Abstract
It is known that certain geometrical-optics predictions often agree well with optical turbulence field observations even though theoretical constraints for ignoring diffraction may be violated. Geometrical optics assumptions can simplify analyses, and ray optics can significantly reduce simulation computation time. Here, an investigation into angle-of-arrival fluctuations is presented involving wave optics and geometrical (ray) optics computer simulations of a plane wave of visible light propagating through a turbulent refractive-index field. The simulation and Rytov-based theory results for the variances of aperture-filtered angle-of-arrival fluctuations generally agree well for weak scattering (Rytov variance, σR2≲0.2), but for increasing Rytov variance, the simulation results demonstrate a positive slope that can be significantly shallower than that predicted by the theory. For weak-to-moderate scattering regimes (σR2≲2.67), a comparison of the ray and wave results show they match for aperture diameters greater than about two Fresnel lengths. This result is consistent with a previous theoretical analysis by Cheon and Muschinski. For the strongest scattering case studied (σR2=26.7), the wave and ray simulations match for aperture diameters greater than about 10 Fresnel lengths. For smaller apertures, we attribute the disparity between the wave and ray simulation results to a Fresnel filtering effect.
Highlights
Understanding the physics of optical wave front distortions caused by turbulence in the atmospheric refractive-index field is essential for the design and operation of optical systems used in free-space optical communication, surveillance, navigation, remote sensing, astronomy, and directed-energy technologies.[1,2,3,4,5,6] If particle scattering, absorption, coupling, and depolarization effects are negligible there are two phenomena that determine the optical field: refraction and diffraction
Shown for comparison are: (1) ray-density fields obtained from the ray simulation and (2) intensity and phase fields obtained from the wave optics simulation
Diffraction effects are negligible in the weak- and moderate-scattering regime if the aperture diameter is larger than about twice the Fresnel length. This result is consistent with the theoretical analysis by Cheon and Muschinski[20] who found that for weak scattering, the AOA variance predicted by geometrical optics deviates from that predicted by the Rytov theory by 1.65
Summary
Understanding the physics of optical wave front distortions caused by turbulence in the atmospheric refractive-index field is essential for the design and operation of optical systems used in free-space optical communication, surveillance, navigation, remote sensing, astronomy, and directed-energy technologies.[1,2,3,4,5,6] If particle scattering, absorption, coupling, and depolarization effects are negligible there are two phenomena that determine the optical field: refraction and diffraction. Since the 1980s, powerful phase-screen techniques have been developed[16,17,18,19] that allow optical propagation through atmospheric turbulence to be accurately simulated well into the “strong-scattering” regime, that is, for scenarios where the negligibility of diffraction effects and the “weak-scattering” assumption become invalid, or at least questionable. Such simulations can be used to test and refine the Optical Engineering. The path length assumed is L 1⁄4 2 km and the refractive-index structure parameter C2n ranges from 10−16 m−2∕3 to 10−13 m−2∕3, which corresponds to
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