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An improved programmable grating array based wavefront sensor (GAWS) is proposed which is capable of estimating the incident wavefront more accurately, by generating an array of uniform intensity +1 order spots with negligible contribution from unwanted higher order spots. The duty cycle of each grating element of the proposed sensor is effectively varied in order to independently control the intensity of each +1 order spot. Furthermore, random binarisation technique is implemented on the diffraction grating array to reduce the contribution from undesirable higher order spots by disintegrating them into noise. Proof-of-principle simulation results are presented to demonstrate the working of the proposed GAWS in comparison to the conventional GAWS, for non-uniform intensity of the +1 order spots.

The biocompatible hydrogel materials are used for the fabrication of artificial cornea. The refractive index of the hydrogel material can affect the optical quality of the artificial cornea. It is challenging to measure the refractive index of the hydrogel material because it is between solid and liquid status during the fabrication processes. In this work, we propose using the Shack–Hartmann wavefront sensor system to measure the wavefront images and to calculate the corresponding aberrations of optical material. The wavefront aberrations can be used to estimate the refractive index of the optical material. To prove the concept, the standard N-BK7 bi-convex lenses with focal lengths of 100 and 150 mm at wavelengths 780, 532, and 450 nm have been measured, and the results show that the relative errors of the average measured refractive indices are about 0.1%–0.17%. We used this proposed method to measure the refractive index of the P407-DA hydrogel material. We found that the average measured refractive indices of the P407-DA hydrogel material are 1.3175, 1.3516, and 1.3645 at wavelengths 780, 532, and 450 nm, respectively. Compared with the refractive index of human cornea, the P407-DA hydrogel can be used as the artificial cornea material. The Cauchy dispersion formula can be used to calculate the Abbe number of the P407-DA hydrogel material, which is about 11, lower than the human cornea’s Abbe number. It means that the P407-DA hydrogel material has higher chromatic dispersion than the human cornea material. Moreover, the elasmobranch (shark) corneas, which are known as the suitable material for xenotransplantation, are measured by the Shack–Hartmann wavefront sensor system. The ABBE refractometer (ATAGO DR-M4) is used to measure the flat P407-DA hydrogel sample and it is found that the refractive index is slightly different when compared with the measured results by the Shack–Hartmann wavefront sensor system. The refractive indices of other biocompatible hydrogels poly(acrylic acid) (PAA) and poly(2-hydroxyethyl methacrylate) (PHEMA) are also measured by our proposed method, and it is found that the refractive index of P407-DA is more suitable for designing corneal samples than other hydrogels. It is found that the refractive indices of the elasmobranch corneas are similar to the P407-DA material. Our proposed method to measure the refractive index of hydrogel material using the Shack–Hartmann wavefront sensor system is useful. It is a non-contact, minimally invasive, and time-efficient method for shaped structures. The results can be used as a guide for developing better biocompatible material for artificial cornea.

Based on wavefront sensor images an objective and quantitative method is presented for characterising cataract. By separating direct and scattered light in the focal plane of the microlenses, the new procedure is able to make two-dimensional maps of the spatial variation of scattering properties in the crystalline lens, and also provides a single figure descriptive for the whole eye. The developed evaluation algorithm successfully quantifies cataract, especially that of nuclear type. To demonstrate its operation, a custom-built measurement setup was constructed using a Shack-Hartmann wavefront sensor with $$40 \times 32$$40×32 microlenses to capture 12-bit images of the pupil plane, and a superluminescent diode of 830 nm wavelength as a light source. Slit-lamp clinical measurements served as reference for calibration and to estimate the accuracy of the new method. The tests were carried out on 78 eyes with cataract in different progression state ranging from healthy to above 5 on the LOCS III scale. The residual error of the calibration (i.e. the standard deviation of difference between clinical reference and our algorithmic characterisation) turned out to be $$\pm 0.29$$±0.29 category on the LOCS III N scale, which approximates the $$\pm 0.33$$±0.33 precision of classic cataract measurements carried out with the greatest care.

