Showing papers on "Wavefront sensor published in 2017"
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TL;DR: This work proposes an optimal linear approach based on the Empirical Orthogonal Functions (EOF) framework commonly employed for atmospheric predictions that offers increased robustness and significant performance advantages over previously proposed wavefront prediction algorithms.
Abstract: Atmospheric wavefront prediction based on previous wavefront sensor measurements can greatly enhance the performance of adaptive optics systems. We propose an optimal linear approach based on the Empirical Orthogonal Functions (EOF) framework commonly employed for atmospheric predictions. The approach offers increased robustness and significant performance advantages over previously proposed wavefront prediction algorithms. It can be implemented as a linear pattern matching algorithm, which decomposes in real time the input (most recent wavefront sensor measurements) into a linear sum of previously encountered patterns, and uses the coefficients of this linear expansion to predict the future state. The process is robust against evolving conditions, unknown spatio-temporal correlations and non-periodic transient events, and enables multiple sensors (for example accelerometers) to contribute to the wavefront estimation. We illustrate the EOFs advantages through numerical simulations, and demonstrate filter convergence within 1 minute on a 1 kHz rate system. We show that the EOFs approach provides significant gains in high contrast imaging by simultaneously reducing residual speckle halo and producing a residual speckle halo that is spatially and temporally uncorrelated.
55 citations
TL;DR: The coronagraphic modal wavefront sensor (cMWS) as discussed by the authors was proposed for real-time NCPE correction by combining the Apodizing Phase Plate (APP) coronagraph with a holographic modal sensor.
Abstract: The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental Non-Common Path Errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10-6 to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor suitable for real-time NCPE correction. This combines the Apodizing Phase Plate (APP) coronagraph with a holographic modal wavefront sensor to provide simultaneous coronagraphic imaging and focal-plane wavefront sensing with the science point-spread function. We first characterise the baseline performance of the cMWS via idealised closed-loop simulations, showing that the sensor is able to successfully recover diffraction-limited coronagraph performance over an effective dynamic range of ±2.5 radians root-mean-square (rms) wavefront error within 2–10 iterations, with performance independent of the specific choice of mode basis. We then present the results of initial on-sky testing at the William Herschel Telescope, which demonstrate that the sensor is capable of NCPE sensing under realistic seeing conditions via the recovery of known static aberrations to an accuracy of 10 nm (0.1 radians) rms error in the presence of a dominant atmospheric speckle foreground. We also find that the sensor is capable of real-time measurement of broadband atmospheric wavefront variance (50% bandwidth, 158 nm rms wavefront error) at a cadence of 50 Hz over an uncorrected telescope sub-aperture. When combined with a suitable closed-loop adaptive optics system, the cMWS holds the potential to deliver an improvement of up to two orders of magnitude over the uncorrected QSS floor. Such a sensor would be eminently suitable for the direct imaging and spectroscopy of exoplanets with both existing and future instruments, including EPICS and METIS for the E-ELT.
44 citations
TL;DR: In this article, a method combining a coronagraph with a simple phase-shifting interferometer was proposed to detect the presence of a substellar companion using the coherence properties of light.
Abstract: Residual speckles due to aberrations arising from optical errors after the split between the wavefront sensor and the science camera path are the most significant barriers to imaging extrasolar planets. While speckles can be suppressed using the science camera in conjunction with the deformable mirror, this requires knowledge of the phase of the electric field in the focal plane. We describe a method which combines a coronagraph with a simple phase-shifting interferometer to measure and correct speckles in the full focal plane. We demonstrate its initial use on the Stellar Double Coronagraph at the Palomar Observatory. We also describe how the same hardware can be used to distinguish speckles from true companions by measuring the coherence of the optical field in the focal plane. We present results observing the brown dwarf HD49197b with this technique, demonstrating the ability to detect the presence of a companion even when it is buried in the speckle noise, without the use of any standard ‘calibration’ techniques. We believe this is the first detection of a substellar companion using the coherence properties of light.
41 citations
01 Sep 2017
TL;DR: In this paper, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned for the WFIRST flight coronagraph instrument, which consists of two coronagraph modes, SHP and HLC, and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront errors.
Abstract: To maintain the required performance of WFIRST Coronagraph in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The LOWFS/C uses a Zernike wavefront sensor (ZWFS) with the phase shifting disk combined with the starlight rejecting occulting mask. For wavefront error corrections, WFIRST LOWFS/C uses a fast steering mirror (FSM) for line-of-sight (LoS) correction, a focusing mirror for focus drift correction, and one of the two deformable mirrors (DM) for other low order wavefront error (WFE) correction. As a part of technology development and demonstration for WFIRST Coronagraph, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned. With its configuration similar to the WFIRST flight coronagraph instrument the OMC testbed consists of two coronagraph modes, Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC), a low order wavefront sensor (LOWFS), and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront error that would be induced by the WFIRST telescope’s vibration and thermal changes. In this paper, we will introduce the concept of WFIRST LOWFS/C, describe the OMC testbed, and present the testbed results of LOWFS sensor performance. We will also present our recent results from the dynamic coronagraph tests in which we have demonstrated of using LOWFS/C to maintain the coronagraph contrast with the presence of WFIRST-like line-of-sight and low order wavefront disturbances.
