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Optics Letters

Spectral shifts produced by source correlations

Dean Faklis, G. Michael Morris

Published Jan 1, 1988• 91 citations
Publisher

Abstract

Statistical correlations of the light emitted by a partially coherent source can produce frequency shifts in the spectrum observed in the far field if the correlation function of the emitted radiation does not satisfy a certain scaling law. A Fourier achromat is used to generate a secondary source in which the degree of spectral coherence is independent of wavelength; i.e., it violates the scaling law. The spectrum detected in the far zone of the secondary source is, in general, found to be displaced in frequency and distorted relative to the spectrum measured at the secondary source. The displacement can be toward the higher frequencies or the lower frequencies depending on the direction of observation.

DOI
10.1364/ol.13.000004

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Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
Statistical correlations of the light emitted by a partially coherent source can produce frequency shifts in the spectrum observed in the far field if the correlation function of the emitted radiation does not satisfy a certain scaling law. A Fourier achromat is used to generate a secondary source in which the degree of spectral coherence is independent of wavelength; i.e., it violates the scaling law. The spectrum detected in the far zone of the secondary source is, in general, found to be displaced in frequency and distorted relative to the spectrum measured at the secondary source. The displacement can be toward the higher frequencies or the lower frequencies depending on the direction of observation.
We recently showed that the spectrum of light emitted by a source depends not only on the spectrum of the source distribution but also on the degree of spectral coherence of the source fluctuations. In this note we show that with a degree of spectral coherence of certain kind, specified by two parameters, the spectrum of the emitted light will be displaced relative to the source spectrum. The displacement will be either toward the lower or toward the higher frequencies, depending on the choice of the parameters.
It has recently been demonstrated both theoretically1-3 and experimentally4 that contrary to general belief, the spectrum of light emitted by a steady-state source depends, in general, not only on the source spectrum but also on the degree of correlation between the source fluctuations. When the source spectrum consists of a single line centered at frequency wo , the source correlation may cause the spectrum of the emitted light to be a line centered on a lower frequency < wo or at a higher frequency w'o > wA, depending on the particular form of the degree of correla?ion. Thus source correlations provide a new physical mechanism for redshifts or blueshifts of spectral lines. This newly discovered phenomenon is obviously-of particular interest for astronomy, where line shifts observed in the spectra of radiation from stellar objects have up to now always been assumed to be caused either by the motion of the stellar object relative to the observer (the Doppler effect) or as being due to gravitation. Examples of redshifted lines due to source correlations are shown in Fig. 1.
Abstract : It is generally taken for granted that the spectrum of light does not change on propagation. This belief is implicit in all spectroscopy. In the last few years it was discovered that while such spectral invariance holds for radiation from commonly used sources it does not hold, in general, because the spectrum of emitted radiation depends not only on the source spectrum but also on the coherence properties of the source. It was found that in some cases the induced changes may be appreciable, possibly resulting in frequency shifts of spectral lines. It has also been demonstrated that similar spectral changes may be produced by scattering on random media whose dielectric susceptibility fluctuations are appropriately correlated. Very recently it was shown that under certain circumstances the frequency shifts generated by dynamic scattering may imitate the Doppler effect, even though the source, the scatterer and the observer are all at rest relative to each other. (emk)
It is generally taken for granted that the spectrum of light does not change on propagation. This belief is implicit in all spectroscopy. In the last few years it was discovered that while such spectral invariance holds for radiation from commonly used sources it does not hold, in general, because the spectrum of emitted radiation depends not only on the source spectrum but also on the coherence properties of the source. It was found that in some cases the induced changes may be appreciable, possibly resulting in frequency shifts of spectral lines. It has also been demonstrated that similar spectral changes may be produced by scattering on media whose dielectric susceptibility is appropriately correlated.