Dynamic eye model for adaptive optics testing

An artificial dynamic eye model is proposed. The prototype enabled us to introduce temporal variations in defocus and spherical aberration, resembling those typically found in the human eye. The eye model consisted of a meniscus lens together with a modal liquid crystal lens with controllable focus. A diffuser placed at a fixed distance from the lenses acted as the artificial retina. Developed software allowed the user to precisely control the dynamic generation of aberrations. In addition, different refractive errors could simultaneously be emulated by varying the distance between the components of the model. The artificial eye was first used as a dynamic generator of both spherical aberration and defocus, imitating the behavior of a real eye. The artificial eye was implemented in an adaptive optics system designed for the human eye. The system incorporated an electrostatic deformable mirror and a Hartmann-Shack wavefront sensor. Results with and without real time closed-loop aberration correction were obtained. The use of the dynamic artificial eye could be quite useful for testing and evaluating adaptive optics instruments for ophthalmic applications.     

全光路像差校正自适应光学系统中共模波前传感器标定方法研究

根据光波在介质中的传播规律,首先详细分析了全光路像差校正自适应光学系统的工作原理,然后对常规方法标定共模波前传感器后的系统校正残余误差作了分析,并从校正残余误差和操作的简易程度两个方面分析了常规方法标定共模波前传感器存在的缺点,最后针对全光路像差校正自适应光学系统的特点,提出了两种新的共模波前传感器标定方法,详细推导了两种方法标定共模波前传感器后全系统的校正残余误差.结果显示,两种方法标定共模波前传感器后,其校正残余误差只与常规方法标定共模波前传感器后系统的校正残余误差中的一种误差有关.     

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.     

High-resolution adaptive optics scanning laser ophthalmoscope with dual deformable mirrors

Adaptive optics scanning laser ophthalmoscopes have been used to produce noninvasive views of the human retina. However, the range of aberration compensation has been limited by the choice of deformable mirror technology. We demonstrate that the use of dual deformable mirrors can effectively compensate large aberrations in the human eye while maintaining the quality of the retinal imagery. We verified experimentally that the use of dual deformable mirrors improved the dynamic range for correction of the wavefront aberrations compared with the use of the micro-electro-mechanical-system mirror alone and improved the quality of the wavefront correction compared with the use of the bimorph mirror alone. We also demonstrated that the large-stroke bimorph deformable mirror improved the capability for axial sectioning with the confocal imaging system by providing an easier way to move the focus axially through different layers of the retina.     

Adaptive optics in microscopy

The imaging properties of optical microscopes are often compromised by aberrations that reduce image resolution and contrast. Adaptive optics technology has been employed in various systems to correct these aberrations and restore performance. This has required various departures from the traditional adaptive optics schemes that are used in astronomy. This review discusses the sources of aberrations, their effects and their correction with adaptive optics, particularly in confocal and two-photon microscopes. Different methods of wavefront sensing, indirect aberration measurement and aberration correction devices are discussed. Applications of adaptive optics in the related areas of optical data storage, optical tweezers and micro/nanofabrication are also reviewed.     

Open-loop correction of horizontal turbulence: system design and result

Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.2 lambda (lambda=0.633 microm) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the system's diffraction-limited resolution of 0.65 arc sec.     

Adaptive optics and vision

It has long been recognised that the optical quality of the human eye is far from diffraction limited. This affects our visual acuity and severely limits the resolution at which images of the living retina can be obtained. Adaptive optics is a technique that can correct for the eye's aberrations and provide diffraction limited resolution. The origins of the technique lie in astronomy, but it was successfully adapted to the human eye just over 10 years ago. Since then there have been rapid developments in the field of adaptive optics and vision science. In vivo images of the retina can now be routinely achieved with unprecedented resolution. Sophisticated experiments can be performed to gain a deeper knowledge of the interaction of neural retinal architecture and visual perception. This article presents the theory behind adaptive optics for the human eye and reviews the developments in this field to date.     

Super-resolution pupil filtering for visual performance enhancement using adaptive optics

Ocular aberration correction can significantly improve visual function of the human eye. However, even under ideal aberration correction conditions, pupil diffraction restricts the resolution of retinal images. Pupil filtering is a simple super-resolution (SR) method that can overcome this diffraction barrier. In this study, a 145-element piezoelectric deformable mirror was used as a pupil phase filter because of its programmability and high fitting accuracy. Continuous phase-only filters were designed based on Zernike polynomial series and fitted through closed-loop adaptive optics. SR results were validated using double-pass point spread function images. Contrast sensitivity was further assessed to verify the SR effect on visual function. An F-test was conducted for nested models to statistically compare different CSFs. These results indicated CSFs for the proposed SR filter were significantly higher than the diffraction correction (p < 0.05). As such, the proposed filter design could provide useful guidance for supernormal vision optical correction of the human eye.     