37 citations
TL;DR: It is inferred that adaptive optics performance rapidly increases with the loop frequency up to 50 Hz, with gain being more limited at higher rates, and the residual wavefront error evolution as a function of the number of corrected modes and of the adaptive optics loop frame rate.
Abstract: We formed a database gathering the wavefront aberrations of 50 healthy eyes measured with an original custom-built Shack-Hartmann aberrometer at a temporal frequency of 236 Hz, with 22 lenslets across a 7-mm diameter pupil, for a duration of 20 s. With this database, we draw statistics on the spatial and temporal behavior of the dynamic aberrations of the eye. Dynamic aberrations were studied on a 5-mm diameter pupil and on a 3.4 s sequence between blinks. We noted that, on average, temporal wavefront variance exhibits a n−2 power-law with radial order n and temporal spectra follow a f−1.5 power-law with temporal frequency f. From these statistics, we then extract guidelines for designing an adaptive optics system. For instance, we show the residual wavefront error evolution as a function of the number of corrected modes and of the adaptive optics loop frame rate. In particular, we infer that adaptive optics performance rapidly increases with the loop frequency up to 50 Hz, with gain being more limited at higher rates.
30 citations
TL;DR: In this article, the Zernike unit for segment phasing (ZEUS) was used for fine cophasing with diffraction-limited images, which is not suitable for fine phasing with see-only images.
Abstract: Context. Segmented aperture telescopes require an alignment procedure with successive steps from coarse alignment to monitoring process in order to provide very high optical quality images for stringent science operations such as exoplanet imaging. The final step, referred to as fine phasing, calls for a high sensitivity wavefront sensing and control system in a diffraction‐limited regime to achieve segment alignment with nanometric accuracy. In this context, Zernike wavefront sensors represent promising options for such a calibration. A concept called the Zernike unit for segment phasing (ZEUS) was previously developed for ground‐based applications to operate under seeing‐limited images. Such a concept is, however, not suitable for fine cophasing with diffraction‐limited images.
Aims. We revisit ZELDA, a Zernike sensor that was developed for the measurement of residual aberrations in exoplanet direct imagers, to measure segment piston, tip, and tilt in the diffraction‐limited regime.
Methods. We introduce a novel analysis scheme of the sensor signal that relies on piston, tip, and tilt estimators for each segment, and provide probabilistic insights to predict the success of a closed‐loop correction as a function of the initial wavefront error.
Results. The sensor unambiguously and simultaneously retrieves segment piston and tip‐tilt misalignment. Our scheme allows for correction of these errors in closed‐loop operation down to nearly zero residuals in a few iterations. This sensor also shows low sensitivity to misalignment of its parts and high ability for operation with a relatively bright natural guide star.
Conclusions. Our cophasing sensor relies on existing mask technologies that make the concept already available for segmented apertures in future space missions.
25 citations
TL;DR: In this article, an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband extreme ultraviolet (XUV) radiation is presented, which yields a focal spot of 3.6 × 4.0 µm2 full width at half maximum.
Abstract: We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband extreme ultraviolet (XUV) radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 × 4.0 µm2 full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 × 3.2 µm2. This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime.
25 citations
TL;DR: By introducing the concept of the incremental random sampling into wavefront sensing, the adaptability of phase retrieving based on the diffraction intensities is significantly improved and the feasibility of the method is demonstrated.
Abstract: A wavefront sensing method based on a spatial light modulator (SLM) and an incremental binary random sampling (IBRS) algorithm is proposed. In this method, the recording setup is built just by a transmittance SLM and an image sensor. The tested wavefront incident to the SLM plane can be quantitatively retrieved from the diffraction intensities of the wavefront passed through the SLM displaying a IBRS pattern. Because only two modulation states (opaque and transparent) of the SLM are used, the method does not need to know the concrete modulation function of the SLM in advance. In addition by introducing the concept of the incremental random sampling into wavefront sensing, the adaptability of phase retrieving based on the diffraction intensities is significantly improved. To the best of our knowledge, no previous study has used this concept for the same purpose. Some experimental results are given for demonstrating the feasibility of our method.
24 citations
TL;DR: A wavefront metrology setup based on the X-ray grating interferometry technique for spatially resolved, quantitative, in situ and at-wavelength measurements of the wavefront at synchrotron radiation and hardX-ray free-electron laser beamlines is reported.