Although ultrasonically controlled liquid crystal lens may play in important role in future optical systems, many optical properties in the liquid crystal layer are still unclear. Moreover, most of the previously conducted studies control the liquid crystal molecular orientation by an applied electric field using transparent electrodes. In this study, using multichannel piezoelectric transducer, standing wave and traveling wave actuation mechanism were applied to the liquid crystal lens and further reported about its optical characteristics using a Shack-Hartmann wavefront sensor. The surface data obtained from the sensor was analyzed and the optical aberrations measured. Traveling wave actuation showed small optical aberrations when compared to the standing wave.

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.

The Shack–Hartmann wavefront sensor (SHWFS) has been used to measure wavefronts. However, the dynamic range and measurement accuracy limits the application of the SHWFS. A new centroid estimation algorithm based on image segmentation and related techniques to expand the dynamic range based on search strategy is presented in this paper. In this study, each spot is segmented from a spot array image and the centroid of each spot is calculated only in regions of segmented spots. To compute the slope between the spot centroid and the sub-aperture centre, the calculated centroids are assigned to their corresponding sub-apertures using a search method. The proposed method overcomes the limitations that the centroid must be calculated in its corresponding detection area and reduces the measurement errors caused by noise, expanding the dynamic range significantly without any change to the SHWFS setup as well as improving the measurement accuracy under strong noise. Moreover, the proposed method provides a unit linear coefficient response of centroid estimation. The dynamic range for the proposed method is evaluated by numerical simulations and experiments. Applications include measurement accuracy and linearity.

A technique for nonlinear refractive index measurement of a single trapped nanoparticle is investigated using Shack–Hartmann sensor. In this technique, trapping laser is used as a probe beam and an interacting green laser is illuminated as a pumping beam. Optical path difference and effective height of particle is measured with dual wavelength unwrapping technique. Using the absorption relation, we obtain the profile of nonlinear refractive index. Also, the results prove the quadric relation of the nonlinear refractive index to the nonlinear absorption. This result may have an enormous effect in investigating and characterization of nonlinear refractive index of Nano and Micro-sized particles. Also, measuring the thermal profile of the trapping site after illuminating the high power trapping laser leads to measuring the gradient intensity that could have an important role in nonconservative forces measurements. Another advantage is in having the effective thickness of particle which is important while geometry of the particle is crucial for determining trap stiffness. This measurement may lead to adaptive trap shape control and potential well measurement.

In the ophthalmic Shack-Hartmann wavefront sensor beacon light originates from various depths within the retina. We model the retina of the human and mouse eyes as formed by two backscattering planes, showing that the use of lenslets with low Fresnel number and/or too small search boxes in the centroiding algorithm will produce artifactual aberrations. We evaluate the impact of these errors for four common beacon illumination strategies: full circular, annular, small circular on-axis and small circular off-axis. We find that artifactual aberrations are larger for annular and off-axis beacon illumination, dominated by defocus plus spherical aberration and defocus plus coma, respectively. These artifactual aberrations can be almost completely eliminated by selecting the minimum centroid search box size based on a simple Gaussian optics model and ocular biometry, provided the lenslet Fresnel number is sufficiently large to introduce minimal cross-talk between images of adjacent lenslets.

With sub-diffraction resolution in three dimensions and good optical sectioning capability, three-dimensional superresolution structured illumination microscopy (3D-SRSIM) can provide eight-fold more information than conventional widefield microscopy. However, the application of SR-SIM is limited to single cells due to optical aberrations in thick tissues. The destructive impacts of aberrations include the decrease in spatial resolution and signal-to-noise ratio (SNR), the distortion of sample morphology, and, even worse, the failure of SIM reconstruction. There are several adaptive optics (AO) methods to correct the optical aberration, including direct wavefront sensing using a Shack Hartmann wavefront sensor (SHWFS). The SHWFS possesses good wavefront measurement accuracy and high-speed response but works best with an isolated guide-star. Therefore, combining SHWFS with widefield microscopy poses difficulties and remains challenging. To effectively apply the direct wavefront sensing method, we built a reconfigurable microscopy system that can switch to a confocal setup for measuring the wavefront where the fluorescence light emitted from the confocal illumination spot is used as the “guide-star” for wavefront measurements. We experimentally demonstrate that the confocal illumination based direct wavefront sensing AO method can precisely correct the sample induced optical aberrations and help to improve the image quality and fidelity of 3D-SIM imaging in thick samples, exhibiting enormous potential for in vivo biomedical research.

