Proceedings Volume 5869

Optical Manufacturing and Testing VI

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Proceedings Volume 5869

Optical Manufacturing and Testing VI

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Volume Details

Date Published: 13 September 2005
Contents: 9 Sessions, 43 Papers, 0 Presentations
Conference: Optics and Photonics 2005 2005
Volume Number: 5869

Table of Contents

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Table of Contents

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  • Projects
  • Optical Fabrication I: FJP
  • Optical Fabrication II: Polishing
  • Optical Fabrication III: MRF et al.
  • Optical Testing I: Absolute Calibration and Algorithms
  • Optical Testing II: Error Analysis and Instrumentation
  • Optical Testing III: Test Setups and Shack-Hartmann
  • Optical Testing IV: Profilers and Coarse Metrology
  • Fiber and Nano-Fabrication Posters
Projects
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Progress toward a third Gemini M2 mirror
Roland Geyl, Eric Ruch, Serge Primet, et al.
This paper is intended to give a report after the successful lightweighting of a mirror substrate dedicated to become a third spare secondary mirror for the Gemini 8-m telescopes installed one in Chili and one Hawaii.
Status of ZERODUR mirror blank production at SCHOTT
Thorsten Doehring, Peter Hartmann, Ralf Jedamzik, et al.
SCHOTT has a history of more than 35 years with the production of the zero expansion glass ceramic material ZERODUR. More than 250 ZERODUR mirror blanks were already delivered to the large segmented telescopes KECK I, KECK II, HET, GTC, and LAMOST. The increasing worldwide demand on large ZERODUR components for LCD display lithography machines is similar to the expected demand for an Extremely Large Telescope. Last year SCHOTT has ramped up its ZERODUR production capacity. These recent investments in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared also for a future production of mirror blanks for Extremely Large Telescopes. The present status of the production capacity and the mass production of ZERODUR mirror blanks for industrial applications are discussed.
Fabrication and metrology of freeform aluminum mirrors for the SCUBA-2 instrument
Ian J. Saunders, Leo Ploeg, Michiel Dorrepaal, et al.
TNO Science and Industry was contracted by the UK Astronomy Technology Center to deliver nine aluminum freeform mirrors for a new sub-millimeter instrument called SCUBA-2. With a much larger field-of-view and sky-background limited sensitivity, SCUBA-2 will map large areas of sky up to 1000 times faster than the original SCUBA camera. The requirement for freeform optics on SCUBA-2 is due to the fact that conventional spherical optic systems do not fit into the allotted space. TNO, who has developed a number of enabling technologies for freeform fabrication and metrology, applied its skills to the mechanical design and analysis of the mirrors. Due to the diverging requirements of the nine mirrors in size, form, and accuracy, TNO needed to spread the fabrication and metrology over additional organizations. The National Aerospace Laboratory of the Netherlands machined and hand polished the five largest mirrors (dimensions exceeding 1 meter and mass exceeding 150kg). They obtained form accuracies between 10 and 15μm peak to valley and roughness down to 40nm RMS. TNO fabricated the smaller and more accurate mirrors (dimensions between 300 and 700mm) on a Precitech Nanoform 350 with a slow tool servo. Measurements by the Dutch National Metrology Institute indicated form accuracies between 4 and 6μm peak to valley and roughness below 20nm RMS for the smaller mirrors.
The prototype of ultralight mirror system
P. Schovanek, M. Palatka, M. Hrabovsky, et al.
We have used our experience in the production of ultralight glass mirrors for the realization of a segmented LIDAR mirror. For non-imaging type of LIDAR, which is meant for monitoring of optical properties of the atmosphere, we designed tessellated mirror from nine segments. The diameter of the whole mirrors system is 1000 mm.
Optical Fabrication I: FJP
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Prepolishing and finishing of optical surfaces using fluid jet polishing
Wilhelmus A. C. M. Messelink, Reto Waeger, Torsten Wons, et al.
