相位不连续点对自适应光学的影响

 分析了激光在湍流大气中传输时，若光束波前中出现相位不连续点，自适应光学校正能力降低的原因，通过数值模拟计算了当光束波前中出现相位不连续点时对自适应光学校正的影响，表明在传输过程中出现不连续点的情况下，为了提高自适应光学的校正能力，必须要考虑不连续相位的影响。     

Real-time blind deconvolution of retinal images in adaptive optics scanning laser ophthalmoscopy

With the use of adaptive optics (AO), the ocular aberrations can be compensated to get high-resolution image of living human retina. However, the wavefront correction is not perfect due to the wavefront measure error and hardware restrictions. Thus, it is necessary to use a deconvolution algorithm to recover the retinal images. In this paper, a blind deconvolution technique called Incremental Wiener filter is used to restore the adaptive optics confocal scanning laser ophthalmoscope (AOSLO) images. The point-spread function (PSF) measured by wavefront sensor is only used as an initial value of our algorithm. We also realize the Incremental Wiener filter on graphics processing unit (GPU) in real-time. When the image size is 512 × 480 pixels, six iterations of our algorithm only spend about 10 ms. Retinal blood vessels as well as cells in retinal images are restored by our algorithm, and the PSFs are also revised. Retinal images with and without adaptive optics are both restored. The results show that Incremental Wiener filter reduces the noises and improve the image quality.     

In situ roughness monitoring of sputtered Pt thin film under dynamic turbulence using adaptive optics

A new optical monitoring system for rapid and in situ surface roughness measurement of Pt film on silicon is developed in this study. The in-process measurement is achieved by combining an optical probe of laser-scattering phenomena and adaptive optics for aberration correction. Platinum (Pt) thin film is selected due to the extensive utilization in semiconductor industry and excellent chemical inertness. The aim of this study was to demonstrate the necessity for adaptive optics (AO) compensation in regions containing room-temperature turbulences. Measurement results of eight Pt films (roughness ranging from 58 to 83 nm) sputtered on top of P-type silicon wafer demonstrate excellent correlation between the peak power and average roughness with a correlation coefficient (R²) of 0.9962 and a trend equation for predicting the surface roughness of Pt thin films is obtained as y = 9E07x^−3.783. Roughness average (Ra) of Pt thin films (x) of can be directly determined from the peak power (y) using the proposed method under dynamic disturbance. Furthermore, the proposed AO-assisted system is in good agreement with stylus method and less than 1.1% error values are obtained for the aforementioned average sample roughness.     

Coherent beam combination of an optical array using adaptive fiber optics collimators

A novel adaptive-fiber-optics-collimator (AFOC) compensating both piston-type and tip/tilt-type phase errors of output beam is introduced, and has been employed in experiments of coherent beam combination (CBC) of a delta distributed fiber array. Feedback control is realized using stochastic parallel gradient descent (SPGD) algorithm. Excellent CBC effect has been achieved when piston and tip/tilt errors among beamlets corrected. The necessity of wavefront tip/tilt control in CBC is verified. Experimental results exhibit great potential applications of this kind of AFOC in fiber amplifier arrays.     

Experimental explorations of the high-order Gaussian mode transformation based on blind-optimization adaptive optics

We present research on high-order Gaussian laser beam transformation by using adaptive optics (AO) technique. Numerical simulation for a mode transformation system based on stochastic parallel gradient descent (SPGD) algorithm was accomplished, along with experimental results obtained for a transformation of TEM₂₀ mode with 37 control channels. The far-field energy centrality of TEM₂₀ mode was advanced and the evaluation function increased more than a factor of 6 after transformation. The experiments results indicate the feasibility of blind-optimization AO in mode transformation system.     

High sensitivity adaptive optics control of laser beam based on interferometric phase-front detection

We present an upgrade of an adaptive optics (AO) system for the control of geometrical fluctuations in a laser beam, based on the interferometric detection of phase front. Acoustic isolation and suitable design of optical system make the present system very sensitive to laser beam geometrical fluctuations and can be an interesting step toward the active correction of the small perturbations of the input beam of gravitational wave interferometric detectors.     

