Aspheric Optics Research Articles

Ultra-smooth finishing of aspheric surfaces using CAST technology

Abstract Growing applications for astronomical ground-based adaptive systems and air-born telescope systems demand complex optical surface designs combined with ultra-smooth finishing. The use of more sophisticated and accurate optics, especially aspheric ones, allows for shorter optical trains with smaller sizes and a reduced number of components. This in turn reduces fabrication and alignment time and costs. These aspheric components include the following: steep surfaces with large aspheric departures; more complex surface feature designs like stand-alone off-axis-parabola (OAP) and free form optics that combine surface complexity with a requirement for ultra-high smoothness, as well as special optic materials such as lightweight silicon carbide (SiC) for air-born systems. Various fabrication technologies for finishing ultra-smooth aspheric surfaces are progressing to meet these growing and demanding challenges, especially Magnetorheological Finishing (MRF) and ion-milling. These methods have demonstrated some good success as well as a certain level of limitations. Amongst them, computer-controlled asphere surface-finishing technology (CAST), developed by Precision Asphere Inc. (PAI), plays an important role in a cost effective manufacturing environment and has successfully delivered numerous products for the applications mentioned above. One of the most recent successes is the Gemini Planet Imager (GPI), the world’s most powerful planet-hunting instrument, with critical aspheric components (seven OAPs and free form optics) made using CAST technology. GPI showed off its first images in a press release on January 7, 2014 . This paper reviews features of today’s technologies in handling the ultra-smooth aspheric optics, especially the capabilities of CAST on these challenging products. As examples, three groups of aspheres deployed in astronomical optics systems, both polished and finished using CAST, will be discussed in detail.

Implementation of a Gaussian Beam Laser and Aspheric Optics for High Spatial Resolution MALDI Imaging MS

We have investigated the use of a Gaussian beam laser for MALDI Imaging Mass Spectrometry to provide a precisely defined laser spot of 5μm diameter on target using a commercial MALDI TOF instrument originally designed to produce a 20μm diameter laser beam spot at its smallest setting. A Gaussian beam laser was installed in the instrument in combination with an aspheric focusing lens. This ion source produced sharp ion images at 5μm spatial resolution with signals of high intensity as shown for images from thin tissue sections of mouse brain.

Investigation of diffraction-based measurement errors in optical testing of aspheric optics with digital micromirror devices

Spatial light modulators (SLMs) have shown to be versatile tools for displays, but also for various applications in optical metrology since light can be individually directed and customized and thus they may serve as flexible masks or holograms. In contrast to SLMs based on liquid crystal on silicon or liquid crystal displays (LCDs), which allow tailoring phase and/or amplitude of a wavefront in transmission or reflection, digital micromirror devices (DMDs) offer highest reflectivity and allow precise as well as fastest guidance of light rays based on the fundamental reflection law. Here, we present and compare different approaches for simulating the diffractive pattern when applying a micromirror device for wavefront readout. The different simulation methods for calculating the diffraction pattern are based on Monte-Carlo simulations in combination with nonsequential ray tracing, on Fourier optics methods (Fourier transform, FT) and on complex digital holographic wavefront propagation. The wavefront measurement concept with the DMDs is based on selecting single subapertures of the wavefront under test and on measuring the wavefront slope consecutively in a scanning procedure. In contrast, in LCD-based approaches already shown in literature, the selection of subapertures and thus the scanning procedure is performed in transmission. The measurement concept and diffraction-based measurement errors of this method will be demonstrated for aspheric optics. Furthermore, different approaches for the prediction and reduction of the diffractive pattern—also based on holographic complex wave front propagation—will be described and characterized.

