Aberration Correction Research Articles

Correcting spatial-spectral crosstalk and chromatic aberrations in broadband line-scan spectral-domain OCT images.

Digital correction of optical aberrations allows for high-resolution imaging across the full depth range in optical coherence tomography (OCT). Many digital aberration correction (DAC) methods have been proposed in the past to evaluate and correct monochromatic error in OCT images. However, other factors that deteriorate the image quality have not been fully investigated. Specifically, in a broadband line-scan spectral-domain OCT system (LS-SD-OCT), photons with different wavelengths scattered from the same transverse location and in the imaged object will be projected onto different spatial coordinates onto the 2D camera sensor, which in this work is defined as spatial-spectral crosstalk. In addition, chromatic aberrations in both axial and lateral directions are not negligible for broad spectral bandwidths. Here we present a novel approach to digital recovery of the spatial resolution in images acquired with a broadband LS-SD-OCT, which addresses these two main factors that limit the effectiveness of DAC for restoring diffraction-limited resolution in LS-SD-OCT images. In the proposed approach, spatial-spectral crosstalk and chromatic aberrations are suppressed by the registration of monochromatic sub-band tomograms that are digitally corrected for aberrations. The new method was validated by imaging a standard resolution target, a microspheres phantom, and different biological tissues. LS-SD-OCT technology combined with the proposed novel image reconstruction method could be a valuable research tool for various biomedical and clinical applications.

Spatial frequency-based correction ofthe spherical aberration inliving brain imaging.

Optical errors, including spherical aberrations, hinder high-resolution imaging of biological samples due to biochemical components and physical properties. We developed the Deep-C microscope system to achieve aberration-free images, employing a motorized correction collar and contrast-based calculations. However, current contrast-maximization techniques, such as the Brenner gradient method, inadequately assess specific frequency bands. The Peak-C method addresses this issue, but its arbitrary neighbor selection and susceptibility to the noise limit its effectiveness. In this paper, we emphasize the importance of a broad spatial frequency range for accurate spherical aberration correction and propose Peak-F. This spatial frequency-based system utilizes a fast Fourier transform as a bandpass filter. This approach overcomes Peak-C's limitations and comprehensively covers the low-frequency domain of image spatial frequencies.

On-chip aberration correction for planar nanofocusing x-ray lenses by focused ion-beam milling

Aberration-free x-ray optics are a prerequisite for nondestructive scanning x-ray microscopy with highest spatial resolution in order to understand complex material systems and processes. Nevertheless, due to highly challenging manufacturing requirements, even state-of-the-art x-ray optics often still suffer from residual lens aberrations, and diffraction-limited performance can often only be achieved by inserting additional corrective optical elements. Here, the concept of tailor-made refractive x-ray phase plates is expanded by integrating these corrective optical elements into the focusing device directly. In this case, planar nanofocusing x-ray lenses out of silicon are corrected for aberrations by structuring the phase plate into the lens chip via focused ion-beam milling. The concept is demonstrated by focusing x-rays with an energy of 18 keV into a diffraction-limited focal spot with a size of 50×65 nm2 full-width at half-maximum and a reduction in residual intensity outside the focus by a factor of well over three.

Adaptive Optical Two-Photon Fluorescence Microscopy Probes Cellular Organization of Ocular Lenses In Vivo.

The mammalian ocular lens is an avascular multicellular organ that grows continuously throughout life. Traditionally, its cellular organization is investigated using dissected lenses, which eliminates in vivo environmental and structural support. Therefore, in vivo optical imaging methods for studying lenses in their native context in live animals are urgently needed. Here, we demonstrated that two-photon fluorescence microscopy can visualize lens cells in vivo. To maintain subcellular resolution at depth, we used adaptive optics to correct aberrations owing to ocular and lens tissues, which led to substantial signal and resolution improvements. Imaging lens cells up to 980 µm deep, we observed novel cellular organizations including suture-associated voids, enlarged vacuoles, and large cavities, contrary to the conventional view of a highly ordered organization. We tracked these features longitudinally over weeks and observed the incorporation of new cells during growth. Taken together, noninvasive longitudinal in vivo imaging of lens morphology using adaptive optics two-photon fluorescence microscopy will allow us to observe the development or alterations of lens cellular organization in living animals directly.

