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.     

Configuration optimization of laser guide stars and wavefront correctors for multi-conjugation adaptive optics

Multi-conjugation adaptive optics(MCAOs) have been investigated and used in the large aperture optical telescopes for high-resolution imaging with large field of view(FOV).The atmospheric tomographic phase reconstruction and projection of three-dimensional turbulence volume onto wavefront correctors,such as deformable mirrors(DMs) or liquid crystal wavefront correctors(LCWCs),is a very important step in the data processing of an MCAO's controller.In this paper,a method according to the wavefront reconstruction performance of MCAO is presented to evaluate the optimized configuration of multi laser guide stars(LGSs) and the reasonable conjugation heights of LCWCs.Analytical formulations are derived for the different configurations and are used to generate optimized parameters for MCAO.Several examples are given to demonstrate our LGSs configuration optimization method.Compared with traditional methods,our method has minimum wavefront tomographic error,which will be helpful to get higher imaging resolution at large FOV in MCAO.     

Impact of telescope seeing on laser adaptive optics

Atmospheric turbulence in the internal light path of a telescope produces noncommon path phase differences that degrade the image quality of adaptive-optics systems employing laser guide stars. The spatial covariance and rms wavefront errors produced by this effect are derived. It is found that the tilt-removed wavefront error ranges from about 12 nm for a representative 30 m telescope to 59 nm for a 100 m telescope of similar optical design. For adaptive-optics systems that aim to derive tip-tilt information from the laser guide stars the error is considerably larger, ranging from 22 nm for the 30 m telescope to 256 nm for the 100 m telescope. This effect can be a significant impediment to the performance of such systems.     

Wavefront sensing issues in MCAO

The problematic of wavefront sensing in Multiconjugate Adaptive Optics (MCAO) is addressed in this article. We first focus on the sky coverage estimation which drives, in particular, the choice between natural and laser guide stars and therefore has a major impact on wavefront sensor design. Then a comparison between star oriented and layer oriented concepts is proposed. Analytical developments and optimization of the concepts are derived in the simplest MCAO case: the ground layer AO system. From this study, advantages and drawbacks of each concept are highlighted. To cite this article: T. Fusco et al., C. R. Physique 6 (2005).     

Adaptive optics microscopy with direct wavefront sensing using fluorescent protein guide stars

We introduce a direct wavefront sensing method using structures labeled with fluorescent proteins in tissues as guide stars. An adaptive optics confocal microscope using this method is demonstrated for imaging of mouse brain tissue. A dendrite and a cell body of a neuron labeled with yellow fluorescent protein are tested as guide stars without injection of other fluorescent labels. Photobleaching effects are also analyzed. The results shows increased image contrast and 3× improvement in the signal intensity for fixed mouse tissues at depths of 70?μm.     

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.     

FALCON: multi-object AO

FALCON is a wide-field, multi-object integral field spectrograph equipped with adaptive optics. It is dedicated to the study of the formation process of primordial galaxies. The AO system uses natural guide stars, and the high sky coverage required for these studies is obtained using tomographic techniques for the wavefront analysis. The structure of the OA system is very new, and particularly suited for a future implementation on extremely large telescopes. To cite this article: E. Gendron et al., C. R. Physique 6 (2005).     

自适应光学系统的最优斜率复原算法

分析了自适应光学系统的波前斜率校正效果，综合大气湍流扰动和测量噪声的统计特性，在最小二乘复原算法的基础上推导得出一种形式简洁、易于实际操作的最优斜率复原算法。在大气湍流波前扰动条件下，对这种最优斜率复原算法的补偿效果和工作稳定性进行了计算研究。     

Experimental investigation of a flowing fluid layer on metal surface roughness measurement and aberration correction using adaptive optics

