Closed-loop Adaptive Optics System Research Articles

Calibration approach of non-modulated pyramid wavefront sensors for improving the dynamic range

The pyramid wavefront sensor (PWFS) can provide high sensitivity in demanding adaptive optics applications. However, its exquisite sensitivity has a limited dynamic range, which relies on the use of beam modulation to improve the dynamic range of the PWFS—despite this task being achieved by moving optical components in the PWFS, increasing the complexity of the system. An attractive idea to simplify the optical and mechanical design of a PWFS is to work without any dynamic modulation. This paper proposes a new method, to the best of our knowledge, called the pyramid wavefront sensor with truncated axicon (TA-PWFS), which is used in the non-modulated PWFS in the closed-loop adaptive optics system with high sensitivity and high dynamic range without the need for cumbersome modulation. The new approach uses a diffractive element placed at a conjugated Fourier plane of the pyramid prism to shape part of the incident light in a ring pattern around the pyramid pin. The radius of the circular-shaped image is identical to that of the modulation radius in the sinusoidal modulation, offering a high dynamic range, while the remaining portion of the light continues un-diffracted, producing high sensitivity through a spot on the pyramid pin. Simulation results with different kinds of aberrations containing large global tilts reveal that the use of the TA-PWFS has a noticeable improvement in the aberration estimation performance. With a highly simplified structure, elements without mechanical modulation, and aberration reconstruction without precorrection of global tilt, the TA-PWFS could present an innovative method for the design of non-modulated pyramid wavefront sensors that can better suit the higher requirements of adaptive optics applications.

Image-to-image translation for wavefront and point spread function estimation

We develop and evaluate a new approach to phase estimation for observational astronomy that can be used for accurate point spread function reconstruction. Phase estimation is required where a terrestrial observatory uses an adaptive optics (AO) system to assist astronomers in acquiring sharp, high-contrast images of faint and distant objects. Our approach is to train a conditional adversarial artificial neural network architecture to predict phase using the wavefront sensor data from a closed-loop AO system. We present a detailed simulation study under different turbulent conditions, using the retrieved residual phase to obtain the point spread function of the simulated instrument. Compared to the state-of-the-art model-based approach in astronomy, our approach is not explicitly limited by modeling assumptions, e.g., independence between terms, such as bandwidth and anisoplanatism—and is conceptually simple and flexible. We use the open-source COMPASS tool for end-to-end simulations. On key quality metrics, specifically the Strehl ratio and Halo distribution in our application domain, our approach achieves results better than the model-based baseline.

Distortion Correction In Particle Image Velocimetry For Measurements Through Fluctuating Interfaces Using A Deep Neural Network And Wavefront Sensing

Imaging-based flow measurements like Particle Image or Particle Tracking Velocimetry re-quire an undistorted optical access to the measurement location. Changes of the refractive index in the light path can cause image deterioration and lead to severe measurement errors. While static aberration as e.g. from curved walls can be corrected by of means static correction optics or dedicated light ray models, this is hardly possible if the aberration change with time at high bandwidth. Many cases of this kind exists in fluid mechanics as e.g. free liquid jets, bubbles, droplets or film flows. These interfaces are related to several open research questions that concern the interaction between the media on both sides of the interface. Hardware-based corrections adapted from astronomy that employs a deformable membrane mirror, a wavefront sensor and a fast control electronics are a viable approach for solving this measurement problem. However, these closed-loop control schemes require extensive and dedicated hardware and are limited by the technical specifications of the adaptive optical mirror in terms of correction bandwidth, stroke and spatial resolution. Here we present an approach based on Multiple-Input deep convolutional neural network that works with a wavefront sensor only and spares the adaptive optical component. Based on a model experiment and a PIV measurement, we demonstrate that this system is able to correct for the errors that are induced by an oscillating water surface by 82 %. The technique can be an alternative to classical closed-loop adaptive optical systems where the performance of the actuators is not sufficient. This can open a perspective towards measurement of interface-related flows that were not accessible before.

