Off-axis Aberrations Research Articles

Enhancement of PZT embedded swing arm actuator using integrated read/write performance and actuation

The current technological challenges associated with the rotary-type seesaw arm actuator for small-form-factor (SFF) disk drives are the small skew focusing, which causes off-axis aberrations and reduces optical quality. This article develops a novel design for a seesaw arm actuator and suspension assembly that is based on a micro-PZT actuator to position pickup head. Dual-stage actuation relaxes the dynamic requirement on the focusing stroke. The proposed actuator with a tilt-compensation mechanism uses a PZT bender to drive pickup head focusing. This combined system, optical module and dual stage actuator, is effectively self-aligned the optical axis for read/write performance in both experiment and simulation result. Finite element modeling and dynamic measurements reveal significant improvements in the actuator bandwidth with and without micro-PZT actuator compensation.

Wide two-dimensional field laser ray-tracing aberrometer

There is an increasing interest in measuring the peripheral optical quality of the eye. Optical aberrations have been studied extensively in the center of the visual field due to the development of Hartmann-Shack wavefront sensor. However, experimental data of the peripheral field of view are still scarce, partly due to the fact that this evaluation presents various challenges. Here, we propose a novel device based on the laser ray-tracing (LRT) aberrometer, which is well suited for measuring the off-axis aberrations. The proposed instrument is able to measure a wide (±40°) 2D visual field and is based on three main design principles: spiral-shaped sampling of the visual field, real-time detection of the eye's entrance pupil, and automatic shaping and delivering of the ray bundle that optimally samples the eye pupil. We present experimental data obtained on 11 healthy subjects and a novel analysis based on a 2D quadratic model of the aberrations as a function of visual field and azimuth. The obtained results are consistent with previous findings.

Near-perfect Collimation of Wide-Field Cassegrain Telescopes

ABSTRACTI describe a simple new method for collimating a Cassegrain telescope that gives near-perfect correction over a wide field of view. This method was used to collimate the 6.5 m MMT in 2002 with remarkably good results. The first step is to point the primary mirror accurately at an object. When the M1 optical axis points exactly toward an object, it cannot contribute pointing error, coma, or anamorphic aberration to the image field. If these are present, they must come from M2. Adjusting M2 to correct both pointing error and coma yields perfect collimation, canceling off-axis anamorphic aberrations. I present an analytical basis for this method and compare it with methods that measure off-axis image aberrations directly. In most cases, the technique described here gives better results. I describe the collimation procedure we used at MMT and suggest an easier way to adapt this method to other telescopes.

Field scanner design for MUSTANG of the Green Bank Telescope

MUSTANG is a bolometer camera for the Green Bank Telescope (GBT) working at a frequency of 90 GHz. The detector has a field of view of 40 arcseconds. To cancel out random emission change from atmosphere and other sources, requires a fast scanning reflecting system with a few arcminute ranges. In this paper, the aberrations of an off-axis system are reviewed. The condition for an optimized system is provided. In an optimized system, as additional image transfer mirrors are introduced, new aberrations of the off-axis system may be reintroduced, resulting in a limited field of view. In this paper, different scanning mirror arrangements for the GBT system are analyzed through the ray tracing analysis. These include using the subreflector as the scanning mirror, chopping a flat mirror and transferring image with an ellipse mirror, and chopping a flat mirror and transferring image with a pair of face-to-face paraboloid mirrors. The system analysis shows that chopping a flat mirror and using a well aligned pair of paraboloids can generate the required field of view for the MUSTUNG detector system, while other systems all suffer from larger off-axis aberrations added by the system modification. The spot diagrams of the well aligned pair of paraboloids produced is only about one Airy disk size within a scanning angle of about 3 arcmin.

