Use Of Adaptive Optics Research Articles

The possibilities of using adaptive optics in modern ophthalmology

Until recently, the assessment of individual retinal cells was possible only with the help of histological examination, since such retinal imaging methods as scanning laser ophthalmoscopy and optical coherence tomography had low resolution to obtain images of structures at the cellular level, which was mainly due to aberrations caused by the optics of the eye. Adaptive optics technology has improved the performance of optical systems by correcting optical wavefront aberrations. Adaptive optics allows noninvasively visualizing the retina at the microscopic level in vivo, providing the opportunity to analyze individual structures such as photoreceptors, blood vessels, nerve fibers, ganglion cells and a lattice plate. Adaptive optics imaging in patients with diabetic retinopathy makes it possible to accurately determine the spatial distribution of cones, a decrease in which is associated with the presence of diabetic retinopathy and an increase in the severity of the disease. The detection of differences in cone distribution density between the control group and patients with diabetes mellitus without clinical signs of diabetic retinopathy may contribute to its early diagnosis, as well as a deeper understanding of the consequences of changes in the photoreceptor apparatus. Adaptive optics imaging methods are able to identify disorders of photoreceptor cells and assess the degree of progression of age-related macular degeneration, which definitely expands diagnostic capabilities at the early stages of its detection. Assessment of the condition of nerve fiber bundles through the use of Adaptive optics helps to identify changes associated with glaucoma, and also provides the ability to visualize details that cannot be evaluated using optical coherence tomography. Adaptive optics imaging allows you to directly measure the wall of retinal vessels and the diameter of their lumen. The ratio of wall thickness to vessel lumen and the cross-sectional area of the vessel wall directly reflect the remodeling process and can be used for the purpose of early diagnosis and monitoring of hypertension.

Convolutional neural network for improved event-based Shack-Hartmann wavefront reconstruction

Shack-Hartmann wavefront sensing is a technique for measuring wavefront aberrations, whose use in adaptive optics relies on fast position tracking of an array of spots. These sensors conventionally use frame-based cameras operating at a fixed sampling rate to report pixel intensities, even though only a fraction of the pixels have signal. Prior in-lab experiments have shown feasibility of event-based cameras for Shack-Hartmann wavefront sensing (SHWFS), asynchronously reporting the spot locations as log intensity changes at a microsecond time scale. In our work, we propose a convolutional neural network (CNN) called event-based wavefront network (EBWFNet) that achieves highly accurate estimation of the spot centroid position in real time. We developed a custom Shack-Hartmann wavefront sensing hardware with a common aperture for the synchronized frame- and event-based cameras so that spot centroid locations computed from the frame-based camera may be used to train/test the event-CNN-based centroid position estimation method in an unsupervised manner. Field testing with this hardware allows us to conclude that the proposed EBWFNet achieves sub-pixel accuracy in real-world scenarios with substantial improvement over the state-of-the-art event-based SHWFS. An ablation study reveals the impact of data processing, CNN components, and training cost function; and an unoptimized MATLAB implementation is shown to run faster than 800 Hz on a single GPU.

Investigation of PZT Materials for Reliable Piezostack Deformable Mirror with Modular Design.

This article presents a study of the electrophysical properties of a piezoceramic material for use in adaptive optics. The key characteristics that may be important for the manufacturing of piezoelectric deformable mirrors are the following: piezoelectric constants (d31, d33, d15), capacitance, elastic compliance values s for different crystal directions, and the dielectric loss tangent (tgδ). Based on PZT ceramics, the PKP-12 material was developed with high values of the dielectric constant, piezoelectric modulus, and electromechanical coupling coefficients. The deformable mirror control elements are made from the resulting material-piezoceramic combs with five individual actuators in a row. In this case, the stroke of the actuator is in the range of 4.1-4.3 microns and the capacitance of the actuator is about 12 nF.

Evaluation of Morphological Changes in Retinal Vessels in Type 1 Diabetes Mellitus Patients with the Use of Adaptive Optics.

