Adaptive Optics Correction Research Articles

Defocused contrast sensitivity function in peripheral vision.

Human peripheral detection performance is affected by optical factors such as defocus and higher order aberrations. From optical theory, we would expect defocus to produce local depressions (notches) in the contrast sensitivity function (CSF). However, such notches have not been observed in peripheral vision, and it is unknown whether human peripheral vision can detect local depressions (notches) in the CSF, such as those produced by monochromatic defocus when all monochromatic ocular aberrations are corrected. The purpose of the study was to identify such notches. Participants were three adult emmetropes. Following full adaptive optics correction, on-axis and 20° nasal visual field detection CSFs in monochromatic light were measured for the right eye with a 7mm diameter pupil, both without and with ±2 D defocus, and with separate determinations for horizontal and vertical gratings. Defocused CSFs were compared with predictions based on theoretical modulation transfer functions. Notches in the monochromatic defocused CSFs were identified for peripheral vision at optically predicted spatial frequencies with other monochromatic ocular aberrations corrected, provided that there was adequate spatial frequency sampling. The spatial frequencies of notches were similar to those predicted from optical theory, but their depths (0.3 to 0.9 log unit) were smaller than predicted. With fine spatial frequency sampling, notches were identified in defocused monochromatic CSFs when all other monochromatic ocular aberrations were corrected, both on-axis and at 20° eccentricity. Unless recognised as such, notches may contribute to noise in through-focus detection measurements of peripheral visual performance.

Debris collision mitigation from the ground using laser guide star adaptive optics at mount Stromlo observatory

The Australian National University (ANU) and industry partners EOS Space Systems and Lockheed Martin are part of the Space Environment Research Centre (SERC), an Australian-Government funded cooperative research centre headquartered in Canberra, Australia. SERC aims to mitigate the threat of collisions with space debris using a ground-based high power (10-20kW) infrared (IR) laser to modify the debris orbit should a collision be predicted. To that end, the ANU has equipped the EOS 1.8m laser tracking station telescope on the ANU Mount Stromlo campus with a state-of-the-art Laser Guide Star Adaptive Optics (LGS AO) system to pre-compensate the IR laser prior to propagation through earth's atmosphere. The ANU LGS AO system is currently being integrated with the EOS telescope and the high-power IR laser. This paper provides a brief overview of the LGS AO system and focus more specifically on the design, integration and commissioning of the first LGS facility ever constructed in Australia. The LGS Facility includes two sodium guide star laser sources which can be either propagated separately or combined so as to produce a brighter reference star to maximise adaptive optics correction. The two lasers, their beam combining optics, their common beam transfer optics, and the laser launch telescope are hereby described.

Effects of adaptive optics on bit error rate of M-ary PPM oceanic optical wireless communication systems with aperture averaging in strong turbulence

Scintillation is the result of oceanic turbulence reducing the bit error rate (BER) performance of oceanic optical wireless communication (OWC) systems. The scintillation, also known as intensity fluctuations, occurs due to the turbulence-induced wavefront deformations. The correction of deformations by adaptive optics (AO) reduces the scintillation effect of turbulence and results in improved BER performance. In this paper, an oceanic OWC (OOWC) system that has a Gaussian laser beam at the transmitter, finite-sized circular aperture at the receiver, employing M-ary pulse position modulation (PPM) and operating in strong oceanic turbulence, is considered. Improvement in the BER performance of the OOWC system is examined with the implementation of AO correction. Comparison of BER performances between the AO and non-adaptive optics OOWC systems is shown by calculating the metric defined. BER of M-ary PPM OOWC links is evaluated over gamma–gamma fading channels. The modified Rytov theory together with the Zernike filter functions is used to find the AO corrected aperture averaged scintillation index where extended Huygens–Fresnel technique is used to obtain the average received signal power.

Experimental determination of shift-less aberration bases for sensorless adaptive optics in nonlinear microscopy.

