Adaptive Optics System Research Articles

Adaptive-optics-based turbulence mitigation in a 400 Gbit/s free-space optical link by multiplexing Laguerre–Gaussian modes varying both radial and azimuthal spatial indices

In general, atmospheric turbulence can degrade the performance of free-space optical (FSO) communication systems by coupling light from one spatial mode to other modes. In this Letter, we experimentally demonstrate a 400 Gbit/s quadrature-phase-shift-keyed (QPSK) FSO mode-division-multiplexing (MDM) coherent communication link through emulated turbulence using four Laguerre Gaussian (LG) modes with different radial and azimuthal indices (LG10, LG11, LG−10, and LG−11). To mitigate turbulence-induced channel cross talk and power loss, we implement an adaptive optics (AO) system at the receiver end. A Gaussian beam at a slightly different wavelength is co-propagated with the data beams as the probe beam. We use a wavefront sensor (WFS) to measure the wavefront distortion of this probe beam, and this information is used to tune a spatial light modulator (SLM) to adaptively correct the four distorted data-beam wavefronts. Using this adaptive-optics approach, the power loss and cross talk are reduced by ∼10 and ∼18 dB, respectively. © 2023 Optical Society of America

Pupil calibration for pyramid wavefront sensors based on a wavefront corrector.

The pyramid wavefront sensor (PWS) presents numerous advantages, such as high energy utilization, exceptional spatial resolution, and adjustability. Precise calibration of the pupil's position and size in advance is essential for accurately extracting wavefront slope information from the captured pupil image by the PWS. What we believe to be a novel calibration method is proposed using a wavefront corrector to enhance the sharpness of the pupil images in the PWS. An experimental setup using a crystal spatial light modulator (SLM) is established to validate this method. Both physical experiments and simulated results demonstrate that our proposed method can achieve accurate calibration of the pupil image with an error within 4 pixels for pupil size and not exceeding 3 pixels for position, meeting practical application requirements. The proposed PWS calibration method exhibits excellent repeatability and robustness, making it directly applicable in astronomical adaptive optics systems.

Poster Session II: The impact of eye's longitudinal chromatic aberration on visual acuity and accommodation response.

Chromatic composition of displays might affect vision and accommodation,possibly influencing myopia development.We investigated differences in visual acuity(VA) and accommodative lag(AccL) for steady accommodative demands (up to 5D,1D steps) with visual stimuli illuminated by monochromatic wavelengths (mono-λ 480,555&630nm,3nm bandwidth) and white light (WL).Data was obtained on 3 young emmetrope using an Adaptive Optics system with a supercontinuum laser,a DMD for stimuli,and a optunable lens to change vergence.Best focus for far was set at 555nm.VA was measured using QUEST(tumbling E).AccL was obtained from the peak shift of through-focus Visual Strehl,calculated from HS aberrometry.All subjects showed myopic shifts in blue consistent with longitudinal chromatic aberration.However,the response to wavelength differed across subjects.S#1 showed a sustained VA across distances(average VA standard deviation 0.054,-0.005logMAR),low AccL(slope:0.3 D/D) and systematic SA negative shift (slope:-0.04um/D),similar across mono-λ and WL.S#2 showed a more sustained VA,lower AccL and higher SA change in blue (0.09logMAR std,0.3D/D,-0.02um/D) than in WL(0.14logMAR std,0.52D/D,-0.013um/D).S#3 showed a steeper decrease in VA at near and higher AccL for mono-λ(0.218logMAR std,0.78D/D,on average)than in WL(0.05logMAR std,0.38D/D).Different subjects use chromatic cues in different ways to accommodate,likely affected by the interplay of chromatic blur,depth-of-focus and defocus sign perception.