Introduction. Diabetes mellitus contributes to the development of microvascular complications in the eye. Moreover, it affects multiple end organs, including brain damage, leading to premature death. The use of adaptive optics technique allows to perform non-invasive in vivo assessment of retinal vessels and to identify changes in arterioles about 100 μm in diameter. The retinal vasculature may be a model of the cerebral vessels both morphologically and functionally. Aim. To evaluate morphological parameters of retinal arterioles in patients with type 1 diabetes mellitus (DM1). Material and methods. The study included 22 DM1 patients (13 females) aged 43.00 ± 9.45 years with a mean diabetes duration of 22.55 ± 10.05 years, and 23 healthy volunteers (10 females) aged 41.09 ± 10.99 years. Blood pressure, BMI, waist circumference, and metabolic control markers of diabetes were measured in both groups. Vascular examinations were performed using an rtx1 adaptive optics retinal camera (Imagine Eyes, Orsay, France); the vessel wall thickness (WT), lumen diameter (LD), wall-to-lumen ratio (WLR), and vascular wall cross-sectional area (WCSA) were assessed. Statistical analysis was performed with the application of IMB SPSS version 23 software. Results. The DM1 group did not differ significantly in age, BMI, waist circumference, blood pressure, or axial length of the eye compared to the control group. Intraocular pressure (IOP) in both groups was normal, but in the DM1 group it was significantly higher. The DM1 group had significantly higher WT, WLR, and WCSA. These parameters correlated significantly with the duration of diabetes, but not with IOP. Conclusions. The presented study demonstrates the presence of significant morphological changes in retinal vessels in DM1 patients without previously diagnosed diabetic retinopathy. Similar changes may occur in the brain and may be early indicators of cardiovascular risk, but further investigation is required to confirm that.

Atmospheric turbulence forecasting using two-stage variational mode decomposition and autoregression towards free-space optical data-transmission link

Free space optical communication (FSOC) is a promising technology for satellite-to-earth communication systems, where vector beams, especially orbital angular momentum (OAM), can further increase the capacity of the optical link. However, atmospheric turbulence along the path can introduce intensity scintillation, wavefront aberrations and severe distortion of spatial patterns, leading to data degradation. Forecasting atmospheric turbulence allows for advanced scheduling of satellite-to-earth data transmission links, as well as the use of adaptive optics (AO) to compensate for turbulence effects and avoid data transmission link performance degradation. Therefore, atmospheric turbulence forecasting is critical for practical applications. In this work, we proposed a hybrid atmospheric turbulence forecasting model based on a two-stage variational mode decomposition (TsVMD) and autoregression model. The variational mode decomposition (VMD) algorithm is first used, to our best knowledge, to denoise the observed atmospheric turbulence dataset, and then is used again to decompose the datasets into several intrinsic mode functions (IMFs). Finally, the autoregression model is used to predict each IMF independently. And the predictions of each IMF are combined to obtain the final atmospheric turbulence predictions. Experiments employing the observed turbulence datasets and two additional methodologies were carried out to verify the performance of the proposed model. The experimental results show that the performance of the proposed model is much superior to that of the comparative methods.

Improvement of the focusability of petawatt laser pulses after nonlinear post-compression

The nonlinear nature of laser pulse post-compression inevitably leads to wavefront distortions. These distortions are nonstationary, i.e., they vary during the pulse, which significantly complicates the use of adaptive optics. We propose four approaches to compensation of wavefront distortions by means of standard adaptive mirrors, three of which can be implemented experimentally. Numerical simulations show that these approaches allow achieving a focal spot intensity close to the case of a perfectly flat wavefront.

Impact of Arterial Hypertension on the Eye: A Review of the Pathogenesis, Diagnostic Methods, and Treatment of Hypertensive Retinopathy

The number of patients with arterial hypertension is continually increasing. Hypertension can cause organ complications, called hypertension-mediated organ damage (HMOD). One example is hypertensive retinopathy, in which high blood pressure (BP) damages both the retinal microcirculation and the retinal nerve fiber layer (RNFL). This can result in progressive and painless vision deterioration in some groups of patients. Unlike anywhere else in the human body, the microvasculature of the retina can be observed in vivo, and the progression of changes can be closely monitored. The harmful effect of increased BP on the eye is not only limited to hypertensive retinopathy, but can also lead to an exacerbation of diabetic retinopathy (DR) and to an increase in intraocular pressure (IOP), and it can also trigger the formation of thromboembolic lesions.This review presents an update on the pathogenesis of hypertensive retinopathy and the use of adaptive optics (AO) combined with optical coherence tomography (OCT) to evaluate the retinal microvasculature. The latest progress and directions of research in the field of hypertensive retinopathy are also discussed.

Generation and compression of 10-fs deep ultraviolet pulses at high repetition rate using standard optics.