HIGH RESOLUTION PROBE-FORMING SYSTEM WITH SPHERICAL ABERRATION CORRECTION FOR NUCLEAR MICROPROBE

A probe-forming system based on a separate orthomorphic quadruplet of magnetic quadrupole lenses with a short working distance is considered, allowing the system demagnification to be significantly increased. Three magnetic octupole lenses are used to correct spherical aberrations. The parameters of the octupole corrector are calculated using the matricant method. The focusing properties of the system are determined by means of a probe formation optimization process based on the value of the maximum reduced collimated acceptance. The calculations performed showed the feasibility of such a probe forming system for microanalysis and proton beam writing technique.

Method to construct the initial structure of optical systems based on full-field aberration correction.

To meet full-field image quality requirements for extremely low aberration optical systems, an initial structure construction method for reflective optical systems based on full-field aberration correction is proposed. The aberration of the full field is used as the main evaluation criterion in this method. A multi-field evaluation function is established using the aberration values of multiple characteristic field points to represent the full-field imaging quality, and spatial ray tracing is introduced to constrain the optical system structure. Multi-objective optimization of the evaluation function is performed using a combinatorial nondominated sorting and metaheuristics algorithm; an initial optical system with a reasonable structure and corrected third-order aberrations over the full field is subsequently obtained. After optimization, an extreme ultraviolet lithography objective with a numerical aperture of 0.33 and root-mean-square wavefront error of 0.128 nm ($1/105\lambda, \lambda = 13.5\;{\rm nm}$) is obtained.

Deep learning-based adaptive optics for light sheet fluorescence microscopy.

Light sheet fluorescence microscopy (LSFM) is a high-speed imaging technique that is often used to image intact tissue-cleared specimens with cellular or subcellular resolution. Like other optical imaging systems, LSFM suffers from sample-induced optical aberrations that decrement imaging quality. Optical aberrations become more severe when imaging a few millimeters deep into tissue-cleared specimens, complicating subsequent analyses. Adaptive optics are commonly used to correct sample-induced aberrations using a deformable mirror. However, routinely used sensorless adaptive optics techniques are slow, as they require multiple images of the same region of interest to iteratively estimate the aberrations. In addition to the fading of fluorescent signal, this is a major limitation as thousands of images are required to image a single intact organ even without adaptive optics. Thus, a fast and accurate aberration estimation method is needed. Here, we used deep-learning techniques to estimate sample-induced aberrations from only two images of the same region of interest in cleared tissues. We show that the application of correction using a deformable mirror greatly improves image quality. We also introduce a sampling technique that requires a minimum number of images to train the network. Two conceptually different network architectures are compared; one that shares convolutional features and another that estimates each aberration independently. Overall, we have presented an efficient way to correct aberrations in LSFM and to improve image quality.

Design of a Prism-Grating Wide Spectral Range Transmittance Imaging Spectrometer.

As spectroscopic detection technology rapidly advances, back-illuminated InGaAs detectors with a wider spectral range have emerged. Compared to traditional detectors such as HgCdTe, CCD, and CMOS, InGaAs detectors offer a working range of 400-1800 nm and exhibit a quantum efficiency of over 60% in both the visible and near-infrared bands. This is leading to the demand for innovative designs of imaging spectrometers with wider spectral ranges. However, the widening of the spectral range has led to the presence of significant axial chromatic aberration and secondary spectrum in imaging spectrometers. Additionally, there is difficulty in aligning the system optical axis perpendicular to the detector image plane, resulting in increased challenges during post-installation adjustment. Based on chromatic aberration correction theory, this paper presents the design of a wide spectral range transmission prism-grating imaging spectrometer with a working range of 400-1750 nm using Code V. The spectral range of this spectrometer covers both the visible and near-infrared regions, which is beyond the capability of traditional PG spectrometers. In the past, the working spectral range of transmission-type PG imaging spectrometers has been limited to 400-1000 nm. This study's proposed chromatic aberration correction process involves selecting optical glass materials that match the design requirements and correcting the axial chromatic aberration and secondary spectrum, ensuring that the system axis is perpendicular to the detector plane and easy to adjust during installation. The results show that the spectrometer has a spectral resolution of 5 nm, a root-mean-square spot diagram less than 8 μm over the full field of view, and an optical transfer function MTF greater than 0.6 at a Nyquist frequency of 30 lp/mm. The system size is less than 90 mm. Spherical lenses are employed in the system design to reduce manufacturing costs and complexity while meeting the requirements of wide spectral range, miniaturization, and easy installation.

