The optical aperture of cat-eye target has the aperture averaging effect to the active detecting laser of active laser detection system, which can be used to identify optical targets. The echo scintillation characteristics of the transmission-type lens target have been studied in previous work. Discussing the differences of the echo scintillation characteristics between the transmission-type lens target and Cassegrain lens target can be helpful to targets classified. In this paper, the echo scintillation characteristics of Cat-eye target’s caliber with Cassegrain lens has been discussed . By using the flashing theory of spherical wave in the weak atmospheric turbulence, the annular aperture filter function and the Kolmogorov power spectrum, the analytic expression of the scintillation index of the cat-eye target echo of the horizontal path two-way transmission was given when the light is normal incidence. Then the impact of turbulence inner and outer scale to the echo scintillation index and the analytic expression of the echo scintillation index at the receiving aperture were presented using the modified Hill spectrum and the modified Von Karman spectrum. Echo scintillation index shows the tendency of decreasing with the target aperture increases and different ratios of the inner and outer aperture diameter show the different echo scintillation index curves. This conclusion has a certain significance for target recognition in the active laser detection system that can largely determine the target type by largely determining the scope of the cat-eye target which depending on echo scintillation index.
Active laser detection technique has a broad application prospect in antimissile and air defense, however the aerodynamic flow field around the planes and missiles cause serious distortion effect on the detecting laser beams. There are many computational fluid dynamics(CFD) codes that can predict the air density distribution and also the density fluctuations of the flow field, it’s necessary for physical optics to be used to predict the distortion properties after propagation through the complex process. Aiming at the physical process of laser propagation in “Cat-eye” lenses and aerodynamic flow field for twice, distortion propagation calculation method is researched in this paper. In the minds of dividing the whole process into two parts, and tread the aero-optical optical path difference as a phase distortion, the incidence and reflection process are calculated using Collins formula and angular spectrum diffraction theory respectively. In addition, turbulent performance of the aerodynamic flow field is estimated according to the electromagnetic propagation theory through a random medium, the rms optical path difference and Strehl ratio of the turbulent optical distortion are obtained. Finally, Computational fluid mechanics and aero-optical distortion properties of the detecting laser beams are calculated with the hemisphere-on-cylinder turret as an example, calculation results are showed and analysed.
The analytical expression of beam-width of distorted cat-eye reflected beam under far-field condition is deduced using the approximate three-dimensional analytical formula for oblique detection laser beam passing through cat-eye optical lens with center shelter, and using the definition of second order moment, Gamma function and integral functions. The laws the variation of divergence angle and astigmatism degree of the reflected light with incident angle, focal shift, aperture size, and center shelter ratio are established by numerical calculation, and physical analysis. The study revealed that the cat-eye reflected beam is like a beam transmitted and collimated by the target optical lens, and has the same characteristics as that of Gaussian beam. A proper choice of positive focal shift would result in a divergence angle smaller than that of no focal shift. The astigmatism is mainly caused by incidence angle.
Cat’s eye lens make the laser beam retro-reflected exactly to the opposite direction of the incidence beam, called cat’s
eye effect, which makes rapid acquiring, tracking and pointing of free space optical communication possible. Study the
influence of cat’s eye effect to cat’s eye retro-reflector at large incidence angle is useful. This paper analyzed the process
of how the incidence angle and focal shit affect effective receiving area, retro-reflected beam divergence angle, central
deviation of cat’s eye retro-reflector at large incidence angle and cat’s eye effect factor using geometrical optics method,
and presented the analytic expressions. Finally, numerical simulation was done to prove the correction of the study. The
result shows that the efficiency receiving area of cat’s eye retro-reflector is mainly affected by incidence angle when the
focal shift is positive, and it decreases rapidly when the incidence angle increases; the retro-reflected beam divergence
and central deviation is mainly affected by focal shift, and within the effective receiving area, the central deviation is
smaller than beam divergence in most time, which means the incidence beam can be received and retro-reflected to the
other terminal in most time. The cat’s eye effect factor gain is affected by both incidence angle and focal shift.
Restrain background clutter in infrared image is the first step before detecting and tracking. As infrared sensor is easily affected by atmosphere hot radiation, long distance and sensor noise, the detected targets in infrared images often present like small targets and drowned in noise. So it’s difficulty to detect the small target in complex background, especially doing it with single frame. A new method of infrared small targets detection based on local third order moment is proposed. Firstly, the local third order moment of target and background are analyzed. The result is that the third order moment of target and background has distinctively difference in value. The local third order moment of background tends toward zero or less than zero, while that of target is greater than one. Secondly, the global normalization is carried out after the third order moment operation to single frame image. With this algorithm, the background magnitude is almost near zero, and the potential targets magnitude is near to one. So the background clutter in the infrared images is suppressed effectively. And the target can be detected with fixed threshold. At last, false target judgment criterion is used, which can eliminate remained big bright object boundary effectively. The experiment results proved that this arithmetic has robust performance to restrain different background clutter; even in the situation the background clutter have high grey-level than the small target.
Based on the cat-eye effect of optical system, free space optical communication based on cat-eye modulating
retro-reflector can build communication link rapidly. Compared to classical free space optical communication system,
system based on cat-eye modulating retro-reflector has great advantages such as building communication link more
rapidly, a passive terminal is smaller, lighter and lower power consuming. The incident angle is an important factor of
cat-eye effect, so it will affect the retro-reflecting communication link. In this paper, the principle and work flow of free
space optical communication based on cat-eye modulating retro-reflector were introduced. Then, using the theory of
geometric optics, the equivalent model of modulating retro-reflector with incidence angle was presented. The analytical
solution of active area and retro-reflected light intensity of cat-eye modulating retro-reflector were given. Noise of PIN
photodetector was analyzed, based on which, bit error rate of free space optical communication based on cat-eye
modulating retro-reflector was presented. Finally, simulations were done to study the effect of incidence angle to the
communication. The simulation results show that the incidence angle has little effect on active area and retro-reflected
light intensity when the incidence beam is in the active field angle of cat-eye modulating retro-reflector. With certain
system and condition, the communication link can rapidly be built when the incidence light beam is in the field angle,
and the bit error rate increases greatly with link range. When link range is smaller than 35Km, the bit error rate is less