Present work demonstrates an experimental approach to evaluate the optical performance of the Infrared (IR) microlens based on the Shack-Hartmann wavefront sensing at 10.6 µm wavelength. A Germanium microlens has been evaluated to validate the consistency and accuracy of the developed experimental methodology. The proposed IR optical setup has the capability to measure optical parameters like focal length, transmitted wavefront errors, Zernike coefficients, and PSF of a microlens with the diameter ranging from 500 to 2500 µm and the focal length ≥1 mm.

espanolEn este trabajo presentamos el analisis de la calidad de imagen de un objetivo de microscopio antiguo datado ca 1850 mediante una tecnica basada en el uso de un sensor de frente de onda Shack-Hartmann. Se describen la base del metodo utilizado y los experimentos realizados, y los resultados se describen para probar la validez de dicho metodo. La tecnica presentada y utilizada en este trabajo supone una herramienta util e interesante para simular el proceso de formacion de imagenes en color. Se comenta, tambien, el potencial uso del trabajo con fines educativos. EnglishIn this work we report the image quality analysis of an antique microscope objective from ca 1850 by using a non-contact technique based on a Shack-Hartmann wavefront sensor. The method and the experiment are described, and results are fully discussed. The technique provides useful and interesting tools for colour image formation process simulations. Application of this work for educational purposes is also commented.

This paper provides a status update on the Natural Guidestar (NGS) Adaptive Optics (AO) system being built for Castor, the meter class telescope at the Starfire Optical Range. We present a radiometric case study for a range of variable parameters such as source brightness, number of Shack-Hartmann sub-apertures, AO and Track loop frame rate and bandwidth. We gauge system performance by contrast and adapt the error budget to allow detection of a dim object near a bright star. We present wave band splits between the different AO components such as track sensor, wavefront sensor, scoring camera, and science camera. We show the different configurations that allow to switch between dim object and bright object tracking. The opto-mechanical design of the AO system is also presented.

This paper investigates the performance and optimization of the Geometric Wavefront Sensor (GWFS) in openloop wavefront sensing, along with the Curvature Wavefront Sensor (CWFS) and Shack-Hartmann Wavefront Sensor (SH-WFS). The GWFS uses a ray tracing process to calculate the displacement of intensity fluctuations from two defocused point source images. Various parameters within the GWFS – such as the signal-to-noise ratio (SNR) sensitivity, the number of Radon angles, the virtual propagation distance, and the number of reconstruction modes – are explored on a laboratory test bench. We found that the GWFS wavefront estimate error experiences an inverse relationship to the SNR, a minimum of 5 Radon angles is required to accurately estimate the single Zernike mode wavefronts (Z4 – Z15), the virtual propagation distance is confined by ray crossing and Fresnel diffraction effects, and the number of reconstruction Zernike modes is limited by noise amplification and over-fitting. This paper demonstrates the capabilities of the GWFS, illustrates the resulting wavefront estimates, and confirms the superior performance of the GWFS compared to the CWFS. The optimized GWFS will be utilized at Mt. John University Observatory (MJUO) in New Zealand for satellite and space debris imaging and tracking

A measurement platform including a Shack-Hartmann Wavefront Sensor (SHWFS) has been designed, integrated and tested at Imagine Optic, Orsay France and delivered to the Rubin Observatory (previously known as the LSST). This instrument will be used for the initial on-axis optical alignment and testing of the LSST telescope. The optical configuration of the Rubin Telescope without the presence of the main LSST camera requires advanced capabilities in term of quantities of aberrations to measure, linearity, accuracy and sensitivity for usage on a natural star. The HASO 128 GE 2, available from Imagine Optic, was identified as the most relevant WFS to meet all those requirements. In that paper, we provide details about the simulations that led to the choice of the HASO 128 GE 2. We also provide the specifics regarding the opto-mechanical design allowing the relay imaging of the pupil compatible with the fast aperture of the telescope, a reference source for the on-site calibration of the system and the optical output made available for a viewer camera.

