Intensity fluctuations in forward scattering and temporal coherence.
The problem of forward scattering by a nonergodic system of identical particles is considered. The analysis of statistical characteristics of intensity fluctuations arising as a result of finite observation time shows that the mean dispersion of intensity fluctuations can be expressed as a function of the wavelength, diffusion coefficient, scattering angle, and observation time. The relation between the temporal correlation function of the scattered radiation and its second-order intensity moments is established. Quantitative estimates of the effect with the infrared laser beam propagating through fog give results comparable to the dispersion due to refractive index fluctuations in a pure atmosphere (Tatarskii).
Coherence-loss of laser light propagated through simulated coastal waters
Research is currently underway to investigate the physical mechanisms which cause loss of coherence in scattered light. It has been shown that, for Brownian motion of particles in a scattering medium, the coherence of incident light decreases rapidly with path length and diffusion coefficient. Experiments confirm that laser light scattered by a water column loses coherence as a function of path length and water turbidity. An experiment has been performed which measures coherence length versus temperature, optical path length, and water quality. The results are reported here. Based on these results and research into the causes of spectral broadening, experiments are proposed to measure each type of broadening mechanism with a much higher degree of accuracy.
On the Propagation of Mutual Coherence
Section 1 presents a brief history of the development of coherence theory and discusses some of the advantages of Wolf’s general formulation of the theory in terms of Gabor’s analytic signals.Section 2 contains an analysis of the limiting cases of coherence and incoherence, showing for these extremes the form of the mutual coherence function in a quasi-monochromatic field. In particular it is shown that in such a field coherence is characterized by a mutual coherence function which, apart from a simple periodic factor, is expressible as the product of a wave function with its complex conjugate, each factor depending on the coordinates of one point only. It is also shown that an incoherent field cannot exist in free space.The mutual coherence function obeys rigorously two wave equations, and in Sec. 3 these equations are solved with the help of appropriate Green’s functions to find the field produced by a general plane polychromatic source. The solution is simplified by the quasi-monochromatic approximation; and the limiting cases are discussed in detail showing that a coherent source always gives rise to a coherent field, and that a well known theorem of Van Cittert and Zernike represents an approximation to the incoherent limit of the quasi-monochromatic solution.