The footprint of the Fluid Jet Polishing process is determined by the shape of the nozzle as well as by the orientation of the slurry beam with respect to the local surface normal. Besides, no tool wear occurs and the footprint remains constant during the manufacturing process allowing shape corrections in a deterministic way. To that aim, FJP has been implemented on a CNC machine and applied for both shaping of previously polished aspheres and polishing of fine ground a-spheres. In this paper, results will be presented showing the application of FJP as a sub-aperture shape correction method. Besides, experimental data will be reported demonstrating FJP's capability of polishing previously fine ground surfaces. The wear rate depends on the sharpness of the abrasives and their kinetic energy. It can thus be adjusted by various parameters, among others the applied pressure, slurry concentration and abrasive sizes. In this paper, an additional process parameter is identified allowing the application of the same polishing compound for wear rates ranging from nanometers to micrometers. This large wear range is achieved by mixing a well controlled amount of gas into the slurry flow allowing the abrasives to travel at higher speeds.
Jet-induced high-precision finishing of challenging optics
Conformal, free form and steep concave optics are important classes of optics that are difficult to finish using conventional techniques due to mechanical interference and steep local slopes. The problem becomes more complicated when the optics approach millimeter size. In this presentation we will discuss some results from finishing such challenging optics to high precision using newly developed jet-based techniques.
Development and optimization of FJP tools and their practical verification
This article presents the recent achievements with Jules Verne, a sub-aperture polishing technique closely related to Fluid Jet Polishing [1]. Whereas FJP typically applies a nozzle stand-off distance of millimeters to centimeters, JV uses a stand-off distance down to 50 μm. The objective is to generate a non-directional fluid flow parallel to the surface, which is specifically suited to reduce the surface roughness [2, 3]. Different characteristic Jules Verne nozzle geometries have been designed and numerically simulated using Computational Fluid Dynamics (CFD). To verify these simulations, the flow of fluid and particles of these nozzles has been visualized in a measurement setup developed specifically for this purpose. A simplified JV nozzle geometry is positioned in a measurement setup and the gap between tool and surface has been observed by an ICCD camera. In order to be able to visualize the motion of the abrasives, the particles have been coated with fluorescence. Furthermore, these nozzles have been manufactured and tested in a practical environment using a modified polishing machine. The results of these laboratory and practical tests are presented and discussed, demonstrating that the CFD simulations are in good agreement with the experiments. It was possible to qualitatively predict the material removal on the processed glass surface, due to the implementation of appropriate erosion models [4, 5] in the CFD software.
New results in hydrodynamic radial polishing using HyDRa
E. Sohn, E. Ruiz, E. Luna, et al.
New results using hydrodynamic radial polishing techniques on assorted materials, using HyDRa are presented. This tool performs corrective lapping and fine polishing by means of a low-cost, foamy abrasive flux. The functioning principle is based on the generation of a grazing, high-velocity, low-pressure, rotational, variable density, abrasive flux with radial geometry. It is currently possible to polish aspheres and free-form optics on diverse materials and sizes. This tool is particularly useful for polishing thin substrates such as membranes and semiconductors since it can be biased for a non-interactive action on the work piece. This process also has the advantage of achieving high removal rates. In order to achieve high degrees of accuracy and repeatability in the HyDRa finishing process, fully automated bias and slurry supply units must be incorporated to the polishing system. The air and slurry supply systems are described, as well as operational tool parameters for optimal polishing performance.
Optical Fabrication II: Polishing
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Cycle time and cost reduction in large-size optics production
Optical fabrication process steps have remained largely unchanged for decades. Raw glass blanks have been rough-machined, generated to near net shape, loose abrasive or fine bound diamond ground and then polished. This set of processes is sequential and each subsequent operation removes the damage and micro cracking induced by the prior operational step. One of the long-lead aspects of this process has been the glass polishing. Primarily, this has been driven by the need to remove relatively large volumes of glass material compared to the polishing removal rate to ensure complete damage removal. The secondary time driver has been poor convergence to final figure and the corresponding polish-metrology cycles. The overall cycle time and resultant cost due to labor, equipment utilization and shop efficiency is increased, often significantly, when