ULTIMATE-Subaru Tomography Adaptive optics Research experimenT (ULTIMATE-START) is a laser tomography AO project on the Subaru telescope. The project is planned to achieve high Strehl Ratio AO correction in NIR bands, and moderate AO correction in visible bands above 600nm. An asterism of 4 laser guide stars (LGSs) will be launched from the laser launching telescope behind the secondary mirror. The tomography wavefront sensing unit with four 32$times$32 Shack-Hartmann wavefront sensors will be installed behind the current facility LGS AO system, AO188. The deformable mirror of AO188 will be upgraded to a 64$times$64 element DM. The corrected light will be fed to the optical integral field spectrograph, 3DII, and NIR camera and spectrograph, IRCS, through a beam switching optics for IR-side Nasmyth focus instruments under development. The first light of the laser launching system and wavefront sensing unit is planned in 2021.

High-resolution imaging with large ground-based telescopes is limited by atmospheric turbulence. The observed images are usually blurred with unknown point spread functions (PSFs) defined in terms of the wavefront distortions of light. To effectively remove the blur, numerical postprocessing with a good approximation of the wavefront is required. The gradient measurements of the wavefront recorded by Shack–Hartmann wavefront sensor (WFS) can be used to estimate the wavefront. A gradients measurement model for Shack–Hartmann WFS is built. This model is based on the frozen flow hypothesis and uses a least-squares-fit of tip and tilt across the subaperture in the WFS to genarate the averaged gradient measurements. Then a high-resolution wavefront reconstruction method using multiframe Shack–Hartmann WFS measurements is presented. The method uses high cadence WFS data in a Bayesian framework and takes into account the available a priori information of the wavefront phase. Numerical results show that the method can effectively improve the spatial resolution and accuracy of the reconstructed wavefront in different seeing conditions.

The results of resolution enhancement of the stellar imaging system by using a Rayleigh laser guide star are presented. The estimation of residual phase error value of adaptive correction is carried out at various position of a laser guide star.

The dynamical phase correction of laser radiation distorted by atmospheric turbulence in the adaptive optical system (AOS) with Shack-Hartmann wavefront sensor is numerically investigated. The criterions of correction efficiency have been obtained depending on the relation between the bandwidths of AOS and turbulence. The ideal as well as actual adaptive mirrors are considered.

Numerical investigation of laser beam phase correction in the turbulent atmosphere by adaptive optical system with Shack-Hartmann wavefront sensor has been performed. The criterions of correction efficiency depending on the adaptive mirror and wavefront sensor spatial resolution have been determined for various values of Fried parameter.

Optical parts, such as lenses or mirrors, are often qualified by surface shape metrology. Wavefront measurements allow for a more direct measurement of the performance of an optical part. However, the dynamic range of current wavefront sensors is not sufficient for measuring the wavefront generated by high-NA optical parts. In this paper, a wavefront sensor is combined with a mechatronic positioning system to overcome these limitations by repositioning and reorienting the wavefront sensor. Feedback loops are implemented to ensure a tangential orientation of the wavefront sensor in every measurement position. An experimental setup is constructed and a measurement strategy that compensates for tip and tilt between wavefront and sensor is developed. It is demonstrated that this setup is capable of directly measuring the wavefront of a high-NA optic.

We presented a new method to measure the topological charge (TC) of the vortex beam (VB) by use a Shack-Hartmann wavefront sensor with sector microlens array (SMLA). The SMLA is circularly symmetric, which perfectly corresponds to helical phase and ring intensity of the VB. On the focal plane, there are spot array with circular distribution due to the special arrangement of the SMLA. In this letter, we also analyzed the error, and concluded that the error of the outer ring is smaller. As a result, we only employed the data of the outermost ring. This method easily can obtain the sign and magnitude of the TC by calculating offset direction and offset of the spots respectively. The simulation results show that our method can determine large TCs and has high accuracy. While this method has a simple structure and can be used in optical communications in the future.

