Electro-optical (EO) tracking systems, while exhibiting strong nonlinear characteristics, are difficult to accurately model. Nonlinear resistance torque is proposed to describe the system’s nonlinear phenomenon and the genetic algorithm is used to identify model parameters. The model’s root-mean-square error (RMSE) was reduced using nonlinear resistance torque by 2.5 times compared to the Stribeck friction model and by 12 times compared to the linear model. Under the identified model, the system’s nonlinearity was effectively compensated. The results demonstrate the feasibility of the proposed method for the identification of EO tracking systems.
An experimental platform was built for the atmospheric turbulence simulation upon a liquid crystal spatial light modulator. The phase-only spatial light modulator is driven by a series of dynamic phase screens that have non-Kolmogorov turbulence characteristics. After passing through the simulated non-Kolmogorov turbulence, the angle-of-arrival (AoA) and light intensity of the laser beam were analyzed statistically. The variation trends of the AoA fluctuation and the scintillation index exhibited by the experimental results are in accordance with the theoretical predictions. Although there was some divergence between the experiments and theory that needs to be improved further, the experimental data demonstrate the feasibility of using the platform to simulate the turbulence influence on laser communication in the lab.
Communication experiments of free space optical system which was laid on the vibration platform were implemented. In the communication experiments, the transmitting terminal is static and the receiving terminal is laid on the vibration platform. The vibration platform was used to validate the performance of the APT (acquiring, pointing and tracking) system. The analytic results of experimental data prove that the APT system can effective restrain the destabilization of laser beam caused by the vibration platform. The communication distance is 3.4 km and the communication rate is 2.5 Gbps.
Increasingly importance has been taken seriously for high frame rate CMOS camera to optical communication acquisition pointing and tacking (APT) system, with its compact structure, easy to developed and adapted to beacon light spot detection in atmospheric channel. As spot position accuracy directly determines the performance of space optical communication, it is very important to design a high precision spot center algorithm. Usually spot location algorithm uses gravity algorithm, shape center capturing algorithm or self-adaption threshold algorithm. In experiments we analyzed the characteristics of the spots which transmitted through atmospheric turbulence and studied light transmission characteristics in turbulent channel. We carried out a beacon light detection experiments in a distance of 3.4km, collected the beacon spots on CMOS camera and signal light power. We calculated spot position with two different algorithm and compared the calculation accuracy between field dispersive spot and ideal Gaussian laser spot. Experiment research show that, gravity center algorithm should be more suitable for beacon beam spot which accuracy can be improved about 1.3 pixels for a Gaussian spot. But the shape center algorithm has higher precision. The reasons were analyzed which made an important preparation for subsequent testing.
A simulation platform is established for target motion using a liquid crystal (LC) spatial light modulator as a nonmechanical beam steering control device. By controlling the period and orientation of the phase grating generated by the spatial light modulator, the platform realizes two-dimensional (2-D) beam steering using a single LC device. The zenith and azimuth angle range from 0 deg to 2.89 deg and from 0 deg to 360 deg, respectively, with control resolution of 0.0226 deg and 0.0300 deg, respectively. The response time of the beam steering is always less than 0.04 s, irrespective of steering angle. Three typical aircraft tracks are imitated to evaluate the performance of the simulation platform. The correlation coefficients between the theoretical and simulated motions are larger than 0.9822. Results show that it is highly feasible to realize 2-D target motion simulation using the LC spatial light modulator.
A simulation platform is set up to study the ship-borne laser communications. A liquid crystal spatial light modulator is used as a non-mechanical beam steering control device to simulate beam motions of moving platforms. Because of its light weight, low power and compact form, the liquid crystal spatial light modulator is superior to its mechanical counterparts. The theory of beam steering is introduced firstly. And then the performance of the simulation platform is researched and evaluated. The steering angles range from -2.89° to 2.89° with the control precision of ~ 0.02°. The simulated sinusoidal frequency of the spatial light modulator can reach 4 Hz in maximum. Finally, the beam motions of the ship platforms with different natural rolling periods are simulated based on the platform. The correlation coefficients between the theoretical and the simulated motion curves are greater than 0.9805. Results show that it is feasible to realize the beam motions simulation of specific moving platforms using the liquid crystal spatial light modulator.
The dynamic atmospheric turbulence is simulated in the laboratory upon the phase-only liquid crystal spatial light
modulator. Dynamic phase screens are generated by the spline function method and the frozen turbulence method. The
average cross-correlation coefficient of the Zernike coefficients between these two methods is 0.6608. Moreover, the
laser atmospheric transition experiment is carried on under different turbulence intensities. The logarithmic light
intensity probability density distribution is close to the normal distribution, and the fitting determine coefficient is above
0.9. In the weak turbulence (r0=0.1m), the standard deviation of the arrival angle fluctuation is approximately 30 to 40
μrad, while it is 40 to 50 μrad in the moderate turbulence (r0=0.01m). The simulation result is compliant with the
turbulence theory. Compared with the frozen method, the arrival angle fluctuation spectrum in high frequency upon the
spline method is smoother. It reveals that although two methods have a good consistency with each other, the dynamic
simulation of the spline method is supposed to be more favorable in the researching of the free-space laser propagation.
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