Measurement And Analysis Of The Temporal And Spatial Characteristics Of Distorted Wavefront Simulated By Atmosphere Turbulence Generators

Atmospheric turbulence is an irregular random motion, so statistical turbulence theory is commonly used to study the atmospheric turbulence. The mathematical model that is mainly used Kolmogorov and non-Kolmogorov turbulence model. Generally spatial feature and temporal feature reflect the characteristics of the wavefront aberrations. The spatial features are atmospheric coherence length and phase structure function while the temporal features are time constant, Greenwood frequency and power spectral density of phase. Adaptive Optics System(AOS) is used for correcting stochastic wavefront aberrations caused by atmospheric turbulence, wavefront sensor detects phase aberrations of the light-wave then compensated by the wavefront corrector with an efficient control algorithm. We need to use Laser Guide Star(LGS) when the light emitted from the target is too weak or there is no bright enough star in the nearby area to detect wavefront aberrations.The temporal and spatial characteristics of atmospheric turbulence determine correction effects of AOS. A laboratory platform of AOS is built up, in order to study the correction methods that match with the temporal and spatial characteristics of atmospheric turbulence under the LGS. The turbulence simulator is one of the major parts of the LGS-based adaptive optics simulation platform. Two ways to generate controlled laboratory atmospheric turbulence are delivered in this paper, one is rotation phase screen and the other is hot-wind turbulence simulator. But the temporal and spatial characteristics of turbulence produced by these simulators haven’t been researched deeply. The purpose of this paper is to measure and analyze the temporal and spatial characteristics of the distorted wavefront caused by turbulence simulators accurately, and to estimate correction effect of AOS under different turbulence conditions.For the rotation phase screen turbulence simulator, we obtained atmospheric coherence length and distribution curve of the variance of Zernike coefficients under different speeds, results show that the low-order aberrations of rotation phase screen atmospheric turbulence simulator are bigger than Kolmogorov turbulence. The time constant decreases and the Greenwood frequency increases as the phase screen accelerated. Experimental results show that the temporal and spatial characteristics of turbulence produced by rotation stochastic phase screen are in line with the statistical theory of Kolmogorov turbulence.For the HT200 hot wind turbulence simulator, the influence of wind speed and temperature on the strength of turbulence has been analyzed. It can be found that as the temperature increases the turbulence becomes violent when the wind speed is fixed, as the wind speed increases the turbulence becomes weakening when temperature is fixed. As for the HT200, the high-order aberrations are bigger and the low-order is smaller than Kolmogorov turbulence, through the analysis of phase structure function and distribution curve of the variance of Zernike coefficients. The exponent of the phase structure function is about 4/3 which is different from the 5/3 corresponding to Kolmogorov turbulence. The turbulence generated by HT200 is non-Kolmogorov turbulence. Based on the Taylor frozen hypothesis, the relationship between space translation and time delay of turbulence aberrations under moving condition has been analyzed. An image translation prediction algorithm is proposed. With the method of moving reconstruction image to simulate advanced angle of LGS, we have studied the feasibility of improving correction effect that using advanced angle of LGS under movement conditions. Experiment results show that the residual wavefront reduced by 10% of the hot wind turbulence simulator compared with 50% of the rotation phase screen. It is the difference of temporal and spatial characteristics of turbulence that leads to the result.In the thesis, we measured temporal and spatial characteristics of turbulence simulated by rotation phase screen and hot wind simulator, and estimated the correction effect of AOS under different simulation turbulence conditions. We will strengthen the theoretical exploration and carry out correction experiment research on LGS AOS under simulation turbulence in the future.