Simultaneous correction of primary aberrations through wearing silicone hydrogel contact lens

Simultaneous correction of primary aberrations through wearing silicone hydrogel contact lens

Multislice Electron Tomography Using Four-Dimensional Scanning Transmission Electron Microscopy

Electron tomography offers useful three-dimensional (3D) structural information, which cannot be observed by two-dimensional imaging. By combining annular dark-field scanning transmission electron microscopy (ADF STEM) with aberration correction, the resolution of electron tomography has reached atomic resolution. However, tomography based on ADF STEM inherently suffers from several issues, including a high electron-dose requirement, poor contrast for light elements, and artifacts from image-contrast nonlinearity. Here, we develop an alternative method called multislice electron tomography (MSET) based on four-dimensional STEM tilt series. Our simulations show that multislice-based 3D reconstruction can effectively reduce undesirable reconstruction artifacts from the nonlinear contrast, allowing precise determination of atomic structures with improved sensitivity for low-Z elements, at considerably low electron-dose conditions. We expect that the MSET method can be applied to a wide variety of materials, including radiation-sensitive samples and materials containing light elements whose 3D atomic structures have never been fully elucidated due to electron-dose limitations or nonlinear imaging contrast.

Multi-element metasurface system for imaging in the near-infrared

Metasurfaces are optically thin 2D arrays of subwavelength scatterers that modify scalar and vector properties of incident electromagnetic fields. Metasurface lenses are of particular interest for imaging applications for their flat form factor, compatibility with CMOS fabrication processes, and potential for correcting aberrations with a small number of elements. We advance this capability by realizing a millimeter-diameter, polarization-independent metalens triplet system with chromatic aberration correction over the wavelength range of 1.30–1.60 μm and monochromatic aberration correction enabling a field of view of 50°.

Design and manufacturing of aspherical GaAs-SILs for a versatile optical semiconductor failure analysis system

In this study, we have developed an aspherical solid immersion lens (SIL) made of gallium arsenide (GaAs) for semiconductor failure analysis. A SIL is an optical component to dramatically increase the numerical aperture and the spatial resolution of a microscopy system by attaching it to the backside of the device-under-test. To further enhance the spatial resolution of the optical microscopy system, a shorter wavelength light source has also been introduced. In the past, we have developed a spherical SIL made of silicon (Si), which is the same material as the device under test. However, Si is not transparent for wavelengths shorter than around 1100 nm. To enable the use of a SIL at shorter wavelengths, we chose GaAs as SIL material, which is transparent at shorter wavelengths than Si and has a refractive index very close to that of Si. However, a spherical GaAs SIL features a more pronounced spherical aberration due to the difference in refractive index between the device-under-test (Si) and that of the SIL (GaAs) deteriorating the optical resolution of the microscopy system. We have therefore designed an aspherical GaAs-SIL which makes it possible to correct spherical aberration. We applied diamond tooling as a technique for manufacturing the SIL shape with a required surface irregularity below 100 nm peak-to-valley, resulting in a diffraction limited performance of the optical system for semiconductor failure analysis with a performance at around 0.18 μm cutoff.

Wavefront single-pixel imaging using a flexible SLM-based common-path interferometer

Wavefront single-pixel imaging (WSPI) is an important tool for simultaneously measuring the amplitude and phase of an unknown field, especially suitable for wavefront detecting in some special wavelengths where array detectors are immature or even unavailable. Here, a passive-detection-based WSPI method using a flexible SLM-based common-path interferometer is proposed, in which by detecting the zero-order diffraction of the SLM directly, the common-path phase-shift or off-axis technique can be flexibly realized by a “sandwich” structure consisting of a half-wave plate, a SLM, and a polarizer. The reference light can be conveniently chosen as the direct reflected light from the SLM, the unmodulated light from the pixels of the SLM, or the light from both of them. We experimentally verify the feasibility and advantages of the proposed method. We further demonstrate the method for wavefront measurement of a dragonfly wing, which suggests it can be used for quantitative phase imaging. It provides a high-resolution spatial wavefront passive detection technique, with a compact, stable, and flexible setup, which has lots of applications involving three-dimensional imaging, aberration correction, digital holographic measurement, and digital microscopy.

