This paper illustrates continuing theoretical and simulation work examining the use of different methods to control a deformable mirror in an adaptive optics system without a wavefront sensor. Two independent techniques, stochastic optimization, or SPGD, and a newly developed functional approximation method, are discussed. Specific results from simulation work performed at SAIC are presented.
We report experimental results that demonstrate compensation of extended turbulence and thermal-blooming of high-energy lasers using target-in-the-loop techniques in a scaled laboratory environment. For these experiments the deformable mirror figure was controlled by an algorithm designed to maximize the target-plane intensity as measured by a camera at the transmitter. Results using this TIL configuration were compared under identical conditions to results obtained under control of a Hartmann wavefront sensor and least-squares reconstructor. Experiments were performed for a variety of propagation scenarios anticipated for tactical HEL applications and in all cases the TIL system was seen to outperform the conventional Hartmann-driven adaptive-optics system. We will discuss the details of the the target-in-the-loop algorithm, the laboratory configuration, and the experimental results.
Since the beginning of High Energy Laser systems, simulations have been used to predict performance, do parameter trades, and assist in troubleshooting. Today, simulations benefit from higher speed computers with more memory, but they are also being asked to do more. New types of HEL devices are being proposed, more hardware details are being incorporated, beam control systems are becoming more complex, innovative new systems are being designed to work under conditions of strong turbulence, and more types of targets are being considered. There are three types of physics level codes: resonator, beam control, and lethality. All three are slow running and require a high level of expertise to use. Scaling law codes are much easier to use and much faster running. These codes are based on analytical predictions and anchored to the wave optics simulations and to experiments. Scaling law codes can quickly predict performance, weight, and volume for various scenarios and conditions. Now that HEL systems are closer to reality, there is more interest in incorporating the scaling law codes into engagement codes, which predict overall system effectiveness in battle situations.
The estimation accuracy of wavefront sensors in strong scintillation is examined. Wave optical simulation is used to characterize the performance of several wavefront sensors in the absence of measurement noise. The estimation accuracy of a Schack-Hartmann sensor is shown to be poor in strong scintillation due primarily to the presence of branch points in the phase function. The estimation accuracy of a unit-shear, shearing interferometer is found to be significantly better than that of a Hartmann sensor in strong scintillation. The estimation accuracy of a phase shifting point diffraction interferometer is shown to be invariant with scintillation.
A wave-optics simulation has been developed which can model and optimize the performance of three-dimensional laser guidestar systems. Important parameters of such a system are discussed. The simulation is described and used to illustrate the difference between zero-, two- and three-dimensional beacons. A number of runs are made to show the effect of varying various simulation and system parameters.
A wave optics simulation has been used to compare the performance of adaptive optics systems in the presence of near-field and distributed turbulence. For a given total turbulence strength, the Greenwood frequency and the log-amplitude variance will degrade a phase-only adaptive optics system both because of the uncorrectable amplitude variations and because the scintillations will cause errors in the wavefront sensor and reconstructor.