The Improvement in Turbulence-Degraded Beam Quality Obtained with a Tilt-Correcting Aperture

In this report, the degradation in the focal-plane irradiance distribution due to atmospheric turbulence, and the potential improvement realizable by employing a wavefront tilt-correcting aperture is calculated. It has been shown that, when the aperture diameter is of the order of the outer scale of turbulence, virtually no improvement is realized relative to the uncompensated case. For the case when the long-term coherence length is small compared with the diameter, there is considerable improvement over the long-term case; however, the focal-point intensity can still be several decibels down from its vacuum value, implying no better than several times diffraction-limited performance. When the coherence length is not much smaller than the diameter, close to diffraction-limited performance can be expected. Comparisons have also been made of the reduction of on-axis intensity with no compensation, tilt- correction, and a full phase-compensating aperture It is shown that the effective coherence length of the compensated aperture due to the residual amplitude fluctuations is greater than the long-term coherence length by a factor of the square--root of the Fresnel number of the aperture; for high Fresnel number systems, this larger coherence length results in considerable increases in on-axis intensity.