Temporal coherence envelope function of field emission in electron microscopy

Imaging in electron microscopy is adversely affected by partial electron spatial and temporal coherence. Temporal coherence has been treated theoretically in the past using the method pioneered fifty years ago by Hanßen and Trepte, who assumed a Gaussian energy distribution. However, state-of-the-art instruments employ field emission (FE) sources that emit electrons with a non-Gaussian energy distribution. We have updated the treatment of temporal coherence to describe the effects of an arbitrary energy distribution on image formation. The updated approach is implemented in Fourier optics simulations to explore the effect of FE on image formation in conventional, non-aberration-corrected (NAC) and aberration-corrected (AC) low energy electron microscopy. It is found that the resolution that can be achieved for the FE distribution is only slightly degraded compared to a Gaussian distribution with the same energy spread. FE also produces a focus offset. These two effects are weaker for AC than for NAC microscopy. These and other insights may be relevant to the selection of the aperture size that optimizes resolution and to analyses that make use of focal image series. The approach developed here is also applicable to transmission electron microscopy.

Design and Study of a Reflector-Separated Light Dispersion-Compensated 3D Microscopy System.

The secondary-phase grating-based tomographic microscopy system, which is widely used in the biological and life sciences, can observe all the sample multilayer image information simultaneously because it has multifocal points. However, chromatic aberration exists in the grating diffraction, which seriously affects the observation of the image. To correct the chromatic aberration of the tomographic microscope system, this paper proposes a system that adopts blazed gratings and angle-variable reflectors as chromatic aberration correction devices according to the principle of dispersion compensation and Fourier phase-shift theory. A reflector-separated light dispersion-compensated 3D microscopy system is presented to achieve chromatic aberration correction while solving the problem of multilayer image overlap. The theoretical verification and optical design of the system were completed using ZEMAX software. The results show that the proposed system reduced the chromatic aberration of ordinary tomographic microscopy systems by more than 90%, retaining more wavelengths of light information. In addition, the system had a relatively wide range in the color difference compensation element installation position, reducing the difficulty of dispersion compensation element installation. Overall, the results indicate that the proposed system is effective in reducing chromatic aberration in grating diffraction.

Apochromatic X-ray focusing

Achromatic doublets are combinations of two individual lenses designed to focus different wavelengths of light in the same position. Apochromatic optics are improved versions of the achromatic schemes which extend the wavelength range significantly. Both achromatic and apochromatic optics are well-established for visible light. However, X-ray achromatic lenses did not exist until very recently, and X-ray apochromatic lenses have never been experimentally demonstrated. Here, we create an X-ray apochromatic lens system using an appropriate combination of a Fresnel zone plate and a diverging compound refractive lens with a tuned separation distance. The energy-dependent performance of this apochromat was characterized at photon energies between 6.5 and 13.0 keV by ptychographic reconstruction of the focal spot and scanning transmission X-ray microscopy of a resolution test sample. The apochromat delivered a reconstructed focal spot size of 940 × 740 nm2. The apochromatic combination shows a four-fold improvement in the chromatic aberration correction range compared to an achromatic doublet configuration. Thus, apochromatic X-ray optics have the potential to increase the focal spot intensity for a wide variety of X-ray applications.

Characterization of thick and contact lenses using an adaptive Shack–Hartmann wavefront sensor: Limitations and solutions

Lens characterization is a key challenge for reliable optical and imaging techniques. Shack–Hartmann wavefront sensor (SHWFS) is frequently used as an efficient tool for characterizing optical elements. However, the aberrations arising from the incident wavefront, utilized for the characterization process, cause imprecision in the spherical power measurement. Accordingly, the aim of the present study is to construct an experimental configuration that consists of SHWFS incorporated with a deformable mirror (DM) to provide a correction for the incident wavefront aberrations which yields an ideal plane wave. Different types of thick lenses with a wide range of focal lengths and three single-vision contact lenses were tested before and after aberrations compensation and compared to the reference-based method. The comparison was based on measuring the spherical powers for the tested lenses. The results indicate that the accuracy of measurements is influenced by the incident wavefront aberrations, which has been improved by the proposed compensation system. Finally, the limitations are investigated and it was concluded that the system in its current configuration failed to accurately measure some thick lenses. This issue was solved by suggesting a compound lens correction methodology that extended the measurement range of spherical powers beyond the designated measurable range of the system.

