Phase gradient averaging for holographic aperture LIDAR in the presence of turbulence

Abstract

The resolution of an image is dictated by the size and quality of the imaging system. The imaging system has a physical limit to the resolution dictated by the diffraction limited resolution. This limit can be improved by making the aperture larger on the imaging system. The increase in the physical aperture size can only be practical to a certain extent. However, to get beyond these physical constraints it is possible to use synthetic aperture methods to allow for the aperture to appear to be increased. Synthetic apertures are created by adding apertures coherently together to create a larger aperture that increases the diffraction limited resolution. To sum the aperture coherently the phase information needs to be available. One way to have access to the phase information is to capture the image as hologram. These holograms are captured by using a coherent light source with a reference beam to create an interference pattern that contains the phase information of the target. Holographic apertures can be used in a synthetic aperture method called Holographic Aperture Lidar (HAL). A problem that can arise while capturing images is turbulence in the atmosphere. Turbulence is a change in the index of refraction caused by a change in the temperature and pressure of the atmosphere. This causes the phase of the light to distort dynamically as it propagates making HAL imaging difficult. This thesis will cover a method to restore the original phase of the signal that has passed through turbulence so that it can be used in digital holography and HAL. This method uses averaging of the phase gradient to remove the dynamic turbulence and keep the phase information of the static target. The improvements observed in actual experiments were small, but the basics of the method worked, and the reason for only small improvement are discussed.