Shack-Hartmann wavefront sensor with large dynamic range by adaptive spot search method

Large-Dynamic-Range Ocular Aberration Measurement Based on Deep Learning with a Shack-Hartmann Wavefront Sensor

The Shack-Hartmann wavefront sensor (SHWFS) is widely utilized for ocular aberration measurement. However, large ocular aberrations caused by individual differences can easily make the spot move out of the range of the corresponding sub-aperture in SHWFS, rendering the traditional centroiding method ineffective. This study applied a novel convolutional neural network (CNN) model to wavefront sensing for large dynamic ocular aberration measurement. The simulation results demonstrate that, compared to the modal method, the dynamic range of our method for main low-order aberrations in ocular system is increased by 1.86 to 43.88 times in variety. Meanwhile, the proposed method also has the best measurement accuracy, and the statistical root mean square (RMS) of the residual wavefronts is 0.0082 ± 0.0185 λ (mean ± standard deviation). The proposed method generally has a higher accuracy while having a similar or even better dynamic range as compared to traditional large-dynamic schemes. On the other hand, compared with recently developed deep learning methods, the proposed method has a much larger dynamic range and better measurement accuracy.

Real-Time Wavefront Sensing at High Resolution with an Electrically Tunable Lens

We have designed, assembled, and evaluated a compact instrument capable of capturing the wavefront phase in real time, across various scenarios. Our approach simplifies the optical setup and configuration, which reduces the conventional capture and computation time when compared to other methods that use two defocused images. We evaluated the feasibility of using an electrically tunable lens in our camera by addressing its issues and optimizing its performance. Additionally, we conducted a comparison study between our approach and a Shack-Hartmann sensor. The camera was tested on multiple targets, such as deformable mirrors, lenses with aberrations, and a liquid lens in movement. Working at the highest resolution of the CMOS sensor with a small effective pixel size enables us to achieve the maximum level of detail in lateral resolution, leading to increased sensitivity to high-spatial-frequency signals.

Ultra-thin light-weight laser-induced-graphene (LIG) diffractive optics

The realization of hybrid optics could be one of the best ways to fulfill the technological requirements of compact, light-weight, and multi-functional optical systems for modern industries. Planar diffractive lens (PDL) such as diffractive lenses, photonsieves, and metasurfaces can be patterned on ultra-thin flexible and stretchable substrates and be conformally attached on top of arbitrarily shaped surfaces. In this review, we introduce recent research works addressed to the design and manufacturing of ultra-thin graphene optics, which will open new markets in compact and light-weight optics for next-generation endoscopic brain imaging, space internet, real-time surface profilometry, and multi-functional mobile phones. To provide higher design flexibility, lower process complexity, and chemical-free process with reasonable investment cost, direct laser writing (DLW) of laser-induced-graphene (LIG) is actively being applied to the patterning of PDL. For realizing the best optical performances in DLW, photon-material interactions have been studied in detail with respect to different laser parameters; the resulting optical characteristics have been evaluated in terms of amplitude and phase. A series of exemplary laser-written 1D and 2D PDL structures have been actively demonstrated with different base materials, and then, the cases are being expanded to plasmonic and holographic structures. The combination of these ultra-thin and light-weight PDL with conventional bulk refractive or reflective optical elements could bring together the advantages of each optical element. By integrating these suggestions, we suggest a way to realize the hybrid PDL to be used in the future micro-electronics surface inspection, biomedical, outer space, and extended reality (XR) industries.

Advanced Optical Wavefront Technologies to Improve Patient Quality of Vision and Meet Clinical Requests

Adaptive optics (AO) is employed for the continuous measurement and correction of ocular aberrations. Human eye refractive errors (lower-order aberrations such as myopia and astigmatism) are corrected with contact lenses and excimer laser surgery. Under twilight vision conditions, when the pupil of the human eye dilates to 5-7 mm in diameter, higher-order aberrations affect the visual acuity. The combined use of wavefront (WF) technology and AO systems allows the pre-operative evaluation of refractive surgical procedures to compensate for the higher-order optical aberrations of the human eye, guiding the surgeon in choosing the procedure parameters. Here, we report a brief history of AO, starting from the description of the Shack-Hartmann method, which allowed the first in vivo measurement of the eye's wave aberration, the wavefront sensing technologies (WSTs), and their principles. Then, the limitations of the ocular wavefront ascribed to the IOL polymeric materials and design, as well as future perspectives on improving patient vision quality and meeting clinical requests, are described.