Dual-camera Three-dimensional Automatic Tracking for a Human Eye Adaptive Optics System

Dual-camera Three-dimensional Automatic Tracking for a Human Eye Adaptive Optics System

Real-time Experimental Demonstrations of a Photonic Lantern Wave-front Sensor

The direct imaging of an Earth-like exoplanet will require sub-nanometric wave-front control across large light-collecting apertures to reject host starlight and detect the faint planetary signal. Current adaptive optics systems, which use wave-front sensors that reimage the telescope pupil, face two challenges that prevent this level of control: non-common-path aberrations, caused by differences between the sensing and science arms of the instrument; and petaling modes: discontinuous phase aberrations caused by pupil fragmentation, especially relevant for the upcoming 30 m class telescopes. Such aberrations drastically impact the capabilities of high-contrast instruments. To address these issues, we can add a second-stage wave-front sensor to the science focal plane. One promising architecture uses the photonic lantern (PL): a waveguide that efficiently couples aberrated light into single-mode fibers (SMFs). In turn, SMF-confined light can be stably injected into high-resolution spectrographs, enabling direct exoplanet characterization and precision radial velocity measurements; simultaneously, the PL can be used for focal-plane wave-front sensing. We present a real-time experimental demonstration of the PL wave-front sensor on the Subaru/SCExAO testbed. Our system is stable out to around ±400 nm of low-order Zernike wave-front error and can correct petaling modes. When injecting ∼30 nm rms of low-order time-varying error, we achieve ∼10× rejection at 1 s timescales; further refinements to the control law and lantern fabrication process should make sub-nanometric wave-front control possible. In the future, novel sensors like the PL wave-front sensor may prove to be critical in resolving the wave-front control challenges posed by exoplanet direct imaging.

About Some Aspects of Use of Optical Sensors for Monitoring the Aquatic Environment

Multi-channel polarization optical technology is increasingly used for prompt monitoring of water systems. Optical devices during the assessment of water quality determine the intensity of light through the studied aquatic environment. Spectrophotometric devices measure the spectrum of weakening of light through the aquatic environment. Spectroellipsometric devices receive spectra in vertical and horizontal polarizations. The presented article develops an adaptive optical hardware and image system for monitoring water bodies. The system is combined. It consists of 2 parts: 1) automated spectrophotometer-refractometer, and 2) adaptive spectroellipsometer. The system is equipped with a corresponding algorithmic and software, including algorithms for identifying spectral curves, databases and knowledge of spectral curves algorithms for solving reverse problems. The presented system is original since it differs from modern foreign systems by a new method of spectrophotometric and spectroellipsometric measurements, an original elemental base of polarization optics and a comprehensive mathematical approach to assessing the quality of a water body. There are no rotating polarization elements in the system. Therefore, this makes it possible to increase the signal-to-noise ratio and, as a result, improve measurement stability and simplify multichannel spectrophotometers and spectroellipsometers. The proposed system can be used in various water systems where it is necessary to assess water quality or identify the presence of a certain set of chemical elements.

Optical wavefront phase-tilt measurement using Si-photonic waveguide grating couplers.

Silicon photonic wavefront phase-tilt sensors for wavefront monitoring using surface coupling grating arrays are demonstrated. The first design employs the intrinsic angle dependence of the grating coupling efficiency to determine local wavefront tilt, with a measured sensitivity of 7 dB/°. The second design connects four gratings in an interferometric waveguide circuit to determine incident wavefront phase variation across the sensor area. In this device, one fringe spacing corresponds to 2° wavefront tilt change. These sensor elements sample a wavefront incident on the chip surface without the use of bulk optic elements, fiber arrays, or imaging arrays. Both sensor elements are less than 60 µm across and are potential unit cell sensor elements for large arrays that monitor wavefront shape across an image or pupil plane in adaptive optics systems for free-space optical communications, astronomy, and beam pointing applications.

