Tomographic Coherent Ladar Based Atmospheric Turbulence Profile Characterization System

ABSTRACT: This Small Business Innovative Research (SBIR) Phase I project will investigate, develop, and determine the feasibility of using a coherent FMCW ladar measurement system and tomographic methods to provide characterization of atmospheric turbulence profiles. By coherently tracking a small array of point targets from multiple receivers in a known geometry, the transverse and longitudinal structure of refractive index fluctuations can be estimated along several intersecting paths. These integrated path measurements can be combined to form an incoherent or coherent tomographic reconstruction of the atmospheric turbulence. The use of high range resolution FMCW chirped ladar allows point targets to be identified and isolated by their range even when individual targets cannot be optically resolved in the transverse dimension due to broadening by strong optical turbulence. The use of passive retro-reflecting targets or possibly laser guide stars illuminating on a diffuse, opaque target makes the technique well suited to characterization of down-looking and slant-path turbulence profiles from an airborne platform. The proposed technique uses methods and ideas from Synthetic Aperture Ladar (SAL) imaging. The Phase I work will provide algorithmic and experimental proof-of-concept demonstrations and analysis to determine the feasibility of a long-range atmospheric turbulence characterization system. BENEFIT: The United States Air Force currently deploys and anticipates more widespread deployment of a variety of active electro-optical sensors and systems including laser altimeters, wind lidar, free-space optical communications, vibrometry, and coherent imaging sensors including synthetic aperture and holographic ladar imaging. The performance of all of these systems is impacted by atmospheric turbulence. Atmospheric turbulence has been extensively studied both theoretically and experimentally, but remains largely an intractable problem particularly in low altitude slant-path applications due to its inherent randomness, chaotic unpredictability, and strong effects. Typically, turbulence is characterized by the path-integrated structure constant or the Fried parameter, whereas the performance of electro-optic systems depends on the structure constant as a function of distance. Bridger Photonics, Inc. and Montana State University Spectrum Lab propose to develop a system to characterize the atmospheric turbulence as a function of distance but also in a spatially and temporally resolved manner. The success of such an approach would represent a significant leap forward in the characterization of atmospheric turbulence and may unlock a deeper understanding of a fundamentally chaotic problem and potentially perpetrate improved electro-optic, coherent imaging and coherent communications systems.