The Three-Dimensional Point Spread Function of Aberration-Corrected Scanning Transmission Electron Microscopy

Aberration correction reduces the depth of field in scanning transmission electron microscopy (STEM) and thus allows three-dimensional (3D) imaging by depth sectioning. This imaging mode offers the potential for sub-Ångstrom lateral resolution and nanometer-scale depth sensitivity. For biological samples, which may be many microns across and where high lateral resolution may not always be needed, optimizing the depth resolution even at the expense of lateral resolution may be desired, aiming to image through thick specimens. Although there has been extensive work examining and optimizing the probe formation in two dimensions, there is less known about the probe shape along the optical axis. Here the probe shape is examined in three dimensions in an attempt to better understand the depth resolution in this mode. Examples are presented of how aberrations change the probe shape in three dimensions, and it is found that off-axial aberrations may need to be considered for focal series of large areas. It is shown that oversized or annular apertures theoretically improve the vertical resolution for 3D imaging of nanoparticles. When imaging nanoparticles of several nanometer size, regular STEM can thereby be optimized such that the vertical full-width at half-maximum approaches that of the aberration-corrected STEM with a standard aperture.

Peripheral aberrations in the human eye for different wavelengths: off-axis chromatic aberration

The interest in the eye’s off-axis aberrations has increased strongly. On-axis the conversion of the aberration magnitude between different wavelengths is well known. We verified if this compensation is correct also for off-axis measurements by building a wavelength tunable peripheral Hartmann–Shack sensor and measuring 11 subjects out to ±30° in the horizontal visual field. At the fovea, an average longitudinal chromatic aberration of 1 D between red (671 nm) and blue (473 nm) light was found, and it increased slightly with eccentricity (up to 1.2 D). A similar trend was measured for astigmatism as a function of wavelength (increase ∼0.15 D). Computational ray tracing in model eyes showed that the origin of the small increase of chromatic aberrations with eccentricity is the change of the oblique power of the refractive surfaces in the eye. Factors related to increase of axial length and refractive index of the eye were found to have a very small influence.

Radiation Boundary Conditions for Computational Fluid Dynamics Models of High-Temperature Cavity Receivers

Rigorous computational fluid dynamics (CFD) codes can accurately simulate complex coupled processes within an arbitrary geometry. CFD can thus be a cost-effective and time-efficient method of guiding receiver design and testing for concentrating solar power technologies. However, it can be computationally prohibitive to include a large multifaceted dish concentrator or a field of hundreds or thousands of heliostats in the model domain. This paper presents a method to allow the CFD code to focus on a cavity receiver domain alone, by rigorously transforming radiance distributions calculated on the receiver aperture into radiance boundary conditions for the CFD simulations. This method allows the incoming radiation to interact with participating media such as falling solid particles in a high-temperature cavity receiver. The radiance boundary conditions of the CFD model can take into consideration complex beam features caused by sun shape, limb darkening, slope errors, heliostat facet shape, multiple heliostats, off-axis aberrations, atmospheric effects, blocking, shading, and multiple focal points. This paper also details implementation examples in ansys fluent for a heliostat field and a dish concentrator, which are validated by comparison to results from delsol and the ray-tracing code asap, respectively.

Broadband FUV imaging spectrometer: advanced design with a single toroidal uniform-line-space grating

Performances of a far-ultraviolet (FUV) imaging spectrometer in an advanced design are presented with a toroidal uniform-line-space (TULS) grating. It provides high spatial resolution and spectral resolution for a broadband and a wide field of view. A particular analysis for the grating aberrations, including all the high-order coefficients neglected by previous existing designs, was generated for indicating their significance. The analysis indicates that these high-order off-axis aberrations would have a remarkable influence on the design results. The transcendental equations composed of these aberration coefficients do not have analytic solutions in algebra. To solve the problem, the past designs always do some simplified calculation which only suits a narrow field of view and waveband. Thus, the optimization of the genetic algorithm is introduced to propose reasonable ranges of optical parameters. Then ZEMAX software is used to obtain the final optical system from these ranges. By comparing different design results of the same example, our advanced TULS design performs better than conventional TULS design and spherical varied-line-space grating design, and as well as the toroidal varied-line-space design. It is demonstrated that aberrations are minimized when the TULS design is operated by our method. The advanced design is low-cost, easy to fabricate, and more suitable for FUV observations.