Astroclimatic parameters characterization at lenghu site with ERA5 products

ABSTRACT Atmospheric turbulence and wind speed distributions are essential in determining the quality of astronomical sites and implementing adaptive optics systems. In this study, the optical turbulence with a general turbulence model and the wind speed characteristics at Lenghu are investigated by employing the 21-yr European Centre for Medium-Range Weather Forecasts’ fifth set of reanalysis (ERA5) data. The vertical distributions and seasonal behaviours of the wind speed and atmospheric turbulence are analysed. The wind speed values at 200 hPa pressure level (V200) are low in spring and summer except for June, and high in autumn and winter. The highest and lowest values of V200 at Lenghu are 39.93 m s−1 in September and 27.48 m s−1 in April, respectively. Additionally, atmospheric conditions are relatively stable above 20 km throughout the year. The locations of the peaks in the $C_n^2$ profiles correspond to the tropopause and the jet wind stream regions at the Lenghu site. Furthermore, the median seeing value from ERA5 is 0.72 arcsec, generally consistent with the statistically measured value of 0.75 arcsec. Moreover, the astronomical parameters (r0, θAO, and τAO) were initially provided at the Lenghu site. The median values of r0, ε, θAO, and τAO are 15.17 cm, 0.72 arcsec, 1.19 arcsec, and 2.83 ms, respectively, thus providing the potential reference for astronomical applications.

Comparison of AO, Schatzker, and three-column classification systems in tibial plateau fractures: Impact on functional outcomes.

This study aimed to compare the AO, Schatzker, and Three-Column classification systems for tibial plateau fractures, focusing on their prognostic and functional outcome prediction and influence on clinical decisions across different trauma types. In this retrospective study, we examined 49 patients (36 males, 11 females; mean age: 40.6±11.8 years; range, 19 to 67 years) with tibial plateau fractures between January 2011 and January 2017. The fractures were classified using the AO, Schatzker, and three-column systems. The main outcome measurements included functional scores (Knee Injury and Osteoarthritis Outcome Score [KOOS], Hospital for Special Surgery [HSS]), range of motion (ROM), duration of hospitalization, thigh atrophy, operation time, and the development of osteoarthritis. The impact of smoking was also assessed. According to the AO classification, type B fractures obtained higher KOOS and HSS scores compared to type C fractures (p=0.013 and p=0.007, respectively). According to the Schatzker classification low-energy fractures achieved higher KOOS and HSS scores than high-energy fractures (p=0.013 and p=0.026, respectively). One-column fractures had higher KOOS and HSS scores compared to two-column and three-column fractures (p=0.007 and p=0.001, respectively). Two-column fractures had a lower ROM compared to other column fractures (p=0.022). Shorter hospital stays were recorded for Schatzker low-energy fractures (p=0.016), whereas higher thigh atrophy was found in Schatzker high-energy fractures (p=0.022) and AO type C fractures (p=0.018). Longer operation times were observed in AO type C fractures (p=0.037) and Schatzker high-energy fractures (p=0.017). According to the Kellgren-Lawrence classification, AO type C fractures and three-column fractures yielded worse outcomes (p=0.039 and p=0.001, respectively). Smoking had a negative impact on functional KOOS and HSS scores across all groups (p=0.022 and p=0.001, respectively). This study highlights the predictive value of the AO, Schatzker, and Three-Column classification systems in determining functional outcomes and clinical data in tibial plateau fractures. Each system provides unique insights into different outcomes, suggesting their concurrent application may yield a more comprehensive prognosis.

Nonlinear Wave Front Reconstruction from a Pyramid Sensor using Neural Networks

The pyramid wave front sensor (PyWFS) has become increasingly popular to use in adaptive optics (AO) systems due to its high sensitivity. The main drawback of the PyWFS is that it is inherently nonlinear, which means that classic linear wave front reconstruction techniques face a significant reduction in performance at high wave front errors, particularly when the pyramid is unmodulated. In this paper, we consider the potential use of neural networks (NNs) to replace the widely used matrix vector multiplication (MVM) control. We aim to test the hypothesis that the NN's ability to model nonlinearities will give it a distinct advantage over MVM control. We compare the performance of a MVM linear reconstructor against a dense NN, using daytime data acquired on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) instrument. In a first set of experiments, we produce wavefronts generated from 14 Zernike modes and the PyWFS responses at different modulation radii (25, 50, 75, and 100 mas). We find that the NN allows for a far more precise wave front reconstruction at all modulations, with differences in performance increasing in the regime where the PyWFS nonlinearity becomes significant. In a second set of experiments, we generate a data set of atmosphere-like wavefronts, and confirm that the NN outperforms the linear reconstructor. The SCExAO real-time computer software is used as baseline for the latter. These results suggest that NNs are well positioned to improve upon linear reconstructors and stand to bring about a leap forward in AO performance in the near future.