Abstract: A wavefront metrology setup based on the X-ray grating interferometry technique for spatially resolved, quantitative, in situ and at-wavelength measurements of the wavefront at synchrotron radiation and hard X-ray free-electron laser beamlines is reported. Indeed, the ever-increasing demands on the optical components to preserve the wavefront shape and the coherence of the delivered X-ray beam call for more and more sensitive diagnostic instruments. Thanks to its angular sensitivity, X-ray grating interferometry has been established in recent years as an adequate wavefront-sensing technique for quantitatively assessing the quality of the X-ray wavefront under working conditions and hence for the in situ investigation of X-ray optical elements. In order to characterize the optical elements at any given beamline by measuring the aberrations introduced in the wavefront, a transportable X-ray grating interferometry setup was realised at the Swiss Light Source (SLS). The instrument, which is expected to be a valuable tool for investigating the quality of the X-ray beam delivered at an endstation, will be described hereafter in terms of the hardware setup and the related data analysis procedure. Several exemplary experiments performed at the X05DA Optics beamline of the SLS will be presented.
24 citations
TL;DR: In this paper, a fiber-optic interferometer is proposed and demonstrated experimentally for ultrasonic detection, which consists of a compact Michelson interferometers (MI), which is fixed in a tilted-tube end-face (45°).
23 citations
TL;DR: It is shown that a Shack-Hartmann wavefront sensor can be used for the quantitative measurement of the specimen optical path difference in an ordinary incoherent optical microscope, if the spatial coherence of the illumination light in the plane of the specimens is larger than the microscope resolution.
Abstract: In this Letter, we show that a Shack-Hartmann wavefront sensor can be used for the quantitative measurement of the specimen optical path difference (OPD) in an ordinary incoherent optical microscope, if the spatial coherence of the illumination light in the plane of the specimen is larger than the microscope resolution To satisfy this condition, the illumination numerical aperture should be smaller than the numerical aperture of the imaging lens This principle has been successfully applied to build a high-resolution reference-free instrument for the characterization of the OPD of micro-optical components and microscopic biological samples
TL;DR: In this article, an accelerometer-based disturbance feedforward control (DFF) was proposed to enlarge the effective servo bandwidth of the NGS wavefront sensor exposure time.
Abstract: Adaptive Optics (AO) systems in large telescopes do not only correct atmospheric phase disturbances, but they also telescope structure vibrations induced by wind or telescope motions. Often the additional wavefront error due to mirror vibrations can dominate the disturbance power and contribute significantly to the total tip-tilt Zernike mode error budget. Presently, these vibrations are compensated for by common feedback control laws. However, when observing faint natural guide stars (NGS) at reduced control bandwidth, high-frequency vibrations (>5 Hz) cannot be fully compensated for by feedback control. In this paper, we present an additional accelerometer-based disturbance feedforward control (DFF), which is independent of the NGS wavefront sensor exposure time to enlarge the "effective servo bandwidth". The DFF is studied in a realistic AO end-to-end simulation and compared with commonly used suppression concepts. For the observation in the faint (>13 mag) NGS regime, we obtain a Strehl ratio by a factor of two to four larger in comparison with a classical feedback control. The simulation realism is verified with real measurement data from the Large Binocular Telescope (LBT); the application for on-sky testing at the LBT and an implementation at the E-ELT in the MICADO instrument is discussed.
TL;DR: A new class of sensor is introduced, the Coded Wavefront Sensor, which provides high spatio-temporal resolution and phase accuracy better than 0.1 wavelengths at reconstruction rates of 50 Hz or more, thus opening up many new applications from high-resolution adaptive optics to real-time phase retrieval in microscopy.
Abstract: Wavefront sensors and more general phase retrieval methods have recently attracted a lot of attention in a host of application domains, ranging from astronomy to scientific imaging and microscopy. In this paper, we introduce a new class of sensor, the Coded Wavefront Sensor, which provides high spatio-temporal resolution using a simple masked sensor under white light illumination. Specifically, we demonstrate megapixel spatial resolution and phase accuracy better than 0.1 wavelengths at reconstruction rates of 50 Hz or more, thus opening up many new applications from high-resolution adaptive optics to real-time phase retrieval in microscopy.
TL;DR: Two novel algorithms for wavefront reconstruction from pyramid-type wavefront sensor data are presented and it is shown that the novel algorithms can be applied in various scientific fields such as astronomy, ophthalmology, and microscopy.
