基于分区域耦合的随机并行梯度下降自适应光学

基于随机并行梯度下降(SPGD)方法的自适应光学(AO)系统通过直接优化系统的性能评价函数来控制波前校正器以补偿光束中存在的波前畸变。为了提高这种无模型优化自适应光学系统的收敛速度, 提出了基于分区域耦合的新方法以改进传统随机并行梯度下降自适应光学系统的工作方式。将波前校正器光学孔径分成多块子区域, 每块子区域对应着的所有驱动器作为一个整体控制单元, 从形式上可以得到一个空间分辨率较低的分区域波前校正器。该校正器与原校正器同步工作, 并采用随机并行梯度下降算法对同一个性能评价函数进行优化, 从而构成了双校正器的耦合工作结构。对256单元分立活塞式波前校正器建立了自适应成像系统的数值模型, 结果表明这种分区域耦合的随机并行梯度下降自适应光学系统比传统随机并行梯度下降自适应光学系统具有更快的收敛速度和更好的渐近态。     

Experimental validation of single-iteration multigrid wavefront reconstruction at the Palomar Observatory

Single-iteration multigrid (SIMG) wavefront reconstruction schemes were implemented and validated on the adaptive optics system at the Hale 5.1 m telescope at the Palomar Observatory. Results indicate that even the simplest such method produces a performance indistinguishable from that of the standard least-squares reconstructor for both bright and dim guide stars. SIMG provides a dramatic reduction in computational cost when compared to vector-matrix multiplication and can be implemented in parallel, making it the obvious choice for reconstruction in future large-scale adaptive optics systems.     

纯相位液晶空间光调制器拟合泽尼克像差性能分析

纯相位液晶空间光调制器作为波前校正器构成的高分辨率、低能耗、价格低廉、易于控制的自适应光学系统受到越来越多的关注.作为一种新型波前校正器件,它对波前像差的校正能力是反映其在自适应光学系统中应用的一个重要的指标,因此有必要仔细地研究它对各种像差的校正能力,以确定其可能的应用范围.波前校正器对各阶泽尼克像差的拟合效果有效地反映了该器件对不同像差的校正能力.利用256×256像素的纯相位液晶空间光调制器(LC-SLM)产生不同系数的前36项泽尼克像差分析LC-SLM对不同像差的校正能力.讨论了填充因子、离散像素 关键词： 液晶空间光调制器 相位调制 自适应光学 泽尼克多项式     

自适应光学系统的实时模式复原算法

 分析了自适应光学系统中实时模式复原算法的基本原理，建立了一种新型的传感器本征模复原算法。与常用的直接斜率法相比，这种模式复原算法可以有效减小探测噪声对复原计算过程的影响，提高系统的闭环稳定性和校正效果。在61单元自适应光学系统上实现了这种模式复原算法，并在实际大气湍流中对传感器本征模复原算法和直接斜率法进行了实验对比研究。     

自适应光学系统随机并行梯度下降控制算法实验研究

随机并行梯度下降算法是一种极具应用潜力的自适应光学系统控制算法,具有不依赖波前传感器直接对系统性能指标进行优化的特点。基于32单元变形镜、CCD成像器件等建立自适应光学系统随机并行梯度下降控制算法实验平台。考察算法增益系数和扰动幅度对校正效果和收敛速度的影响,验证随机并行梯度下降算法的基本原理。实验结果表明参量选取合适的情况下,随机并行梯度下降控制算法对静态或慢变化的畸变波前具有较好的校正能力。根据实验结果分析了影响随机并行梯度下降算法校正速度的主要因素。     

Iterative wave-front reconstruction for open-loop metrology applications: Shack-Hartmann and shearing interferometer wave-front sensing using Fourier or Multigrid reconstructors

Numerous techniques exist to reconstruct the wave-front in Shack-Hartmann and shearing interferometer wave-front sensors. Two of these techniques involve solving the least-squares wave-front reconstruction via Fourier transform or Multigrid methods. These two techniques operate on a square grid and are sensitive to both nonlinearities and to the methods used to extend the gradients beyond the aperture to the edges of the square grid. Both of these sensitivities can lead to large residual wave-front errors when reconstructing large aberrations. In this article we introduce an iterative computational loop in the reconstruction process which is designed to improve the reconstruction variances introduced by these parameters. This technique allows for accurate reconstruction of large phases which can arise in applications such as open-loop compensation of atmospheric turbulence, aberrometers and metrology. In this article, it is demonstrated that large aberrations can be accurately reconstructed utilizing this iterative method with Fourier or Multigrid reconstructors.     

