A fast adaptive optical system, operating at frequencies up to 2000 Hz (frames per second), was used to analyze turbulence created in the laboratory by using a fan heater. The turbulent distortion bandwidth was approximately 100 Hz. The expansion of the wavefront in terms of Zernike polynomials was used when processing the raw data. Then the statistical analysis was performed separately for each polynomial. As a result, the degree of predominance of definite aberrations in the wavefront of laser radiation was obtained. Taylor's hypothesis is confirmed: low-order aberrations are slower than high-order ones. The dependence of the correction quality on the number of corrected Zernike polynomials is also shown.
The results of experiments carried out on the setup of a fast adaptive optical system are presented. A fan heater was used as a source of wavefront aberrations, the air flow of which was directed perpendicular to the laser beam. The processing of the experimental data consisted in determining the spectral characteristics of the disturbing effect from the dynamics of oscillations of the coordinates of the focal point of the lens array. To ensure sufficient resolution in the frequency domain, a sample of the original data was recorded for 10 s, which provided a resolution along the frequency axis of 1/10 Hz. The graphs of the spectral energy for the full set of wavefront aberrations calculated from the fluctuations of the lens array focal spot are shown. For a more detailed consideration, the wavefront aberrations were expanded in terms of Zernike polynomials, after which a spectral analysis of each aberration was carried out. It is shown that more than 90% of all turbulence energy is concentrated in lower-order aberrations, which makes it possible to use a bimorph mirror as a wavefront corrector, which reproduces well the lower-order aberrations.
The results of experiments conducted on a laboratory setup of a fast adaptive optical system based on the use of FPGA as the main control element and a bimorph mirror as a wavefront corrector are presented. The adaptive system bandwidth ranged from a dozen Hertz to 2,000 Hertz. For independent control of the quality of correction the intensity distribution in the far field was recorded. It is shown that for a good correction of the wavefront the system bandwidth should be an order of magnitude higher than the upper boundary of the spectrum of wavefront distortions caused by turbulence. A comparison of the model and experimental data is also presented.
One of the main problems in tasks of laser beam propagation though Earth’s atmosphere is decrease the efficiency of the optic-electronic systems operation due to atmospheric turbulence influence that leads to laser beam’s wavefront distortions. Use of fast adaptive optical system are suggested to solve this problem. It allows to compensate the wavefront distortions, which upper bound of the spectrum is up to 150 Hz, in real time. Owing to the fact that adaptive optical system is discrete (it’s defined by digital camera included in the system), the sampling rate shall be at least 1500 Hz (frames per second).
An adaptive optical system that implements a phase conjugation algorithm designed to compensate for the effect of atmospheric turbulence the propagating laser beam is presented. The system allows compensating for the influence of atmospheric disturbances up to 200 Hz (in terms of sine). To achieve the compensation effect system operates at a frequency of 2000 Hz (in terms of fps - frames per second). Such high performance can be achieved only when using FPGA as the master control element of the system. The results of correction of disturbances obtained by using a heat fan, simulating the turbulence to frequencies of 200 Hz, are presented.
In tasks related to free-space communications, a significant role has a turbulent atmosphere which influences lead to a decrease in the efficiency of systems. Since the characteristic turbulence spectrum rarely exceeds 100 Hz for typical paths, it is proposed to use a discrete adaptive optical system with a frequency of 1500 frames per second to reduce the influence of the atmosphere. The structure of the system based on the use of FPGA as a computing device as well as the main results associated with the correction of both static and dynamic components of aberrations are presented.
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