Biomedical subjects

Ari T Friberg

Publications and source records attributed to Ari T Friberg.

At least 19 recordsLinked to original sources

PubMed2006

Contrasts of Stokes parameters in Young's interference experiment and electromagnetic degree of coherence.

We analyze the modulation of the Stokes parameters in Young's two-pinhole interference experiment with a random electromagnetic beam. We demonstrate that the electromagnetic (spectral) degree of coherence put forward in Opt. Lett.29, 328 (2004) [or its space-time analog in Opt. Express 11, 1137 (2003)] is physically related to the contrasts of modulation in the four Stokes parameters. More explicitly, the electromagnetic degree of coherence is a measure of both the visibility of the intensity fringes and the modulation contrasts of the three polarization Stokes parameters. We also show that by using suitable wave plates the modulation in any Stokes parameter can be transformed into the form of intensity variation, and hence the electromagnetic degree of coherence can be obtained experimentally by four visibility measurements.

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PubMed2006

Stokes parameters and polarization contrasts in Young's interference experiment.

We derive a spectral interference law that governs the behavior of the four Stokes parameters in Young's two-pinhole experiment with a random electromagnetic beam. In addition to the visibility of intensity fringes, we introduce three new contrast parameters that describe the interference-induced changes in the field's state of partial polarization. The polarization modulation depends on the electric field correlations at the pinholes and is closely related to the two-point Stokes parameters. The results are expected to be particularly useful in polarization interferometry and electromagnetic coherence theory. The formalism is demonstrated with specific examples.

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PubMed2006

Spatial coherence effects in light scattering from metallic nanocylinders.

We study the scattering of a partially coherent electromagnetic beam from metallic nanocylinders and analyze the effects of plasmon resonances on the coherence and polarization properties of the optical near field. We employ the coherent-mode representation for the incident field and solve the scattering problem independently for each mode by using a boundary-integral method. Our results show that the plasmon resonances may significantly affect the coherence and polarization characteristics of the near field and that partial coherence influences the energy flow in nanocylinder arrays.

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PubMed2006

Quasi-stationary plane-wave optical pulses and the van Cittert-Zernike theorem in time.

We study the properties of quasi-stationary, partially coherent, plane-wave optical pulses in the space-time and space-frequency domains. A generalized van Cittert-Zernike theorem in time is derived to describe the propagation of the coherence function of quasi-stationary pulses. The theory is applied to rectangular pulses chopped from a stationary light source, and the evolution characteristics of such pulse trains with different states of coherence are discussed and illustrated with numerical examples.

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PubMed2005

Effect of absorption on the spatial coherence in scalar fields generated by statistically homogeneous and isotropic sources.

We investigate the behavior of the spectral degree of coherence of fluctuating scalar wave fields generated by statistically homogeneous and isotropic sources within absorbing media. In the limit of negligible losses, the degree of coherence is shown to be of a known universal form (sinc function), whereas in the limit of infinite losses it is shown to asymptotically approach the degree of coherence of the source. The intermediate cases are studied for fields produced by sources with Gaussian and sinc-type coherence functions. These studies show that the concept of negligible losses is meaningful only when defined with respect to specific source distribution statistics, and that even for arbitrarily small nonzero losses, the coherence length of the field can be arbitrarily short.

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PubMed2005

Influence of the short-range coordination order of erbium ions on excitation migration and upconversion in multicomponent glasses.

Applying a pair-correlation function of erbium ions that is known from x-ray absorption fine structure spectroscopy, we develop a new statistical model of migration-assisted upconversion in erbium-doped glasses. The results of modeling show that short-range coordination order of erbium ions contributes to the upconversion rate for both low and high population inversion. In addition, we report the methods of suppression of the short-range order of erbium ions in multicomponent glasses.

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PubMed2005

Faraday rotation in a two-dimensional photonic crystal with a magneto-optic defect.

We study the magneto-optic (MO) Faraday rotation in a two-dimensional square-lattice photonic crystal with a central MO defect layer in the optical wavelength range. We show that when a TM plane wave is incident upon a photonic crystal, an enhancement of Faraday rotation takes place in a region where a resonance peak appears in the photonic bandgap. In this region the mode conversion is high.

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PubMed2005

Coherent-mode representation of a statistically homogeneous and isotropic electromagnetic field in spherical volume.

It is known that statistically stationary, homogeneous, and isotropic source distributions generate, in an unbounded low-loss medium, an electromagnetic field whose electric cross-spectral density tensor is proportional to the imaginary part of the infinite-space Green tensor. Using the recently established electromagnetic theory of coherent modes, we construct, in a finite spherical volume, the coherent-mode representation of the random electromagnetic field having this property. The analysis covers the fundamental case of blackbody radiation but is valid more generally; since a thermal equilibrium condition is not invoked, the electromagnetic field may have any spectral distribution. Within the scalar theory of coherent modes, which has been available for more than two decades, the analogous formulation results in the first explicit three-dimensional coherent-mode representation.

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PubMed2005

Electromagnetic coherence theory of laser resonator modes.