Adaptive optics (AO) systems for retinal imaging have a small field of view (FoV) of 1°-2°. To increase the FoV we developed a multi-conjugated AO-system. By choosing different scanning patterns, aberrations introduced by the eye at different locations within the FoV can be measured by a Shack Hartmann wavefront sensor. A second deformable mirror that is conjugated to a plane anterior to the retina corrects for varying aberrations across the field. Retinal imaging using spectral-domain optical coherence tomography is performed with 4o x 4o FoV and imaging data including the visualization of the cone mosaic is presented.

Grating array based zonal wavefront sensor (GAZWFS), using an array of gratings implemented with a liquid crystal spatial light modulator (LCSLM) to display the gratings, offers a flexible measurement approach. But the wavefront with a large slope may not accurately be sensed by the sensor like any Shack- Hartmann type sensor. In this paper, we propose a zone wise scanning method to improve the dynamic range of GAZWFS by blocking and unblocking the individual gratings. The estimation of the wavefront and expansion of the dynamic range of GAZWFS by the zone wise scanning method is demonstrated by numerical simulation and a proof-of-principle experimental implementation.

Light Sheet Microscopy has developed rapidly over the past decade and is the ideal approach for imaging model organisms such as zebrafish and other thick tissue specimens. Despite the superior optical sectioning capability, high imaging speed, and large field of view, the performance of light sheet microscopy still suffers from optical aberrations. We have implemented a scene-based Shack-Hartmann wavefront sensor for directly measuring the optical aberrations on the emission side of the light-sheet microscope. In this work, we show that our system is capable of AO correction using sensor based and sensorless based approaches. We demonstrate correction up-to one hundred microns deep in zebrafish and fruitfly embryos.

This paper proposes an efficient approach to decrease the computational costs of correlation-based centroiding methods used for point source Shack–Hartmann wavefront sensors. Four typical similarity functions have been compared, i.e. the absolute difference function (ADF), ADF square (ADF 2 ), square difference function (SDF), and cross-correlation function (CCF) using the Gaussian spot model. By combining them with fast search algorithms, such as three-step search (TSS), two-dimensional logarithmic search (TDL), cross search (CS), and orthogonal search (OS), computational costs can be reduced drastically without affecting the accuracy of centroid detection. Specifically, OS reduces calculation consumption by 90%. A comprehensive simulation indicates that CCF exhibits a better performance than other functions under various light-level conditions. Besides, the effectiveness of fast search algorithms has been verified.

Existing wavefront reconstruction methods are usually low in resolution, restricted by structure characteristics of the Shack Hartmann wavefront sensor (SH WFS) and the deformable mirror (DM) in the adaptive optics (AO) system, thus, resulting in weak homodyne detection efficiency for free space optical (FSO) communication. In order to solve this problem, we firstly validate the feasibility of liquid crystal spatial light modulator (LC SLM) using in an AO system. Then, wavefront reconstruction method based on wavelet fractal interpolation is proposed after self-similarity analysis of wavefront distortion caused by atmospheric turbulence. Fast wavelet decomposition is operated to multiresolution analyze the wavefront phase spectrum, during which soft threshold denoising is carried out. The resolution of estimated wavefront phase is then improved by fractal interpolation. Finally, fast wavelet reconstruction is taken to recover wavefront phase. Simulation results reflect the superiority of our method in homodyne detection. Compared with minimum variance estimation (MVE) method based on interpolation techniques, the proposed method could obtain superior homodyne detection efficiency with lower operation complexity. Our research findings have theoretical significance in the design of coherent FSO communication system.

Results of an experimental study of the density distribution in a small-size (1–2 mm in diameter) supersonic gas jet in vacuum are reported. The measurements are performed by the developed Shack–Hartmann wavefront sensor, which consists of a microlens array with 100 × 100 elements and a video camera with a resolution of 2048 × 2048 pixels. The reliability of measurements in terms of the space and time resolution, as well as in terms of the minimum levels of phase changes induced by the tested object, is analyzed.