Generic Misalignment Aberration Patterns in Wide-Field Telescopes

ABSTRACTAxially symmetric telescopes produce well-known Seidel off-axis third-order aberration patterns: coma, astigmatism, curvature of field, and distortion. When axial symmetry is broken by the small misalignments of optical elements, additional third-order aberration patterns arise: one each for coma, astigmatism, and curvature of field and two for distortion. Each of these misalignment patterns is characterized by an associated two-dimensional vector, each of which in turn is a linear combination of the tilt and decenter vectors of the individual optical elements. For an N-mirror telescope, 2(N - 1) patterns must be measured to keep the telescope aligned. Alignment of the focal plane may require two additional patterns. For N = 3, as in a three-mirror anastigmat, there is a two-dimensional “subspace of benign misalignment” over which the misalignment patterns for third-order coma, astigmatism, and curvature of field are identically zero. One would need to measure at least one of the two distortion patterns to keep the telescope aligned. Alternatively, one might measure one of the fifth-order misalignment patterns, which are derived herein. But the fifth-order patterns are rather insensitive to misalignments, even with moderately wide fields, rendering them of relatively little use in telescope alignment. Another alternative would be to use telescope pointing as part of the alignment solution.

Optical design and imaging performance testing of a 96-mm diameter femtosecond laser microsurgery probe

We present the optical design of a 9.6-mm diameter fiber-coupled probe for combined femtosecond laser microsurgery and nonlinear optical imaging. Towards enabling clinical use, we successfully reduced the dimensions of our earlier 18-mm microsurgery probe by half, while improving optical performance. We use analytical and computational models to optimize the miniaturized lens system for off-axis scanning aberrations. The optimization reveals that the optical system can be aberration-corrected using simple aspheric relay lenses to achieve diffraction-limited imaging resolution over a large field of view. Before moving forward with custom lenses, we have constructed the 9.6-mm probe using off-the-shelf spherical relay lenses and a 0.55 NA aspheric objective lens. In addition to reducing the diameter by nearly 50% and the total volume by 5 times, we also demonstrate improved lateral and axial resolutions of 1.27 µm and 13.5 µm, respectively, compared to 1.64 µm and 16.4 µm in our previous work. Using this probe, we can successfully image various tissue samples, such as rat tail tendon that required 2-3 × lower laser power than the current state-of-the-art. With further development, image-guided, femtosecond laser microsurgical probes such as this one can enable physicians to achieve the highest level of surgical precision anywhere inside the body.

Enhancement of Optical Adaptive Sensing by Using a Dual-Stage Seesaw-Swivel Actuator with a Tunable Vibration Absorber

Technological obstacles to the use of rotary-type swing arm actuators to actuate optical pickup modules in small-form-factor (SFF) disk drives stem from a hinge’s skewed actuation, subsequently inducing off-axis aberrations and deteriorating optical quality. This work describes a dual-stage seesaw-swivel actuator for optical pickup actuation. A triple-layered bimorph bender made of piezoelectric materials (PZTs) is connected to the suspension of the pickup head, while the tunable vibration absorber (TVA) unit is mounted on the seesaw swing arm to offer a balanced force to reduce vibrations in a focusing direction. Both PZT and TVA are designed to satisfy stable focusing operation operational requirements and compensate for the tilt angle or deformation of a disc. Finally, simulation results verify the performance of the dual-stage seesaw-swivel actuator, along with experimental procedures and parametric design optimization confirming the effectiveness of the proposed system.