Adaptive optics can improve the performance of optical systems and devices by correcting phase aberrations. While in most applications wavefront sensing is employed to drive the adaptive optics correction, some microscopy methods may require sensorless optimization of the wavefront. In these cases, the correction is performed by describing the aberration as a linear combination of a base of influence functions, optimizing an image quality metric as a function of the coefficients. The influence functions base is generally chosen to either efficiently represent the adaptive device used or to describe generic wavefronts in an orthogonal fashion. A rarely discussed problem is that most correction bases have elements which introduce, together with a correction of the aberration, a shift of the imaging field of view in three dimensions. While simple methods to solve the problem are available for linear microscopy methods, nonlinear microscopy techniques such as multiphoton or second harmonic generation microscopy require non-trivial base determination. In this paper, we discuss the problem, and we present a method for calibrating a shift-less base on a spatial light modulator for two-photon microscopy.

High-resolution structural and functional deep brain imaging using adaptive optics three-photon microscopy

Multiphoton microscopy has become a powerful tool with which to visualize the morphology and function of neural cells and circuits in the intact mammalian brain. However, tissue scattering, optical aberrations and motion artifacts degrade the imaging performance at depth. Here we describe a minimally invasive intravital imaging methodology based on three-photon excitation, indirect adaptive optics (AO) and active electrocardiogram gating to advance deep-tissue imaging. Our modal-based, sensorless AO approach is robust to low signal-to-noise ratios as commonly encountered in deep scattering tissues such as the mouse brain, and permits AO correction over large axial fields of view. We demonstrate near-diffraction-limited imaging of deep cortical spines and (sub)cortical dendrites up to a depth of 1.4 mm (the edge of the mouse CA1 hippocampus). In addition, we show applications to deep-layer calcium imaging of astrocytes, including fibrous astrocytes that reside in the highly scattering corpus callosum.

Study on Aberration Correction of Adaptive Optics Based on Convolutional Neural Network

The existence of aberrations has always been an important limiting factor in the imaging field. Especially in optical microscopy imaging, the accumulated aberration of the optical system and the biological samples distorts the wavefront on the focal plane, thereby reducing the imaging resolution. Here, we propose an adaptive optical aberration correction method based on convolutional neural network. By establishing the relationship between the Zernike polynomial and the distorted wavefront, with the help of the fast calculation advantage of an artificial intelligence neural network, the distorted wavefront information can be output in a short time for the reconstruction of the wavefront to achieve the purpose of improving imaging resolution. Experimental results show that this method can effectively compensate the aberrations introduced by the system, agarose and HeLa cells. After correcting, the point spread function restored the doughnut-shape, and the resolution of the HeLa cell image increased about 20%.

Retinal adaptive optics imaging with a pyramid wavefront sensor.

The pyramid wavefront sensor (P-WFS) has replaced the Shack-Hartmann (SH-) WFS as the sensor of choice for high-performance adaptive optics (AO) systems in astronomy. Many advantages of the P-WFS, such as its adjustable pupil sampling and superior sensitivity, are potentially of great benefit for AO-supported imaging in ophthalmology as well. However, so far no high quality ophthalmic AO imaging was achieved using this novel sensor. Usually, a P-WFS requires modulation and high precision optics that lead to high complexity and costs of the sensor. These factors limit the competitiveness of the P-WFS with respect to other WFS devices for AO correction in visual science. Here, we present a cost-effective realization of AO correction with a non-modulated P-WFS based on standard components and apply this technique to human retinal in vivo imaging using optical coherence tomography (OCT). P-WFS based high quality AO imaging was successfully performed in 5 healthy subjects and smallest retinal cells such as central foveal cone photoreceptors are visualized. The robustness and versatility of the sensor is demonstrated in the model eye under various conditions and in vivo by high-resolution imaging of other structures in the retina using standard and extended fields of view. As a quality benchmark, the performance of conventional SH-WFS based AO was used and successfully met. This work may trigger a paradigm shift with respect to the wavefront sensor of choice for AO in ophthalmic imaging.

Adaptive optics with spatio-temporal lock-in detection for temporal focusing microscopy.

Wavefront distortion in temporal focusing microscopy (TFM) results in a distorted temporal profile of the excitation pulses owing to spatio-temporal coupling. Since the pulse duration is dramatically changed in the excitation volume, it is difficult to correct the temporal profile for a thick sample. Here, we demonstrate adaptive optics (AO) correction in a thick sample. We apply structured illumination microscopy (SIM) to an AO correction in wide-field TFM to decrease the change in the pulse duration in the signal detection volume. The AO correction with SIM was very successful in a thick sample for which AO correction with TFM failed.

