COAT/target-signature interactions

Amplitude modulation effects which can occur when target return speckle patterns move by a receiver aperture, have been studied in adaptive optical systems that use multidither coherent optical adaptive techniques (COAT). Good agreement is found between experimental, analytical, and computer simulation studies made to characterize the degree to which COAT systems can be affected by speckle. Both experimental and computer simulation results show that in the worst-case, the convergence-level (Strehl ratio) of an 18-element COAT system is reduced to 30% of that obtained with no spurious speckle modulations. Calculations using an analysis developed here, predict an average covergence level of 0.18, in reasonable agreement with experiment, but somewhat pessimistic. In the presence of speckle noise, the COAT system convergence rate is unaffected as is the convergence level until it reaches the 'average' level that is set by the speckle noise. The convergence level then oscillates in a random fashion between well-defined maximum and minimum values. For any given multidither COAT system, we have developed analytical and computer simulation tools that can accurately predict the average convergence level and oscillation extremes once the power spectrum of speckle-induced modulations is known. We have observed that reducing the servo gain (bandwidth) reduces the effect of speckle modulations nearly to zero, but at the cost of increased convergence time.