Adaptive optics system for a short wavelength mid-IR laser based on a Shack-Hartmann wavefront sensor and analysis of thermal noise impacts

We present an adaptive optics (AO) system for a 1.94-µm laser source. Our system consists of a home-made Shack-Hartmann wavefront sensor and silver-coated bimorph deformable mirror operating in a closed-loop control scheme. The wavefront sensor used an uncooled vapor phase deposition PbSe focal-plane array for the actual light sensing. An effect of thermal afterimage was found to be reducing the centroid detection precision significantly. The effect was analyzed in detail and finally has been dealt with by updating the background calibration. System stability was increased by reduction of control modes. The system functionality and stability were demonstrated by improved focal spot quality. By replacing some of the used optics, the range of the demonstrated mid-IR AOS could be extended to cover the spectral range of 1-5 µm. To the best of our knowledge, it is the first AO system built specifically for mid-IR laser wavefront correction.

Telephoto-lens-based Optical Differentiation Wavefront Sensor for freeform metrology

We report an Optical Differentiation Wavefront Sensor based on a telephoto lens system and binary pixelated filters. It provides a five-fold reduction in the system length compared to a 4f system with identical effective focal length. Measurements of phase plates with this system are compared to measurements performed with a commercial low-coherence interferometer. The telephoto-lens-based system can measure wavefronts with accuracy better than λ/10 Root Mean Squared (RMS) at λ=633 nm. Experimental investigation shows that the system has a high tolerance to components alignment errors.

Extended-aperture Hartmann wavefront sensor with raster scanning

In this paper, we propose an extended-aperture Hartmann wavefront sensor (HWFS) based on raster scanning. Unlike traditional HWFS, where there is a trade-off between the dynamic range and spatial resolution of wavefront measurement, our extended-aperture HWFS breaks the trade-off and thus could achieve a large dynamic range and high spatial resolution simultaneously. By applying a narrow-beam raster-scanning scheme, the detection aperture of our HWFS is extended to 40 × 40 mm2 without using the enlarging 4f relay system. The spatial resolution of our setup depends on the scanning step, the pinhole size, and the wavelength. The sensitivity and dynamic range can be adjusted flexibly by varying the axial distance between the pinhole plane and the imaging sensor plane, because our decoupled large dynamic range could be reasonable traded-off to achieve better sensitivity. Furthermore, compared with tradition HWFS, our method does not need to compute the positions of a two-dimensional spots array where complicated spots tracking algorithms are necessary to achieve high dynamic range, thus remarkably reduces the spots aliasing issue and the computational cost. It should be noted that this scheme is not only applicable for HWFS but also for Shack-Hartmann wavefront sensor (SHWFS) with microlens array to achieve higher accuracy and better power efficiency. Experiments were performed to demonstrate the capability of our method.

Extended aperture line-scanning Hartmann wavefront sensor

We first propose a line-scanning Hartmann wavefront sensor (LS-HWS) with extended aperture. In the LS-HWS, a line-scanning imaging sensor was driven by a motor and scanning behind a large-area Hartman mask. Compared to the traditional Hartman wavefront sensor with two-dimensional imaging sensors, our method can significantly enlarge the aperture because of the larger imaging area with line-scanning imaging sensors. Cross correlation registration was adopted to reduce the scanning error. Experiments on two single spherical lenses and a free-form lens were performed to demonstrate the capability of the LS-HWS method. The results show that our method can achieve an aperture of 17.5×37.5mm² with the prototype system, which could be further extended easily and is limited only by the size of the line-scanning imaging sensor and the scanning range of the motor. We believe that the LS-HWS method is promising for many wavefront sensing applications where a large aperture is preferred.

Shack-Hartmann wavefront sensor optical dynamic range

The widely used lenslet-bound definition of the Shack-Hartmann wavefront sensor (SHWS) dynamic range is based on the permanent association between groups of pixels and individual lenslets. Here, we formalize an alternative definition that we term optical dynamic range, based on avoiding the overlap of lenslet images. The comparison of both definitions for Zernike polynomials up to the third order plus spherical aberration shows that the optical dynamic range is larger by a factor proportional to the number of lenslets across the SHWS pupil. Finally, a pre-centroiding algorithm to facilitate lenslet image location in the presence of defocus and astigmatism is proposed. This approach, based on the SHWS image periodicity, is demonstrated using optometric lenses that translate lenslet images outside the projected lenslet boundaries.