Application of the Gaussian modeling algorithm to a Shack-Hartmann wavefront sensor for daylight adaptive optics.

Daytime use of Shack-Hartmann wavefront sensor (SHWS)-based adaptive optics (AO) at a telescope is hindered by the fluctuating bright sky background. Therefore, we propose a Gaussian modeling centroid extraction algorithm that performs real-time daylight AO closed-loop corrections. The algorithm is executed in the process of modeling and updating each pixel of the consecutive SHWS images. The simulation results show that our method can provide a lower centroid estimation error (CEE) for bright sky background conditions. We also performed a field experiment on a 2 m ground-based telescope and achieved a daylight AO correction of 2.1 times diffraction limit for a star with a visual magnitude of 4.7 (HIP113116) when the elevation angle of the sun was 7.7°. This represents the first publicly reported daylight natural guide star AO correction result on a 2 m class telescope.

Research progress of adaptive optics in wireless optical communication system for Xi’an University of Technology

In chronological order, this paper summarizes the research progress and technical classification of adaptive optics technology in the application of wireless optical communication system at home and abroad. Then it introduces the work of Xi’an University of Technology in this field, including adaptive optics system with wavefront measurement, adaptive optics system with wavefront-less measurement, wavefront correction of liquid crystal spatial light modulator, wavefront correction of the combination of tilt mirror and deformable mirror, spatial optical fiber coupling adaptive optical wavefront correction, etc. The adaptive optics technology can effectively correct the distorted wavefront caused by atmospheric turbulence and improve the coupling efficiency and communication performance in wireless optical communication. Although these methods are not perfect in theoretical analysis and engineering practice, they can be regarded as useful exploration in this field.

Blind restoration of solar images via the Channel Sharing Spatio-temporal Network

Context. Due to the presence of atmospheric turbulence, the quality of solar images tends to be significantly degraded when observed by ground-based telescopes. The adaptive optics (AO) system can achieve partial correction but stops short of reaching the diffraction limit. In order to further improve the imaging quality, post-processing for AO closed-loop images is still necessary. Methods based on deep learning (DL) have been proposed for AO image reconstruction, but the most of them are based on the assumption that the point spread function is spatially invariant. Aims. Our goal is to construct clear solar images by using a sophisticated spatially variant end-to-end blind restoration network. Methods. The proposed channel sharing spatio-temporal network (CSSTN) consists of three sub-networks: a feature extraction network, channel sharing spatio-temporal filter adaptive network (CSSTFAN), and a reconstruction network (RN). First, CSSTFAN generates two filters adaptively according to features generated from three inputs. Then these filters are delivered to the proposed channel sharing filter adaptive convolutional layer in CSSTFAN to convolve with the previous or current step features. Finally, the convolved features are concatenated as input of RN to restore a clear image. Ultimately, CSSTN and the other three supervised DL methods are trained on the binding real 705 nm photospheric and 656 nm chromospheric AO correction images as well as the corresponding speckle reconstructed images. Results. The results of CSSTN, the three DL methods, and one classic blind deconvolution method evaluated on four test sets are shown. The imaging condition of the first photospheric and second chromospheric set is the same as training set, except for the different time given in the same hour. The imaging condition of the third chromospheric and fourth photospheric set is the same as the first and second, except for the Sun region and time. Our method restores clearer images and performs best in both the peak signal-to-noise ratio and contrast among these methods.

Signal-to-noise ratio with adaptive optics compensation in non-Kolmogorov weak turbulent atmosphere

This study investigates the average signal-to-noise ratio ⟨ SNR ⟩ at the shot-noise limited receiver of an optical wireless communication system (OWC) that uses collimated Gaussian beam with adaptive optics correction in non-Kolmogorov weak turbulent atmosphere. ⟨ SNR ⟩ and the variation in the percentage ⟨ SNR ⟩ are calculated and the results are presented against various adaptive optics correction modes, non-Kolmogorov power-law exponent, receiver aperture size, Gaussian beam source size, photodetector quantum efficiency, electronic bandwidth, link length, and turbulence strength. Adaptive optics correction is analyzed for tilt, defocus, astigmatism, coma and trefoil modes and their variations. The effects of adaptive optics compensation are examined that cause an increase in SNR, hence improve the performance of an OWC system using collimated Gaussian beam and operating in the non-Kolmogorov weak turbulent atmosphere.