Application of the General Extended-Object Wavefront Sensor into Aberration Correction of Lasers

Thermal aberrations are the main impediments for high-energy transverse multimode lasers to achieve high brightness and beam qualities. To address this issue using adaptive optics, traditional wavefront sensing methods have great limitations for aberration correction in high-energy transverse multimode lasers, because these lasers are no longer point sources. The general extended-object wavefront sensor (GEWFS) invented by the authors can work with extended sources in real time, and it is highly suitable to solve this problem. The application of the GEWFS into the compensation of thermal aberrations in a high-energy laser is preliminarily investigated by means of a simulation experiment. In this simulation experiment, a transverse multimode laser diode simulates a high-energy laser, a and phase plate thermal aberrations, which is compensated by a closed-loop adaptive optics system mainly composed of the GEWFS and a membrane deformable mirror. Simulation experimental results show that the peak intensity of the laser beam image is improved about 2.35 times, and the approximate Strehl ratio of the laser system attained 0.90. The results indicate that the GEWFS is simple and accurate enough for wavefront sensing from extended sources, and it will greatly expand the application fields of adaptive optics.

Real-Time Correction of a Laser Beam Wavefront Distorted by an Artificial Turbulent Heated Airflow

This paper presents a FPGA-based closed-loop adaptive optical system with a bimorph deformable mirror for correction of the phase perturbation caused by artificial turbulence. The system’s operating frequency of about 2000 Hz is, in many cases, sufficient to provide the real-time mode. The results of the correction of the wavefront of laser radiation distorted by the airflow formed in the laboratory conditions with the help of a fan heater are presented. For detailed consideration, the expansion of the wavefront by Zernike polynomials is used with further statistical analysis based on the discrete Fourier transform. The result of the work is an estimation of the correction efficiency of the wavefront distorted by the turbulent phase fluctuations. The ability of the bimorph adaptive mirror to correct for certain aberrations is also determined. As a result, it was concluded that the adaptive bimorph mirrors, together with a fast adaptive optical system based on FPGA, can be used to compensate wavefront distortions caused by atmospheric turbulence in the real-time mode.

Investigation of spatial and temporal characteristics of turbulent-distorted laser radiation during its dynamic phase correction in an adaptive optical system

We report the results of the analysis of experiments on dynamic phase correction of laser radiation distorted by a turbulent airflow in an adaptive optical system (AOS) with a Shack–Hartmann wavefront sensor (WFS). A field-programmable gate array is used as the main AOS control element, which provides a closed loop AOS bandwidth of up to 2000 Hz. The WFS is used to estimate the characteristic bandwidth of the generated turbulence ν turb. Changes in the spatial and temporal spectra of the laser radiation phase, as well as the quality of the beam during its dynamic phase correction at various AOS frequencies, are analysed. It is shown experimentally and by calculation that to ensure a high efficiency of the wavefront correction, the AOS frequency should be at least 20 times higher than ν turb.

Distortion correction for particle image velocimetry using multiple-input deep convolutional neural network and Hartmann-Shack sensing.

Aberrations degrade the accuracy of quantitative, imaging-based measurements, like particle image velocimetry (PIV). Adaptive optical elements can in principle correct the wavefront distortions, but are limited by their technical specifications. Here we propose an actuator-free correction based on a multiple-input deep convolutional neural network which uses an additional input from a wavefront sensor to correct time-varying distortions. It is applied for imaging flow velocimetry to conduct measurements through a fluctuating air-water phase boundary. Dataset for neural network is generated by an experimental setup with a deformable mirror. Correction performance of trained model is estimated in terms of image quality, which is improved significantly, and flow measurement results, where the errors induced by the distortion from fluctuating phase boundary can be corrected by 82 %. The technique has the potential to replace classical closed-loop adaptive optical systems where the performance of the actuators is not sufficient.

Global tip-tilt and high-order aberration correction with plenoptic wavefront sensors in closed-loop AO systems.