自适应光学系统时域特性分析

分析了激光导星发出的波前通过湍流大气所造成的波前相位畸变的时间功率谱 ;根据湍流相位时间功率谱和自适应光学系统的传递函数 ,比较了使用自然导星和激光导星时自适应光学系统的时间响应误差 ;讨论了使用自然导星时自适应光学系统的时域优化问题。     

Improve the loop frequency of liquid crystal adaptive optics by concurrent control technique

A method to improve the loop frequency of liquid crystal adaptive optics systems (LC AOSs) was investigated. Based on open-loop control, a concurrent control scheme was proposed and the correction frequency of the LC AOS was increased from 85 Hz to 212 Hz. A simulated correction analysis illustrates that the distortion rejection ability of our LC AOS was improved with −1.1 dB by using the concurrent control technique. A 500 m horizontal turbulence correction experiment was done to validate our considerations. Firstly, 500 m horizontal turbulence was measured and its Greenwood frequency and atmospheric coherence length is 31.5 Hz and 4-5 cm, respectively. Then, 500 m horizontal turbulence was corrected in real time by using consecutive and concurrent control methods. The corrected results indicate that the resolution and contrast of the image was significantly improved by the concurrent control method. These results show that the concurrent control method can be used for LC AOSs to improve the correction frequency.     

Shaft diameter measurement using a digital image

In this paper, a non-contact and high-precision method based on the processing of a digital image is presented to measure the diameter of a shaft. The method mainly involves three steps: first, the camera is calibrated by an improved approach, which only uses the feature points in the measurement area of the image to optimise the local camera model; second, with the help of the parameters of the model in the first step, a measurement method for determining the shaft diameter is proposed; finally, to embody the spatial attitude of the shaft accurately, the extrinsic parameters are re-calibrated by measuring a shaft whose diameter is known, and then the precision of measurement is improved by means of the new extrinsic parameters. The experimental data demonstrate that the proposed method exhibits high precision, with relative errors of approximately 0.005 mm.     

液晶空间光调制器在自适应光学中的应用

本文主要包括两方面的内容,一方面描述用液晶光阀产生空间与时间性能可控的小尺寸薄相位屏;另一方面则讨论用液晶空间光调制器实现波前相位畸变校正的可能性。从而表明,这类价格低廉、结构简单而性能可靠的非线性光学器件可望在高性能新型自适应光学系统中得到广泛应用。     

Experimental validation of closed-loop adaptive optics based on a self-referencing interferometer wavefront sensor and a liquid-crystal spatial light modulator

A closed-loop adaptive optics system based on a self-referencing interferometer wavefront sensor (SRI WFS) using phase-shifting point-diffraction technique and an electrically addressed phase-only liquid-crystal spatial light modulator (LC SLM) is built and validated experimentally. The wavefront aberration of incoming beam is directly measured by analyzing four frames phase-shifted interferograms captured by a single CCD camera in two camera shots, and then by loading the conjugate function onto the SLM the wavefront aberration is corrected. The proposed scheme does not rely on any complicated control algorithm or wavefront-reconstruction algorithm and can achieve high-resolution and high-accuracy correction. Closed-loop correction results of single order Zernike aberrations and a Kolmogorov turbulence phase screen show that considerable improvements in the Strehl ratio (of greater than 0.94) is achieved.     

Performance simulation of an adaptive optics system with liquid crystal corrector under closed-loop and open-loop control

The adaptive optics system (AOS) often operates in a discrete sampling process with finite closed-loop frequency. Reconstruction, detection, and time lag induced errors are the main correction errors of the system. An AOS that is based on a liquid crystal (LC) benefits from the LC’s high correction precision, thus the reconstruction error can be ignored. The primary error will be induced by the time lag from the time of detection to the time of compensation. In this paper, some theoretical simulations are introduced in order to evaluate the correction precision of AOS with an LC corrector. The main purpose is to compare the correction precision between the open-loop and closed-loop control. We attempt to find a method to ascertain the exact precision of the open-loop control and show whether it improves the correction precision. The conclusion is thus reached that the actual error rejection bandwidth for the closed-loop was lower than the −3 dB error rejection bandwidth measured in practice. The increased refresh frequency of the open-loop control can improve the imaging performance to nearly −3 dB bandwidth of the detector measured, which is the maximum possible bandwidth due to the time lag.     

