Rao Changhui

Hybrid deconvolution of adaptive optics retinal images from wavefront sensing

Adaptive optics can be used to compensate for the wave aberration of the human eyes to achieve high-resolution imaging in real time. However the correction is partial due to the limitation of hardware. We propose a kind of hybrid image post-processing method, which uses the blind deconvolution combined with the residual data in wavefront sensor to restore the partially adaptive optics corrected retinal image. This method is applied in the image restoration of the vivid human retinal images. The results show that it is effective to improve the retinal image quality.

Thermal stability test and analysis of a 20-actuator bimorph deformable mirror

One of the important characteristic of adaptive mirrors is the thermal stability of surface flatness. In this paper, the thermal stability from 13 degrees C to 25 degrees C of a 20-actuator bimorph deformable mirror is tested by a Shack-Hartmann wavefront sensor. Experimental results show that, the surface P-V of bimorph increases nearly linearly with ambient temperature. The ratio is 0.11 mu m/degrees C and the major component of surface displacement is defocused, compared with which, astigmatism, coma and spherical aberration contribute very small. Besides, a finite element model is built up to analyse the influence of thickness, thermal expansion coefficient and Youngs modulus of materials on thermal stability. Calculated results show that bimorph has the best thermal stability when the materials have the same thermal expansion coefficient. And when the thickness ratio of glass to PZT is 3 and Youngs modulus ratio is approximately 0.4, the surface instability behaviour of the bimorph manifests itself most severely.

The co-phasing detection method for sparse optical synthetic aperture systems

Co-phasing between different sub-apertures is important for sparse optical synthetic aperture telescope systems to achieve high-resolution imaging. For co-phasing detection in such a system, a new aspect of the systems far-field interferometry is analysed and used to construct a novel method to detect piston errors. An optical setup is built to demonstrate the efficacy of this method. Experimental results show that the relative differences between measurements by this method and the criterion are less than 4%, and their residual detecting errors are about 0.01 lambda for different piston errors, which makes the use of co-phasing detection within such a system promising.

A modified phase diversity wavefront sensor with a diffraction grating

The phase diversity wavefront sensor is one of the tools used to estimate wavefront aberration, and it is often used as a wavefront sensor in adaptive optics systems. However, the performance of the traditional phase diversity wavefront sensor is limited by the accuracy and dynamic ranges of the intensity distribution at the focus and defocus positions of the CCD camera. In this paper, a modified phase diversity wavefront sensor based on a diffraction grating is proposed to improve the ability to measure the wavefront aberration with larger amplitude and higher spatial frequency. The basic principle and the optics construction of the proposed method are also described in detail. The noise propagation property of the proposed method is also analysed by using the numerical simulation method, and comparison between the diffraction grating phase diversity wavefront sensor and the traditional phase diversity wavefront sensor is also made. The simulation results show that the diffraction grating phase diversity wavefront sensor can obviously improve the ability to measure the wavefront aberration, especially the wavefront aberration with larger amplitude and higher spatial frequency.

Modified moment-matching method for estimating pointing parameters in the presence of atmospheric turbulence

An accurate pointing system is required in free-space optical (FSO) communication links. Low energy-transmission efficiency caused by pointing errors would decline the communication system’s performance. The statistics of the detected signal or return signal values could be used to estimate the pointing parameters, whereas atmospheric turbulence brings in serious challenges. A modified moment-matching estimation method is presented in this paper. The irradiance fluctuation caused by the atmospheric turbulence is considered, and the probability density function (PDF) in a weak turbulence condition is assumed to be lognormal. This modified approach is evaluated with wave-propagation simulation data and shows significant improvement over the conventional approach. The estimation accuracy and the properties of this new approach are also discussed. Although our method is based on lognormal irradiance PDF under a weak turbulence condition, the irradiance PDF would tend to be lognormal with aperture averaging effect under moderate to strong turbulence, and the ideas can be extended with appropriate PDF models to satisfy different conditions.An accurate pointing system is required in free-space optical (FSO) communication links. Low energy-transmission efficiency caused by pointing errors would decline the communication systems performance. The statistics of the detected signal or return signal values could be used to estimate the pointing parameters, whereas atmospheric turbulence brings in serious challenges. A modified moment-matching estimation method is presented in this paper. The irradiance fluctuation caused by the atmospheric turbulence is considered, and the probability density function (PDF) in a weak turbulence condition is assumed to be lognormal. This modified approach is evaluated with wave-propagation simulation data and shows significant improvement over the conventional approach. The estimation accuracy and the properties of this new approach are also discussed. Although our method is based on lognormal irradiance PDF under a weak turbulence condition, the irradiance PDF would tend to be lognormal with aperture averaging effect under moderate to strong turbulence, and the ideas can be extended with appropriate PDF models to satisfy different conditions.

Development of solar adaptive optics

对太阳大气进行大视场高分辨力光学成像观测是开展太阳物理、空间天气等基础与应用研究的重要前提。对于地基太阳望远镜而言，为了消除地球大气湍流对光学系统的影响，自适应光学是高分辨力成像观测必备的技术手段，与此同时，为了突破大气非等晕性对传统自适应光学校正视场的限制，近年来多层共轭自适应光学技术等大视场自适应光学得到极大发展。本文首先梳理国外太阳自适应光学系统研制情况，重点介绍国内太阳自适应光学技术发展及应用情况，并进一步介绍了后续大视场太阳自适应光学技术发展情况以及目前所取得的成果。

Prediction Control Algorithm for Close-Loop Adaptive Optical System

Abstract To reduce the effect of servo lag error in the close-loop adaptive optics system (AOS), a prediction control algorithm (PCA) is proposed in this paper. We describe a linear predictive controller that uses the historical voltages of deformable mirror to predict their future voltages in advance. The parameters of the predictor are optimized in real time by a modified Recursive Least-Square (RLS) algorithm. Numerical simulations are carried out to show the significant improvements brought by the predictor. The close-loop Strehl Ratio of the residual wave-front error and the power spectrum density of the residual voltages of deformable mirror are calculated as the evaluation criterions. The comparison results between the PCA and the classical proportional integral (PI) control algorithm show that the residual errors caused by servo lag in the system could be more effectively reduced by using the PCA than using the classical PI control algorithm. In addition, the control bandwidth of AOS is improved by using the PCA.