Comparative Analysis of Image-shift Measurement Algorithms for Solar Shack–Hartmann Wavefront Sensors

Observations of the Sun provide unique insights into its structure, evolution, and activity, with significant implications for space weather forecasting and solar energy technologies. Ground-based telescopes offer cost-effective and flexible solutions for high-resolution solar observations, but image quality can be affected by atmospheric turbulence. Adaptive optics (AO) systems equipped with Shack–Hartmann wave front sensors (SH-WFS) enable real-time image correction to mitigate these effects. The accuracy of SH-WFS relies on correlation algorithms that measure wave front shifts, but reaching consistent conclusions regarding their accuracy remains challenging. In this study, we conducted an evaluation and comparison of standard correlation algorithms (the Square Difference Function, Normalized Cross-Correlation, Absolute Difference Function, Absolute Difference Function-Squared, and the Covariance Function in the frequency domain (CFF)) using simulated and authentic solar images. We optimized the algorithms through pre-processing techniques and carefully selected the most suitable window function for the CFF algorithm. Additionally, we analyzed the influence of various factors, such as shift ranges, bias, and the size of live images on the accuracy of algorithms. The consistent findings revealed that the CFF algorithm demonstrates superior measurement accuracy and robustness compared to the others. Choosing the CFF algorithm for solar observations can significantly enhance measurement accuracy, AO system performance, and the overall quality of solar research findings, thereby providing crucial support for space weather forecasting and other related scientific fields.

A Real-Time Measurement System for Atmospheric Turbulence Intensity and Distribution Based on the GLAO System

Measuring the intensity and distribution of atmospheric optical turbulence at large-aperture astronomical telescope sites is crucial to optimizing turbulence correction for different layers. A real-time measurement of turbulence distribution in large-aperture telescopes would be valuable for the parameter optimization of adaptive optics (AO) systems, especially for large field-of-view AO systems such as multi-conjugate adaptive optics (MCAO) and ground-layer adaptive optics (GLAO). Based on the GLAO system of NVST at FSO, a real-time measurement system was deployed to assess the site’s atmospheric turbulence intensity and distribution. This system is, to our knowledge, the first real-time turbulence parameter measurement system in the world with an AO system. We adopt pseudo-open loop methods to restore the turbulence information from the close-loop data of GLAO and measure the turbulence strength and distribution. Multiple subaperture pairs are used instead of a pair of subapertures for fitting calculation to increase the measurement accuracy. Two conventional measurement algorithms, SLODAR and S-DIMM+, are compared with the data from the open-source simulator SOAPY, to cross-verify the correctness of our calculation based on the data process of pseudo-open loop data and multiple subaperture pairs. The simulation results show that for two layers’ turbulence input, approximately 93% of the turbulence is correctly detected with the SLODAR method and the given parameters of wavefront sensors and correctors, while the S-DIMM+ is 87%. Real-time measurements of atmospheric turbulence at the NVST site were carried out on 28 May 2023. The observation results indicated that approximately 80% of the turbulence was located below an altitude of 2000 m; only a few appear in the upper height.

Enhanced-resolution Shack-Hartmann wavefront sensing for extended objects.

Adaptive optics systems for large-aperture solar telescopes, especially multiconjugate adaptive optics systems, suffer from a fundamental trade-off between wavefront sampling rate and sub-aperture resolution. We introduce an enhanced-resolution Shack-Hartmann wavefront sensing method that decouples sub-aperture resolution from the desired wavefront sampling rate. We experimentally verified the validity of this method. Results show that by synthesizing multiple low-spatial samplings, this method is capable to sense higher-frequency aberrations beyond any low-spatial sampling involved in the synthesis, and it allows higher sub-aperture resolution and higher operating bandwidths, which can better fulfill the needs of solar adaptive optics.