The generation and characterization of ultrashort laser pulses in the deep ultraviolet spectral region is challenging, especially at high pulse repetition rates and low pulse energies. Here, we combine achromatic second harmonic generation and adaptive pulse compression for the efficient generation of sub-10 fs deep ultraviolet laser pulses at a laser repetition rate of 200 kHz. Furthermore, we simplify the pulse compression scheme and reach pulse durations of ≈10 fs without the use of adaptive optics. We demonstrate straight-forward tuning from 250 to 320 nm, broad pulse spectra of up to 63 nm width, excellent stability and a high robustness against misalignment. These features make the approach appealing for numerous spectroscopy and imaging applications.

Feature issue introduction: applications of adaptive optics.

This feature issue of Optics Express follows the 2020 Imaging and Applied Optics Congress and comprises of articles on the development and use of adaptive optics across the broad range of domains in which the technique has been applied - including atmospheric correction, ophthalmology, vision science, microscopy, optical communications and beam control. This review provides a basic introduction to adaptive optics and a summary of the multidisciplinary articles included in this issue.

Towards a high resolution view of infrared line formation

Observing in the infrared has many benefits, such as seeing through the interstellar dust in the galaxy, easier use of adaptive optics, and better flux ratios for direct observations of exoplanets. Before the infrared spectral range can be utilised for highly accurate stellar spectroscopy there is a need for a better understanding of both stellar modelling and the atomic physics that go into forming spectral lines. In this thesis I aim to evaluate to what extent abundances derived from infrared spectra agree with optical values, and which aspects of the line formation cause eventual discrepancies. The aspects investigated in this study are: macroturbulence determination; astrophysical linestrengths; NLTE corrections; and hyperfine structure splitting of atomic energy levels. For this purpose I have analysed H band (1.49 to 1.80 $\mu m$) spectra of 34 K giants, taken with the spectrometer IGRINS. The elemental abundances derived from these infrared spectra are benchmarked against results from high resolution optical spectra of the same stars. This study uses stellar parameters derived from the same optical spectra, as techniques for determining them from infrared spectra are still being developed. My results for the elemental abundances of C, Na, Mg, Ca, Ti, V, Cr, Co, Ni, Cu and Ce show good agreement with the optical values. Significant differences are seen in the abundances of Al, Si, Mn and Nd. I have also measured the abundances of P, S, K and Zn, elements which lack optical benchmark values. No measurements have been possible for Ge and Rb. Different factors affecting the analysis have been studied in more detail. I show that the method used for determining macroturbulence is an important factor in abundance determination, especially for stars with supersolar metallicity. The lack of accurate measurements of spectral linestrength is addressed in the thesis by my astrophysical measurements of the quantity, using a method developed in the thesis that accounts for the relative strengths of fine and hyperfine structure lines. For elements with NLTE corrections from Amarsi et al. (2020) I show how these corrections improve the agreement between optical and infrared results. For elements with data on the hyperfine structure splitting of the energy levels, I have assessed the impact of including the additional transitions, showing the importance of doing so.

MUSE narrow field mode observations of the central kinematics of M15

ABSTRACT We present observations of the stellar kinematics of the centre of the core collapsed globular cluster M15 obtained with the MUSE integral field spectrograph on the Very Large Telescope operating in narrow field mode. Thanks to the use of adaptive optics, we obtain a spatial resolution of 0.1 arcsec and are able to reliably measure the radial velocities of 864 stars within 8 arcsec of the centre of M15, thus providing the largest sample of radial velocities ever obtained for the innermost regions of this system. Combined with previous observations of M15 using MUSE in wide field mode and literature data, we find that the central kinematics of M15 are complex with the rotation axis of the core of M15 offset from the rotation axis of the bulk of the cluster. While this complexity has been suggested by previous work, we confirm it at higher significance and in more detail.

Final design and on-sky testing of the iLocater SX acquisition camera: broad-band single-mode fibre coupling

ABSTRACT Enabling efficient injection of light into single-mode fibres (SMFs) is a key requirement in realizing diffraction-limited astronomical spectroscopy on ground-based telescopes. SMF-fed spectrographs, facilitated by the use of adaptive optics (AO), offer distinct advantages over comparable seeing-limited designs, including higher spectral resolution within a compact and stable instrument volume, and a telescope independent spectrograph design. iLocater is an extremely precise radial velocity (EPRV) spectrograph being built for the Large Binocular Telescope (LBT). We have designed and built the front-end fibre injection system, or acquisition camera, for the SX (left) primary mirror of the LBT. The instrument was installed in 2019 and underwent on-sky commissioning and performance assessment. In this paper, we present the instrument requirements, acquisition camera design, as well as results from first-light measurements. Broad-band SMF coupling in excess of 35 per cent (absolute) in the near-infrared (0.97–1.31 ${\mu {\rm m}}$) was achieved across a range of target magnitudes, spectral types, and observing conditions. Successful demonstration of on-sky performance represents both a major milestone in the development of iLocater and in making efficient ground-based SMF-fed astronomical instruments a reality.

