The aperture averaged scintillation of the intensity of a Gaussian laser beam propagated through strong optical turbulence and reflected by various targets

The normalized variance of irradiance of an optical signal propagating through atmospheric turbulence is a measure of the strength of intensity fluctuations, called scintillation, of such signal as seen by a point receiver. The strength of these fluctuations reflects the impact of the optical turbulence on the beam and consequently on the information being transmitted by the optical signal. For a double passage situation, such as associated with laser radar, the scintillation of the reflected beam depends not only on the atmospheric characteristics but also on the beam, receiver and reflector properties as well.; Assuming a Gaussian-beam wave incident on a point target, and using our recently developed filter function, we have calculated the scintillation of the reflected beam intensity as measured by a point receiver and compared our results with experimental data. The fact that the actual receiver is usually of a finite size that exceeds the correlation length of the reflected beam, especially in strong turbulence, leads to a reduction of the received scintillation known as aperture averaging. This effect was previously studied for plane and spherical waves. Here this effect is considered in detail and calculated as a function of the diameter of the receiving aperture for a Gaussian beam reflected by a point target.; The scintillation of a spherical wave reflected by a rough surface is calculated and light is shed on the resemblance between the scintillation due to rough surface reflector and that due to an extended receiver. Moreover, experimental results indicate that the scintillation of a laser beam intensity reflected from a rough surface behaves like that of an extended incoherent source.; The new theory employed in this work utilizes a new filter function that results from adapting a modulation idea in which we assume that the intensity of the propagating laser beam is a product of two quantities; one is related to the large scale eddies and the other is related to the small scale eddies. The new filter function consists of two separate parts, each related to only one of the above scales.