Abstract: In this paper, we present two novel algorithms for wavefront reconstruction from pyramid-type wavefront sensor data. An overview of the current state-of-the-art in the application of pyramid-type wavefront sensors shows that the novel algorithms can be applied in various scientific fields such as astronomy, ophthalmology, and microscopy. Assuming a computationally very challenging setting corresponding to the extreme adaptive optics (XAO) on the European Extremely Large Telescope, we present the results of the performed end-to-end simulations and compare the achieved AO correction quality (in terms of the long-exposure Strehl ratio) to other methods, such as matrix-vector multiplication and preprocessed cumulative reconstructor with domain decomposition. Also, we provide a comparison in terms of applicability and computational complexity and closed-loop performance of our novel algorithms to other methods existing for this type of sensor.
TL;DR: In this paper, the vertical profiles of C^2_N and the outer scale at Maunakea were estimated using a SLOpe Detection And Ranging (SLODAR) method from on-sky telemetry taken by a multi-object adaptive optics (MOAO) demonstrator, called RAVEN, on the Subaru telescope.
Abstract: Prior statistical knowledge of atmospheric turbulence is essential for designing, optimizing and evaluating tomographic adaptive optics systems. We present the statistics of the vertical profiles of C^2_N and the outer scale at Maunakea estimated using a SLOpe Detection And Ranging (SLODAR) method from on-sky telemetry taken by a multi-object adaptive optics (MOAO) demonstrator, called RAVEN, on the Subaru telescope. In our SLODAR method, the profiles are estimated by fitting the theoretical autocorrelations and cross-correlations of measurements from multiple Shack–Haltmann wavefront sensors to the observed correlations via the non-linear Levenberg–Marquardt Algorithm (LMA). The analytical derivatives of the spatial phase structure function with respect to its parameters for the LMA are also developed. From a total of 12 nights in the summer season, a large ground C^2_N fraction of 54.3 per cent is found, with median estimated seeing of 0.460 arcsec. This median seeing value is below the results for Maunakea from the literature (0.6–0.7 arcsec). The average C^2_N profile is in good agreement with results from the literature, except for the ground layer. The median value of the outer scale is 25.5 m and the outer scale is larger at higher altitudes; these trends of the outer scale are consistent with findings in the literature.
TL;DR: In this article, a 6-in. f/2.2 dual-wavelength transmission sphere (TS) based on the Fizeau interferometer was verified at a wavelength of 632.8nm and is compatible for measuring the transmitted wavefront error of an i-line lithography lens.
Abstract: This paper presents the validation of the design of a 6-in. f/2.2 dual-wavelength transmission sphere (TS) based on the Fizeau interferometer. The TS was verified at a wavelength of 632.8 nm and is compatible for measuring the transmitted wavefront error of an i-line lithography lens. The achromatic design is imperative for a dual wavelength TS and requires more lenses for correcting the chromatic focal shift. Because the overall weight of the TS should be controlled within the load range of the piezoelectric transducers of the interferometer, the fabrication and mounting of lenses with high aspect ratios are challengeable. The mounting of the reference surface based on three-point mounting was successful for the residual reference wavefront error under peak-to-valley (PV) λ/10. Furthermore, the reference wavefront is typically restricted within PV 5λ to avoid distorted interference fringes. Therefore, we built a double-pass interferometer model for tolerance analysis, and the error budget facilitated decision-making regarding the suitable specifications of lens manufacturing and assembly for cost efficiency. The test results demonstrated that the deformation of the reference wavefront and the residual reference wavefront error met the critical specification at 632.8 nm and that the achromatic TS is compatible for measuring i-line lithography lenses.
TL;DR: In this paper, a Lyot-stop low-order wavefront sensor (LLOWFS) was introduced to calibrate and measure these aberrations for focal-plane phase mask coronagraphs.
Abstract: Detecting faint companions in close proximity to stars is one of the major goals of current/planned ground- and space-based high-contrast imaging instruments. High-performance coronagraphs can suppress the diffraction features and gain access to companions at small angular separation. However, the uncontrolled pointing errors degrade the coronagraphic performance by leaking starlight around the coronagraphic focal-plane mask, preventing the detection of companions at small separations. A Lyot-stop low-order wavefront sensor (LLOWFS) was therefore introduced to calibrate and measure these aberrations for focal-plane phase mask coronagraphs. This sensor quantifies the variations in wavefront error decomposed into a few Zernike modes by reimaging the diffracted starlight rejected by a reflective Lyot stop. The technique was tested with several coronagraphs on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system at the Subaru Telescope. The wavefront was decomposed into 15 and 35 Zernike modes with an occulting and focal-plane phase mask coronagraph, respectively, which were used to drive a closed-loop correction in the laboratory. Using a 2000-actuator deformable mirror, a closed-loop pointing stability between 10−3–10^(−4) λ/D was achieved in the laboratory in H-band, with sub nanometer residuals for the other Zernike modes (Noll index >4). On-sky, the low-order control of 10+ Zernike modes for the phase-induced amplitude apodization and the vector vortex coronagraphs was demonstrated, with a closed-loop pointing stability of 10^(-4) λ /D under good seeing and 10^(-3) λ/D under moderate seeing conditions readily achievable.