Effect of an aspheric intraocular lens on the ocular wave‐front adjusted for pupil size and capsulorhexis size

To compare the ocular wave-front of eyes with silicone Intraocular lens (IOLs) with aspheric and spherical optics after cataract surgery, taking into account the patient's pupil size under reading conditions and after pupil dilatation. In this institutional prospective, randomized, controlled, patient and examiner masked, bilateral trial with intra-individual comparison, 60 eyes of 30 patients with bilateral age-related cataract were included. Each patient received a spherical IOL (CeeOn Edge, 911A, AMO, Santa Ana, CA, USA) in one eye and an aspheric IOL (Tecnis, Z9000, AMO) in the contra-lateral eye. Exclusion criteria were other ocular pathologies, capsular changes or zonular weakness. The main outcome variable was spherical aberration of the ocular wave-front under mesopic pupil conditions measured 2 years after surgery. Additional outcome variables were visual acuity and photopic and mesopic contrast sensitivity. There was no significant difference in visual acuity between the two IOL types under physiological pupil conditions and also not after pupil dilation. However, spherical aberrations were significantly lower with the aspheric IOL (SA: spherical 0.38 μm, SD: 0.11 μm; aspheric 0.10 μm, SD: 0.13 μm; p < 0.01), and there was a significant difference in contrast sensitivity at 12 cycles/degree. This is the first randomized and masked trial on visual function and ocular wave-front after implantation with this silicone aspheric IOL, taking the patients' own pupil size into account. The effect on visual function was detectable for mesopic contrast sensitivity, but there was no difference in visual acuity. The SA was found to be significantly lower under physiological pupil conditions as well as when recalculated for the rhexis size and under pharmacological dilatation.

Research on the Residual Error Evaluation Method for Deterministic Polishing of Aspheric Optics

Residual error evaluation method for deterministic polishing of aspheric optics is studied. Two residual error evaluation methods, which are axis-direction error method and normal error method respectively, are researched theoretically. It's inferred that the residual error of aspheric surface should be evaluated by normal error method. A new approach is proposed to calculate normal direction residual error on the basis of the axis-direction residual error of the aspheric surface. There exists difference between these two kinds of error which increases from the center of the aspheric optic to the edge through the comparison of them. Taking bonnet polishing and numerical controlled small tool polishing as examples, experiments are made to quantitatively prove that using axis-direction error method to evaluate residual error in deterministic polishing would introduce different degrees of processing error. It's found that the processing error is positively correlated with the relative aperture of aspheric optics, which is the ratio of the optic's aperture and vertex's curvature radius. Therefore, it is recommended to use axis-direction error method instead of normal error method as the evaluation method of the residual error during deterministic polishing aspheric optics with relatively small relative aperture; the opposite is the other way around.

Effect of curve groove on the spectral resolution for soft X-ray holographic flat-field gratings

The soft X-ray spectrograph is an important instrument for plasma diagnostics. As the core optical element of spectrograph, holographic flat-field grating is fabricated by aspheric wave-front recording optics, so grooves on the surface are curve. The curve grooves of the grating would affect the spectral image properties, thus influencing spectral resolutions. In the design of recording optics, only the groove density distribution on the surface in meridian line should be guaranteed, so optimized recording optics is not unique. Thus gratings with different curvatures of grooves but with expected groove density distribution could be obtained. For holographic flat-field gratings used in a 0.8-6 nm region, we analyze the influences of different curve grooves on the spectral image by ray tracing, and find that the almost straight grooves which are obtained by means of cylinder mirror can obtain the better spectral images. The theoretical results show that the spectral resolutions of grating with almost straight grooves are obviously improved compared with curve grooves, the theoretical spectral resolutions increase from 626 to 953 at 3 nm and from 635 to 1222 at 5 nm, respectively.