The induced optical aberration of laser beam passing through a transparent flowing fluid layer on a metal specimen is experimentally and empirical formula studied. The proposed study presents an experimental investigation of metal surface roughness measurement by combining an optical probe of laser-scattering phenomena and adaptive optics (AO) for aberration correction. In the absence of the AO correction scheme, induced flow velocity of 0.278 m/s can severely degrade the residual wavefront root mean square (RMS) error to 0.58 μm and decrease the scattered laser intensity. Real-time AO correction in closed-loop at a sampling rate of 8Hz can reduce the wavefront RMS error to 0.19 μm and improve the attenuation of scattered laser intensity. The maximum relative error of the estimated roughness (R _a) is less than 7.8% compared with the stylus method. The experimental results show satisfactory correction in the presence of a flowing fluid layer using the AO system.     

自适应光学系统的模式法数值模拟

建立了利用模式法笃自适应光学系统进行数值模拟的理论模型，编制了计算程序，并与激光大气传输计算程序衔接起来，进行了大量数值模拟计算。首次发现：存在泽尼特多项式展开的最佳项数。大于一定项数的展开式的效果迅速变坏，竖排和斜排经特面式展开有类似的结果。文献中认为可以采用的１５项经特多项式展开的效果不好，最佳项数随着横向风速的增加而减小，在风速较大时最佳项数下的模范地结果稍好于直接斜率控制法的结果。     

Optimizing Rayleigh laser guide star range-gate depth during initial loop closing

The use of a Rayleigh laser guide star (LGS) as a wavefront reference for adaptive optics (AO) allows complete control over the distance to, and depth of, the slice of the Rayleigh plume that is selected. By altering the LGS range-gate depth (RGD) and distance at three defined stages during closed-loop operations, the LGS performance can be optimized. For the given example of a 20 km LGS on a 4.2 m telescope the application of this technique would increase the permissible RGD from 200to930 m after optimization. This can allow either an increase in LGS altitude or an increase in wavefront sensor frame rate, thereby increasing AO system performance.     

Experimental validation of Fourier-transform wave-front reconstruction at the Palomar Observatory

Wave-front reconstruction with use of the Fourier transform has been validated through theory and simulation. This method provides a dramatic reduction in computational costs for large adaptive (AO) systems. Because such a reconstructor can be expressed as a matrix, it can be used as an alternative in a matrix-based AO control system. This was done with the Palomar Observatory AO system on the 200-in. Hale telescope. Results of these tests indicate that Fourier-transform wave-front reconstruction works in a real system. For both bright and dim stars, a Hudgin-geometry Fourier-transform method produced performance comparable to that of the Palomar Adaptive Optics least squares. The Fried-geometry method had a noticeable Strehl ratio performance degradation of 0.043 in the K band (165-nm rms wave-front error added in quadrature) on a dim star.     

自适应光学系统的数值模拟：直接斜率控制法

采用直接斜率控制法完整地实现了对自适应光学（ＡＯ）系统的数值模拟，系统研究了带自适应光学校正的激光大气传输规律。提出了对计算出的位相进行了“剪接”的办法，解决了残余位相方差与自适应光学的校正效果没有对应关系的问题。证明可以把快速傅里叶变换法（ＦＦＴ）用于透镜成像的聚焦计算，与积分法得到相同的结果，报道了对于一定的延迟时间，当大气横向风速大于一个阈值时，自适应光学补偿比安全相位补偿的效果还要好，表明     

湍流大气中哈特曼传感器的模式波前复原误差

 分别采用Zernike和Karhunen-loeve两种模式波前复原法,分析了子孔径斜率测量不受噪声影响的理想情况下,哈特曼传感器对大气湍流畸变波前的模式复原误差与大气湍流相干长度、传感器的结构尺寸、模式复原阶数等的关系。结果表明Karhunen-loeve模式法比Zernike模式法的波前复原误差更小些。     

On practical aspects of Laser Guide Stars

The Gemini Observatory has developed an extensive Adaptive Optics (AO) program, including Classical AO, Laser Guide Star (LGS) AO, Multi-Conjugate AO (MCAO), extreme AO (eXAO) and Ground Layer AO (GLAO). Most of these instruments use one or several LGSs. A laser has been in operation at Gemini since May 2005. Most of the laser related systems (beam transport, launch, safety systems) have been developed in house. These are major components, requiring a development effort not to be underestimated. In this article, we propose to share the Gemini experience in terms of practical issues and calibration requirements associated with the use of lasers in AO. To cite this article: F. Rigaut, C. d'Orgeville, C. R. Physique 6 (2005).     