Fast scanning peripheral wave-front sensor for the human eye

We designed and built a fast scanning peripheral Hartmann-Shack (HS) wave-front sensor to measure the off-axis wave-front aberrations in the human eye. The new instrument is capable of measuring the optical quality over the central 80° horizontal visual field in 1.8 seconds with an angular resolution of 1°. The subject has an open field of view without any moving elements in the line-of-sight and the head is kept in place by a head-chin rest. The same efficiency, reliability and measurement quality as the current static HS sensor were found but with much higher acquisition speed and comfort for the patients. This instrument has the potential to facilitate and improve future research on the peripheral optical quality of the eye in large groups of subjects.

Deep-tissue access with confocal fluorescence microendoscopy through hypodermic needles

We report on the design and implementation of a gradient-index microendoscope suitable for accessing tissues deep within the body using confocal fluorescence imaging. The 350-μm diameter microendoscope has a length of 27 mm, which enables it to be inserted through a 22-gauge hypodermic needle. A prototype imaging system is demonstrated to obtain images of tissue samples at depths of ~15 mm with a lateral resolution of ~700 nm. To the best of our knowledge, this is the highest resolution and imaging depth reported for a confocal probe of these dimensions. We employ a scanning arrangement using a lensed fiber that can conveniently control the input beam parameters without causing off-axis aberrations typically present in the optical relay lenses used in galvanometer-mirror scanning systems.

Proton beam writing of long, arbitrary structures for micro/nano photonics and fluidics applications

The last decade has seen proton beam writing maturing into a versatile lithographic technique able to produce sub-100nm, high aspect ratio structures with smooth side walls. However, many applications in the fields of photonics and fluidics require the fabrication of structures with high spatial resolution that extends over several centimetres. This cannot be achieved by purely magnetic or electrostatic beam scanning due to the large off-axis beam aberrations in high demagnification systems. As a result, this has limited us to producing long straight structures using a combination of beam and stage scanning. In this work we have: (1) developed an algorithm to include any arbitrary pattern into the writing process by using a more versatile combination of beam and stage scanning while (2) incorporating the use of the ubiquitous AutoCAD DXF (drawing exchange format) into the design process. We demonstrate the capability of this approach in fabricating structures such as Y-splitters, Mach-Zehnder modulators and microfluidic channels that are over several centimetres in length, in polymer. We also present optimisation of such parameters as scanning speed and scanning loops to improve on the surface roughness of the structures. This work opens up new possibilities of using CAD software in PBW for microphotonics and fluidics device fabrication.

折/衍混合自由曲面式头戴显示器光学系统设计

To resolve the contradictions between wide Field of View(FOV),large exit pupil and compact,lightweight,a hybrid refractive/diffractive optical system using Free Form Surfaces(FFSs) for a Head Mount Display(HMD) was designed.The dispersion characteristics of diffractive elements were used to correct the chromatic aberration,and the appropriate types of FFS were selected to correct the off-axial aberrations(astigmatism,coma and distortion).Furthermore,the FFS prism was made of plastic materials to form an optical structure without rotational symmetry to achieve compact and easy to align functions.Obtained optical system can offer the eye relief of 17.4 mm,exit pupil of 4 mm and the FOV of 55°.Moreover,the RMS diameters of spot diagram are less than 30 μm,the MTF are higher than 0.1 at the spatial frequency of 30 lp/mm across the entire visual field,and the maximum distortion is less than 10%.Experimental results indicate that the optical system has perfect performance and can meet the demands of eyepieces for aberration characteristics.