the optical prescription is aspheric. In addition to the long polishing cycle times, the duration of the polishing time is often very difficult to predict given that current polishing processes are not deterministic processes. This paper will describe a novel approach to large optics finishing, relying on several innovative technologies to be presented and illustrated through a variety of examples. The cycle time reductions enabled by this approach promises to result in significant cost and lead-time reductions for large size optics. In addition, corresponding increases in throughput will provide for less capital expenditure per square meter of optic produced. This process, comparative cycles time estimates and preliminary results will be discussed.
New results extending the Precessions process to smoothing ground aspheres and producing freeform parts
D. D. Walker, A. T. H. Beaucamp, V. Doubrovski, et al.
Zeeko's Precession polishing process uses a bulged, rotating membrane tool, creating a contact-area of variable size. In separate modes of operation, the bonnet rotation-axis is orientated pole-down on the surface, or inclined at an angle and then precessed about the local normal. The bonnet, covered with standard polishing cloth and working with standard slurry, has been found to give superb surface textures in the regime of nanometre to sub-nanometre Ra values, starting with parts directly off precision CNC aspheric grinding machines. This paper reports an important extension of the process to the precision-controlled smoothing (or 'fining') operation required between more conventional diamond milling and subsequent Precession polishing. The method utilises an aggressive surface on the bonnet, again with slurry. This is compared with an alternative approach using diamond abrasives bound onto flexible carriers attached to the bonnets. The results demonstrate the viability of smoothing aspheric surfaces, which extends Precessions processing to parts with inferior input-quality. This may prove of particular importance to large optics where significant volumes of material may need to be removed, and to the creation of more substantial aspheric departures from a parent sphere. The paper continues with a recent update on results obtained, and lessons learnt, processing free-form surfaces, and concludes with an assessment of the relevance of the smoothing and free-form operations to the fabrication of off-axis parts including segments for extremely large telescopes.
UltraForm finishing process for optical materials
E. Fess, J. Schoen, M. Bechtold, et al.
A new compliant sub-aperture optical finishing technique is being investigated for the removal of mid-spatial frequency artifacts and smoothing of hard polycrystalline infrared ceramics for aspheric applications and conformal shaped optics. The UltraForm concept was developed by OptiPro Systems, Ontario, NY, and is a joint process development effort with the Center for Optics Manufacturing (COM). The latest version of the UltraForm tool "V3" is of a belted design whereby a belt of finishing material is passed over a toroidal elastomeric wheel. Finishing materials used include a wide variety of pad materials and abrasive selections. Experimentation has been conducted using both slurry mixes and fixed abrasive bands. The toroidal wheel is rotated while the compliant tool is compressed into contact with the optical surface. Presented will be the current results in optical glasses and crystalline ceramics such as ALON, Spinel and Polycrystalline Alumina.
A new approach to predict computer controlled polishing results
A novel approach to handle and quantify a computer controlled polishing process will be introduced. This approach will be compared to real data. This comparison indicates the correctness of this approach. Based on it a formula has been developed to predict the results of a computer controlled polishing process. The formula will be used to predict real polishing processes and the results will be compared to the real results. The limits when using this formula will be shown along with suggestions when the formula would be useful. This rough prediction of the computer controlled polishing results may be used to enhance the automation of a computer controlled polishing process. Also a way to improve the formula itself will be introduced. It is the opinion of the author that by further stabilizing of the whole computer controlled polishing process the whole system becomes more robust, the prediction more accurate and the whole system improves in reliability and the results become better.
Improvement of thickness uniformity of quartz crystal wafer by numerically controlled plasma CVM
Masafumi Shibahara, Kazuya Yamamura, Yasuhisa Sano, et al.