The plenoptic wavefront sensor (PlWFS) has been proposed as being suitable for operating on extended objects in adaptive optics (AO) systems. We propose a new self-referencing method that uses the cell image of PlWFS as a correlation reference and enables the simultaneous measurement of high-order aberrations and tilt. Simulations have been performed for different operating conditions to verify the feasibility of the proposed method. These show that using the cell image results in better AO performance compared to existing reference generation techniques and allows stable wavefront tilt control with a small residual variance.

Reconstruction of the High Resolution Phase in a Closed Loop Adaptive Optics System

Adaptive optics is a commonly used technique to correct the phase distortions caused by the Earth's atmosphere to improve the image quality of the ground-based imaging systems. However, the observe...

Efficient implementation of pseudo open-loop control for adaptive optics on Extremely Large Telescopes

Closed-loop adaptive optics systems that use minimum mean square error wavefront reconstruction require the computation of pseudo open-loop wavefront slopes. These techniques incorporate a knowledge of atmospheric statistics that must therefore be represented within the wavefront slope measurements. These pseudo open-loop slopes are computed from the sum of the measured residual slopes and the reconstructed slopes that would be given if the deformable mirror was flat, generally involving the multiplication of an interaction matrix with actuator demands from the previous time-step. When using dense algebra, this multiplication is computationally expensive for Extremely Large Telescopes, requiring a large memory bandwidth. Here, we show that this requirement can be significantly reduced, maintaining mathematical correctness and significantly reducing system complexity. This therefore reduces the cost of these systems and increases robustness and reliability.N

Modulation-nonmodulation pyramid wavefront sensor with direct gradient reconstruction algorithm on the closed-loop adaptive optics system.

A modulation-nonmodulation pyramid wavefront sensor with direct gradient reconstruction algorithm using on the adaptive optics is reported in this paper. This means that the interaction matrix of the system is obtained by using a modulated pyramid and the closed-loop control process is performed with a nonmodulated pyramid. The theoretical basis and simulation analysis of the direct gradient reconstruction algorithm are described in detail, and laboratory results show that the pyramid wavefront sensor based on this algorithm can work as expected in a closed-loop adaptive optics system.

High brightness laser based on Yb:YAG MOPA chain and adaptive optics system at room temperature.

We demonstrate a master oscillator power amplifier (MOPA) architecture based on Yb:YAG amplifiers and adaptive optics (AO) systems with a high power and high beam quality laser output. With two conduction cooled, dual-end-pumped Yb:YAG zigzag-slab amplifiers at room temperature, the fiber laser of 300 W was scaled to 11.9 kW. Moreover, AO system positioned downstream was utilized to correct wavefront of amplified laser. The beam quality β at maximum output power was 2.8 times diffraction limited with closed-loop AO system.

Ground-based adaptive optics coronagraphic performance under closed-loop predictive control

The discovery of the exoplanet Proxima b highlights the potential for the coming generation of giant segmented mirror telescopes (GSMTs) to characterize terrestrial—potentially habitable—planets orbiting nearby stars with direct imaging. This will require continued development and implementation of optimized adaptive optics systems feeding coronagraphs on the GSMTs. Such development should proceed with an understanding of the fundamental limits imposed by atmospheric turbulence. Here, we seek to address this question with a semianalytic framework for calculating the postcoronagraph contrast in a closed-loop adaptive optics system. We do this starting with the temporal power spectra of the Fourier basis calculated assuming frozen flow turbulence, and then apply closed-loop transfer functions. We include the benefits of a simple predictive controller, which we show could provide over a factor of 1400 gain in raw point spread function contrast at 1 λ/D on bright stars, and more than a factor of 30 gain on an I=7.5 mag star such as Proxima. More sophisticated predictive control can be expected to improve this even further. Assuming a photon-noise limited observing technique such as high-dispersion coronagraphy, these gains in raw contrast will decrease integration times by the same large factors. Predictive control of atmospheric turbulence should therefore be seen as one of the key technologies that will enable ground-based telescopes to characterize terrestrial planets.

Dynamic changes in higher-order aberrations after correction of lower-order aberrations with adaptive optics in myopic and emmetropic eyes.