人眼像差哈特曼测量仪的重复性测试

人眼像差哈特曼测量仪利用 Hartmann-Shack 波前传感器探测波前,并用 Zernike 多项式实现波前的重建。测试结果的重复性对该仪器来说是至关重要的。此仪器对模拟人眼静态像差和活体人眼动态像差分别进行单帧采集和连续采集,所得的模拟人眼静态像差 Zernike 系数的标准偏差的最大值,PV 与 RMS 依次为 0.016?,0.061?和 0.006?;活体人眼动态像差 Zernike 系数的标准偏差的最大值,PV 与 RMS 依次为 0.045?,0.167? 和 0.030?,这些指标都小于 ?/5,表明该仪器具有较高的重复性。     

Adaptive optics for peripheral vision

Understanding peripheral optical errors and their impact on vision is important for various applications, e.g. research on myopia development and optical correction of patients with central visual field loss. In this study, we investigated whether correction of higher order aberrations with adaptive optics (AO) improve resolution beyond what is achieved with best peripheral refractive correction. A laboratory AO system was constructed for correcting peripheral aberrations. The peripheral low contrast grating resolution acuity in the 20° nasal visual field of the right eye was evaluated for 12 subjects using three types of correction: refractive correction of sphere and cylinder, static closed loop AO correction and continuous closed loop AO correction. Running AO in continuous closed loop improved acuity compared to refractive correction for most subjects (maximum benefit 0.15?logMAR). The visual improvement from aberration correction was highly correlated with the subject's initial amount of higher order aberrations (p?=?0.001, R ^2?=?0.72). There was, however, no acuity improvement from static AO correction. In conclusion, correction of peripheral higher order aberrations can improve low contrast resolution, provided refractive errors are corrected and the system runs in continuous closed loop.     

Adaptive Control of a Micromachined Continuous Membrane Deformable Mirror for Aberration Compensation

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基于脉动阵列的自适应光学实时波前处理机设计

针对自适应光学系统对波前处理机高计算量、高实时性的要求,本文提出了一种基于脉动阵列的自适应光学实时波前处理方法.该方法将脉动阵列的概念引入波前处理机设计,完成了波前斜率计算、复原运算和控制运算向脉动阵列的映射,合理地建立了数据的深度流水线,同时分析了以FPGA技术实现时系统的计算延时.对于48个子孔径,61单元的自适应光学系统,以一片Xilinx Virex-Ⅱ XC2V3000芯片实现了基于脉动阵列的实时波前处理机,实验测得计算延时仅8.6μs,结果表明该方法能极大地提高系统的实时性,集成度、通用性和扩展性.     

Efficient parallel adaptive aberration correction

Previous studies have shown that the parallel adaptive receive compensation algorithm (PARCA) can improve clinical images of the abdomen degraded by aberrations. Corrected images show both improved contrast resolution and overall image quality. However, PARCA is very computationally intensive and, consequently, cannot be easily implemented in real-time. In this study, modifications to PARCA for possible real-time implementation are explored. A modified algorithm, PARCA2 (parallel adaptive receive compensation algorithm 2), is introduced to perform aberration correction much more efficiently. Results from tissue-mimicking phantoms and human images show that corrections with PARCA2 rival that of PARCA.     

Adaptive optics technique to overcome the turbulence in a large-aperture collimator

A collimator with a long focal length and large aperture is a very important apparatus for testing large-aperture optical systems. But it suffers from internal air turbulence, which may limit its performance and reduce the testing accuracy. To overcome this problem, an adaptive optics system is introduced to compensate for the turbulence. This system includes a liquid crystal on silicon device as a wavefront corrector and a Shack-Hartmann wavefront sensor. After correction, we can get a plane wavefront with rms of about 0.017 lambda (lambda=0.6328 microm) emitted out of a larger than 500 mm diameter aperture. The whole system reaches diffraction-limited resolution.     

Microlens characterization by digital holographic microscopy with physical spherical phase compensation

Microlenses have been characterized by a digital holographic microscopy system, which is immune to the inherent wavefront aberration. The digital holographic microscopy system takes advantage of fiber optics and uses the light emitted directly from a single-mode fiber as the recording reference wave. By using such a reference beam, which is quasi-identical to the object beam, the inherent wavefront aberration of the digital holographic microscope is removed. The alignment of the optical setup can be optimized with the help of numerical reconstruction software to give the system phase with the off-axis tilt removed. There is one, and only one, reference fiber point position to give a reference wavefront that is quasi-identical to the object wavefront where the system is free of wavefront aberration and directly gives the quantitative phase of the test object without the need for complicated numerical compensation.     