单双层共轭系统的共轭高度对等晕角的影响

 简要介绍了传统自适应光学系统的局限性和多层共轭自适应光学基本原理。模拟了单双层共轭校正系统的共轭高度,并结合平程与垂程(HV模型)两种传输状态对系统等晕角增益作了进一步的分析。对单层共轭系统,在20 km的传输距离内,在平程中整个区域都属于等晕角放大区,共轭高度的最佳位置在传输距离的中间（约10 km处）,等晕角取极大值,增益效果较好;但垂程中增益效果变差,且等晕角放大区也仅在3.6 km之内。对双层共轭系统来说,第1层共轭高度的变形镜主要对近距离畸变波前进行校正,并对整个传输距离的等晕角影响很大,是双层共轭系统的关键因素;第2层共轭高度对远距离等晕角影响较大。     

自适应光学进展及展望

1953年Babcock首先提出自适应光学的概念,近四十年来自适应光学在理论、系统及单元技术研究方面取得突出进展,正步入成熟应用期,当今世界上任何大型光学系统的研究均离不开自适应光学的支持,其应用范围正向各个门类的光学仪器扩展。     

Experimental investigation of segmented adaptive optics for spatial laser array in an atmospheric turbulence condition

We describe a concept of using segmented adaptive optics (AO) to correct the higher-order wavefront aberrations (HOWAs) in an atmospheric turbulence condition. The effects of piston and tip-tilt corrections are analyzed by numerical simulation. An experiment test-bed is built for verification. Seven spatial beamlets are combined into a hexagonal array by use of a pyramid, and an active segmented mirror is used for phase modulation. The AO control is implemented by stochastic parallel gradient descent (SPGD) algorithm. Experimentally, after segmented AO is enabled, the HOWA in warmed air environment has been effectively corrected and the peak intensity of far-field has been enhanced by 5 times.     

微变形镜在自适应光学中的应用与发展

自适应光学系统主要用于校正由大气湍流等引起的波前畸变,经过半个多世纪的发展,自适应光学系统已经应用到了许多领域。传统自适应光学系统中的变形镜在许多情况下已经不能满足自适应光学新发展的需要。随着微制造技术的发展,用微制造技术制作的新型微变形镜大大提高了致动单元的集成度,简化了电源,提高了可靠性,降低了自适应光学系统的体积和造价。综述了微变形镜技术的发展情况及其在自适应光学中的应用,并且讨论了其发展前景。     

用于白天自适应光学的波前探测方法分析

提出了两种视场偏移哈特曼波前传感器，用于探测白天条件下强背景中目标信号的波前信息.给出了哈特曼波前传感器子孔径中焦面上天光背景非均匀性分布特征的实验结果，详细分析了基于分光棱镜的双焦面视场偏移哈特曼波前传感器的探测误差，并通过基于倾斜镜的单焦面视场偏移哈特曼波前传感器实验给出了波前测量的结果，证明视场偏移哈特曼波前传感器能够精确、稳定的测量白天条件下目标信号的畸变波前信息，同时给出了其在白天条件下探测能力的估算结果及其受限因素，估算结果表明云南天文台的自适应光学系统在10 W/m²·sr天光背景条件下应用视场偏移哈特曼波前传感器能对5等星(可视星等值)目标进行准确的波前探测. 关键词： 自适应光学 波前传感器 白天工作 误差分析     

自适应光学系统的带宽及稳定性

自适应光学系统的带宽及稳定性与 Hartmann- Shack波前传感器的积分时间τs、信号采集与处理时间τd 及环路增益 g有关。自适应光学系统的校正带宽约为 1/ (10τs) ,τd及 g对系统带宽有一定影响 ;系统对于 1/ (2 0τs)以下频率范围内的扰动具有较好的拟制效果 ;τd ≥τs时 ,系统不稳定 ,减小 g可使系统稳定     

Scattering by random rough surfaces: Study of direct and inverse problem

In order to study the problems of scattering by rough metallic surfaces, we have used Maxwell’s equations in covariant form within the framework of a non-orthogonal coordinates system adapted to the geometry of the problem. Electromagnetic fields are written in Fourier’s integral form. The solution is found by using a perturbation method applied to the smooth surface problem; this is fully justified when the defects are of small magnitude.For the direct problem, the mean value of diffraction intensity is obtained for random rough surfaces of finite conductivity by computer simulation.In the case of the inverse problem, the reconstruction of the profile of the metal surface from values of the diffraction intensity, obtained by simulation, is found using an iterative algorithm.     

Coma measurement using a PSM and transmission image sensor

As feature size decreases, especially with the use of resolution enhancement technique, requirements for the coma aberrations in the projection lenses of the lithographic tools have become extremely severe. So, fast and accurate in situ measurement of coma is necessary. In the present paper, we present a new method for characterizing the coma aberrations in the projection lens using a phase-shifting mask and a transmission image sensor. By measuring the image positions at multiple NA and partial coherence settings, we are able to extract the coma aberration. The simulation results show that the accuracy of coma measurement increases approximately 20% compared to the previous straightforward measurement technique.