Sensorless Adaptive Optics Research Articles

Two-photon imaging of the mammalian retina with ultrafast pulsing laser

Noninvasive imaging of visual system components in vivo is critical for understanding the causal mechanisms of retinal diseases and for developing therapies for their treatment. However, ultraviolet light needed to excite endogenous fluorophores that participate in metabolic processes of the retina is highly attenuated by the anterior segment of the human eye. In contrast, 2-photon excitation fluorescence imaging with pulsed infrared light overcomes this obstacle. Reducing retinal exposure to laser radiation remains a major barrier in advancing this technology to studies in humans. To increase fluorescence intensity and reduce the requisite laser power, we modulated ultrashort laser pulses with high-order dispersion compensation and applied sensorless adaptive optics and custom image recovery software and observed an over 300% increase in fluorescence of endogenous retinal fluorophores when laser pulses were shortened from 75 fs to 20 fs. No functional or structural changes to the retina were detected after exposure to 2-photon excitation imaging light with 20-fs pulses. Moreover, wide bandwidth associated with short pulses enables excitation of multiple fluorophores with different absorption spectra and thus can provide information about their relative changes and intracellular distribution. These data constitute a substantial advancement for safe 2-photon fluorescence imaging of the human eye.

Handheld Adaptive Optics Scanning Laser Ophthalmoscope.

Adaptive optics scanning laser ophthalmoscopy (AOSLO) has enabled in vivo visualization and enhanced understanding of retinal structure and function. Current generation AOSLOs have a large footprint and are mainly limited to imaging cooperative adult subjects. To extend the application of AOSLO to new patient populations, we have designed the first portable handheld AOSLO (HAOSLO) system. By incorporating a novel computational wavefront sensorless AO algorithm and custom optics, we have miniaturized our HAOSLO to weigh less than 200 grams. HAOSLO imaged the cones closest to the fovea with a handheld probe in adults and captured the first AO-enhanced image of cones in infants.

A rapid hybrid wave front correction algorithm for sensor-less adaptive optics in free space optical communication

Abstract In free-space optical communication (FSOC) system, wave front sensor-less adaptive optics (AO) technology is widely used to compensate wave front distortions caused by atmospheric turbulences. There are different kinds of compensation methods with both advantages and disadvantages. The conventional and often considered model-free algorithm, stochastic parallel gradient descent (SPGD) algorithm, requires a lot of iterations to obtain optimal solution, which may lead to a poor real-time performance. The newly developed model based algorithms have certain constraints and difficulties for the construction of the models. Therefore, in order to develop a rapid wave front distortion compensation algorithm with better performance, we present a hybrid method by a suitable combination of SPGD algorithm and a model-based algorithm presented by Martin J. Booth. Simulations under our built experiment platform show that this hybrid method can compensate wave front aberrations fitted by the Zernike polynomials in higher speed than that of SPGD algorithm at higher SR and lower RMS with strong turbulence, which means it can improve communication quality.

Adaptive wavefront interferometry for unknown free-form surfaces

The primary problem of conventional wavefront interferometers is limited dynamic range. Unknown free-form surface figure error with large amplitude or slope is not measurable for too dense or invisible fringes. To troubleshoot this problem, we propose adaptive wavefront interferometry (AWI). AWI utilizes a wavefront sensor-less adaptive optics (AO) subsystem to intelligently speculate and compensate the unknown free-form surface figure error. In this subsystem, adaptive null optics is utilized to iteratively generate adaptive wavefronts to compensate the unknown severe surface figure error. The adaptive null optics is close-loop controlled (i.e., wavefront sensor-less optimization algorithms are utilized to control it by real time monitoring the compensation effects to guarantee convergence of the iteration). Ultimately, invisible fringes turn into resolvable ones, and null test is further realized. To demonstrate the feasibility of AWI, we designed one spatial light modulator (SLM) based AWI modality as an example. The system is based on a commercial interferometer and is easy to establish. No other elements are required besides the SLM. Principle, simulation, and experiments for the SLM based AWI are demonstrated.

Fast Restoration of Aberration-Degraded Extended Object Based on Local Region Abstraction

The compensating capability of wavefront sensorless adaptive optics (WFSless AO) system for extended object images degraded by aberration is severely limited to correction speed due to largely sized images and low frame rate of camera. To improve the correction capability of WFSless AO system, a method based on local region abstraction (LRA) is presented according to the sparse characteristics of extended object imaging. The experiment results of image restoration with WFSless AO system based on LRA show that the correction speed of this method is 8–20 times faster than that of the previous method for the static aberration, the image restoration speed is improved by about 3.5 times under the dynamic aberration.

