SPATIAL COHERENCE OF A GAUSSIAN-BEAM WAVE IN WEAK AND STRONG OPTICAL TURBULENCE

A generalized Huygens - Fresnel integral, valid for optical wave propa gation through random inhomogeneities in the presence of any complex o ptical system characterized by an ABCD ray matrix, is used to derive a general expression for the mutual coherence function (MCF) associated with a Gaussian-beam wave in the weak-fluctuation regime. The mean ir radiance obtained from this expression shows excellent agreement with all known asymptotic relations. By introducing a pair of effective bea m parameters THETA(t) and LAMBDA(t) that account for additional diffra ction on the receiving aperture, resulting from turbulence, the normal ized MCF and the related degree of coherence are formally extended int o the regime of strong fluctuations. Results for the normalized MCF fr om this heuristic approach compare well with numerical calculations ob tained directly from the formal solution of the parabolic equation. Al so, the implied spatial coherence length from this analysis in moderat e-to-strong-fluctuation regimes generally agrees more closely with num erical solutions of the parabolic equation than do previous approximat e solutions. All calculations are based on the modified von Karman spe ctrum for direct comparison with established results.