Optimization of the Hartmann-Shack microlens array

In this work we propose to optimize the microlens-array geometry for a Hartmann-Shack wavefront sensor. The optimization makes possible that regular microlens arrays with a larger number of microlenses are replaced by arrays with fewer microlenses located at optimal sampling positions, with no increase in the reconstruction error. The goal is to propose a straightforward and widely accessible numerical method to calculate an optimized microlens array for a known aberration statistics. The optimization comprises the minimization of the wavefront reconstruction error and/or the number of necessary microlenses in the array. We numerically generate, sample and reconstruct the wavefront, and use a genetic algorithm to discover the optimal array geometry. Within an ophthalmological context, as a case study, we demonstrate that an array with only 10 suitably located microlenses can be used to produce reconstruction errors as small as those of a 36-microlens regular array. The same optimization procedure can be employed for any application where the wavefront statistics is known.     

Wavefront response matrix for closed-loop adaptive optics system based on non-modulation pyramid wavefront sensor

Pyramid wavefront sensor (PWFS) is a kind of wavefront sensor with high spatial resolution and high energy utilization. In this paper an adaptive optics system with PWFS as wavefront sensor and liquid-crystal spatial light modulator (LC-SLM) as wavefront corrector is built in the laboratory. The wavefront response matrix is a key element in the close-loop operation. It can be obtained by measuring the real response to given aberrations, which is easily contaminated by noise and influenced by the inherent aberration in the optical system. A kind of analytic solution of response matrix is proposed, with which numerical simulation and experiment are also implemented to verify the performance of closed-loop correction of static aberration based on linear reconstruction theory. Results show that this AO system with the proposed matrix can work steadily in closed-loop operation.     

自适应光学系统的数值模拟：噪音和探测误差的效应

噪音和探测误差是影响自适应光学系统性能的三个主要因素之一。噪音和探测误差使哈特曼－夏克（Hartmann-Shack)波前传感器所测得的华斜量产生误差,进而影响整个自适应光学系统的性能,建立了对噪声和探测误差对哈特曼－夏克波前传感器的影响进行数值模拟的理论模型,编制了计算程序,与已有的激光大气传输与自适应光学系统的计算程序相衔接,进行了模拟计算,对有限的离散采样,读出噪音和光子噪音的效应作了数值模拟研究,获得了一些对于实际的自适应光学系统的最佳设计有价值的结果。     

采用阵列波带片波前传感的激光波前重构

 在高功率激光系统，激光束小尺度相位畸变波前探测是实现激光束中频段波前补偿与控制的重要前提。采用阵列菲涅耳波带片作为激光束波前传感的子孔径分割器，利用区域重构的Southwell模型进行波前重构。并针对波前传感器子孔径数目、小尺度相位畸变波前的参数对波前重构精度影响进行分析与模拟计算，给出了阵列菲涅耳波带片合理的子孔径数目，为20×20～30×30；对小尺度相位畸变，激光束波前的重构误差小于十分之一波长。     

波荡器位相误差对自由电子激光

对波荡器位相误差对自由电子激光小信号增益的影响进行了分析讨论。对低增益情况解析地给出了波荡器位相误差引起的增益降低因子。结果表明:与自发辐射类似增益的降低与位相误差的方差成简单的指数关系，位相误差的线性变化部分引起最大增益的位移，还表明波荡器位相误差存在时Madey定理仍然成立。对高增益情况也给出了波荡器位相误差引起的增益降低因子。