高分辨力哈特曼传感器的快速波面重构和拼接

建立哈特曼传感器的模型,证明在高空间分辨力下,可以采用Hudgin模型进行波面重构,避免了采用Fried模型带来的复杂性。对哈特曼子孔径缺失破坏连续性的问题进行了分析,介绍了相应的边缘处理算法。完成了基于离散傅里叶变换的波面重构算法数值模拟,实现了波面的无损重构。针对实际应用中输入波面在被遮挡处不连续的问题,提出了基于最小二乘解的拼接方案,实现了非连续波面拼接。分析了影响波面重构速度的主要因素,提出了提高波面重构性能的方法。     

Beam cleanup of a 532-nm pulsed solid-state laser using a bimorph mirror

A successful beam cleanup of a 5-mJ/200-μs pulsed solid-state laser system operating at 532-nm wavelength is demonstrated. In this beam cleanup system, a wave-front sensor-less adaptive optics (AO) system is set up with a 20-element bimorph mirror (BM), a high-voltage amplifier, a charge-coupled device camera, and a control software implementing the stochastic parallel gradient descent (SPGD) algorithm. The brightness of the laser focal spot is improved because the wave-front distortions have been compensated. The performance of this system is presented and the experimental results are analyzed.     

自适应光学系统几种随机并行优化控制算法比较

 直接对系统性能指标进行优化是自适应光学系统中一种重要的波前畸变校正方法,选择合适的随机并行优化控制算法是该技术成功实现的关键。以32单元变形镜为校正器,基于多种随机并行优化算法建立自适应光学系统仿真模型。从算法的收敛速度、校正效果、局部极值3个方面对遗传算法、单向扰动随机并行梯度下降、双向扰动随机并行梯度下降及模拟退火算法进行了比较。仿真结果表明,遗传算法收敛速度太慢,不适用于需要实时控制的自适应光学系统;双向扰动随机并行梯度下降算法收敛速度、校正效果要优于单向扰动随机并行梯度下降,且能够适应各种情况下的扰动电压;模拟退火几乎以概率1收敛到全局极值附近,且收敛速度是上述算法中最快的。     

Wave-front reconstruction from multidirectional phase derivatives generated by multilateral shearing interferometers

To increase the accuracy of wave-front evaluation, we propose to exploit the natural capability of multiple lateral shearing interferometers to measure simultaneously more than two orthogonal phase derivatives. We also describe a method, based on Fourier-transform analysis, that uses this multiple information to reconstruct the wave-front under study.     

Adaptive-optics ultrahigh-resolution optical coherence tomography

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.     

高质量数字全息波面重建系统研究

基于阿贝成像理论对数字全息波面重建系统进行研究. 结果表明,利用成像系统对物光场进行变换,可以让物光场的高频角谱分量到达电荷耦合器件（CCD）探测器.为研究重建物光场质量,导出了CCD在光学系统后任意位置波面重建的计算公式,研究了物光通过傍轴光学系统到达CCD时波面重建系统的脉冲响应. 理论及实验研究表明,将CCD平面置于物光场像平面附近可以对物光场进行高质量重建. 关键词： 信息光学 数字全息 波面重建     

Free-space laser communications with adaptive optics: Atmospheric compensation experiments

Refractive index inhomogeneities of the turbulent air cause wave-front distortions of optical waves propagating through the atmosphere, leading to such effects as beam spreading, beam wander, and intensity fluctuations (scintillations). These distortions are responsible for severe signal fading in free-space optical communications systems and therefore compromise link reliability. Wave-front distortions can be mitigated, in principle, with adaptive optics, i.e., real-time wave-front control, reducing the likeliness of signal fading. However, adaptive optics technology, currently primarily used in astronomical imaging, needs to be adapted to the requirements of free-space optical communication systems and their specific challenges.In this chapter we discuss a non-conventional adaptive optics approach that has certain advantages with respect to its incorporation into free-space optical communication terminals. The technique does not require wave-front measurements, which are difficult under the strong scintillation conditions typical for communication scenarios, but is based on the direct optimization of a performance quality metric, e.g., the communication signal strength, with a stochastic parallel gradient descent (SPGD) algorithm.We describe an experimental adaptive optics system that consists of a beam-steering and a higher-resolution wave-front correction unit with a 132-actuator MEMS piston-type deformable mirror controlled by a VLSI system implementing the SPGD algorithm. The system optimizes the optical signal that could be coupled into a single-mode fiber after propagating along a 2.3-km near-horizontal atmospheric path. We investigate characteristics of the performance metric under different atmospheric conditions and evaluate the effect of the adaptive system. Experiments performed under strong scintillation conditions with beam-steering only as well as with higher-resolution wave-front control demonstrate the mitigation of wave-front distortions and the reduction of signal fading.     