Brillouin micro-spectroscopy through aberrations via sensorless adaptive optics.

Brillouin spectroscopy is a powerful optical technique for non-contact viscoelastic characterizations which has recently found applications in three-dimensional mapping of biological samples. Brillouin spectroscopy performances are rapidly degraded by optical aberrations and have therefore been limited to homogenous transparent samples. In this work, we developed an adaptive optics (AO) configuration designed for Brillouin scattering spectroscopy to engineer the incident wavefront and correct for aberrations. Our configuration does not require direct wavefront sensing and the injection of a “guide-star”; hence, it can be implemented without the need for sample pre-treatment. We used our AO-Brillouin spectrometer in aberrated phantoms and biological samples and obtained improved precision and resolution of Brillouin spectral analysis; we demonstrated 2.5-fold enhancement in Brillouin signal strength and 1.4-fold improvement in axial resolution because of the correction of optical aberrations.

Optimal model-based sensorless adaptive optics for epifluorescence microscopy.

We report on a universal sample-independent sensorless adaptive optics method, based on modal optimization of the second moment of the fluorescence emission from a point-like excitation. Our method employs a sample-independent precalibration, performed only once for the particular system, to establish the direct relation between the image quality and the aberration. The method is potentially applicable to any form of microscopy with epifluorescence detection, including the practically important case of incoherent fluorescence emission from a three dimensional object, through minor hardware modifications. We have applied the technique successfully to a widefield epifluorescence microscope and to a multiaperture confocal microscope.

Fast correction approach for wavefront sensorless adaptive optics based on a linear phase diversity technique.

Wavefront sensorless (WFSless) adaptive optics (AO) systems have been widely studied in recent years. To reach optimum results, such systems require an efficient correction method. This paper presents a fast wavefront correction approach for a WFSless AO system mainly based on the linear phase diversity (PD) technique. The fast closed-loop control algorithm is set up based on the linear relationship between the drive voltage of the deformable mirror (DM) and the far-field images of the system, which is obtained through the linear PD algorithm combined with the influence function of the DM. A large number of phase screens under different turbulence strengths are simulated to test the performance of the proposed method. The numerical simulation results show that the method has fast convergence rate and strong correction ability, a few correction times can achieve good correction results, and can effectively improve the imaging quality of the system while needing fewer measurements of CCD data.

Multiscale sensorless adaptive optics OCT angiography system for in vivo human retinal imaging

We present a multiscale sensorless adaptive optics (SAO) OCT system capable of imaging retinal structure and vasculature with various fields-of-view (FOV) and resolutions. Using a single deformable mirror and exploiting the polarization properties of light, the SAO-OCT-A was implemented in a compact and easy to operate system. With the ability to adjust the beam diameter at the pupil, retinal imaging was demonstrated at two different numerical apertures with the same system. The general morphological structure and retinal vasculature could be observed with a few tens of micrometer-scale lateral resolution with conventional OCT and OCT-A scanning protocols with a 1.7-mm-diameter beam incident at the pupil and a large FOV (15 deg× 15 deg). Changing the system to a higher numerical aperture with a 5.0-mm-diameter beam incident at the pupil and the SAO aberration correction, the FOV was reduced to 3 deg× 3 deg for fine detailed imaging of morphological structure and microvasculature such as the photoreceptor mosaic and capillaries. Multiscale functional SAO-OCT imaging was performed on four healthy subjects, demonstrating its functionality and potential for clinical utility.

Increasing the field of view of adaptive optics scanning laser ophthalmoscopy.

An adaptive optics scanning laser ophthalmoscope (AO-SLO) set-up with two deformable mirrors (DM) is presented. It allows high resolution imaging of the retina on a 4°×4° field of view (FoV), considering a 7 mm pupil diameter at the entrance of the eye. Imaging on such a FoV, which is larger compared to classical AO-SLO instruments, is allowed by the use of the two DMs. The first DM is located in a plane that is conjugated to the pupil of the eye and corrects for aberrations that are constant in the FoV. The second DM is conjugated to a plane that is located ∼0.7 mm anterior to the retina. This DM corrects for anisoplanatism effects within the FoV. The control of the DMs is performed by combining the classical AO technique, using a Shack-Hartmann wave-front sensor, and sensorless AO, which uses a criterion characterizing the image quality. The retinas of four healthy volunteers were imaged in-vivo with the developed instrument. In order to assess the performance of the set-up and to demonstrate the benefits of the 2 DM configuration, the acquired images were compared with images taken in conventional conditions, on a smaller FoV and with only one DM. Moreover, an image of a larger patch of the retina was obtained by stitching of 9 images acquired with a 4°×4° FoV, resulting in a total FoV of 10°×10°. Finally, different retinal layers were imaged by shifting the focal plane.

