SCINTILLATION OF INITIALLY CONVERGENT GAUSSIAN BEAMS IN THE VICINITY OF THE GEOMETRIC FOCUS

As an initially convergent Gaussian beam enters the vicinity of the ge ometric focus, weak fluctuation theory predicts a drop in the longitud inal component of the log-irradiance variance and an increase in the r adial component off the beam center. The phenomenon intensifies as the beam nears the geometric focus, also with decreasing magnitude of the focusing parameter. Precisely at the geometric focus, first-order wea k fluctuation theory further predicts that as the initial beam size co ntinues to increase, the longitudinal component of the log-irradiance variance decreases toward zero, while the radial component increases w ithout bound. This eventually entails a rapid change in scintillation across the beam surface that has yet to be verified experimentally, to our knowledge. We demonstrate that when diffraction like effects prod uced by optical turbulence are introduced, predicted log-irradiance va riance exhibits such extremes in behavior only in the case of weak tur bulence. Also, at the exact geometric focus, scintillation does not va nish with increasing initial beam size but achieves a value determined by and growing with turbulence strength and nearly independent of ini tial beam size. The radial component of log-irradiance quickly loses s ignificance as turbulence strength increases. In fact, general extrema l behavior of the log-irradiance variance in the vicinity of the geome tric focus is drastically curtailed. Differences across the diffractiv e beam surface become small and exhibit only a modest dependence on th e initial beam size. (C) 1995 Optical Society of America