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Chapter 10:
Laser Radar Systems: Imaging through Turbulence
Electro-optics (EO) basically refers to systems that involve both electronics and optical fields [1-7]. Part of the general area of EO includes diverse systems such as EO sensors, optical communication systems, and optical signal processing systems. The EO systems of interest here are those that respond to wavelengths from 0.4 μm to around 2 μm (see Fig. 7.1). When such systems interact with a propagating optical field, they exhibit characteristics that can be classified as linear (or quasilinear). Therefore, with appropriate assumptions, these EO systems can be treated with the powerful mathematical methods associated with linear systems analysis involving temporal and frequency relations between input and output signals. That is, the body of knowledge available for describing general linear system performance (such as an electric circuit) is also available for many EO systems, particularly imaging systems, which constitute our main focus. Atmospheric effects on optical seeing has long been recognized by astronomers. Resolution is generally limited by turbulence rather than by the optical design and optical quality of the telescope. In particular, atmospheric turbulence restricts the angular resolution of large ground-based telescopes to about 1 arcsec at visible wavelengths. There is a large body of literature available that is concerned with the study of atmospheric turbulence on incoherent imaging and compensation methods, much of which has been directed at the astronomical problem [8]. On the other hand, there has been much less attention given to the study of coherent imaging through turbulence. This is due in part to the fact that the analysis of imaging through turbulence when the object is coherently illuminated is more complicated than the incoherent case [9,10]. Nonetheless, imaging laser radars (ladar) that provide their own illumination have an advantage over passive imaging devices that must rely on outside sources of illumination or on radiation from the target itself. The use of ladar seekers for autonomous vehicle identification and targeting from short-range expendable munitions has seen an increase of interest due to the inherently high resolution shape data and the relatively low cost of the sensor [11]. A number of exploratory programs over the last decade or so have proven the value of ladar for tactical applications.
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