Shack–Hartmann wavefront sensor (SHWS) is a traditional method of obtaining the phase information of the incident light. As a classical method, SHWS has the merit of simple structure, good usability, real-time monitoring and wide waveband. Traditional SHWS depends mainly on a lenslet array which can split the incident wavefront and produce an array of distorted foci, which reflect the change of wavefront curvature under test. Compared to the mono-focal lenslet in SHWS, here self-interference Chinese Taiji lenslet have been introduced into the wavefront sensing in order to improve the precision of the centroid position by means of multiple foci. Wavefront sensing experiments on self-interference Taiji-lenslet array with bi- and quad-foci were sequentially carried out to verify the validity of our proposed method. More foci-splitter would bring more accuracy. Taiji lens can also be designed with amplitude modulation such as Taiji sieve, which is very suitable for wavefront measurement in the region of short wavelength, especially EUV and soft X-ray.

An adaptive optics (AO) system has advantages of avoiding waffle modes and ease to assemble, when the lenslets of the Shack-Hartmann wavefront sensor (SH-WFS) and the actuators of the deformable mirror (DM) are configured according to the Southwell geometry. However, an AO system which follows the Southwell geometry suffers from the calibration difficulty and from the poor control stability. In this paper, an AO system based on the Southwell configuration is built up and experimentally demonstrated. The calibration problem is solved by adopting a DM which has an appropriate inter-actuator coupling, so that the movement of an actuator can be measured by the neighboring lenslets. The control stability is evaluated by the error propagators, and then, the control stability can be improved by filtering out smaller singular values of the response matrix. It is also shown that a tradeoff between the bandwidth error and the fitting error of an AO system can be made according to the error propagator. Finally, the performance measurement experiments indicate the reasonable ranges for the number of reserved singular values of the response matrix and the integrator gain, i.e. from 81 to 89, and from 0.2 to 0.5, respectively.

The results of studies of the optics of the 1-m Zeiss-1000 telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) by the Shack–Hartmann (SH) method are presented. Using a Shack–Hartmann wavefront sensor (SH WFS) we have adjusted the telescope optical system by means of shifts and tilts of the secondary mirror. The procedure has significantly reduced the aberrations that appeared during the long-term instrument operation. A new method to investigate the surface quality of the mirrors of the Zeiss-1000 being applied, characteristics close to the diffraction limit are achieved. In general, the entire opto-mechanical telescope system provides an image quality of about 0.5″ at 80%energy level.

The method of the dynamic range expansion of the Shack–Hartmann wavefront sensor is discussed. It is based on the use of nonlinear dual focus holographic lenslet arrays with the aberration precompensation. The data concerning the optical setup and the technique of adaptive lenslet array producing based on nonlinear holographic recording phenomenon are represented. On the example of spherical wavefronts it is shown, that the use of three lenslet arrays with different amount of the aberration precompensation allows expanding approximately in five times the dynamic range of the sensor four times greater with preserving the specified sensitivity in comparison with the corresponding refractive lenslet array.

The conventional adaptive optics scanning laser ophthalmoscopy (AOSLO) arrangement is specifically designed to capture the confocal (directly backscattered) light by placing a physical pinhole conjugate to a chosen layer in the retina. This arrangement can be used to generate high contrast images of the photoreceptor mosaic by limiting the light from other retinal layers, such as the retinal pigment epithelium. However, there is growing demand for the study of different retinal features that has led to the development of different off-axis techniques to collect the non-confocal (multiply scattered) light. In this paper, we replace the physical pinhole of the AOSLO with a reconfigurable aperture to simultaneously collect the directly backscattered light, generating confocal images, as well as the multiply scattered light, generating non-confocal images. The reconfigurable aperture pattern is implemented with a digital micromirror device (DMD) and is optimised based on the information collected from Shack Hartmann wavefront sensor data. We present preliminary experimental results with a human eye to illustrate our findings.