Computer Simulation for the Effects of Optical Aberrations on Solar Images Using Karhunen-Loeve polynomials

Numerical simulations were carried out to evaluate the effects of different aberrations modes on the performance of optical system, when observing and imaging the solar surface. Karhunen-Loeve aberrations modes were simulated as a wave front error in the aperture function of the optical system. To identify and apply the appropriate rectification that removes or reduces various types of aberration, their attribute must be firstly determined and quantitatively described. Wave aberration function is well suitable for this purpose because it fully characterizes the progressive effect of the optical system on the wave front passing through the aperture. The Karhunen-Loeve polynomials for circular aperture were used to describe wave front deviations and to predict the initial state of adaptive optics corrections. The results showed that increasing the aberration modes causes an increase in the blurring of the observed image. Also, we conclude that the optical phase error is increased significantly when aperture’s radii are increased.

Fully Convolutional Approaches for Numerical Approximation of Turbulent Phases in Solar Adaptive Optics

Information on the correlations from solar Shack–Hartmann wavefront sensors is usually used for reconstruction algorithms. However, modern applications of artificial neural networks as adaptive optics reconstruction algorithms allow the use of the full image as an input to the system intended for estimating a correction, avoiding approximations and a loss of information, and obtaining numerical values of those correlations. Although studied for night-time adaptive optics, the solar scenario implies more complexity due to the resolution of the solar images potentially taken. Fully convolutional neural networks were the technique chosen in this research to address this problem. In this work, wavefront phase recovery for adaptive optics correction is addressed, comparing networks that use images from the sensor or images from the correlations as inputs. As a result, this research shows improvements in performance for phase recovery with the image-to-phase approach. For recovering the turbulence of high-altitude layers, up to 93% similarity is reached.

A correlation-locking adaptive filtering technique for minimum variance integral control in adaptive optics

We propose the correlation-locking optimization scheme (CLOSE), a real-time adaptive filtering technique for adaptive optics (AO) systems controlled with integrators. CLOSE leverages the temporal autocorrelation of modal signals in the controller telemetry and drives the gains of the integral command law in a closed servo-loop. This supervisory loop is configured using only a few scalar parameters and automatically controls the modal gains to closely match transfer functions achieving minimum variance control. This optimization is proven to work throughout the range of noise and seeing conditions relevant to the AO system. This technique was designed while preparing the high-order AO systems for extremely large telescopes, in particular to tackle the optical gain (OG) phenomenon. This is a sensitivity reduction induced by on-sky residuals and is a prominent issue with pyramid wavefront sensors (PWFS). CLOSE follows upon the linear modal compensation approach to OG that was previously demonstrated to substantially improve AO correction with high-order PWFS systems. Operating on modal gains through multiplicative increments, CLOSE naturally compensates for the recurring issue of unaccounted sensitivity factors throughout the AO loop. We present end-to-end simulations of the MICADO instrument single-conjugate AO to demonstrate the performances and capabilities of CLOSE. We demonstrate that a single configuration provides an efficient and versatile optimization of the modal integrator while accounting for OG compensation and while providing significant robustness to transient effects impacting the PWFS sensitivity.

Experimental study on adaptive optical wavefront correction with dual mirrors in free space optical communication

The wavefront of light wave contains a tilt component after atmospheric turbulence, which increases the wavefront distortion and reduce the quality of laser communication. A fast steering mirror (FSM) must be introduced to correct the wavefront tilt. The adaptive optical closed-loop control algorithm based on Zernike mode decomposition is used to correct the wavefront distortion of light wave after indoor (0.5 m), 600 m, 1 km, 5 km, 10 km, and 100 km transmission. The experimental results show that, under the above-mentioned six different distances, the wavefront phase decreased from 9.5 µm, 15 µm, 12.5 µm, 22 µm, 35 µm, and 450–5 µm, 10 µm, 7.5 µm, 8 µm, 20 µm, and 300 µm, respectively, only after the closed-loop of the FSM; only after the closed-loop of the deformable mirror (DM), the wavefront phase decreased to 8 µm, 9 µm, 9 µm, 20 µm, 15 µm, and 300 µm, respectively. Further, when the FSM and DM are closed simultaneously, the wavefront phase decreases to 0.2 µm, 2.1 µm, 0.4 µm, 3.1 µm, 5 µm, and 200 µm. The correction effect of closed-loop of FSM and DM at the same time is better than that of either FSM or DM only.

