Laser Propagation in Random Media.

The theory of wave propagation through a medium such as the atmosphere, with random fluctuations of refractive index, is tested by a controlled laser experiment. Simultaneous measurements of the angular spectrum of plane waves and intensity statistics were conducted under practically homogeneous and isotropic conditions. Assuming Taylor's hypothesis (the medium does not change while the wave passes through it), the measured angular spectrum provides the wave structure function, which is sufficient information to determine (in principle) the spatial statistics of the intensity fluctuations. It is this connection that permits the test of the theory. The wave structure function is given by. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). where k is the wave number of the propagating wave, and D(,n)(s,z) = <{n(r,z)-n(r+s,u+z)}('2)> is the refractive index structure function. Here, s = (x,y) is a vector perpendicular to the propagation axis z. Our measurements show that D'(s)(PROPORTIONAL)s('2) for s<s(,i) and D'(s)(PROPORTIONAL)s('5/3) for s>s(,i) as expected from turbulence theory. The inner scale s(,i) depends on atmospheric conditions: we found that s(,i)(DBLTURN)3mm. The measured intensity statistics do not agree with theory based on zero inner scale. The scintillation index is too large in strong scattering and too small in weak scattering. In very strong scattering two scales of intensity fluctuations are observed, as predicted by asymptotic theory. However, neither agrees quantitatively with the theory based on zero inner scale. Including the inner scale in the theoretical calculation of weak scattering produces better agreement with the data. In order to investigate the effects of the inner scale in strong scattering, an analytic solution for the problem of a plane wave passing through a thin screen has been developed. Evaluation of this expression yields intensity statistics that are in qualitative agreement with measurements. A physical picture of the effects of the inner scale on the scintillation process is presented. Enhancement of the scintillation index is explained as focusing of radiation by irregularities which have dimensions comparable to the inner scale. The implications of the thin screen solution to the extended medium problem is discussed.