Publications
Volume 5, Number 1
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Adaptive Optics Research at Lincoln Laboratory Adaptive optics is a technique for measuring and correcting optical aberrations in real time. It is particularly useful for atmospheric compensation—the correction of aberrations incurred by light propagating through the atmosphere. For more than two decades Lincoln Laboratory has been a leader in adaptive optics research and has performed seminal experiments in atmospheric compensation, including the first thermal-blooming compensation of a high-energy laser beam, the first compensation of a laser beam propagating from ground to space, and the first compensation using a synthetic beacon. In this article we describe the fundamental concepts of adaptive optics for atmospheric compensation and briefly review more than 20 years of Lincoln Laboratory work in the field. Atmospheric-Turbulence Compensation Experiments Using Cooperative Beacons Lincoln Laboratory recently completed the three-year Short-Wavelength Adaptive Techniques (SWAT) field program, designed to investigate the performance of adaptive optics in a variety of scenarios. This article describes the operation of the SWAT adaptive optics system working with cooperative beacons. Examples are presented of system operation in compensating star images and in propagating a compensated laser beam to a satellite. Sodium-layer Synthetic Beacons for Adaptive Optics Using adaptive optics to compensate for atmospherically induced wavefront distortions requires a remote beacon. In astronomical imaging the beacon can be the object of interest or a nearby bright star. For a satellite, the beacon can be a retroreflector illuminated by a ground-based laser. Unfortunately, dim stars don't always have bright neighbors, and we cannot place retroreflectors on satellites belonging to unfriendly nations. Synthetic beacons, generated by laser backscatter from the atmosphere, offer a solution to this problem. In 1984, Lincoln Laboratory performed the first measurements on wavefronts propagated through atmospheric turbulence from a synthetic beacon in the mesospheric sodium layer. Lincoln Laboratory has been highly active in the development and evaluation of synthetic beacons since that time. Although military applications initially stimulated the development of synthetic-beacon technology, current interest in synthetic beacons has expanded to include the astronomical community. Atmospheric-Turbulence Compensation Experiments Using Synthetic Beacons Atmospheric turbulence limits the ability of ground-based telescopes to form images of astronomical or orbiting objects. Turbulence effects also decrease the on-axis intensity of laser beams propagated from ground to space. Conventional adaptive optics systems typically compensate for the deleterious effects of the atmosphere by sensing the wavefront of light originating at or near the object of interest, and correcting for it in real time. If the object is dim, however, conventional adaptive optics techniques fail. Recent experiments at the Lincoln Laboratory Maui Field Site have shown that Rayleigh backscatter from laser beams focused at altitudes up to 8 km can be used to generate beacons for an adaptive optics system; this process relaxes the requirements on object brightness. Synthetic-beacon adaptive optics technology can be applied to space surveillance, ballistic-missile defense, anti-satellite systems, and ground-based astronomy. Adaptive Optics for Astronomy During the last decade optical astronomy has played an increasingly important role in our understanding of the universe, and many recent discoveries can be directly attributed to revolutionary improvements in telescope design. At least 10 optical systems with apertures exceeding 3.5 m are currently available to the scientific community, and several telescopes in the 8-to-l0-m class are now under construction. Although the light-gathering properties of these new telescopes are remarkable, at visible wavelengths their resolution is ultimately limited by phase distortions associated with atmospheric turbulence. Even under excellent seeing conditions, the imaging quality of these systems in the visible is seldom better than that obtainable from a 20-cm receiver. Recent changes in military security guidelines have now made it possible to apply technology developed for high-energy laser-beam control to the problem of turbulence compensation for astronomy. In particular, the combination of high-bandwidth adaptive optics and synthetic-beacon sources offers the potential for near-diffraction-limited resolution at all optical wavelengths. This article investigates the principal design issues associated with the construction of adaptive optics systems for dim-target imaging, and develops quantitative performance estimates for a 4-m telescope. The SWAT Wavefront Sensor A team of researchers at Lincoln Laboratory built an advanced 241-channel Hartmann wavefront sensor for the adaptive optics system that was used in the Short-Wavelength Adaptive Techniques (SWAT) experiments. This sensor measures the phase of either pulsed or continuous sources of visible light. The instrument uses binary optics lens arrays made at Lincoln Laboratory to generate 16 X 16 subaperture focal spots whose centroids are measured with custom-built 64 X 64-pixel charge-coupled-device (CCD) cameras. The back-illuminated CCD detectors have quantum efficiencies of 85% at 500 nm; the camera has a readout noise of 25 electrons rms at 7000 frames/sec. A special pipeline processor converts the CCD camera data to wavefront gradients in 1.4 μs. The sensor has an accuracy of λ/15 at an input light level of 2000 photons per subaperture. The Theory of Compensated Laser Propagation through Strong Thermal Blooming Thermal blooming is the spreading of a laser beam that results when some of the beam's energy is absorbed by the medium that the beam is propagating through. If left uncorrected, the spreading would significantly reduce the effectiveness of high-power lasers as directed-energy weapons and as devices for beaming power over long paths through the atmosphere. In this article we survey the theory of adaptive compensation for thermal blooming, with an emphasis on research at Lincoln Laboratory since 1985. This work includes the development of MOLLY, a uniquely realistic computer simulation of adaptively compensated laser propagation through turbulence and thermal blooming, and the development of robust experimental signatures for important fundamental processes, most notably phase-compensation instability (PCI). Caused by positive feedback between an adaptive optics system and laser-induced atmospheric heating, PCI can strain the capabilities of adaptive optics hardware. Results from both MOLLY, which was optimized for the Cray-2 supercomputer, and our analysis of PCI have been verified in laboratory and field experiments. In this article, we discuss the physics of uncompensated and compensated thermal blooming, the architecture and capabilities of MOLLY, and an analysis of PCI that takes into account the detailed structure of adaptive optics hardware. Finally, we use MOLLY output to illustrate signatures of PCI, effects that ameliorate PCI, and characteristic trends that make it possible to predict large-system performance from subscale experiments. Thermal-Blooming Laboratory Experiments We conducted a multiphase series of laboratory experiments to explore the adaptive optics compensation of a laser beam distorted by strong thermal blooming. Our experimental approach was to create on a small, low-power beam the same phase distortion that would be experienced by a large, high-power beam propagating through the atmosphere and to apply phase compensation via deformable mirrors. We performed the investigations to lay the foundation for future ground-based laser experiments and their corresponding atmospheric-propagation computer models. The experiments had three primary objectives: to provide a controllable, repeatable means of obtaining data for the verification and benchmarking of computer codes, to investigate the different phase-compensation instability (PCI) mechanisms that were predicted by theory and computer code, and to gain information relevant to the design of adaptive optics systems for ground-based laser propagation. The experiments were successful in realizing all three objectives: we observed the first experimental evidence of PCI and achieved excellent agreement between experimental results and computer-code predictions. In addition, the work provided valuable insight to the role of adaptive optics hardware in influencing high-energy beam propagation and to the importance of incorporating realistic hardware models into propagation codes. |
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