Experimental analysis of adaptive optics correction methods on the beam carrying orbital angular momentum mode through oceanic turbulence

In recent years, as the demand for underwater communication has increased, underwater wireless optical communication (UWOC) has attracted a lot of attentions. Meanwhile, orbital angular momentum (OAM) has been applied in UWOC system to increase communication link capacity. However, the aberrations caused by oceanic turbulence (OT) is unavoidable, which results in serious intermodal crosstalk. Usually, adaptive optics (AO) is used to compensate these distortion aberrations. In this work, we experimentally demonstrate the influence of the distortion caused by OT, and evaluate the performance of AO correction algorithms, including Shack-Hartmann (SH), Stochastic-Parallel-Gradient-Descent (SPGD) and Gerchberg-Saxton (GS) algorithms. During the experiment, OT is simulated by using improved random phase screen model. We discuss the influence of various parameters of OT, including relative strength of temperature and salinity, strength of OT Cn2, and propagation distance, on the compensation effect with and without correction algorithms. The results show that all the three AO algorithms have good compensation effect on the distortion caused by OT, and GS algorithm has a better capability than the other two algorithms. Additionally, when the iteration numbers is less than 100, the compensation effect of GS and SPGD algorithms becomes more obvious as the iteration numbers increases, and GS algorithm is superior to SPGD algorithm. This work is beneficial to aberration correction of OAM-based UWOC system.

Adaptive optics correction in natural turbulent waters.

The recently introduced power spectrum of the refractive index fluctuations of the natural oceanic water turbulence is applied to an underwater communication system in the presence of adaptive optics corrections. The effects of the average temperature (0-30°C), the average salinity (0-40 ppt), the temperature-salinity gradient ratio (0-400°C/ppt), and the wavelength of the source (400-700 nm) on such a system are considered for the first time, to the best of our knowledge. It is revealed that even in the presence of adaptive optics the communication system's operation is severely affected by the regional and seasonal averages and fluctuations in the water's refractive index.

Adaptive optics for structured illumination microscopy based on deep learning.

Structured illumination microscopy (SIM) is widely used in biological imaging for its high resolution, fast imaging speed, and simple optical setup. However, when imaging thick samples, the structured illumination patterns in SIM will suffer from optical aberrations, leading to a serious deterioration in resolution. Therefore, it is necessary to reconstruct structured illumination patterns with high quality and efficiency in deep tissue imaging. Here we demonstrate an adaptive optics (AO) correction method based on deep learning in wide-field SIM imaging system. The mapping between the coefficients of the first 15 Zernike modes and their corresponding distorted patterns is established to train the convolution neural network (CNN). The results show that the optimized CNN can predict the aberration phase within ~10.1 ms with a personal computer. The correlation index between the aberration phases and their corresponding predicted aberration phase is up to 0.9986. This method is highly robust and effective for patterns with various spatial densities and illumination conditions and able to effectively correct the imaging distortion caused by optical aberration in SIM system.

Experimental Demonstration of Adaptive Optics Correction of the External Aberrations for Distributed Fiber Laser Array

A novel method of pre-compensating the phase distortions outside the fiber laser array based on active integrated wavefront sensing is proposed and demonstrated experimentally. The scheme and the working principle are introduced. Pre-compensation experiment of the fiber laser array based on a 7-element adaptive fiber optics collimator (AFOC) array with filled factor of 0.875 is presented. Residual error RMS of $0.11~\mu \text{m}$ is obtained while sensing lower order aberrations of a glass plate is with initial RMS of $0.62~\mu \text{m}$ . Metric of far-field power-in-the-bucket (PIB) is improved about 3.5 times after the phase distortions are pre-compensated. This method could promote the transmission efficiency of the fiber laser array in atmosphere, without limits of array scale and transmission light delay. This method would be upgraded to correct the aberrations located in the whole optical link from laser source to the target in the future. Such method would highly promote the applications of fiber laser array based systems like free space laser communication.