Soft Tissue Aberration Correction for Histotripsy Using Acoustic Emissions From Cavitation Cloud Nucleation and Collapse.

Phase aberration from soft tissue limits the efficacy of histotripsy, a therapeutic ultrasound technique based on acoustic cavitation. Previous work has shown that the acoustic emissions from cavitation can serve as "point sources" for aberration correction (AC). This study compared the efficacy of soft tissue AC for histotripsy using acoustic cavitation emissions (ACE) from bubble cloud nucleation and collapse. A 750-kHz, receive-capable histotripsy array was pulsed to generate cavitation in ex vivo porcine liver through an intervening abdominal wall. Received ACE signals were used to determine the arrival time differences to the focus and compute corrective delays. Corrections from single pulses and from the median of multiple pulses were tested. On average, ACE AC obtained 96% ± 3% of the pressure amplitude obtained by hydrophone-based correction (compared with 71% ± 5% without AC). Both nucleation- and collapse-based corrections obtained >96% of the hydrophone-corrected pressure when using medians of ≥10 pulses. When using single-pulse corrections, nucleation obtained a range of 49%-99% of the hydrophone-corrected pressure, while collapse obtained 95%-99%. The results suggest that (i) ACE AC can recover nearly all pressure amplitude lost owing to soft tissue aberration and that (ii) the collapse signal permits robust AC using a small number of pulses.

Design of a 1-MHz therapeutic ultrasound array for small volume blood-brain barrier opening at cortical targets in macaques.

Focused ultrasound (FUS) can temporarily open the blood brain barrier (BBB) and increase the delivery of chemotherapeutics, viral vectors, and other agents to the brain parenchyma. To limit FUS BBB opening to a single brain region, the transcranial acoustic focus of the ultrasound transducer must not be larger than the region targeted. In this work, we design and characterize a therapeutic array optimized for BBB opening at the frontal eye field in macaques. We used 115 transcranial simulations in four macaques varying f-number and frequency to optimize the design for focus size, transmission, and small device footprint. The design leverages inward steering for focus tightening, a 1 MHz transmit frequency, and can focus to a simulation predicted 2.5 ± 0.3 mm lateral and 9.5 ± 1.0 mm axial full-width at half maximum spot size at the frontal eye field without aberration correction. The array is capable of steering axially 35 mm outward, 26 mm inward, and laterally 13 mm with > 50% the geometric focus pressure. The simulated design was fabricated, and we characterized the performance of the array using hydrophone beam maps in a water tank and through an ex vivo skull cap to compare measurements with simulation predictions, achieving a 1.8 mm lateral and 9.5 mm axial spot size with a transmission of 37 % (transcranial, phase corrected). The transducer produced by this design process is optimized for BBB opening at the frontal eye field in macaques.

Convex optical freeforms using fringe Zernike overlay approach for two-mirror and three-mirror telescopes for space applications

The challenge in manufacturing and testing of convex optical parts is well known in comparison to its concave counterpart and an optimized imaging system invariably features a combination of both. Of late, freeform mirrors find lot of interest in space applications as the optical systems remain compact within the allocated envelope. As is well known, these freeform mirrors are difficult to manufacture and test due to their lack of rotational or translational symmetry and poses additional challenges if it has convex profile. In the current investigations, two new convex freeform optical surfaces are proposed which is based on fringe Zernike overlay technique. The first convex freeform envisaged is for a two-mirror off-axis telescope where the secondary mirror is an off-axis freeform and the second convex freeform for a three-mirror anastigmat telescope where the secondary mirror is an on-axis convex freeform. Substitution of these convex freeform surfaces in the two-mirror and three-mirror telescope systems aid in the correction of asymmetrical aberrations to a wider view field, thereby enhancing the performances of the de-centred and tilted systems in the respective configurations. The procedure adopted for the manufacturing of these convex freeform surfaces using bonnet polishing methods and the interferometric tests using modified-Hindle null corrector and subaperture-stitching techniques for evaluation are described in detail.