Research on echo energy of ‘cat-eye’ target based on laser's character of polarization

Most of the military electro-optical (EO) systems, including optical observing and collimating systems, EO surveillance systems, EO tracking systems, EO search systems, EO fire control systems and EO distance-measuring systems, have reflective elements or partial reflective elements at the focal planes, such as modulate tray, cross image, lattice and photo-sensitive area of photoelectric detector. The collimating echo along the primary route can be formed when one of these systems is irradiated by incidence laser. This echo energy is two to four orders of magnitude greater than normal diffuse reflectors' echo energy in the aspect of order of magnitude. This phenomenon is called “cat-eye” effect and so these electro-optical systems are called “cat-eye” targets. Presently research on transmission and reflective energy of “cat-eye” target from the view of geometrical optics is the main study aspect and a lot of field use these research results into application, such as active laser detection system, retro-reflector, optical communication and target identification. To a certain extent all of the optical systems can change polarization state of laser wave. For example, optical lens or reflecting mirror can cause the incident laser's transformation of polarization state and phase delay. In this paper, the “cat-eye” target is regard as the combination of a thin lens and a reflecting surface. In accordance with path of light beam's propagation in the “cat-eye” target, this target is unfolded along the reflecting surface symmetrically, and forms a 4-f system model. That is to say, a polarization converter is set at the con-focal plane of two lenses. According to the Fresnel's law and the theory of Jones matrix, echo energy's characteristic equations of normal incident polarization laser through “cat-eye” target are theoretically deduced in detail by using ray tracing method. The character of thin-lens system's echo energy is quantitatively analyzed by Matlab and the discipline of different incident laser's azimuth and coordinate is discussed. The simulation results indicate that the output polarization laser's amplitude would be subdued to 0.034 times of the incident laser's amplitude after the refraction and reflection of “cat-eye” target, which has nothing to do with the focal length of “cat-eye” target, while the polarization degree and polarization state are almost not affected. Moreover, as for other diffuse reflecting target, the polarization characteristics of echo laser will be changed obviously. These results are valuable for the active laser detection, target recognition of laser radar and the “cat-eye” retro-reflector in optical communication.