Single-shot higher-order transport-of-intensity quantitative phase imaging based on computer-generated holography

The imaging quality of quantitative phase imaging (QPI) based on the transport of intensity equation (TIE) can be improved using a higher-order approximation for defocused intensity distributions. However, this requires mechanically scanning an image sensor or object along the optical axis, which in turn requires a precisely aligned optical setup. To overcome this problem, a computer-generated hologram (CGH) technique is introduced to TIE-based QPI. A CGH generating defocused point spread function is inserted in the Fourier plane of an object. The CGH acts as a lens and grating with various focal lengths and orientations, allowing multiple defocused intensity distributions to be simultaneously detected on an image sensor plane. The results of a numerical simulation and optical experiment demonstrated the feasibility of the proposed method.

High fill factor microlens array fabrication using direct laser writing and its application in wavefront detection

We fabricate 100% fill factor microlens arrays (MLAs) using femtosecond laser direct writing. The array consists of periodical hexagonal plano-convex microlens units with a diameter of 9 µm. The focusing efficiency of each microlens is measured to be 92%. Combined with a CCD camera, the MLA works as a Shack-Hartmann wavefront sensor. We use it to detect wavefronts of both oblique incident plane beams and vortex beams. The experimental results match well with theoretical ones.

Transport-of-intensity phase imaging with polarization directed flat lenses

A phase imaging technique based on the transport of intensity equation with polarization directed flat lenses is demonstrated. Transport-of-intensity phase imaging enables one to obtain a phase distribution from through-focus intensity distributions by solving the transport of intensity equation. In general, the through-focus intensity distributions are obtained by mechanical scanning of an image sensor or target object. Therefore, a precise alignment of an optical system is required. To solve this issue, the introduction of polarization directed flat lenses is presented. In the proposed method, two intensity distributions at different depth positions on the optical axis are obtained without mechanical scanning by changing polarization states of incident light. The feasibility of the proposed method is confirmed by an optical experiment.

High precision wavefront correction using an influence function optimization method based on a hybrid adaptive optics system

A hybrid adaptive optics (AO) system with an influence function (IF) optimization method is presented for high precision wavefront correction of a traditional Shack-Hartmann AO system. The hybrid AO system consists of a Shack-Hartmann wavefront sensor (SHWFS) and a deflectometry system (DS) to measure the wavefront of the laser beam and the IF of the deformable mirror, respectively. An IF optimization method is used to generate a hybrid IF (H-IF) through a position-calibration algorithm and a resolution-conversion algorithm by use of the original IFs measured by the SHWFS (S-IF) and the DS (D-IF). Configuration of the hybrid AO system is introduced. Principles and calculation results of the IF optimization method are presented. Comparison of the wavefront correction ability between the H-IF and the original IF is carried out in simulation. Closed-loop performance of the hybrid AO system using the H-IF is investigated in experiment. Simulation and experiment results show that for a traditional Shack-Hartmann AO system, the H-IF has better correction ability than the original S-IF and the IF optimization method could help improve closed-loop performance without sacrificing the simplicity of the system structure and the rapidity of the closed-loop correction.

Customised aberration-controlling corrections for keratoconic patients using contact lenses

Technological advancements in the design of soft and scleral contact lenses have led to the development of customised, aberration-controlling corrections for patients with keratoconus. As the number of contact lens manufacturers producing wavefront-guided corrections continues to expand, clinical interest in this customisable technology is also increasing among both patients and practitioners. This review outlines key issues surrounding the measurement of ocular aberrations for patients with keratoconus, with a particular focus on the possible factors affecting the repeatability of Hartmann-Shack aberrometry measurements. This review also discusses and compares the relative successes of studies investigating the design and fitting of soft and scleral customised contact lenses for patients with keratoconus. A series of key limitations that should be considered before designing customised contact lens corrections is also described. Despite the challenges of producing and fitting customised lenses, improvements in visual performance and comfortable wearing times, as provided by these lenses, could help to reduce the rate of keratoplasty in keratoconic patients, thereby significantly reducing clinical issues related to corneal graft surgery. Furthermore, enhancements in optical correction, provided by customised lenses, could lead to increased independence, particularly among young adult keratoconic patients, therefore leading to improvements in quality of life.

