms

One of the main advances of dynamical holography is the possibility to operate with coherent images in real-time experimental situation. Non-degenerate four-wave mixing has some advantages among different holographic methods because it gives the possibility to perform visualization of infra-red images or to convert visible radiation into ultra-violet region. In the work we study different schemes of frequency conversion of coherent images by thin and volume dynamic holograms nonlinearly formed in multilevel resonance media in conditions of non-degenerate four-wave mixing. The theoretical model of dynamic gratings formation in dye solutions based on coupled-wave equations has been developed. Experimental realization of frequency conversion of coherent images from infra-red to the visible and between different wavelengths of visible region has been performed. It has been shown that the criterion of hologram thickness is dependent on the optical density of medium and intensity of input radiation. The diffraction efficiency of dynamic gratings has been measured in conditions of optical bleaching and in the mode of strong absorption, enabling recording of thin gratings.

Formation of polarization dynamic holograms in saturable absorbers and frequency conversion of coherent images

The schemes of degenerate and nondegenerate multiwave mixing in saturable absorbers (Rhodamine 6G and polymethine 3274 U dyes) exhibiting higher-order nonlinearities have been analyzed experimentally. The role of polarization gratings resultant from spatial modulation of the light field polarization state at the orthogonal polarization of the hologram recording waves has been established. Also, it has been found that polarization of the diffracted wave is dependent on the diffraction order. The frequency conversion of coherent images from infrared to the visible and between different wavelengths of the visible region has been realized experimentally.