The combined use of adaptive optics and nonlinear optical wavefront reversal techniques to compensate for turbulent distortions when focusing laser radiation on distant objects

Approaches to constructing a mock-up of a system for focusing laser radiation on distant objects using both adaptive optics elements and nonlinear-optical wavefront reversal methods providing compensation for turbulent distortions are considered. Numerical calculations were preliminarily performed, in which the split-step method was used as a numerical method for solving a second-order partial differential wave equation for the complex amplitude of the wave field of a laser beam. This method, combined with methods of spectral-phase Fourier transforms and statistical tests, is the most effective way to obtain reliable quantitative results for solving engineering problems of atmospheric wave optics. Quantitative data are obtained on the effect of turbulent atmospheric distortions along propagation paths on the main parameters of coherent laser beams – focusing, effective average radius, and the proportion of the beam energy in its diffraction spot. The preliminary results obtained of the system mock-up performance confirm the conclusions of the theory.

Analysis and visualization of the output mode-matching requirements for squeezing in Advanced LIGO and future gravitational wave detectors

The sensitivity of ground-based gravitational wave (GW) detectors will be improved in the future via the injection of frequency-dependent squeezed vacuum. The achievable improvement is ultimately limited by losses of the interferometer electromagnetic field that carries the GW signal. The analysis and reduction of optical loss in the GW signal chain will be critical for optimal squeezed light-enhanced interferometry. In this work we analyze a strategy for reducing output-side losses due to spatial mode mismatch between optical cavities with the use of adaptive optics. Our goal is not to design a detector from the top down, but rather to minimize losses within the current design. Accordingly, we consider actuation on optics already present and one transmissive optic to be added between the signal recycling mirror and the output mode cleaner. The results of our calculation show that adaptive mode-matching with the current Advanced LIGO design is a suitable strategy for loss reduction that provides less than 2% mean output mode-matching loss. The range of actuation required is +47 uD on SR3, +140 mD on OM1 and OM2, +50 mD on the SRM substrate, and -50 mD on the added new transmissive optic. These requirements are within the demonstrated ranges of real actuators in similar or identical configurations to the proposed implementation. We also present a novel technique that graphically illustrates the matching of interferometer modes and allows for a quantitative comparison of different combinations of actuators.

Automated superpixels-based identification and mosaicking of cone photoreceptor cells for adaptive optics scanning laser ophthalmoscope

An automated superpixels identification/mosaicking method is presented for the analysis of cone photoreceptor cells with the use of adaptive optics scanning laser ophthalmoscope (AO-SLO) images. This is an image oversegmentation method used for the identification and mosaicking of cone photoreceptor cells in AO-SLO images. It includes image denoising, estimation of the cone photoreceptor cell number, superpixels segmentation, merging of superpixels, and final identification and mosaicking processing steps. The effectiveness of the presented method was confirmed based on its comparison with a manual method in terms of precision, recall, and F1-score of 77.3%, 95.2%, and 85.3%, respectively.

A 37-actuator polyimide deformable mirror with electrostatic actuation for adaptive optics microscopy

Widespread use of adaptive optics in life-science microscopy is predominantly hindered by the high cost of conventional MEMS deformable mirrors. We present here a potentially low-cost and compact alternative in the form of an electrostatic deformable mirror developed particularly for applications in adaptive optics microscopy. The device is fabricated with standard micro-machining processes using widely available materials to minimize cost, but still offers exceptional post-assembly mirror quality. With an aperture size of 10 mm, the device has a post-fabrication root-mean-square mirror flatness error of <250 nm and an active best-flat of <15 nm. The membrane is made of an aluminum-coated 3.5 m thick polyimide membrane that has high compliance, allowing a maximum peak-to-valley stroke of 15 m at 375 V. Driven using a custom-developed optimization-based open-loop control scheme, we successfully reproduced Zernike modes of up to fifth radial order with high shape fidelity.