The paper discusses an application of Shack-Hartmann wavefront sensor with stroboscopic illumination for measurements of the vibration modes at resonance frequencies of a MEMS devices. The wavefront sensor estimates the local slopes of the wavefront by measuring the displacement of light spots created by microlens array on the camera plane. The goal of the work is to investigate the features of the proposed method. Amplitudes of vibration modes of a MEMS square membrane are measured and compared with data obtained from the stroboscopic interferometry realized in Twyman-Green configuration. The difference in PV values of the vibration amplitude is less than 52 nm (λ/12). It confirms the high accuracy of the measurement. Moreover, in this case the Shack-Hartmann wavefront sensor enables to simplify the system configuration and reducing the impact of the environmental conditions on the measurement result.

High-quality in-depth imaging of three-dimensional samples remains a major challenge in modern microscopy. Selective plane illumination microscopy (SPIM) is a widely used technique that enables imaging of living tissues with subcellular resolution. However, scattering, absorption, and optical aberrations limit the depth at which useful imaging can be done. Adaptive optics (AOs) is a method capable of measuring and correcting aberrations in different kinds of fluorescence microscopes, thereby improving the performance of the optical system. We have incorporated a wavefront sensor adaptive optics scheme to SPIM (WAOSPIM) to correct aberrations induced by optically-thick samples, such as multi-cellular tumor spheroids (MCTS). Two-photon fluorescence provides us with a tool to produce a weak non-linear guide star (NGS) in any region of the field of view. The faintness of NGS, however, led us to develop a high-sensitivity Shack&ndash, Hartmann wavefront sensor (SHWS). This paper describes this newly developed SHWS and shows the correction capabilities of WAOSPIM using NGS in thick, inhomogeneous samples like MCTS. We report improvements of up to 79% for spatial frequencies corresponding to cellular and subcellular size features.

A high-quality flat wave front is usually used to calibrate the Shach-Hartmann wave-front sensors. The article discusses the possibility of calibrating sensors with spherical wave fronts. Special attention is paid to the consideration of calibration in standard laboratory conditions. The mathematical apparatus and the scheme of the experiment are considered.

An adaptive scanning optical microscope with extended depth of field (ASOM-EDOF) is described. The system is based on the ASOM developed previously*, and uses a custom-built Thorlabs 0.21NA objective with a 75mm pupil diameter that allows scanned imaging over a circular region of 40mm diameter using fast steering mirror. The microscope is configured for fluorescence imaging with epi-illumination. A 140 actuator, 5.5µm stroke DM is conjugated to the pupil of the objective, and is used in conjunction with a Shack Hartmann wavefront sensor in an adaptive optics loop to measure and compensate errors of the objective as a function of nominal scan angle. At a given scan angle, the microscope camera forms an image of a 200µm x 200µm region with resolution of about 1.4µm. Images recorded at different scan angles can be stitched together to form a larger image mosaic. At each scan angle, the DM has been calibrated not only to compensate astatic aberrations, but also to perform an axial focal sweep: changing shape from concave to convex at high speed during a single camera exposure. This type of extended depth of field (EDOF) imaging (without aberration compensation) has been reported previously**. By combining these two techniques (ASOM and EDOF), a single recorded camera frame includes in-focus light from objects at depths from the nominal objective focus to depths +/-250µm from that focus, corresponding to an extension of the depth of focus by a factor of 100x for this microscope. The image also includes out-of-focus light from all depths. After a simple deconvolution, one can recover the in-focus light from all swept layers, condensed into a 2D image. Calibration details and performance metrics are described, along with example images from large volumetric samples. *Potsaid B, Wen JTY, “Adaptive scanning optical microscope: large field of view and high-resolution imaging using a MEMS deformable mirror,” Journal of Micro-Nanolithography Mems and Moems, [7], 10, (2008). **Shain WJ, Vickers NA, Goldberg BB, Bifano T, Mertz J, “Extended depth-of-field microscopy with a high-speed deformable mirror,” Optics Letters, [42], 995-998, (2017).