A theory of open laser resonators is formulated within the framework of the electromagnetic coherence theory. It is shown that if only one Fox-Li mode contributes to the field at a given frequency, then the field at that frequency is necessarily completely coherent in view of the space-frequency counterpart of the recently introduced degree of coherence of electromagnetic fields [Opt. Express 11, 1137 (2003)]. It is also shown that the relation between the number of Fox-Li modes and the new degree of coherence is analogous to the relation established in the scalar theory of laser resonator modes. Difficulties that arise with the formerly introduced visibility-based definition of the electromagnetic degree of coherence are briefly discussed.

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PubMed2005

Degree of polarization in tightly focused optical fields.

We analyze the degree of polarization of random, statistically stationary electromagnetic fields in the focal region of a high-numerical-aperture imaging system. The Richards-Wolf theory for focusing is employed to compute the full 3 x 3 electric coherence matrix, from which the degree of polarization is obtained by using a recent definition for general three-dimensional electromagnetic waves. Significant changes in the state of partial polarization, compared with that of the incident illumination, are observed. For example, a wave consisting of two orthogonal and uncorrelated incident-electric-field components produces rings of full polarization in the focal plane. These effects are explained by considering the distribution of the spectral densities of the three electric field components as well as the correlations between them.

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PubMed2004

Intensity fluctuations and degree of polarization in three-dimensional thermal light fields.

The normalized intensity fluctuations of arbitrary electromagnetic wave fields obeying Gaussian statistics are expressed in terms of the three-dimensional degree of polarization. This general formulation implies an important physical result concerning the polarization of planar fields and the dimensionality of the formalism. The results are expected to be particularly useful in intensity interferometry.

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PubMed2004

Simple lens axicon.

We present the design of a cemented doublet-lens axicon made from spherical surfaces only. Compared with diffractive axicons, refractive cone axicons, and earlier lens axicons with aspheric surfaces, this element is inexpensive and easy to manufacture even with large apertures. The lens axicon is based on the deliberate use of the spherical aberration of the surfaces. The design principles of the element and its characterization, numerically and experimentally, are presented in detail. Although performance was traded for simplicity and robustness, the results show that the lens axicon has the main axicon properties: a narrow, extended line focus of relatively constant width.

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PubMed2004

Complete electromagnetic coherence in the space-frequency domain.

We show that the following properties of a random electromagnetic field are equivalent: (i) the field is spatially completely coherent in the sense of the recently introduced electromagnetic degree of coherence and (ii) the electric cross-spectral density tensor factors in the two spatial variables.

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PubMed2004

Theory of partially coherent electromagnetic fields in the space-frequency domain.

We construct the coherent-mode representation for fluctuating, statistically stationary electromagnetic fields. The modes are shown to be spatially fully coherent in the sense of a recently introduced spectral degree of electromagnetic coherence. We also prove that the electric cross-spectral density tensor can be rigorously expressed as a correlation tensor averaged over an appropriate ensemble of strictly monochromatic vectorial wave functions. The formalism is demonstrated for partially polarized, partially coherent Gaussian Schell-model beams, but the theory applies to arbitrary random electromagnetic fields and can find applications in radiation and propagation and in inverse problems.

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PubMed2003

Spatial correlations and degree of polarization in homogeneous electromagnetic fields.

We consider stationary electromagnetic fields modeled as superpositions of unpolarized and angularly uncorrelated plane waves and show that in anisotropic case the electric cross-spectral tensor is proportional to the imaginary part of the Green tensor. This is as for blackbody radiation, but here the field need not be in thermal equilibrium. We also evaluate the degree of polarization for a homogeneous but nonisotropic field for which the plane waves propagate within a cone of angles. The results are compared with the known polarization properties of blackbody radiation.

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PubMed2003

Diffractive axicons in oblique illumination: analysis and experiments and comparison with elliptical axicons.

Axicons in oblique illumination produce broadened focal lines, a problem, e.g., in scanning applications. A compact mathematical description of the focal segment is presented, for the first time, to our knowledge, and the results are compared with elliptical axicons in normal illumination. In both cases, analytical expressions in the form of asteroid curves are obtained from asymptotic wave theory and caustic surfaces. The results are confirmed by direct diffraction simulations and by experiments. In addition we show that at a fixed angle an elliptical axicon can be used to compensate for the adverse effects of oblique illumination.

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PubMed2002

Degree of polarization in near fields of thermal sources: effects of surface waves.

We introduce the concept of degree of polarization for electromagnetic near fields. The approach is based on the generalized Stokes parameters that appear as expansion coefficients of the 3 x 3 coherence matrix in terms of the Gell-Mann matrices. The formalism is applied to optical near fields of thermally fluctuating half-space sources with particular interest in fields that are strongly polarized owing to resonant surface plasmons or phonons. This novel method is particularly useful when assessing the full vectorial characteristics of random evanescent fields, e.g., for near-field spectroscopy and polarization microscopy.

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PubMed2002

Fabrication of advanced fiber Bragg gratings by use of sequential writing with a continuous-wave ultraviolet laser source.

We present a novel scheme based on sequential writing for fabrication of advanced fiber Bragg gratings. As opposed to earlier sequential methods this technique uses a cw UV laser source and allows for very precise control and repetitivity of the formation of the gratings. Furthermore it is possible to use high average irradiances without destroying the fiber, resulting in considerable reduction in fabrication time for complex gratings. The method has been applied to several test gratings, which proved its versatility and quality.

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