Influence of Age on Peripheral Ocular Aberrations

To compare peripheral lower and higher order aberrations across the horizontal (±40°) and inferior (-20°) visual fields in healthy groups of young and old emmetropes. We have measured off-axis aberrations in the groups of 30 younger (24 ± 3 years) and 30 older (58 ± 5 years) emmetropes. The aberrations of OD were measured using the COAS-HD VR Shack-Hartmann aberrometer in 10° steps to ±40° horizontally and -20° inferiorly in the visual field. The aberrations were quantified with Zernike polynomials for a 4 mm pupil diameter. The second-order aberration coefficients were converted to their respective refraction components (M, J45, and J180). Mixed between-within subjects, analysis of variance were used to determine whether there were significant differences in the refraction and aberration components for the between-subjects variable age and the within-subjects variable eccentricity. Peripheral refraction components were similar in both age groups. Among the higher order coefficients, horizontal coma (C3) and spherical aberration (C4) varied mostly between the groups. Coma increased linearly with eccentricity, at a more rapid rate in the older group than in the younger group. Spherical aberration was more positive in the older group compared with the younger group. Higher order root mean square increased more rapidly with eccentricity in the older group. Like the axial higher order aberrations, the peripheral higher order aberrations of emmetropes increase with age, particularly coma and spherical aberration.

Aplanatic imaging systems for the transmission electron microscope

During the last decade aberration correctors have become a well-accepted tool in high-resolution transmission electron microscopy. The available correctors compensate for the spherical aberration Cs of the imaging system. Recently, for instruments with considerably improved information limit also the off-axial aberrations have attracted more attention since these aberrations limit the high-resolution field of view. We have proposed a novel hexapole-type Cs/B3-corrector which corrects for the spherical aberration and the off-axial coma of the imaging system. We discuss the assessment and correction of off-axial aberrations and report about the optical performance of the first prototype instrument.

Electron optics of multi-beam scanning electron microscope

We have developed a multi-beam scanning electron microscope (MBSEM), which delivers a square array of 196 focused beams onto a sample with a resolution and current per beam comparable to a state of the art single beam SEM. It consists of a commercially available FEI Nova-nano 200 SEM column equipped with a novel multi-electron beam source module. The key challenge in the electron optical design of the MBSEM is to minimize the off-axial aberrations of the lenses. This article addresses the electron optical design of the system and presents the result of optics simulations for a specific setting of the system. It is shown that it is possible to design a system with a theoretical axial spot size of 1.2nm at 15kV with a probe current of 26pA. The off-axial aberrations for the outermost beam add up 0.8nm, increasing the probe size to 1.5nm.

A new practical model of an objective lens with assembling clearance for transmission electron microscope

A new objective lens model including magnetic pole pieces, coil windings, magnetic circuit as well as the assembling clearance between the pole piece and magnetic circuit is developed to obtain high simulation precision. The calculation is based on the second-order finite element method (SOFEM) with the measured B– H magnetization curves of the lens materials. The magnetic pole pieces and magnetic circuit are firstly optimized to reduce the lens saturation and obtain minimum spherical aberration coefficient, then modified to release the magnetic flux leakage caused by the increased clearance. In the end, an example is given for a 200 kV TEM with the point resolution of 0.25 nm and off-axis aberration coefficients at the image plane are calculated for dynamic correction. Results show that the magnetic circuit is unsaturated and the saturated area is only 0.8×4.8 mm 2 around the lower pole piece with a maximum magnetic flux density of 2.537 T. This model can reduce the magnetic flux leakage and obtain the point resolution with smaller excitation. The calculation also shows that the off-axis aberrations require correcting dynamically to acquire fine image quality under low system magnification.

Wide Field Scanning Telescope Using MEMS Deformable Mirrors

Traditional optical telescopes can obtain high resolution images, but only over a narrow field of view. To image over a large field, the typical approach is to move the entire telescope to point to a particular area of interest. In this article, we consider a scanning telescope that only moves one of the optical elements in the telescope. Moving the lower inertia of a single optical element instead of the entire telescope means faster scanning, reduced vibration, and more accurate pointing. Inherent in the design is the introduction of optical aberrations due to off-axis imaging. This article presents simulation and experimental results of using deformable mirrors to correct for the off-axis aberrations. Two different types of deformable mirrors are considered: electrostatic and electromagnetic. The mirror apertures are similar in size, but the electromagnetic mirror has about twice the actuator displacement range than the electrostatic mirror. Through simulation and experiments, we demonstrated that the static field of view may be increased by 40 times with the electrostatic mirror and 80 times with the electromagnetic mirror.