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Soviet Journal of Quantum Electronics

Detection of narrow "laser" lines masked by spatially inhomogeneous broadening in radiation emitted from stellar atmospheres

N. N. Lavrinovich, Vladilen S Letokhov

Published Sep 30, 1976• 3 citations

Abstract

Methods for detection of narrow spectral lines in the radiation emitted by a distant source, masked by a spatially inhomogeneous frequency shift, are considered. An analysis is made of the possibility of using, in principle, the Brown-Twiss correlation method and of the optical heterodyne method with independent heterodynes for this purpose. The normalized mutual correlation function is calculated in the plane of two receivers used to investigate a distant source characterized by a spatially inhomogeneous broadening of the spectrum. Estimates are given of the signal/noise ratio for both methods when applied to sources in the form of laser regions in a stellar atmosphere in the case of a strong rotation-induced spatial broadening of the spectrum.

DOI: 10.1070/qe1976v006n09abeh011839

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Methods for detection of narrow spectral lines in the radiation emitted by a distant source, masked by a spatially inhomogeneous frequency shift, are considered. An analysis is made of the possibility of using, in principle, the Brown-Twiss correlation method and of the optical heterodyne method with independent heterodynes for this purpose. The normalized mutual correlation function is calculated in the plane of two receivers used to investigate a distant source characterized by a spatially inhomogeneous broadening of the spectrum. Estimates are given of the signal/noise ratio for both methods when applied to sources in the form of laser regions in a stellar atmosphere in the case of a strong rotation-induced spatial broadening of the spectrum.

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.

It has recently been shown that correlations between source fluctuations throughout the source region can lead to frequency shifts of spectral lines.1,2 We consider the more general problem of modifying the spectrum of the emitted radiation by making a suitable choice of the degree of spatial coherence of the source. To illustrate the potential of this method we consider a simple configuration of two small sources emitting light with identical spectra. We demonstrate that some source correlations can generate multiple spectral lines from a single spectral line of either a Gaussian or Lorentzian profile. Spectral line-shape parameters such as width, amplitude, center frequency, and functional form can also be controlled.

The potentialities of the correlation technique with respect to extracting the homogeneous line profile from an inhomogeneously broadened spectrum are analyzed by computer simulation. The signal-to-noise ratio for autoconvolution of the homogeneous line profile, obtained by statistical processing of an array of realizations of the inhomogeneously broadened spectrum for different experimental conditions and different values of parameters of the system is calculated. A general form of the dependence of the signal-to-noise ratio on the magnitude of the accumulated signal is established. An interpolation formula that expresses this dependence in an analytical form is obtained and a simple procedure is proposed that allows one to plot this dependence for specific experimental situations.

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.