TL;DR: This work proposes a centroid estimator based on stream processing, where the center of gravity calculation window floats with the incoming pixel from the detector, and shows that the proposed estimator simplifies the choice of optimized parameters, provides a unit linear coefficient response, and reduces the influence of background and noise.
Abstract: Using center of gravity to estimate the centroid of the spot in a Shack–Hartmann wavefront sensor, the measurement corrupts with photon and detector noise. Parameters, like window size, often require careful optimization to balance the noise error, dynamic range, and linearity of the response coefficient under different photon flux. It also needs to be substituted by the correlation method for extended sources. We propose a centroid estimator based on stream processing, where the center of gravity calculation window floats with the incoming pixel from the detector. In comparison with conventional methods, we show that the proposed estimator simplifies the choice of optimized parameters, provides a unit linear coefficient response, and reduces the influence of background and noise. It is shown that the stream-based centroid estimator also works well for limited size extended sources. A hardware implementation of the proposed estimator is discussed.
07 Sep 2017
TL;DR: In this article, at-wavelength metrology of 2D parabolic beryllium lenses and a wavefront sensor using a diamond crystal beam splitter were explored in two cases.
Abstract: Single-grating Talbot imaging relies on high-spatial-resolution detectors to perform accurate measurements of X-ray beam wavefronts. The wavefront can be retrieved with a single image, and a typical measurement and data analysis can be performed in few seconds. These qualities make it an ideal tool for synchrotron beamline diagnostics and in-situ metrology. The wavefront measurement can be used both to obtain a phase contrast image of an object and to characterize an X-ray beam. In this work, we explore the concept in two cases: at-wavelength metrology of 2D parabolic beryllium lenses and a wavefront sensor using a diamond crystal beam splitter.
TL;DR: Simulation and experimental results show that the proposed TFD wavefront aberration measurement model can achieve high measurement accuracy.
Abstract: A simple, precise, and innovative technique for wavefront aberration measurement is presented in this paper. This technique is based on transmitted fringe deflectometry (TFD) used to measure wavefront slope and wavefronts. The system measurement error when measuring wavefront aberration is analyzed, including the aberration of the reference wavefront caused by shape distribution and the tilt and decentration of the phase object. Based on this, the system error model is established, and the system error compensation method is also presented. In addition, the system parameters calibration method is discussed in detail. Simulation and experimental results show that the proposed TFD wavefront aberration measurement model can achieve high measurement accuracy.
TL;DR: This system is based on three tunable-focus fluidic lenses and thin-film holographic optical elements to perform automatic refractive error measurement and provide a diagnostic prescription without supervision and can be used in rapid screening of large patient populations especially in the developing countries that lack sufficient facilities and specialist doctors.
Abstract: Using a phoropter to measure the refractive error is one of the most commonly used methods by ophthalmologists and optometrists. Here, we demonstrate design and fabrication of a portable automatic phoropter with no need for patient’s feedback. The system is based on three tunable-focus fluidic lenses and thin-film holographic optical elements to perform automatic refractive error measurement and provide a diagnostic prescription without supervision. Three separate lenses are deployed to correct the defocus and astigmatism. The refractive error is measured using a Shack-Hartmann wavefront sensor that calculates the Zernike values of an infrared wavefront emerging from the eye. Holographic optical elements steer the emerging wavefront into the wavefront sensor, while simultaneously providing an unobstructed view for the subject. The power of each lens is controlled by pumping a liquid in and out of the lens chamber using servo motor actuated diaphragm pumps. Spherical and cylindrical correction range of −10 to +10 diopters with 0.1 diopter increments is achieved in less than 15 seconds using wavefront sensor feedback to the pumps. This system can be used in rapid screening of large patient populations especially in the developing countries that lack sufficient facilities and specialist doctors.
TL;DR: An adaptive aberration correction system to simultaneously compensate for aberrations and reshaping the beams to achieve the highest performance from such a high-power, high-pulse repetition rate Nd:YAG solid-state laser ever built.
Abstract: In this Letter, we present an adaptive aberration correction system to simultaneously compensate for aberrations and reshaping the beams. A low-order aberration corrector is adapted. In this corrector, four lenses are mounted on a motorized rail, whose positions can be obtained using a ray tracing method based on the beam parameters detected by a wavefront sensor. After automatic correction, the PV value of the wavefront is reduced from 26.47 to 1.91 μm, and the beam quality β is improved from 18.42 to 2.86 times that of the diffraction limit. After further correction with a deformable mirror, the PV value of the wavefront is less than 0.45 μm, and the beam quality is 1.64 times that of the diffraction limit. To the best of our knowledge, this is the highest performance from such a high-power, high-pulse repetition rate Nd:YAG solid-state laser ever built.
