Quantitative color analysis for capillaroscopy image segmentation.

This communication introduces a novel approach for quantitatively evaluating the role of color space decomposition in digital nailfold capillaroscopy analysis. It is clinically recognized that any alterations of the capillary pattern, at the periungual skin region, are directly related to dermatologic and rheumatic diseases. The proposed algorithm for the segmentation of digital capillaroscopy images is optimized with respect to the choice of the color space and the contrast variation.

Optical propagation in uniaxial crystals orthogonal to the optical axis: paraxial theory and beyond.

We describe monochromatic light propagation in uniaxial crystals by means of an exact solution of Maxwell's equations. We subsequently develop a paraxial scheme for describing a beam traveling orthogonal to the optical axis. We show that the Cartesian field components parallel and orthogonal to the optical axis are extraordinary and ordinary, respectively, and hence uncoupled.

Soliton-based Y-branch in photorefractive crystals induced through dispersion-shifted optical fiber.

We experimentally demonstrate for what is believed to be the first time that a dispersion-shifted fiber can be used to electro-optically induce a soliton Y-branch structure in a photorefractive centrosymmetric paraelectric crystal (potassium lithium tantalate niobate). The application of a nonstationary external bias field enables us to stabilize the spatially partially coherent behavior of the optical beam at the fiber output. Furthermore, we show the switching capabilities of this soliton-based device in the optical communication field guiding a probe beam at a nonphotorefractive wavelength (1557 nm).

Angular momentum dynamics of a paraxial beam in a uniaxial crystal.

The conservation law governing the dynamics of the radiation angular momentum component along the optical axis (z axis) of a uniaxial crystal is derived from Maxwell's equations; the existence of this law is physically related to the rotational invariance of the crystal around the optical axis. Specializing the obtained general expression for the z component of the angular momentum flux to the case of a paraxial beam propagating along the optical axis, we find that the expression is the same as the corresponding one for a paraxial beam propagating in an isotropic medium of refractive index n(o) (ordinary refractive index of the crystal); besides, we show that the flux is conserved during propagation and that it decomposes into the sum of an intrinsic and an orbital contribution. Investigating their dynamics we demonstrate that they are coupled and, during propagation, an exchange between them exists.

Circularly polarized beams and vortex generation in uniaxial media.

We deduce the expressions for the two circularly polarized components of a paraxial beam propagating along the optical axis of a uniaxial crystal. We find that each of them is the sum of two contributions, the first being a free field and the second describing the interaction with the opposite component. Moreover, we expand both components as a superposition of vortices of any order, thus obtaining a complete physical picture of the interaction dynamics.

Paraxial propagation along the optical axis of a uniaxial medium.

An approach for describing paraxial propagation of light along the optical axis of a uniaxial medium is introduced. Contrary to previous theoretical schemes, our approach directly deals with the propagation of the whole optical field without resorting to the standard decomposition into ordinary and extraordinary parts, thus avoiding some related mathematical difficulties. A paraxial equation governing the field propagation has been derived, and its formal solution has been deduced.

Energy exchange between the Cartesian components of a paraxial beam in a uniaxial crystal.

The evolution of the optical power associated with the Cartesian components of a paraxial beam propagating along the optical axis in a uniaxial crystal is investigated. The energy exchange is found to undergo a saturation that is due to both diffraction and coupling between the chi- and gamma-field components; for linearly polarized circularly symmetric input beams, the asymptotic power exchange always amounts to a quarter of the total power. The general results are applied to the case of astigmatic Gaussian beams, which admits of a fully analytical description.

Laguerre-Gauss and Bessel-Gauss beams in uniaxial crystals.

A simple correspondence between the paraxial propagation formulas along the optical axis of a uniaxial crystal and inside an isotropic medium is found in the case of beams with linearly polarized circularly symmetric boundary distributions. The electric fields of the ordinary and the extraordinary beams are related to the corresponding expressions in a medium with refractive index n(o) and n(e)2/n(o), where n(o) and n(e) are the ordinary and the extraordinary refractive indexes, respectively. Closed-form expressions for Laguerre-Gauss and Bessel-Gauss beams propagating through an anisotropic crystal are given.

Stokes parameters of a Gaussian beam in a calcite crystal.

We derive the analytical expression of the Stokes parameters corresponding to a Gaussian beam propagating along the optical axis of a uniaxial crystal, pointing the simultaneous effects of anisotropy and diffraction out. The theoretical results are compared with experimental measurements at the output of a calcite crystal, showing a good agreement.

Nonparaxial description of reflection and transmission at the interface between an isotropic medium and a uniaxial crystal.

Angular spectra of reflected and transmitted fields, induced by an arbitrary electromagnetic beam passing through the planar interface between a homogeneous medium and a uniaxially anisotropic medium, are derived and related to the incident medium. By using these formulas, we obtain the expressions for paraxial and slightly nonparaxial fields. The reflected paraxial field is related to the incident one by means of Fresnel relations; the transmitted paraxial field is the superposition of an ordinary and an extraordinary beam, multiplied by the Fresnel coefficient.

Propagation of cylindrically symmetric fields in uniaxial crystals.

We investigate the paraxial propagation along the optical axis of a uniaxially anisotropic crystal of a general paraxial beam whose boundary Cartesian components possess cylindrical symmetry. This property allows us to obtain expressions whose dependence on the azimuth angle phi (in cylindrical coordinates) is fully described and very simple. We also find that the beam loses its boundary cylindrical symmetry during propagation, as a consequence of medium anisotropy.