A high speed model-based approach for wavefront sensorless adaptive optics systems

To improve temporal-frequency property of wavefront sensorless adaptive optics (AO) systems, a fast general model-based aberration correction algorithm is presented. The fast general model-based approach is based on the approximately linear relation between the mean square of the aberration gradients and the second moment of far-field intensity distribution. The presented model-based method is capable of completing a mode aberration effective correction just applying one disturbing onto the deformable mirror(one correction by one disturbing), which is reconstructed by the singular value decomposing the correlation matrix of the Zernike functions’ gradients. Numerical simulations of AO corrections under the various random and dynamic aberrations are implemented. The simulation results indicate that the equivalent control bandwidth is 2–3 times than that of the previous method with one aberration correction after applying N times disturbing onto the deformable mirror (one correction by N disturbing).

Adaptive optics compensation of orbital angular momentum beams with a modified Gerchberg–Saxton-based phase retrieval algorithm

Practical orbital angular momentum (OAM)-based free-space optical (FSO) communications commonly experience serious performance degradation and crosstalk due to atmospheric turbulence. In this paper, we propose a wave-front sensorless adaptive optics (WSAO) system with a modified Gerchberg–Saxton (GS)-based phase retrieval algorithm to correct distorted OAM beams. We use the spatial phase perturbation (SPP) GS algorithm with a distorted probe Gaussian beam as the only input. The principle and parameter selections of the algorithm are analyzed, and the performance of the algorithm is discussed. The simulation results show that the proposed adaptive optics (AO) system can significantly compensate for distorted OAM beams in single-channel or multiplexed OAM systems, which provides new insights into adaptive correction systems using OAM beams.

Synchronous model-based approach for wavefront sensorless adaptive optics system.

In this paper, an aberration correction algorithm for wavefront sensorless adaptive optics system is presented, which is based on the approximately linear relation between the mean square of the aberration gradients and the second moment of far-field intensity distribution. To demonstrate algorithm's performance, an experiment system of aberration corrections for Fresnel zone plates imaging is set up. The correction results show the excellent performance of this method in correction speed, and correction capability. Comparing with conventional stochastic parallel gradient descent (SPGD) algorithm, the correction speed with this method is improved about four times for the similar correction effect.

Theoretical analysis of simultaneously improving the light coupling efficiency and bandwidth between two separated grating couplers using integrated distributed Bragg reflectors

The performance of free space optical (FSO) communication system is limited by atmospheric turbulent extremely. Adaptive optics (AO) is the significant method to overcome the atmosphere disturbance. Especially, for the strong scintillation effect, the sensor-less AO system plays a major role for compensation. In this paper, a modified artificial fish school (MAFS) algorithm is proposed to compensate the aberrations in the sensor-less AO system. Both the static and dynamic aberrations compensations are analyzed and the performance of FSO communication before and after aberrations compensations is compared. In addition, MAFS algorithm is compared with artificial fish school (AFS) algorithm, stochastic parallel gradient descent (SPGD) algorithm and simulated annealing (SA) algorithm. It is shown that the MAFS algorithm has a higher convergence speed than SPGD algorithm and SA algorithm, and reaches the better convergence value than AFS algorithm, SPGD algorithm and SA algorithm. The sensor-less AO system with MAFS algorithm effectively increases the coupling efficiency at the receiving terminal with fewer numbers of iterations. In conclusion, the MAFS algorithm has great significance for sensor-less AO system to compensate atmospheric turbulence in FSO communication system.

Modified artificial fish school algorithm for free space optical communication with sensor-less adaptive optics system

Phase-contrast x-ray imaging (PCXI) is an emerging technology which allows for imaging of smaller features in the examined sample than conventional attenuation-based x-ray imaging with lower x-ray dose. In this work, we investigated a relatively simple approach to PCXI using a conventional x-ray grid, which has potential to open the way to further widespread use of the technique into many application fields. We performed a simulation using a useful PCXI software developed in our previous study to investigate the image characteristics. We also performed a preliminary experiment for PCXI using a table-top setup which consists of a focused-linear xray grid having a 200-lines/in strip density, a microfocus x-ray tube having a 55-μm focal spot, and a CMOS-type flat-panel detector having a 49-μm pixel resolution. We successfully extracted absorption, dark-field, and phase-contrast x-ray images of much enhanced visibility at a time from the raw image of the sample by Fourier analysis. Further, we made a composite image by combining the absorption image with the colored dark-field image and compared its image characteristic to that of the absorption image alone.