Shack–Hartmann wavefront sensor (SHWS) is one of the popular zonal wavefront sensor that typically consists of a two-dimensional (2D) array of lenslets and a camera. Another variant of a zonal wavefront sensor is the programmable grating array based wavefront sensor (PGAWS) that replaces the lenslets array of the SHWS with a 2D array of programmable diffraction gratings and a single focusing lens to result in a 2D array of +1 order spots. The ability of a PGAWS to correctly measure the incident wavefront depends on the accuracy with which each +1 order spot centroid position in the camera plane is calculated. However, in practical applications the intensities of the +1 order spots are usually non-uniform and there is presence of higher order spots that lead to erroneous calculation of the centroid position. In the present work, an improved PGAWS is proposed and by exploiting its programmable facility the transmittance function of the grating array is defined to generate an array of uniform intensity +1 order spots with negligible contribution from unwanted higher order spots by breaking them into noise. We present proof-of-concept simulation results that illustrate the generation of uniform intensity +1 order spots and also quantify its uniformity numerically by defining a quality metric. Furthermore, the working of the proposed PGAWS is substantiated through simulation results which demonstrate its accurate wavefront estimation capability in comparison to the conventional PGAWS, in the presence of non-uniform intensity +1 order spots.

Electron multiplying charge-coupled-device (EMCCD) applied in Shack–Hartmann wavefront sensor (S–H WFS) makes the wavefront sensing more efficient for adaptive optics (AO). However when the brightness of the observed target changes in large ranges in a few minutes, a fixed electron multiplying (EM) gain may not be optimum. Thus an auto-gain-control (AGC) method based on the spots image of the S–H WFS is proposed. The designed control value is the average value of the maximum signals of all the light spots in a frame. It has been demonstrated in the experiments that the control value is sensitive to the change of the target brightness, and is stable in the presence of detecting noises and turbulence influence. The goal value for control is predetermined based on the linear relation of the signal with the EM gain and the number of photons collected in sub-apertures. The conditions of the self-protection of the EMCCD are also considered for the goal value. Simulations and experiments indicate that the proposed control method is efficient, and keeps the sensing in a high SNR which reaches the upper SNR limit when sensing with EMCCD. The self-protection of the EMCCD is avoided during the whole sensing process.

Several techniques have been used with Shack–Hartmann wavefront sensors to determine the local wave-front gradient across each lenslet. While the centroid error of Shack–Hartmann wavefront sensor is relatively large since the skylight background and the detector noise. In this paper, we introduce a new method based on sparse representation to extract the target signal from the background and the noise. First, an over complete dictionary of the spot signal is constructed based on two-dimensional Gaussian model. Then the Shack–Hartmann image is divided into sub blocks. The corresponding coefficients of each block is computed in the over complete dictionary. Since the coefficients of the noise and the target are large different, then extract the target by setting a threshold to the coefficients. Experimental results show that the target can be well extracted and the deviation, RMS and PV of the centroid are all smaller than the method of subtracting threshold.

The conventional Shack–Hartmann wavefront sensor (SHWS) requires wavefront slope measurements of every micro-lens for wavefront reconstruction. In this Letter, we applied deep learning on the SHWS to directly predict the wavefront distributions without wavefront slope measurements. The results show that our method could provide a lower root mean square wavefront error in high detection speed. The performance of the proposed method is also evaluated on challenging wavefronts, while the conventional approaches perform insufficiently. This Letter provides a new approach, to the best of our knowledge, to perform direct wavefront detection in SHWS-based applications.

Shack-Hartmann wavefront sensors (SHWFSs) usually have fixed subaperture areas on the detector, in order to fix the minimum and maximum amounts of wavefront departure, or the dynamic range of measurement. We introduce an active approach, named Adaptive Shack Hartmann Wavefront Sensor (A-SHWFS). A-SHWFS is used to reconfigure detection subaperture areas by either blocking or unblocking desired lenslets by using an electronically modulated mask. This mask either increases or decreases the measurable aberration magnitude by placing a liquid crystal display (LCD) panel in front of the lenslet array. Depending on which control signal that is sent to the LCD, the variable, application-dependent blocking pattern (horizontal, vertical, diagonal, uneven) makes this an adaptive and efficient sensor with a variable dynamic range of measurement. This scheme is also useful for regional blocking, which occurs when the wavefront is severely aberrated in a limited region.