Dept. of Applied Physics
Oregon Graduate Center
This study is concerned with the propagation of visible and middle infra-red laser radiation through the turbulent atmosphere. An experimental approach was used to investigate the spatial and temporal disturbances induced on a laserâs intensity distribution. The measurements were made for propagation over very homogeneous paths so that the results could be compared directly with available theories. Included are unique measurements of (I) the scintillation of quasi-point laser sources for large integrated-path turbulence, (II) the propagation effects of a large, passive, focused beam wave, and (III) the characteristics of a large, focused beam wave which has had atmospherically induced wander cancelled through the use of a reciprocity tracker. The large integrated-path turbulence measurements allowed detailed examination of saturation phenomena. The log-amplitude variance at 10.6 Âµm was observed to saturate for the first time. Further, the covariance function at 4880 Ã was discovered to be drastically modified by conditions of strong turbulence. It was experimentally established that the correlation at short distances was drastically reduced with the onset of saturation. Concurrently, a residual correlation appeared at large distances. This change in the covariance function resulted in aperture averaging for "point" receivers, and caused the bandwidth of the electronic processing equipment (which was quite adequate under conditions of weak turbulence) to be much too narrow in this case. Taking into consideration the spatial and temporal filtering, the experimental measurements were found to be in good agreement with recent theoretical models for optical propagation subject to strong atmospheric perturbations. When the large, passive focused beam wave was studied at 4880 Ã , no advantage was found over the fade characteristics of a quasi-point source. In fact, during weak turbulence the beam wave fading was worse than that of the point source. This resulted from atmospherically induced wander which randomly steered the beam (nearly diffraction-limited over a 1.4 km path) about the receiver in the target plane. This fact points out a serious deficiency in the first-order theoretical approaches to beam wave propagation: the wander phenomenon is not included in their descriptions. In addition, later experiments showed that scintillation measurements on the main lobe of a beam wave provided negligible transmitter aperture averaging, even when wander was cancelled. Hence it is suggested that large, beam wave transmitter aperture averaging cannot be achieved at visible wavelengths for reasonable apertures and path lengths. Subsequently, a more comprehensive, unified theory was investigated experimentally. This theory is the extended Huygens-Fresnel method, and its analysis applies not only to beam wave propagation but also to imaging through turbulence and the operation of optical heterodyne receivers in the atmosphere. Data related to the on-axis mean irradiance and fading for the passive beam and for the beam with wander cancellation was collected at 6328 Ã over a 1.6 km path. Comparison of this data with detailed numerical and asymptotic evaluations of the extended Huygens- Fresnel predictions confirmed fundamental dependencies on the ratio of the Gaussian beam diameter to the transverse coherence scale size, D(subscript g) /p(subscript o). The results suggest that D (subscript g) /p (subscript o) of order unity is a special combination of beam-geometry and turbulence conditions that generates maximum beam wandering. Consequently, tracking provided (1) a maximum increase in the on-axis mean irradiance and (2) a maximum decrease in the fading during this condition. Furthermore, this experiment verified that the on-axis mean irradiance and fading are nearly universal functions of D (subscript g) /p (subscript o). These results are extremely useful when considering applications utilizing lasers, imaging, or optical heterodyne receivers over horizontal and vertical paths through the atmosphere.
Dunphy, James R., "Propagation of laser radiation through atmospheric turbulence" (1974). Scholar Archive. 36.