Real-time turbulence profiling with a pair of laser guide star Shack-Hartmann wavefront sensors for wide-field adaptive optics systems on large to extremely large telescopes

Real-time turbulence profiling is necessary to tune tomographic wavefront reconstruction algorithms for wide-field adaptive optics (AO) systems on large to extremely large telescopes, and to perform a variety of image post-processing tasks involving point-spread function reconstruction. This paper describes a computationally efficient and accurate numerical technique inspired by the slope detection and ranging (SLODAR) method to perform this task in real time from properly selected Shack-Hartmann wavefront sensor measurements accumulated over a few hundred frames from a pair of laser guide stars, thus eliminating the need for an additional instrument. The algorithm is introduced, followed by a theoretical influence function analysis illustrating its impulse response to high-resolution turbulence profiles. Finally, its performance is assessed in the context of the Thirty Meter Telescope multi-conjugate adaptive optics system via end-to-end wave optics Monte Carlo simulations.     

Measurement of the repeatability of the prompt flashlamp-induced wavefront aberration on beamlines at the National Ignition Facility

We have undertaken a measurement campaign to determine the repeatability of the prompt flashlamp-induced wavefront aberration on beamlines at the National Ignition Facility (NIF) and determine the extent to which shot-to-shot variations in this aberration may degrade the performance of a proposed adaptive optics system for the short-pulse Advanced Radiographic Capability beamline on NIF. In this paper we will describe the unique NIF configuration that was required to make this measurement, present the results of the experiment, and discuss the implications of these results for the adaptive optics system design.     

Computational performance comparison of wavefront reconstruction algorithms for the European Extremely Large Telescope on multi-CPU architecture

The European Southern Observatory (ESO) is studying the next generation giant telescope, called the European Extremely Large Telescope (E-ELT). With a 42?m diameter primary mirror, it is a significant step from currently existing telescopes. Therefore, the E-ELT with its instruments poses new challenges in terms of cost and computational complexity for the control system, including its adaptive optics (AO). Since the conventional matrix-vector multiplication (MVM) method successfully used so far for AO wavefront reconstruction cannot be efficiently scaled to the size of the AO systems on the E-ELT, faster algorithms are needed. Among those recently developed wavefront reconstruction algorithms, three are studied in this paper from the point of view of design, implementation, and absolute speed on three multicore multi-CPU platforms. We focus on a single-conjugate AO system for the E-ELT. The algorithms are the MVM, the Fourier transform reconstructor (FTR), and the fractal iterative method (FRiM). This study enhances the scaling of these algorithms with an increasing number of CPUs involved in the computation. We discuss implementation strategies, depending on various CPU architecture constraints, and we present the first quantitative execution times so far at the E-ELT scale. MVM suffers from a large computational burden, making the current computing platform undersized to reach timings short enough for AO wavefront reconstruction. In our study, the FTR provides currently the fastest reconstruction. FRiM is a recently developed algorithm, and several strategies are investigated and presented here in order to implement it for real-time AO wavefront reconstruction, and to optimize its execution time. The difficulty to parallelize the algorithm in such architecture is enhanced. We also show that FRiM can provide interesting scalability using a sparse matrix approach.     

Application of eigenmode in the adaptive optics system based on a micromachined membrane deformable mirror

The construction process and characteristics of a deformable mirror eigenmode are introduced. The eigenmode of a 37-element micromachined membrane deformable mirror (MMDM) from OKO, Ltd. is analyzed. The Gaussian-Seidel low-order aberrations are fitted with eigenmodes as basic functions. An experimental adaptive optics (AO) system is constructed with the MMDM as the wavefront corrector, a deformable mirror eigenmode as the wavefront control algorithm, and a Shack-Hartmann wavefront sensor as the wavefront detector. The experimental results demonstrate that the deformable mirror eigenmode can act as the wavefront control algorithm for the AO system based on the MMDM.     

Optimal modal fourier-transform wavefront control

Optimal modal Fourier-transform wavefront control combines the speed of Fourier-transform reconstruction (FTR) with real-time optimization of modal gains to form a fast, adaptive wavefront control scheme. Our modal basis is the real Fourier basis, which allows direct control of specific regions of the point-spread function. We formulate FTR as modal control and show how to measure custom filters. Because the Fourier basis is a tight frame, we can use it on a circular aperture for modal control even though it is not an orthonormal basis. The modal coefficients are available during reconstruction, greatly reducing computational overhead for gain optimization. Simulation results show significant improvements in performance in low-signal-to-noise-ratio situations compared with nonadaptive control. This scheme is computationally efficient enough to be implemented with off-the-shelf technology for a 2.5 kHz, 64 x 64 adaptive optics system.