大口径非球面元件磁流变加工稳定性研究

大口径非球面元件磁流变加工稳定性研究

Magnetorheological finishing removal stability of large aperture aspherical optics

Magnetorheological finishing removal stability of large aperture aspherical optics

Off-Axis Aspheric Surfacing Using Sub-Aperture Tools

The off-axis aspheric surface used in modern optical systems widely can obtain nearly perfect quality, realize both small packet-size and low-mass, and avoid the central obscuration. But it is difficult to fabricate because of asymmetry. There are some key technologies during the testing and fabrication of off-axis asphere. In this article, we proposed a method of the best fit sphere based on non-negative minimized removal criterion. And a measured data fitting algorithm was presented to estimate the geometry parameters of off-axis aspheric surface. Then an off-axis mirror was fabricated, and the interferometrically measured data was corrected to eliminate the distortion introduced by null compensator in every run. Finally, the surface error of off-axis mirror reduced to pv = 0.372λ, rms = 0.046λ, the surface roughness reached 0.72 nm. These methods mentioned in the article are suitable for off-axis aspheric optics.

COB형 LED 보안등을 위한 렌즈 구조 설계

The study carried out in this dissertation focuses on the lens structure design and the light distribution for LED safety luminaires using COB type LED module. Lens structures for LED lights has been designed 1) to induce light diffusion by dual process of internal reflection and refraction, 2) to minimize the inherent LED lights" glittering, and 3) to have uniform brightness. The lens designed with the proposed structures function as diffusers for the divergence of the LED lights so that they form a wide angle of view and adjust the light distribution. We designed of lens with stable uniformity factor and average roughness using aspheric optics property. Finally we made the analysis data of the simulated data.

Etch depth mapping of phase binary computer-generated holograms by means of specular spectroscopic scatterometry

Detailed analysis of etch depth map for phase binary computer-generated holograms intended for testing aspheric optics is a very important task. In particular, diffractive Fizeau null lenses need to be carefully tested for uniformity of etch depth. We offer a simplified version of the specular spectroscopic scatterometry method. It is based on the spectral properties of binary phase multi-order gratings. An intensity of zero order is a periodical function of illumination light wave number. The grating grooves depth can be calculated as it is inversely proportional to the period. Measurement in reflection allows one to increase the phase depth of the grooves by a factor of 2 and measure more precisely shallow phase gratings. Measurement uncertainty is mainly defined by the following parameters: shifts of the spectrum maximums that occur due to the tilted grooves sidewalls, uncertainty of light incidence angle measurement, and spectrophotometer wavelength error. It is theoretically and experimentally shown that the method we describe can ensure 1% error. However, fiber spectrometers are more convenient for scanning measurements of large area computer-generated holograms. Our experimental system for characterization of binary computer-generated holograms was developed using a fiber spectrometer.

Basic Theory and Method of Controllable Compliant Tools for Optic Elements Manufacturing

Aspheric optics can attain excellent optical qualities and image effects,which makes it feasible to get miniaturized arrangements of multi-mirrors.Therefore they are more and more widely applied in fields like aviation,space,national defense and high-tech civilian industries.Conventional optic elements manufacturing technology is far less capable to meet extensive demands of aspheric optics,leading to the transformation to modern advanced optic manufacturing technologies.Herein researches on advanced optic manufacturing technology system and basic theory direct further development of this area.On the basis of reviewing the evolution of aspheric optic manufacturing,a basic idea of controllable compliant tools(CCT) is put forward.Key common theories are composed of the origin and mathematic model of removal function,multi-parameter control strategy,4-dimension numerical control technology and error evolution theory as well as control method and equipments.It is a primary discussion of this theory on the foundation of experiments using magnetorheological finishing(MRF) and the ion beam figuring(IBF).Controllable compliant tools can be regarded as another innovative means to define the third generation of aspheric optics manufacturing,and maybe even become a creative research system in the field of optical manufacturing.

Short spatial filters with spherical lenses for high-power pulsed lasers

We report possible employment of short spatial filters based on spherical lenses in a pulsed laser source (neodymium glass, ). The influence of the spherical aberration on the quality of output radiation and coefficient of conversion to the second harmonics is studied. The ultra-short aberration spatial filter of length with an aperture of is experimentally tested. A considerable shortening of multi-cascade pump lasers for modern petawatt laser systems is demonstrated by the employment of short spatial filters without expensive aspherical optics.

Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing

Fabricating super-smooth aspheric optics for future hard X-ray telescopes will require a new process chain. Whilst diamond turning of electroless nickel plating can generate ~100nm P-V aspheric molding dies, associated turning marks must be removed before replication. An innovative two-step freeform finishing method is presented, that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. The removal rate and surface texture relationship of fluid jet with abrasive type and pressure is documented, and form correction demonstrated down to 27nm P-V. A novel bonnet tool-pathing method called “continuous precessing” is then applied, which delivers super-smooth anisotropic surface texture of 0.28nm rms.

Computerized interferometric surface measurements [Invited

The addition of electronics, computers, and software to interferometry has enabled enormous improvements in optical metrology. This paper discusses four areas in which computerized interferometric measurement improvements have been made in the measurement of surface shape and surface roughness: (a) The use of computer-generated holograms for the testing of aspheric optics, (b) phase-shifting interferometry for getting interferometric data into a computer so the data can be analyzed, (c) computerized interference microscopes, including multiple-wavelength and coherence scanning, for the precision measurement of surface microstructure, and (d) vibration-insensitive dynamic interferometers for enabling precise measurements in noncontrolled environments.

Objective evaluation of spherical aberration induced by soft contact lenses with spherical and aspherical optics and its effect on visual quality

Purpose: To measure, in mesopic luminance, the effects on ocular spherical aberration (OSA) of soft CLs with spherical optic (SOCL) and aspherical optic (AOCL) and to evaluate the impact on visual quality (VQ).

The Control Software of Aspheric Optics in MRF

Based on the mechanism of magnetorheological finishing(MRF), a material removal model was established. The dwell time function was solved by Jansson-Van Cittert algorithm to accomplish the kernel module design. Thus the control software of aspheric optics in MRF was designed. In order to verify accuracy of the processing control software, experiments were carried out on large aperture aspheric optics and the wavefront error of elements was achieved rapidly convergence. It is proved that the software could control the whole polishing process accurately.

Methods of Determining Optimal Fitting Spherical Surface for Off-Axis Aspheric Optics Finishing

Most present methods of determining optimal fitting spherical surface for rotationally symmetrical aspheric optics are either unsuitable for off-axis optics or unable to guarantee “the condition of positive removal” (distances from points on desired concave aspheric surface to the center of fitting sphere are all longer than radius of sphere while distances from points on desired convex aspheric surface to the center of fitting sphere are all shorter than radius of sphere). To surmount the two problems, this paper proposes three methods of determining starting spherical surface in finishing/polishing aspheric optics: method of using the function of “lsqlin” provided in Matlab, the modified method of least squares and the method of exhaustive search of tangent spheres. An example is presented to validate the three methods and to demonstrate all of them gain some advantages over conventional one by comparing attributes (normal deviation distribution, maximum normal deviation, volume of material to be removed, rms of normal deviation distribution, etc.) of their optimal fitting spheres against those of sphere obtained by utilizing conventional method.

Eight-axis-polishing Machine for Large Off-axis Aspheric Optics

For the purpose of fabricating off-axis aspheric optics, we propose an 8-axis-polishing machine combined with a testing tower whose height is up to 9 m. The proposed polishing machine was designed and analyzed by using a well-known finite element method. The eight axes of the machine have a synchronized motion generated by a computer, and each axis was calibrated by a heterodyne laser interferometer or an optical encoder. After calibration, the maximum positioning error of the machine was less than 2 μm within a whole 2 m × 2 m area. A typical fabrication result of a φ1.5 m concave mirror was also described in this manuscript.

61 Comparison of spherical aberration controland visual acuity with aspheric and spherical contact lens optics

61 Comparison of spherical aberration controland visual acuity with aspheric and spherical contact lens optics