TL;DR: Sub-aperture based digital adaptive optics is demonstrated in a fiber based point scanning optical coherence tomography system using a 1060 nm swept source laser.
Abstract: Sub-aperture based digital adaptive optics is demonstrated in a fiber based point scanning optical coherence tomography system using a 1060 nm swept source laser. To detect optical aberrations in-vivo, a small lateral field of view of ~[Formula: see text] is scanned on the sample at a high volume rate of 17 Hz (~1.3 kHz B-scan rate) to avoid any significant lateral and axial motion of the sample, and is used as a "guide star" for the sub-aperture based DAO. The proof of principle is demonstrated using a micro-beads phantom sample, wherein a significant root mean square wavefront error (RMS WFE) of 1.48 waves (> 1[Formula: see text]) is detected. In-vivo aberration measurement with a RMS WFE of 0.33 waves, which is ~5 times higher than the Marechal's criterion of [Formula: see text] waves for the diffraction limited performance, is shown for a human retinal OCT. Attempt has been made to validate the experimental results with the conventional Shack-Hartmann wavefront sensor within reasonable limitations.
TL;DR: A new configuration of solar MCAO based on high order ground layer adaptive optics and low order high altitude correction, which result in a homogeneous correction effect in the whole field of view is proposed.
Abstract: Multi-conjugate adaptive optics (MCAO) is the most promising technique currently developed to enlarge the corrected field of view of adaptive optics for astronomy. In this paper, we propose a new configuration of solar MCAO based on high order ground layer adaptive optics and low order high altitude correction, which result in a homogeneous correction effect in the whole field of view. An individual high order multiple direction Shack-Hartmann wavefront sensor is employed in the configuration to detect the ground layer turbulence for low altitude correction. Furthermore, the other low order multiple direction Shack-Hartmann wavefront sensor supplies the wavefront information caused by high layers’ turbulence through atmospheric tomography for high altitude correction. Simulation results based on the system design at the 1-meter New Vacuum Solar Telescope show that the correction uniform of the new scheme is obviously improved compared to conventional solar MCAO configuration.
TL;DR: This method is designed to be sample-independent thanks to an automatic axial locking on objects of interest through the use of an image-based metric that is designed, and shows an increase of in vivo imaging quality in the hippocampus.
Abstract: Adaptive optics is a promising technique for the improvement of microscopy in tissues. A large palette of indirect and direct wavefront sensing methods has been proposed for in vivo imaging in experimental animal models. Application of most of these methods to complex samples suffers from either intrinsic and/or practical difficulties. Here we show a theoretically optimized wavefront correction method for inhomogeneously labeled biological samples. We demonstrate its performance at a depth of 200 μm in brain tissue within a sparsely labeled region such as the pyramidal cell layer of the hippocampus, with cells expressing GCamP6. This method is designed to be sample-independent thanks to an automatic axial locking on objects of interest through the use of an image-based metric that we designed. Using this method, we show an increase of in vivo imaging quality in the hippocampus. In vivo imaging of neuronal calcium dynamics using two-photon microscopy is an increasingly used method of choice to study neuronal activity at microcircuit level. In the dorsal region CA1 of the hippocampus (the most optically accessible), this technique allows neuronal activity recording, in large fields of view containing hundreds of cells 1. It has led to pioneering discoveries of multineuron dynamics including, for example fear conditioning 2 , spatial navigation 3–5 , epilepsy 6 or quiet rest 7. However, the implementation of this technique remains challenging as it requires, prior to cranial window implantation, surgery to remove the overlaying cortex, which introduces a high variability of " optical access " to the tissue. The main issues are the presence of blood from the capillaries and sometimes from small hemorrhage as well as the quality of the interface between the glass window and the brain surface. The former causes optical absorption and can be reduced by performing the surgery following water restriction to increase the viscosity of the blood 1,5 , while the latter causes optical aberrations. Furthermore, the densely packed layer of CA1 pyramidal neurons is located 200 μ m below the glass window covering the brain; the incoming laser beam is also perturbed by light scattering and optical aberrations during the propagation within the tissue. This problem should be tackled in order to improve detection of calcium probes which is impaired by the lowered contrast of the aberrated images. Even a modest improvement in contrast should lead to the detection of neural activity that otherwise is masked by background fluorescence from brain tissue. Optical aberrations alter the quality of beam focusing, which in turn leads to reduced spatial resolution but also to lower signal and contrast. Thus, even when objects of interest are one order of magnitude larger than the diffraction limited laser focus (e.g. neurons' somata are 10–15 μ m in diameter), the reduction of optical aberrations is critical to increasing the contrast of the fluorescence images. This improvement can be achieved using adaptive optics, a promising tool increasingly used for microscopy 8. Adaptive optics is the process of quantifying optical aberrations through wavefront measurement and correcting them by the use of an adaptive correction element (deformable mirror DM or spatial light modulator SLM). Note that in point-scanning two-photon microscopy the correction is applied on the excitation beam alone and no correction is needed on the detection path. In such microscopes, the wavefront can either be directly measured or indirectly estimated. Direct wave-front measurement relies on introducing a wavefront sensor such as a Shack-Hartmann in the detection part of the microscope. A point source in the sample is then imaged on the sensor. Direct methods have been proposed for two-photon imaging in weakly scattering samples where auto-fluorescence signals can be used to generate a highly localized signal 9,10 , but more complex methods such as coherence gating 11 or near-IR guide stars 12 are required to avoid out-of-focus fluorescence in highly scattering samples. Indirect or sensorless wavefront estimation has the advantage of being easy-to-implement on existing systems as it relies on conventional imaging systems. Indeed, this technique, called image-based adaptive optics, relies on successive image measurements with
01 Jan 2017
TL;DR: In this article, the authors focus on the development of different predictive control strategies that estimate how the wavefront phase uctuations evolve over a time horizon equal to the servo-lag.
