Effect of uplink atmospheric wavefront distortion on image quality of sheared-beam imaging

Abstract

Sheared-beam imaging (SBI) is a non-traditional imaging technique in which utilized are three sheared coherent lasers for illumination, and detector array to receive the intensity of speckle pattern reflected from the target. Finally the image of target can be reconstructed by computer algorithm from the data collected before. The SBI has some advantages in high resolution imaging for long-distance space targets. However, the wavefront distortion caused by atmospheric turbulence is a key factor affecting the imaging quality of SBI. Therefore, this paper focuses on the influence of wavefront distortion caused by atmospheric turbulence on the extraction of target spectral information. Theoretical model of the influence of wavefront distortion on imaging is established. The effects of low-order and high-order atmosphere turbulence on SBI imaging quality are analysed respectively. It turns out that low-order atmosphere turbulence does not result in poor image quality nor low-resolution, and just change the position of target on the image plane. But the image quality can be degraded when the wavefront root mean square (RMS) value at the target plane, caused by high-order atmosphere turbulence, exceeds /20. Beam emitted from larger aperture becomes more susceptible to perturbing effect, thus forming lower-quality wavefront. Considering that after passing through the atmosphere, beam also travels a long distance to reach the target surface. Targets at different heights will obtain different wavefront quality due to the diffraction of light. Thus the final wavefront quality is determined by turbulence intensity, aperture size and target height. Multi-layer phase-screen model and Hufnagel-Valley model are used to simulate the influences of near-earth (25 km) atmosphere on wavefront distortion at target plane with different imaging distances. Simulation results show that the wavefront RMS value rises with the increase of transmitting aperture diameter, and decreases with the increase of imaging distance. Transmitting aperture sizes in a range from 0.2 times r0 to twice r0 have been recommended for effective imaging by Hutchin[Hutchin R A 1993 Proc. SPIE 2029 161]. However, we find in our simulations that beams on the order of 2 r0 may cause significant wavefront error at short range target, and under some circumstances the clear image of target cannot be reconstructed. The imaging results of SBI at different laser transmitting apertures and different imaging distances are obtained, and evaluated by Strehl ratio. Imaging results show that choosing appropriate transmitting aperture size can effectively improve the imaging quality. But for the short-range targets, aperture size selection range presented by Hutchin can be too broad to be practicable. This paper suggests some approaches to choosing suitable aperture size for SBI system, and also providing a reference for the difference analysis of imaging quality for targets in different heights.