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.
KEYWORDS: Target detection, Sensors, Doppler effect, Heterodyning, Velocity measurements, Signal detection, Laser applications, Signal to noise ratio, Linear filtering, Photodiodes
A novel single-source double-heterodyne coherent detection high-speed moving target is presented. Under a limited detector bandwidth condition, the method inherits the characteristics of dual frequency laser coherent detection and transforms the Doppler shift of a high-speed moving target to low frequency using the beat frequency difference method. Compared with the traditional heterodyne detection system, the maximum speed detectable range of the system is greatly increased. The double-heterodyne mixing process is deduced, and the relationship between the beat frequency, detector bandwidth, and maximum detectable speed of the system is obtained through analysis. Thus, the beat frequency can be dynamically adjusted for the system to adapt to different measurement environments. Different signal components and speed information extraction methods of detectors under different motion speeds are analyzed. The feasibility of the method is verified through simulation. Moreover, the data acquisition is minimal, and the system has good real-time performance.
The laser one-dimensional range profile of a target can be approximated as the convolution of the transmitted laser pulse and the range-resolved laser radar cross section. Therefore, the widening effect caused by the laser pulse is eliminated as much as possible by echo deconvolution, and the resolution of the target laser one-dimensional range profile detection can be improved to some extent. To this end, the Richardson-lucy image iterative deconvolution algorithm is introduced into the laser echo deconvolution. Although the algorithm can eliminate the influence of Poisson noise effectively,In order to further improve the deconvolution effect, wavelet noise reduction is first performed to improve the signal-to-noise ratio of the measured data before using the algorithm. The effectiveness of the proposed algorithm is verified by simulation experiments.
To solve the problems existing in the Modulating retro-reflector optical system which can not simultaneously satisfy the wide field of view, reversibility of light path, small F number and all lens are spherical surface, an optical system with wide field of view and near the diffraction limit is designed. The structure of optical system is optimized and analyzed with an optical design software, 30 degree view field angle and F number of 1.3 CMERR optical system has been designed, all lens are spherical lens and near diffraction limit. The results show that within the whole field of view, the lens retro-reflects incidence beam perfectly, and can match the small size modulator. It ensures the contrast ration of the whole system which has advantages of low cost and easy processing.
Based on the laser range profiles (LRPs), the information of target geometry and movement coefficients can be detected and deduced. In this paper, the theoretical expression is given as well as the convolution model of the target’s LRPs. Primarily, a simulation method for obtaining complex target’s LRPs is introduced and illustrated, which is based on the 3-D modeling and OpenGL. The projection and shading process and the procedure of acquiring the normal of model facet are explained. By using this method the LRPs of complex target in different conditions are obtained conveniently and quickly, then the influences of target’s attitudes, shape, scattering characteristics of surface and emitted pulse width on target’s LRPs are simulated and analyzed. This work is useful to produce abundant LRPs data for target recognition, classification and reflection tomography imaging.
By using the geometrical optics and physical optics method, the models of wedge plate interference optical path, Michelson interferometer and Mach Zehnder interferometer thus three different active interference pattern are built. The optical path difference (OPD) launched by different interference patterns, fringe spacing and contrast expression have been derived. The results show that far field interference peak intensity of the wedge plate interference is small, so the detection distance is limited, Michelson interferometer with low contrast affects the performance of detection system, Mach Zehnder interferometer has greater advantages in peak intensity, the variable range of interference fringe spacing and contrast ratio. The results of this study are useful for the theoretical research and practical application of laser active interference detection.
In this paper, laser radar range profile theory and simulation method are investigated. Laser radar range profile theoretical formula is given based on the theory of Gaussian laser beam, BRDF, and target shape. On the purpose of simulating the range profile of certain targets, the geometrical 3D models of some simple and complex objects are constructed respectively, and then the viewpoint coordinate and target coordinate are established. The location information of vertexes and facets can be obtained and exported to utilize. In next step their laser radar range profiles in many postures are acquired and compared sequentially. The influences of Gaussian pulse width, target shape, size and transmit-receive angles on the simulation results are discussed. In this way this paper can provide theoretical and simulation methods and bases for extracting target features and recognizing targets using laser radar range profiles.
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.
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
than 10-16.
As the damage region of a sensor can be regarded as some sort of anomalistic aperture diaphragm, the reflectivity model
of the sensor can be established, where the optical path of a surface damage detection system can be taken as a
propagation process for a plane wave through a defocused optical lens and is reflected back to the return place. Based on
the theory of angular spectrum diffraction, the diffraction transmission formulas for a plane wave through the cat-eye
system can be derived by taking two-dimensional discrete Fourier transfer. The regulations on the variation of the
intensity distribution of the cat-eye reflected light with different damage conditions of the sensor can then be given, and
the effect of the focal shift can be analyzed. Experiment results show the intensity distribution of cat-eye reflected light
has a direct relationship with the profile of the damage region and the amplificatory effect for the small damage region is
of direct ratio with the focal shift. Moreover, the proposed method can be straightforwardly utilized for the surface
damage effect detection of a sensor in optical lens without disconnecting it.
Laser active imaging system, which is of high resolution, anti-jamming and can be three-dimensional (3-D) imaging, has
been used widely. But its imagery is usually affected by speckle noise which makes the grayscale of pixels change
violently, hides the subtle details and makes the imaging resolution descend greatly. Removing speckle noise is one of
the most difficult problems encountered in this system because of the poor statistical property of speckle. Based on the
analysis of the statistical characteristic of speckle and morphological filtering algorithm, in this paper, an improved
multistage morphological filtering algorithm is studied and implemented on TMS320C6416 DSP. The algorithm makes
the morphological open-close and close-open transformation by using two different linear structure elements respectively,
and then takes a weighted average over the above transformational results. The weighted coefficients are decided by the
statistical characteristic of speckle. This algorithm is implemented on the TMS320C6416 DSPs after simulation on
computer. The procedure of software design is fully presented. The methods are fully illustrated to achieve and optimize
the algorithm in the research of the structural characteristic of TMS320C6416 DSP and feature of the algorithm. In order
to fully benefit from such devices and increase the performance of the whole system, it is necessary to take a series of
steps to optimize the DSP programs. This paper introduces some effective methods, including refining code structure,
eliminating memory dependence, optimizing assembly code via linear assembly and so on, for TMS320C6x C language
optimization and then offers the results of the application in a real-time implementation. The results of processing to the
images blurred by speckle noise shows that the algorithm can not only effectively suppress speckle noise but also
preserve the geometrical features of images. The results of the optimized code running on the DSP platform show that
the optimized outcome realizes better instruction-level parallelism and pipeline operation and the program is proved to
be reliable, effective and high real time.
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