Evaluation of the performance of an adaptive optics system in strong scintillation

Recent studies on laser beam propagating over a long distance through even weak atmospheric turbulence have shown that under conditions where scintillation is significant a continuous phase function does not in general exist, owing to the presence of branch points in the complex optical field. The scintillation can create problems when one is using an adaptive-optics system for atmospheric-turbulence compensation. Significant degradation was observed in the adaptive-optics correction through the results of our simulated experiments with a wave-optics code as the scintillation increased when the Fried parameter r0 remained a constant. As result of the measured gradient field (i.e., idealized pointwise phase differences measurements) including two components, one that is expressed as the gradient of a scalar potential and the other that is expressed as the curl of a vector potential, the scalar and vector potentials are both easily reconstructed from the measured gradients. The total potential (including the scalar and vector potentials) and the scalar potential serve as an upper bound on the performance of Hartmann-sensor reconstruction and Hartmann-sensor-based least-squares reconstruction, respectively. The results obtained illustrate that scintillation effects are not significant until the Rytov number is greater than 0.2. When the Rytov number is greater than 0.2, the performance of adaptive-optics correction is obviously degraded. These computational results showed that the compensative effect of adaptive optics is not characterized completely by Fried parameter. The effect of scintillation needs to be considered in compensation of adaptive optics when a laser beam propagated over a long distance through even weak atmospheric turbulence.