Abstract: Current high contrast imaging systems are limited at small angular separations, preventing the direct imaging of Earth-size exoplanets close to their host stars. One primary cause of this is the performance of the extreme adaptive optics (AO) system which is dominated by the servo-lag error at small angular separations. Prediction can be used to help reduce this servo-lag error. Most AO systems today do not use predictive controllers (integral controller), resulting in a phase correction that is proportional to the current measured wavefront and does not take into account the evolution of the wavefront phase uctuations between measurement and correction. To improve upon this, we focus on the development of different predictive control strategies that estimate how the wavefront phase uctuations evolve over a time horizon equal to the servo-lag. Moreover, the statistical properties of the turbulence phase are non-stationary (change in time) which needs to be included in the AO analysis for high contrast imaging systems. First, we show that the non-stationary properties of the atmosphere can be modeled using a von Karman covariance function which can incorporate variations in time of the Fried parameter, outer scale, and wind velocity. Using this new disturbance model of turbulence, the performance of three different predictors-based on a steady state, a recursive, and an adaptive linear-minimum-mean-squareestimator (LMMSE)|is tested under non-stationary conditions. We feed the prediction algorithms wavefront slopes from a 11-by-11 subaperture Shack-Hartmann wavefront sensor. A 97 actuator deformable mirror applies the predictor's phase correction. We present the latest results of our simulations and compare our predictors with the common integrator. We show that under our varying wind conditions, the root-mean-square wavefront error can increase by a factor of two for both the integrator and our predictors. In our simulations, we do not observe an increase in performance using an exponential forgetting factor adaptive LMMSE.
TL;DR: The results of alignment and acceptance tests of the optics system of the 2.5m telescope installed at the Caucausus Mountain Observatory of the Sternberg Astronomical Institute in 2013-2015 are reported in this paper.
Abstract: The results of alignment and acceptance tests of the optics system of the 2.5-m telescope installed at the Caucausus Mountain Observatory of the Sternberg Astronomical Institute in 2013–2015 are reported. The optical elements of the Ritchey–Chretien system of the telescope were manufactured by REOSC (France). Measurements of aberrations were carried out using a specially manufactured Shack–Hartmann wavefront sensor. Adjustment of the load-distribution mechanisms of the primary mirror and automatic correction for bending of the structure have made it possible to achieve the target image quality at all operational positions of the telescope, corresponding to 80% of the energy being concentrated in a circle 0.3″ in diameter. Factory interferograms of the mirrors and methods for measuring their abberation using stellar images are presented.
TL;DR: A method based on geometry optics to simultaneously correct low-order aberrations and reshape the beams of slab lasers using a coaxial optical system with three lenses is presented.
Abstract: In this paper, we present a method based on geometry optics to simultaneously correct low-order aberrations and reshape the beams of slab lasers. A coaxial optical system with three lenses is adapted. The positions of the three lenses are directly calculated based on the beam parameters detected by wavefront sensors. The initial sizes of the input beams are 1.8 mm×11 mm, and peak-to-valley (PV) values of the wavefront range up to several tens of microns. After automatic correction, the dimensions may reach nearly 22 mm×22 mm as expected, and PV values of the wavefront are less than 2 μm. The effectiveness and precision of this method are verified with experiments.
DOI•
15 Mar 2017
TL;DR: The Data-based Online Nonlinear extremum-seeker (DONE) algorithm is developed and the effectiveness of the DONE algorithm compared to other optimization algorithms for WFSL-AO on biological and non-biological samples is demonstrated.