Large dynamic range autorefraction with a low-cost diffuser wavefront sensor

Wavefront sensing with a thin diffuser has emerged as a potential low-cost alternative to a lenslet array for aberrometry. Here we show that displacement of caustic patterns can be tracked for estimating wavefront gradient in a diffuser wavefront sensor (DWFS), enabling large dynamic-range wavefront measurements with sufficient accuracy for eyeglass prescription measurements. We compare the dynamic range, repeatability, precision, and number of resolvable prescriptions of a DWFS to a Shack-Hartmann wavefront sensor (SHWFS) for autorefraction measurement. We induce spherical and cylindrical errors in a model eye and use a multi-level Demon's non-rigid registration algorithm to estimate caustic displacements relative to an emmetropic model eye. When compared to spherical error measurements with the SHWFS using a laser diode with a laser speckle reducer, the DWFS demonstrates a ∼5-fold improvement in dynamic range (-4.0 to +4.5 D vs. -22.0 to +19.5 D) with less than half the reduction in resolution (0.072 vs. 0.116 D), enabling a ∼3-fold increase in the number of resolvable prescriptions (118 vs. 358). In addition to being lower-cost, the unique, non-periodic nature of the caustic pattern formed by a diffuser enables a larger dynamic range of aberration measurements compared to a lenslet array.

Intrasession Repeatability and Intersession Reproducibility Measurements Using VX120 Multidiagnostic Unit

OBJECTIVE

The VX120 multidiagnostic unit is a multidiagnostic instrument that combines several functions: autorefraction, keratometry, corneal topography, aberrometry, pachymetry, and noncontact tonometry. The purpose of this study was to determine the intrasession repeatability and the intersession reproducibility of all parameters measured by the VX120 multidiagnostic unit in a sample of normal healthy eyes.

METHODS

Three repeated measurements in the right eye of the volunteers were taken with VX120. Repeatability of the sphere, cylinder, axis, anterior corneal powers (K flat and K steep, 3-mm pupil), high- and low-order aberration root mean square (HOA-RMS and LOA-RMS, 3-mm pupil), eccentricity, white-to-white (WTW), anterior chamber depth (ACD), and central corneal thickness (CCT) (2-mm central) was analyzed. Within-subject SD (Sw), precision, repeatability, coefficient of variation (CV), and the intraclass correlation coefficient (ICC) were calculated.

RESULTS

The CV was low for K flat and K steep, WTW, ACD, and CCT with a range from 0.34% to 1.16%. The CV was higher for sphere, cylinder, HOA-RMS, and LOA-RMS and eccentricity with a range among 6.92% to 54.24%. The ICC showed high values in all parameters except in HOA-RMS (0.720-0.776) and eccentricity (0.889) in first session with moderate agreement. Comparing the intrasession repeatability of first and second session, statistically significant differences (P<0.01) were found between both sessions just to the CV for all parameters (except cylinder values) measured with VX120. However, nonstatistically significant differences (P>0.13) were found for Sw, precision, and repeatability values.

CONCLUSIONS

The VX120 multidiagnostic unit provides repeatable measurements in anterior corneal power (K flat and K steep), WTW, ACD, and CCT. However sphere, cylinder, HOA-RMS, LOA-RMS, and eccentricity showed worse repeatability. Intersession reproducibility showed good results with little differences between sessions in healthy subjects.

Hartmann-Shack wavefront sensing without a lenslet array using a digital micromirror device

The common Hartmann-Shack wavefront sensor makes use of a lenslet array to sample in-parallel optical wavefronts. Here, we introduce a Hartmann-Shack wavefront sensor that employs a digital micromirror device in combination with a single lens for serial sampling by scanning. Sensing is analyzed numerically and validated experimentally using a deformable mirror operated in closed-loop adaptive optics with a conventional Hartmann-Shack wavefront sensor, as well as with a set of ophthalmic trial lenses, to generate controllable amounts of monochromatic aberrations. The new sensor is free of crosstalk and can potentially operate at kilohertz speed. It offers a reconfigurable aperture that can exclude unwanted parts of the wavefront.