Fast adaptive optics scanning light ophthalmoscope retinal montaging.

The field of view of high-resolution ophthalmoscopes that require the use of adaptive optics (AO) wavefront correction is limited by the isoplanatic patch of the eye, which varies across individual eyes and with the portion of the pupil used for illumination and/or imaging. Therefore all current AO ophthalmoscopes have small fields of view comparable to, or smaller than, the isoplanatic patch, and the resulting images have to be stitched off-line to create larger montages. These montages are currently assembled either manually, by expert human graders, or automatically, often requiring several hours per montage. This arguably limits the applicability of AO ophthalmoscopy to studies with small cohorts and moreover, prevents the ability to review a real-time captured montage of all locations during image acquisition to further direct targeted imaging. In this work, we propose stitching the images with our novel algorithm, which uses oriented fast rotated brief (ORB) descriptors, local sensitivity hashing, and by searching for a ‘good enough’ transformation, rather than the best possible, to achieve processing times of 1–2 minutes per montage of 250 images. Moreover, the proposed method produces montages which are as accurate as previous methods, when considering the image similarity metrics: normalised mutual information (NMI), and normalised cross correlation (NCC).

Image registration for daylight adaptive optics.

Daytime use of adaptive optics (AO) at large telescopes is hampered by shot noise from the bright sky background. Wave-front sensing may use a sodium laser guide star observed through a magneto-optical filter to suppress the background, but the laser beacon is not sensitive to overall image motion. To estimate that, laser-guided AO systems generally rely on light from the object itself, collected through the full aperture of the telescope. Daylight sets a lower limit to the brightness of an object that may be tracked at rates sufficient to overcome the image jitter. Below that limit, wave-front correction on the basis of the laser alone will yield an image that is approximately diffraction limited but that moves randomly. I describe an iterative registration algorithm that recovers high-resolution long-exposure images in this regime from a rapid series of short exposures with very low signal-to-noise ratio. The technique takes advantage of the fact that in the photon noise limit there is negligible penalty in taking short exposures, and also that once the images are recorded, it is not necessary, as in the case of an AO tracker loop, to estimate the image motion correctly and quickly on every cycle. The algorithm is likely to find application in space situational awareness, where high-resolution daytime imaging of artificial satellites is important.

Non-invasive assessment of human cone photoreceptor function.

Vision begins when light isomerizes the photopigments within photoreceptors. Noninvasive cellular-scale observation of the structure of the human photoreceptor mosaic is made possible through the use of adaptive optics (AO) enhanced ophthalmoscopes, but establishing noninvasive objective measures of photoreceptor function on a similar scale has been more difficult. AO ophthalmoscope images acquired with near-infrared light show that individual cone photoreceptor reflectance can change in response to a visible stimulus. Here we show that the intrinsic response depends on stimulus wavelength and intensity, and that its action spectrum is well-matched to the spectral sensitivity of cone-mediated vision. Our results demonstrate that the cone reflectance response is mediated by photoisomerization, thus making it a direct measure of photoreceptor function.

The Proper Motion of Pyxis: The First Use of Adaptive Optics in Tandem with HST on a Faint Halo Object

Abstract We present a proper motion measurement for the halo globular cluster Pyxis, using Hubble Space Telescope/ACS data as the first epoch and GeMS/GSAOI Adaptive Optics data as the second, separated by a baseline of ∼5 years. This is both the first measurement of the proper motion of Pyxis and the first calibration and use of Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a faint, distant halo object. Consequently, we present our analysis of the Adaptive Optics data in detail. We obtain a proper motion of = 1.09 ± 0.31 mas yr−1 and μ δ = 0.68 ± 0.29 mas yr−1. From the proper motion and line-of-sight velocity, we find that the orbit of Pyxis is rather eccentric, with its apocenter at more than 100 kpc and its pericenter at about 30 kpc. We also investigate two literature-proposed associations for Pyxis with the recently discovered ATLAS stream and the Magellanic system. Combining our measurements with dynamical modeling and cosmological numerical simulations, we find it unlikely Pyxis is associated with either system. We examine other Milky Way satellites for possible association using the orbit, eccentricity, metallicity, and age as constraints and find no likely matches in satellites down to the mass of Leo II. We propose that Pyxis probably originated in an unknown galaxy, which today is fully disrupted. Assuming that Pyxis is bound and not on a first approach, we derive a 68% lower limit on the mass of the Milky Way of 0.95 × 1012 M ⊙.