Coherence degradation of a speckle field and turbulence effects on Fourier telescopy imaging system

摘要

We investigate a speckle sensitivity of the receiver in the Fourier telescopy imaging system that has been proposed for high-resolution imaging of Geostationary (GEO) satellites using laser illumination. A theory and numerical results are presented for a speckle bias of the triple-product, which is used in a phase-closure-procedure to remove turbulence-induced low frequency phase distortions on the uplink propagation path. We show that in the far zone of the object and far zone of the turbulence coherence scale, atmospheric turbulence degrades spatial coherence of a reflected field and increases speckle averaging by the receiver. This reduces speckle bias of the phase-closure triple-product by several orders of magnitude and thus reduces speckle sensitivity of the receiver. The Van Cittert-Zernike theorem generalized to randomly inhomogeneous medium is presented and applied to interpretation of the results obtained. Two competing processes, diffraction of the illuminated beam on the satellite and turbulence-induced wavefront distortions of a speckle field in the atmosphere determine the size of the spatial coherence scale at the receiver. On one hand, the coherence scale of a speckle field increases with the distance due to diffraction on the satellite. On the other hand, it decreases due to turbulence-induced wavefront distortions. For the propagation scenario corresponding to a GEO satellite, the second process predominates. Coherence degradation of a speckle field caused by turbulence reduces speckle sensitivity of the receiver in the Fourier telescopy system. Experimental data that validate the generalized Van Cittert Zernike theorem in the turbulent atmosphere are reviewed.