This study investigated the instantaneous changes of higher-order aberrations (HOAs) following lower-order aberrations (LOAs) correction with a closed-loop adaptive optics (AO) system in myopic and emmetropic eyes. Data were analyzed using generalized additive mixed models. Time-related changes in HOAs were modeled with two-piecewise linear regressions and were compared between myopic and emmetropic eyes. Both vertical coma and spherical aberrations shifted to the positive direction immediately after LOA correction. The fluctuations of the above values were significantly faster in myopic than in emmetropic eyes. Understanding these changes in HOAs following LOA correction may help in achieving better visual outcomes.

The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments

The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental non-common path errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10-6 to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor suitable for real-time NCPE correction. This pupil-plane optic combines the apodizing phase plate coronagraph with a holographic modal wavefront sensor, to provide simultaneous coronagraphic imaging and focal-plane wavefront sensing using the science point spread function. We first characterise the baseline performance of the cMWS via idealised closed-loop simulations, showing that the sensor successfully recovers diffraction-limited coronagraph performance over an effective dynamic range of +/-2.5 radians root-mean-square (RMS) wavefront error within 2-10 iterations. We then present the results of initial on-sky testing at the William Herschel Telescope, and demonstrate that the sensor is able to retrieve injected wavefront aberrations to an accuracy of 10nm RMS under realistic seeing conditions. We also find that the cMWS is capable of real-time broadband measurement of atmospheric wavefront variance at a cadence of 50Hz across an uncorrected telescope sub-aperture. When combined with a suitable closed-loop adaptive optics system, the cMWS holds the potential to deliver an improvement in raw contrast of up to two orders of magnitude over the uncorrected QSS floor. Such a sensor would be eminently suitable for the direct imaging and spectroscopy of exoplanets with both existing and future instruments, including EPICS and METIS for the E-ELT.

First on-sky demonstration of the piezoelectric adaptive secondary mirror.

We propose using a piezoelectric adaptive secondary mirror (PASM) in the medium-sized adaptive telescopes with a 2-4m aperture for structure and control simplification by utilizing the piezoelectric actuators in contrast with the voice-coil adaptive secondary mirror. A closed-loop experimental setup was built for on-sky demonstration of the 73-element PASM developed by our laboratory. In this Letter, the PASM and the closed-loop adaptive optics system are introduced. High-resolution stellar images were obtained by using the PASM to correct high-order wavefront errors in May 2016. To the best of our knowledge, this is the first successful on-sky demonstration of the PASM. The results show that with the PASM as the deformable mirror, the angular resolution of the 1.8m telescope can be effectively improved.

적응광학계용 37채널 SiC 변형거울을 이용한 파면 보상

본 논문에서는 37채널을 갖는 적응광학계용 SiC(Silicon Carbide) 변형거울의 파면 보상 성능 검증에 관한 내용을 다룬다. 컴퓨터 시뮬레이션을 통해 SiC 변형거울의 파면 보상 성능을 예측하였고, 실제 closed-loop 적응광학계를 구성하여 파면 보상 성능을 확인 하였다. Closed-loop 적응광학계는 광원, 위상판, SiC 변형거울, 고속 샥-하트만 센서 그리고 제어용 컴퓨터로 구성되어있다. 회전하는 위상판에 의해 왜곡된 파면을 샥-하트만 센서로 측정하고, SiC 변형거울을 이용하여 왜곡된 파면을 보상해주는 시스템이다. 결과적으로 closed-loop 적응광학계에서 500 Hz의 속도로 PV(Peak-to-Valley) <TEX>$0.3{\mu}m{\sim}0.9{\mu}m$</TEX>, RMS(Root-Mean-Square) <TEX>$0.06{\mu}m{\sim}0.25{\mu}m$</TEX>의 왜곡된 파면을 PV <TEX>$0.1{\mu}m$</TEX>, RMS <TEX>$0.03{\mu}m$</TEX>이하로 보상시킬 수 있었다. In this paper, we deal with the wavefront compensation capability of a silicon carbide (SiC) deformable mirror (DM) with 37 actuators for adaptive optics. The wavefront compensation capability of the SiC DM is predicted by computer simulation and examined by actual experiments with a closed-loop adaptive optics system consistsing of a light source, a phase plate, a SiC DM, a high speed Shack-Hartmann sensor, and a control computer. Distortion of wavefront is caused by the phase plate in the closed-loop adaptive optics system. The distorted wavefront has a peak-to-valley (PV) wavefront error of <TEX>$0.3{\mu}m{\sim}0.9{\mu}m$</TEX> and root-mean-square (RMS) error of <TEX>$0.06{\mu}m{\sim}0.25{\mu}m$</TEX>. The high-speed Shack-Hartmann sensor measures the wavefront error of the distortion caused by the phase plate, and the SiC DM compensates for the distorted wavefront. The compensated wavefront has residual errors lower than <TEX>$0.1{\mu}m$</TEX> PV and <TEX>$0.03{\mu}m$</TEX> RMS. Consequently, we conclude that we can compensate for the distorted wavefront using the SiC DM in the closed-loop adaptive optics system with an operating frequency speed of 500 Hz.