Anomalous intensity of pinned speckles at high adaptive correction

Ground-based optical searches for faint stellar or planetary companions about other stars may be limited by speckle noise, which is the rapid intensity fluctuations that are due to motions of remnant atmospheric speckles. Adaptive optics (AO) can reduce residual wave-front phase errors to low values, substantially reducing the unwanted power in the speckle halo. At high correction, however, the noise in the halo will be dominated by anomalously bright "pinned" speckles that have a number of unusual properties. They can have negative intensities and will appear in spatially antisymmetric patterns; they are spatially pinned to Airy rings and have zero mean in a sufficiently long integration. Some of these properties may be used to reduce the unanticipated effect of pinned speckles on companion searches, depending on details of the AO system. But, in short exposures, pinned speckles dominate speckle noise over much of the inner halo for Strehl ratios S as low as 0.6 and over much of the outer halo too as Strehl and deformable-mirror actuator densities increase. I show that these anomalously bright pinned speckles are not included in the traditional expression for speckle power in an image, (1 - S), on which sensitivity estimates of future high-performance AO systems have been based.     

测量噪声对波前重构精度的影响

 Hartmann-Shack波前传感器斜率测量误差是自适应光学系统中影响波前重构精度的一个主要误差来源。针对模式法波前重构算法,推导出了普遍适用的模式法波前重构误差传递系数计算公式;利用61单元自适应光学系统中的Hartmann-Shack波前传感器测量的斜率数据,验证了模式法波前重构误差传递系数计算公式的正确性。     

A high precision phase reconstruction algorithm for multi-laser guide stars adaptive optics

Adaptive optics(AO) systems are widespread and considered as an essential part of any large aperture telescope for obtaining a high resolution imaging at present.To enlarge the imaging field of view(FOV),multi-laser guide stars(LGSs) are currently being investigated and used for the large aperture optical telescopes.LGS measurement is necessary and pivotal to obtain the cumulative phase distortion along a target in the multi-LGSs AO system.We propose a high precision phase reconstruction algorithm to estimate the phase for a target with an uncertain turbulence profile based on the interpolation.By comparing with the conventional average method,the proposed method reduces the root mean square(RMS) error from 130 nm to 85 nm with a 30% reduction for narrow FOV.We confirm that such phase reconstruction algorithm is validated for both narrow field AO and wide field AO.     

Combinational-deformable-mirror adaptive optics system for compensation of high-order modes of wavefront

A new kind of adaptive optics (AO) system, in which several low spatial frequency deformable mirrors(DMs) with optical conjugation relationship are combined to correct high-order aberrations, is proposed.The phase compensation principle and the control method of the combinational AO system are introduced.The numerical simulations for the AO system with two 60-element DMs are presented. The results indicate that the combinational DM in the AO system can correct different aberrations effectively as one single DM with more actuators, and there is no change of control method. This technique can be applied to a large telescope AO system to improve the spatial compensation capability for wavefront by using current DM.     

Multi-reference lens-less Fourier-transform holography with a Greek-ladder sieve array

Lens-less Fourier-transform holography has been actively studied because of its simple optical structure and its single-shot recording.However,a low-contrast interferogram between the reference and object waves limits its signal to noise ratio.Here,multi-reference lens-less Fourier-transform holography with a Greek-ladder sieve array is proposed in the experiment and demonstrated effectively to improve the signal to noise ratio.The key technique in our proposed method is a Greek-ladder sieve array,which acts as not only a wave-front modulator but also a beam splitter.With advantages of the common path,single shot,and no need for a lens,this system has enormous potential in imaging and especially in extreme ultraviolet and soft X-ray holography.     

无零级衍射干扰的彩色数字全息研究

彩色数字全息研究中,用球面波为重建波及角谱衍射公式进行波前重建是一种有效的方法.然而,重建图像上通常伴有强烈的零级衍射干扰.本文通过理论分析,对波前重建过程作了重要改进.用改进后的方法进行彩色数字全息的实验研究表明,重建彩色图像的质量获得显著改善. 关键词： 彩色数字全息 波前重建 零级衍射干扰     

近距离数字全息术记录和再现问题

讨论了记录距离小于菲涅耳衍射要求的近距离数字全息记录和再现问题。对全息记录与再现中高次相位的补偿问题进行了分析,证明了在CCD的参量和记录距离给定后,只要记录时使物体的大小、球面参考光波的位置和距离满足一定的条件,即使在记录距离小于菲涅耳衍射要求的最小距离情况下,也可将高次相位的影响补偿到足够小,使得近距离数字全息的数字再现仍可用快速傅里叶变换算法计算。推导出了满足高次相位补偿的条件和满足补偿条件时的数值再现计算公式。实验结果与理论分析的结论相吻合,并给出了一种修正实际记录的参考光和计算机模拟的理想参考光之间偏差的方法。