Hybrid control method for generating airy beam in wavefront sensorless adaptive optics system

In order to reduce the complexity of Airy beams generation based on adaptive optics with a wavefront sensor, a wavefront sensorless adaptive optics system, which employs a camera to measure the far-field intensity distribution as the feedback for optimizing the control voltages of the deformable mirror, is proposed for generating high-quality Airy beams. A hybrid control method combining hill-climbing algorithm and open-loop control method is developed to correct the aberrations of the system and produce the cubic phases. High-quality tunable Airy beams were generated successfully. The properties of the generated Airy beams, such as non-diffraction, self-acceleration and self-healing, were verified. The experimental results coincide with the theoretical results, showing the proposed method has potential applications such as Airy beam light-sheet microscope and optical manipulation.

Improving the beam quality of single-grating spectrally combined fiber lasers with adaptive optics.

Spectral combination is promising for diffraction-limited beam quality and single aperture beams. Unfortunately, beamlet deviations, linewidth broadening, and thermal aberrations inevitably degrade the beam quality. Many high-power laser systems integrate adaptive optics systems to maintain beam qualities. However, owing to the nature of incoherent combination, there is no well-defined wavefront in the spectrally combined beam, and whether phase compensations can enhance beam quality has not been discussed yet. We present the feasibility of improving the beam quality of spectral combined fiber lasers by adaptive optics. Simulations indicate that common path aberrations can be effectively corrected by adaptive optics, while beam quality degraded by displacement deviations and linewidth broadening cannot be improved. Additionally, the combined beam could be directly used as the beacon light in the propagation tunnel. To our knowledge, this study is the first to demonstrate that adaptive optics can improve the beam quality of spectrally combined fiber lasers and enable a further step toward diffraction-limited beam quality.

SHARPEST: the atmospheric turbulence profiling experiment using Shack–Hartmann sensor at the Subaru telescope

ABSTRACT Atmospheric turbulence profile plays an important role in designing and operating adaptive optics (AO) systems with multiple laser guide stars. To obtain representative free atmospheric profiles and resolved ground layer profiles for future AO systems at the Subaru telescope, we are conducting the Shack–Hartmann Atmospheric tuRbulence Profiling Experiment at the Subaru Telescope project. In this project, we develop a turbulence profiler comprising two Shack–Hartmann (SH) sensors to observe a pair of bright stars through the Subaru telescope with high spatial sampling by 2 cm subapertures. We perform two analyses on the SH spot data: variance analysis on the spot scintillation for free atmospheric profiles, and on the spot slope for ground layer profiles. This paper introduces the initial results of free atmospheric profiles as well as total seeing values and wind profiles obtained by the first two engineering runs. The free atmospheric profiles reconstructed by the two independent SH sensors show good agreement. The results are also consistent with simultaneous measurements by another profiler except for turbulence strength at ∼1 km, which is explained by an overestimation problem of scintillation-based profilers. Measured total seeing values are also smaller than the simultaneous measurements, possibly due to the difference in ground layer turbulence between the two sites. The wind profiles show good consistency with the direct measurements by a rawinsonde. Through this study, we establish a method to constrain the free atmospheric profile, the total seeing, and the wind profile by analysing data from a single SH sensor with fine subapertures.

Wavefront control capability in a modal lens with segmented circular peripheral electrodes.

We report the detailed investigation of the capability of an electrically tunable liquid crystal lens (TLCL) to dynamically generate various wavefront shapes. The TLCL operates in the modal-control mode with a peripheral circular electrode divided into eight individually controlled segments. This segmentation allows producing a rather rich set of influence functions. We characterize these functions and the crosstalk between them by adjusting the voltage and the frequency of electrical signals applied to different electrode segments. Various wavefronts are produced in a closed-loop control mode and described using Zernike polynomials. The dynamical response of the lens is also briefly investigated. Obtained results may be used to design different adaptive optical systems where a dynamic wavefront control is required.