Multichannel optimal control algorithm for adaptive optics system

In this paper we propose a multichannel optimal control algorithm (MOCA) which can be applied to practical real-time closed-loop adaptive optics system (AOS). The MOCA, doesn't like the traditional ones, considers both the effect of atmospheric turbulence and of the time delay. The control parameters are optimized timely through iterative calculations in order to let the system working at the best controlling state. Here we study, by the means of numerical simulations, the multichannel optimal control method with Recursive Least-Square (RLS) algorithm, considering a 61-element AOS as an example. Analyses were made on convergence, control performance and bandwidth of this algorithm. Compared with the classical optimal proportional-integral control algorithm, simulation results show that the residual error caused by the atmospheric turbulence and servo lag in the system are reduced more effectively by using MOCA which has good adaptability and could improve the close-loop bandwidth of the AOS.

Plenoptic camera wavefront sensing with extended sources

The wavefront sensor is used in adaptive optics to detect the atmospheric distortion, which feeds back to the deformable mirror to compensate for this distortion. Different from the Shack–Hartmann sensor that has been widely used with point sources, the plenoptic camera wavefront sensor has been proposed as an alternative wavefront sensor adequate for extended objects in recent years. In this paper, the plenoptic camera wavefront sensing with extended sources is discussed systematically. Simulations are performed to investigate the wavefront measurement error and the closed-loop performance of the plenoptic sensor. The results show that there are an optimal lenslet size and an optimal number of pixels to make the best performance. The RMS of the resulting corrected wavefront in closed-loop adaptive optics system is less than 108 nm (0.2λ) when D/r0 ≤ 10 and the magnitude M ≤ 5. Our investigation indicates that the plenoptic sensor is efficient to operate on extended sources in the closed-loop adaptive optics system.

Comparison of the Shack–Hartmann and plenoptic sensor in closed-loop adaptive optics system

The wavefront sensor is used in adaptive optics (AO) to detect the atmospheric distortion, which feeds back to the deformable mirror to compensate for this distortion. While the Shack–Hartmann sensor has been widely used, the plenoptic sensor was proposed in recent years. The two different wavefront sensing methods have different interpretations and numerical consequences, though they are both slope-based. The plenoptic sensor is compared with the Shack–Hartmann sensor in a closed-loop AO system. Simulations are performed to investigate their performances under closed-loop conditions. The plenoptic sensors both without and with modulation are discussed. The results show that the closed-loop performance of the plenoptic sensor without modulation is worse than that of the Shack–Hartmann sensor when the star for observation is brighter than magnitude 7, but better when the star is fainter. The closed-loop performance of the plenoptic sensor could be improved by modulation, except for the faint star. In summary, the limiting magnitude of the astronomical AO system may be improved by using the plenoptic sensor instead of the Shack–Hartmann sensor, and the modulation of the plenoptic sensor is more suitable for the bright star.