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Physical Review E

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

Kasimir Blomstedt, Tero Setälä, Ari T. Friberg

Published Nov 8, 2005• 1 citation
Publisher

Abstract

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.

DOI
10.1103/physreve.72.056604

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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.
Recent developments in coherence theory have shown that the spectrum of the field produced by a partially coherent source depends not only on the source spectrum but also on the spatial correlations of the source fluctuations.1 In particular, it has been found that some correlations may give rise to frequency shifts of spectral lines.2 Questions have been raised concerning energy conservation in such situations. In this paper we first discuss energy conservation laws pertaining to fields generated by sources of any state of coherence, and we use these fields to demonstrate that conservation laws are not violated when source correlations produce spectral changes in the emitted radiation. We also show that spectral shifts may be generated by isotropic sources, without violating energy conservation.
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Within the validity of the first-order Born approximation, we study the spectrum changes generated by the scattering of light with a tunable spectral degree of coherence (SDOC) from a spatially deterministic medium. Before interacting with the medium, a scalar plane wave is incident upon Young's pinholes, which are employed to produce the incident wave with a tunable SDOC. By assuming that both the scattering potential of the medium and the spectrum of incident plane waves obey Gaussian profiles, the spectrum of the scattered field is derived as a linear combination of Fourier transforms of the scattering potentials. Changes in the spectrum of the scattered field are strongly dependent on the scattering angle, the effective radius of the medium, and Young's pinhole parameter. Moreover, red shifts occurring in the scattered field can be enhanced by either reducing the scattering angle or increasing the effective radius. The results presented here generalize the previous reports, where the light with prescribed functions of the SDOC scattered from a deterministic medium was considered.
This paper presents a review of research, both theoretical and experimental, concerning the influence of coherence properties of fluctuating light sources and of correlation properties of scattering media on the spectra of radiated and scattered fields. Much of this research followed a discovery made in 1986, that the spectrum of light may change on propagation, even in free space. More than 100 papers on this topic have been published to date and many of them are reviewed, or at least mentioned, in this article. After an introduction and a summary of some of the main mathematical results relating to second-order coherence theory of statistically stationary optical fields, spectral changes that may take place on superposing fields produced by two partially correlated sources are discussed. Spectral effects in fields produced by two-dimensional secondary sources and by three-dimensional primary sources are then considered. The section which follows describes spectral changes that may arise when polychromatic light is scattered on media whose physical properties vary randomly either in space and/or in time. A review is also presented of recent research, which has revealed that under certain circumstances the changes in the spectrum of light scattered on random media may imitate the Doppler effect, even though the source, the medium and the observer are all at rest with respect to one another. In the final section a brief review is given of a new emerging technique sometimes called spatial-coherence spectroscopy. It is based on the discovery that it is possible, under certain circumstances, to determine field correlations from spectral measurements.
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