Ultrashort Light Pulses in Optical Augmentation
NAVAL RESEARCH LAB WASHINGTON DC
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The development of ultrashort light pulses has led to the examination of these pulses for specialized applications. One particular application would be in the area of optical augmentation, where one seeks to determine the function of an enemy optical system by remote probes. The major parameters needed to reconstruct such a system are the focal length, number of optical- elements in the system, element spacing, index of refraction, surface curvature, field of view, and f-number. The f-number is a good clue to a systems function. This number can be found in principle by using radar-like, timed, subnanosecond light pulses to measure focal length, and also using multiple or movable detectors to map the distribution of the reflected light intensity, which leads to a measure of the target aperture. It has been found, both theoretically and experimentally, that the focal length of a system can be measured using present technology out to a range of nearly 1 km with a quasi-cw high-repetition-rate laser, and to perhaps 30 km with a high-intensity isolated-pulse laser. This excludes, however, systems utilizing reflecting elements or telephoto lenses. Also, aperture measurement is impractical due to the great extent of the spatial intensity distribution in the detector plane. Consequently, the f-number cannot be readily determined. In conclusion, we believe that this approach will not be profitable because of the limited information it provides and because of the broad classes of systems from which it is excluded.
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