Target reconstruction algorithm for four-beam sheared coherent imaging

Abstract

Sheared-beam imaging, which is a nonconventional coherent laser imaging technique, can be used to better solve the problem of taking pictures with high resolution for remote targets through turbulent medium than conventional optical methods. In the previous research on this technique, a target was illuminated by three coherent laser beams that were laterally arranged at the transmitter plane into an L pattern. In order to obtain a high quality image, a series of time-varying scattered signals is collected to reconstruct speckled images of the same object. To overcome atmospheric turbulence, multiple sets of three-beam laser should be emitted, which increases data acquisition time. In this paper, aiming at the quasi real-time problem of conventional sheared beam imaging technique, we use four-beam laser with rectangular distribution instead of the traditional L type sheared three-beam laser to illuminate the target. According to this, we propose a target reconstruction algorithm for four-beam sheared coherent imaging to reconstruct four target images simultaneously in one measurement, which can acquire high quality images by reducing the amount of measurement and the speckle noise. Meanwhile, it can greatly reduce the amount of beam switching in multi-group emission and improve the imaging efficiency. Firstly, the principle of the four-beam sheared coherent imaging technique is deduced. Secondly, in the algorithm, the speckle amplitude and phase difference frames can be extracted accurately by searching for the accurate positions of the beat frequency components. Based on the speckle phase difference frames, four sets of wavefront phases can be demodulated by the least squares method, and wavefront amplitude can be obtained by algebraic operation of speckle amplitude. The reconstructed wavefront is used for inverse Fourier transform to yield a two-dimensional image. A series of speckled images is averaged to form an incoherent image. Finally, the validity of the proposed technique is verified by simulations. From the simulation results, the image quality of the proposed method is better than that of the traditional method in the same amount of measurement. Furthermore, on the premise of the same image quality, the data acquisition amount of the proposed method is 2-3 times as large as that of the traditional method. In other words, compared with that of the traditional method, the data acquisition time of the proposed method is reduced at least by half and the algorithm processing time is less. It can be concluded that the proposed imaging technique can not only improve the efficiency of target reconstruction, but also present a better way of imaging the remote moving targets.