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Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics

Invariance properties of random pulses and of other random fields in dispersive media

Weijian Wang, E. Wolf

Published Nov 1, 1995• 4 citations
Publisher

Abstract

This paper is concerned with one-dimensional random fields consisting of randomly distributed identical light pulses which propagate in a linear dispersive medium. It is shown that such fields are necessarily stationary and that their power spectra and their longitudinal coherence properties do not change on propagation. The invariance of the power spectrum and of the coherence properties are shown to apply more generally to any one-dimensional stationary field of any state of coherence, propagating in a linear dispersive medium. Some invariance properties of nondiffracting partially coherent fields in dispersive media and of partially coherent fields in lossless fibers are also discussed.

DOI
10.1103/physreve.52.5532

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This paper is concerned with one-dimensional random fields consisting of randomly distributed identical light pulses which propagate in a linear dispersive medium. It is shown that such fields are necessarily stationary and that their power spectra and their longitudinal coherence properties do not change on propagation. The invariance of the power spectrum and of the coherence properties are shown to apply more generally to any one-dimensional stationary field of any state of coherence, propagating in a linear dispersive medium. Some invariance properties of nondiffracting partially coherent fields in dispersive media and of partially coherent fields in lossless fibers are also discussed.
In recent years longitudinal coherence of electron and neutron wave packets has been investigated,1,2 and it was shown that their coherence length remains unchanged even though the length of the wave packet increases on propagation.3,4 As pointed out in Ref. 3, one may expect that a similar result will hold for light that propagates in a dispersive medium. We present some new general theorems relating to an ensemble of identical 1-D optical pulses which propagate in a linear homogeneous medium of any given dispersive properties. In particular we derive a general expression for the mutual coherence function and for the cross-spectral density function of the pulse field in the medium in terms of the Fourier spectrum of a single pulse shape. The analysis brings out clearly the distinction (already noted in Ref. 4) between the coherence length and the coherence time of fields in dispersive media. We also show that the pulse field is spatially completely coherent at each frequency, even though the shape of each pulse changes on propagation. The results have potential application to interferometry with optical pulses and with de Broglie wave packets of elementary particles.
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.
For the nondispersive propagation of waves in one dimension, there is no essential difference between the concepts of longitudinal and temporal coherence, and these terms are often used interchangeably. For the dispersive propagation of waves, such as electrons or neutrons in vacuo, or light in a medium, this is no longer the case. We analyze the coherence properties of a dispersively propagating beam of radiation, and discuss the observation of its spatial and temporal coherence properties by means of a two-beam interferometer.
Optical experiments reveal the fact that real laser pulses have random fluctuations in their amplitude and phase that crucially affect pulse evolution. Statistical optics considers the random nature of light to give a more realistic description of laser pulses behavior. Generation of ultrashort pulses, operating at frequencies close to internal transition frequencies of medium atoms, motivated the growing interest in studying on-resonance light-matter interactions. Moreover, there exist optical phenomena that only occur at optical resonance. Thus, near resonance light-matter interactions are of great importance. In this dissertation, statistical properties of ultrashort pulses propagating in different linear and nonlinear media near optical resonance is studied. Various partially coherent pulses are simulated and effects of initial statistical properties of pulses on their propagation in resonant absorbing and amplifying media are explored. The research performed throughout this thesis leads to following results. 1- Various classes of ultrashort self-similar partially coherent pulses are explored along with closed form expressions for their correlation functions. Also, the evolution of coherence properties of pulses upon their short-distance and long-distance propagation is investigated. 2- Generic partially coherent pulses are simulated and their global and local correlation properties upon propagation in resonant linear absorber media are studied. The evolution of the coherence functions of pulses shows that partially coherent pulses are strongly affected by the medium regarding to their coherence levels. A correlation area theorem is also derived to describe global correlation properties of stochastic pulses propagating in such media. 3- Evolution of small-area pulse coherence properties, propagating in linear amplifiers near optical resonance, is studied. Our simulation results reveal that more coherent pulses are amplified more effectively by the medium than are their less coherent counterparts. 4- Propagation of partially coherent pulses in resonant, inhomogenously broadened nonlinear media is simulated. Stochastic pulses of different coherence levels are generated to investigate effects of initial properties of pulses on their long-term evolution. We also provide evidence of self-induced transparency phenomena and soliton formation for relatively coherent pulses. Also, evolution of the coherence functions reveal that low-coherence pulses lose their coherence level upon propagation faster than do highly coherent pulses.
The propagation of pulses through dispersive media was investigated by solving Maxwell's equations without any approximation. It was demonstrated that the superluminal or subluminal pulse propagation originates essentially from the interference of the different frequency components of the pulse. The coherence of the different frequency components plays an important role in superluminal propagation. When the pulse becomes partially coherent, the propagation changes from superluminal to subluminal. Since superluminality always exists as long as the spectrum of the coherent pulse is within the anomalously dispersive region whatever the shape of the pulse is, such a coherent pulse must be transmitted superluminally.
The concept of the Wigner distribution function is applied to stochastic signals, in particular to partially coherent light. Relations between the Wigner distribution function, the power spectrum and related subjects are formulated. Some inequalities for the Wigner distribution function are derived; one of these inequalities leads to the definition of an overall degree of coherence. The overall degree of coherence of a stochastic signal remains invariant when the signal propagates through a lossless linear system.