Optimizing the metric in sensorless adaptive optical microscopy with fluorescence fluctuations.

Adaptive optics (AO) strategies using optimization-based, sensorless approaches are widely used, especially for microscopy applications. To converge rapidly to the best correction, such approaches require that a quality metric and a set of modes be chosen optimally. Fluorescence fluctuations microscopy, a family of methods that provides quantitative measurements of molecular concentration and mobility in living specimen, is in particular need of adaptive optics, since its results can be strongly biased by optical aberrations. We examined two possible metrics for sensorless AO, measured in a solution of fluorophores diffusing in 3D: the fluorescence count rate and the molecular brightness (or number of photons detected per molecule in the observation volume). We studied their respective measurement noise and sensitivity to aberrations. Then, AO correction accuracy was experimentally assessed by measuring the residual aberration after correcting a known wavefront. We proposed a theoretical framework to predict the correction accuracy, knowing the metric measurement noise and sensitivity. In the small aberration range, the brightness allows more accurate corrections when fluorophores are few but bright, whereas the count rate performs better in more concentrated solutions. When correcting large aberrations, the count rate is expected to be a more reliable metric.

Application of wavefront sensorless adaptive optics in biological imaging

Obtaining high-quality images of organisms is critical to life sciences research, but imaging optical system and organism itself contain aberrations, leading to reduced resolution and contrast. Since aberrations introduced by different organisms are different and may change with time, they cannot be compensated by conventional optical design methods. Adaptive optics, originating in astronomical imaging observations, has the ability to overcome the effects of dynamic aberrations, and can therefore be used to improve imaging quality in biological imaging. Conventional adaptive optics typically has a wavefront sensor (such as Shack-Hartmann sensor) to measure wavefront errors. Recently developed wavefront sensorless adaptive optics (WSLAO) has got rid of dedicated wavefront sensors, so the system structure can be simplified and the cost is reduced. WSLAO can also overcome some limitations of traditional adaptive optics and has good application prospects. In this paper, the working principle of WSLAO is given at first. Then the applications of this technology in fundus imaging and fluorescence microscopy are discussed and the future development trend is proposed.

Wavefront sensorless adaptive optics OCT with the DONE algorithm for in vivo human retinal imaging [Invited

In this report, which is an international collaboration of OCT, adaptive optics, and control research, we demonstrate the Data-based Online Nonlinear Extremum-seeker (DONE) algorithm to guide the image based optimization for wavefront sensorless adaptive optics (WFSL-AO) OCT for in vivo human retinal imaging. The ocular aberrations were corrected using a multi-actuator adaptive lens after linearization of the hysteresis in the piezoelectric actuators. The DONE algorithm succeeded in drastically improving image quality and the OCT signal intensity, up to a factor seven, while achieving a computational time of 1 ms per iteration, making it applicable for many high speed applications. We demonstrate the correction of five aberrations using 70 iterations of the DONE algorithm performed over 2.8 s of continuous volumetric OCT acquisition. Data acquired from an imaging phantom and in vivo from human research volunteers are presented.

模型式无波前探测自适应光学系统抗噪能力分析

模型式无波前探测自适应光学系统抗噪能力分析

Enhanced visualization of peripheral retinal vasculature with wavefront sensorless adaptive optics optical coherence tomography angiography in diabetic patients.

Optical coherence tomography angiography (OCTA) is a promising technique for non-invasive visualization of vessel networks in the human eye. We debut a system capable of acquiring wide field-of-view (>70°) OCT angiograms without mosaicking. Additionally, we report on enhancing the visualization of peripheral microvasculature using wavefront sensorless adaptive optics (WSAO). We employed a fast WSAO algorithm that enabled wavefront correction in <2 s by iterating the mirror shape at the speed of OCT B-scans rather than volumes. Also, we contrasted ∼7° field-of-view OCTA angiograms acquired in the periphery with and without WSAO correction. On average, WSAO improved the sharpness of microvasculature by 65% in healthy eyes and 38% in diseased eyes. Preliminary observations demonstrated that the location of 7° images could be identified directly from the wide field-of-view angiogram. A pilot study on a normal subject and patients with diabetic retinopathy showed the impact of utilizing WSAO for OCTA when visualizing peripheral vasculature pathologies.