Characterization of the performance of a laser imaging system through the atmosphere

Laser scanning and imaging through the Earth's atmosphere is an important topic for analysis and study for several reasons. The system's performance is highly affected by the presence of atmospheric turbulence as well as atmospheric aerosols. The objective of this study is to characterize the influence of the atmosphere on the performance of laser imaging system.;The fundamental theory of a Gaussian beam propagation through vacuum as well as turbulent medium are discussed. The essential aspects of the atmospheric effects on a propagating beam are described analytically. The received beam profile features are characterized by both the point spread function and the optical transfer function.;Wave optics simulations are employed not only to validate the analytic results but also to visualize the performance of the system under investigation. The simulation is based on the use of the discrete Fourier transform and employing the diffraction theory. The essential aspect of a basic imaging system are also discussed with some numerical calculation. Atmospheric turbulence is simulated by phase screens placed along the propagation path. The phase screens are characterized by random phase values associated with the physical features of the turbulent medium. Simulation parameters are selected in such a way that numerical sampling criteria are met and the physical problem is modeled correctly.;The fundamentals aspects of forward scattering due to the aerosol are discussed. The profile of the angular irradiance is transformed directly from the classical expression of the aerosol modulus transfer function. Therefore, the divergence of a collimated beam that propagates through aerosol's medium is determined. We propose a simulation procedure to describe the angular beam profile in the presence of atmospheric aerosols. The aerosol's influence is modeled with particular phase screens distributed through the propagation path. The procedure can be applied to describe homogeneous as well as non homogeneous media. Indeed, the aerosol approach can be used simultaneously with turbulence phase screens.;A particular laser imaging system that employs a Gaussian beam is analyzed through the corresponding optical transfer function. Long and short exposures with the laser propagating through the Fresnel zone are described with a special emphasis on the effect of the beam wander. The investigated system employs a collimated Gaussian beam truncated by a circular aperture to scan a 2-D target. The reflected beam is collected by a single detector, then the final image is constructed by considering the detected irradiance at each picture element. The system parameters that are studied include the angular beam spread, point spread function and modulation transfer function. The performance of the system is characterized through both the ratio and the Nyquist frequency.