Atmospheric Turbulence with Kolmogorov Spectra: Software Simulation, Real-Time Reconstruction and Compensation by Means of Adaptive Optical System with Bimorph and Stacked-Actuator Deformable Mirrors

Atmospheric turbulence causes refractive index fluctuations, which in turn introduce extra distortions to the wavefront of the propagated radiation. It ultimately degrades telescope resolution (in imaging applications) and reduces radiation power density (in focusing applications). One of the possible ways of researching the impact of turbulence is to numerically simulate the spectrum of refractive index fluctuations, to reproduce it using a wavefront corrector and to measure the resultant wavefront using, for example, a Shack–Hartmann sensor. In this paper, we developed turbulence simulator software that generates phase screens with Kolmogorov spectra. We reconstructed the generated set of phase screens using a stacked-actuator deformable mirror and then compensated for the introduced wavefront distortions using a bimorph deformable mirror. The residual amplitude of the wavefront reconstructed by the 19-channel stacked-actuator mirror was 0.26 λ, while the residual amplitude of the wavefront compensated for by the 32-channel bimorph mirror was 0.08 λ.

Highly robust spatiotemporal wavefront prediction with a mixed graph neural network in adaptive optics

The time-delay problem, which is introduced by the response time of hardware for correction, is a critical and non-ignorable problem of adaptive optics (AO) systems. It will result in significant wavefront correction errors while turbulence changes severely or system responses slowly. Predictive AO is proposed to alleviate the time-delay problem for more accurate and stable corrections in the real time-varying atmosphere. However, the existing prediction approaches either lack the ability to extract non-linear temporal features, or overlook the authenticity of spatial features during prediction, leading to poor robustness in generalization. Here, we propose a mixed graph neural network (MGNN) for spatiotemporal wavefront prediction. The MGNN introduces the Zernike polynomial and takes its inherent covariance matrix as physical constraints. It takes advantage of conventional convolutional layers and graph convolutional layers for temporal feature catch and spatial feature analysis, respectively. In particular, the graph constraints from the covariance matrix and the weight learning of the transformation matrix promote the establishment of a realistic internal spatial pattern from limited data. Furthermore, its prediction accuracy and robustness to varying unknown turbulences, including the generalization from simulation to experiment, are all discussed and verified. In experimental verification, the MGNN trained with simulated data can achieve an approximate effect of that trained with real turbulence. By comparing it with two conventional methods, the demonstrated performance of the proposed method is superior to the conventional AO in terms of root mean square error (RMS). With the prediction of the MGNN, the mean and standard deviation of RMS in the conventional AO are reduced by 54.2% and 58.6% at most, respectively. The stable prediction performance makes it suitable for wavefront predictive correction in astronomical observation, laser communication, and microscopic imaging.

The Adaptive Optics System for the Gemini Infrared Multi-Object Spectrograph: Performance Modeling

The Gemini Infrared Multi-Object Spectrograph (GIRMOS) will be a near-infrared, multi-object, medium spectral resolution, integral field spectrograph (IFS) for Gemini North Telescope, designed to operate behind the future Gemini North Adaptive Optics system (GNAO). In addition to a first ground layer Adaptive Optics (AO) correction in closed loop carried out by GNAO, each of the four GIRMOS IFSs will independently perform additional multi-object AO correction in open loop, resulting in an improved image quality that is critical to achieve top level science requirements. We present the baseline parameters and simulated performance of GIRMOS obtained by modeling both the GNAO and GIRMOS AO systems. The image quality requirement for GIRMOS is that 57% of the energy of an unresolved point-spread function ensquared within a 0.1 × 0.1 arcsecond at 2.0 μ m. It was established that GIRMOS will be an order 16 × 16 adaptive optics (AO) system after examining the tradeoffs between performance, risks and costs. The ensquared energy requirement will be met in median atmospheric conditions at Maunakea at 30° from zenith.