Abstract: Optical coherence tomography (OCT) is a technique for non-invasive imaging based on low coherence interferometry. Its main application is found in ophthalmology, where it is used for 3D in vivo imaging of the cornea and the retina. OCT has evolved over the past decade as one of the most important ancillary tests in ophthalmic practice, providing great diagnostic value for disease screening and monitoring. In retinal OCT imaging, the lateral resolution is not determined by the pupil size, but instead it is limited by optical wavefront aberrations of the cornea and lens. These aberrations reduce the OCT image resolution and lower the signal to noise ratio. To obtain high quality OCT images the optical aberrations can be removed using adaptive optics (AO). In general, AO consists of an adaptive optical element and a wavefront sensor. The adaptive element, such as a deformable mirror, is used to reshape the wavefront and remove the undesired aberrations. The wavefront sensor measures the aberrations by reconstructing the phase of the wavefront, which is used to determine the correction on the wavefront applied by the deformable mirror. However, the use of a wavefront sensor has some disadvantages. It requires light being directed out of the imaging path onto the wavefront sensor. This leads to a loss of signal in the imaging path and can result in non-common optical path errors in the aberrations estimation procedure. Additionally, the use of a deformable mirror and a wavefront sensor leads to a bulky and expensive OCT setup. The work presented in this thesis has the goal of reducing the cost and bulkiness of an AO-OCT system. First, we investigate the influence of optical wavefront aberrations to the OCT signal strength. The establishment of the relation between aberrations and the OCT signal strength is key to estimating and correcting the aberrations based on single OCT scans. By using Fresnel optical wave propagation and determining the fiber coupling efficiency, we find that the OCT transfer function, i.e. the function that expresses the relation between the aberrations and OCT signal strength, is quasi-convex. We determine both analytically and experimentally the transfer function for both reflective and scattering media, such as a mirror and Scotch tape sample. Additionally, if the OCT system and its optical properties are well-known we demonstrate a method to correct a defocus aberration in one step. Second, we use the OCT transfer function to develop and determine an efficient wavefront sensorless (WFSL) AO optimization procedure. WFSL-AO methods aim to correct the aberrations without using a wavefront sensor, but instead base the determination of the wavefront on the imaging signal itself. This eliminates the use of the wavefront sensor, its extra cost and its disadvantages from an AO-OCT setup. To keep up with the OCT imaging rate, which is of the order of several tens of kHz, the algorithm has to be computationally efficient. Furthermore, there are no analytic derivatives available for the optimization and the OCT signal is very noisy. Finally, the derivative-free optimization algorithm also has to be able to determine the aberrations accurately when dealing with a minimum number of noisy measurements. We developed the Data-based Online Nonlinear extremum-seeker (DONE) algorithm. Every iteration, the DONE algorithm updates a surrogate function, which is based on random Fourier expansions (RFE) of the OCT transfer function, with a new OCT signal measurement. The optimum of the RFE surrogate function is then found with a well-known (quasi-Newton) optimization method. We demonstrate the effectiveness of the DONE algorithm compared to other optimization algorithms for WFSL-AO on biological and non-biological samples. We conclude that DONE has a smaller convergence error, while maintaining similar or faster convergence speeds compared to the other algorithms. Third, we demonstrate a fully functional WFSL-AO OCT setup for retinal imaging. We use a state-of-the-art deformable lens with 18 actuators, rather than a deformable mirror, which leads to a smaller and more integrated WFSL-AO setup. The WFSL-AO OCT setup is successfully used for in vivo retinal OCT imaging and demonstrates that the DONE algorithm can remove the ocular wavefront aberrations with the deformable lens during in vivo OCT imaging. By developing a new algorithm and exploring the options for adaptive components, we have succeeded in retinal WFSL-AO OCT. In a broader perspective, we show that the DONE algorithm is suitable for other applications than WFSL-AO OCT. We demonstrate that the DONE derivative-free optimization algorithm is robust towards noisy measurements for applications in robotics, microscopy and optical beam forming networks.
TL;DR: The technique provides an efficient and simple way to measure retardance of less than 1 nm with a diffraction-limited spatial resolution in transparent samples, and coupled to thermomechanical simulations it gives access to birefringence distribution in the sample.
Abstract: A technique that provides quantitative and spatially resolved retardance measurement is studied for application to laser-induced modification in transparent materials. The method is based on the measurement of optical path differences between two wavefronts carrying different polarizations, measured by a wavefront sensor placed in the image plane of a microscope. We have applied the technique to the investigation of stress distribution induced by CO2 laser processing of fused silica samples. By comparing experiments to the results of thermomechanical simulations we demonstrate quantitative agreement between measurements and simulations of optical retardance. The technique provides an efficient and simple way to measure retardance of less than 1 nm with a diffraction-limited spatial resolution in transparent samples, and coupled to thermomechanical simulations it gives access to birefringence distribution in the sample.