To improve the thickness uniformity of thin quartz crystal wafer, new machining process which utilizing an atmospheric pressure plasma was developed. In an atmospheric pressure plasma process, since the kinetic energy of ions which impinge to the wafer surface is small and the density of reactive species is large, high efficiency machining without damage is realized. In this process, thickness distribution of the quartz crystal wafer is corrected by numerically controlled machining which consists of two steps. First, long spatial wavelength component of thickness error is corrected by using a cylindrical rotary electrode. And next, short wavelength component is corrected by using small size pipe electrode. By using our two step correcting process, thickness distribution of an AT cut wafer was improved from 108 nm (p-v : peak to valley) to 14 nm (p-v). And spurious mode in the resonance curve was reduced by improving the parallelism of the quartz crystal wafer.
Optical Fabrication III: MRF et al.
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Analysis of thermal sources in a magnetorheological finishing (MRF) process
Magnetorheological finishing (MRF) is a computer controlled polishing (CCP) technique for high quality surfaces. The process uses a magnetorheological fluid which stiffens in a magnetic field and thus acts as the polishing tool. At the University of Applied Sciences Deggendorf thermal sources in a MRF polishing unit have been analysed using an infrared camera. The result of the research is a warming of the fluid in the fluid conditioner caused by the mixer motor. The existing cooling is therefore essential, in order to ensure a constant polishing tool characteristic during polishing runs. A new fluid conditioner, which was developed at the University of Applied Sciences Deggendorf, with the aim of an extended fluid lifetime may be used without cooling, because an increase of the fluid temperature in the conditioner could not been detected. Furthermore, a warming of the workpiece during the polishing process was not ascertainable.
Effects of nanodiamond abrasive friability in experimental MR fluids with phosphate laser glass LHG-8 and other optical glasses
Jessica E. DeGroote, Anne E. Marino, John P. Wilson, et al.
Research is currently being conducted to better understand the role that nanodiamond abrasives play in the removal process of Magnetorheological Finishing (MRF). The following presents removal rate data for a set of six optical glasses that were spotted (not polished out) with four different MR fluids, as well as texturing/smoothing data for phosphate laser glass LHG-8. Three of the fluids contained nanodiamonds with varying friability levels and the fourth fluid was an abrasive-free fluid that was used as a baseline for comparison. The medium friability nanodiamonds were found to be the most efficient in removing material on LHG-8, and the three silicate glasses, FS, BK-7 and FD-60. The high friability nanodiamond fluid was the most efficient for removal with the titanium and fluro- phosphate glasses EFDS-1 and FCD-1. With this nanodiamond the removal rates of all six glasses followed a mechanical figure of merit. The presence of nanodiamonds in the MR fluid greatly affected the surface texture of LHG-8. The abrasive-free MR fluid caused severe pitting that was either reduced or eliminated once the nanodiamonds were added to the fluid.
New viscosity measurement for magnetorheological polishing fluid
In Magnetorheological Finishing (MRF) a magnetic field is applied to a stream of abrasive magnetorheological fluid, in order that the fluid behaves as the polishing tool. The process may be used to finish the surface of high quality optical lenses. The fluid viscosity is one important parameter the polishing tool characteristic depends on. At the University of Applied Sciences Deggendorf a new viscosity measurement, which uses the inductance of the fluid had been tested. The result of the research is a close relationship between viscosity and inductance. The new viscosity measurement is not an absolute, but a comparative system, based on inductance of the flowing fluid and the fluid age.
Analysis of the ductile-brittle mode transition in loose abrasive grinding
Through single scratch experiments on glass, the transition from ductile to brittle mode grinding is analysed and the ductile regime dependent on load and coolant type is determined. The influence of different coolants on the grinding mode is discussed and certain manufacturing "tricks" used in the optical shop are explained. The results are verified by loose abrasive grinding experiments.
Effect of humidity on optical fiber-adhesive bonding
A. Priyadarshi, H. Kaur, Anand K. Asundi, et al.
Optical adhesives serve a means of structural attachment as well as provide an optical path between connecting elements in an optical modules. The aim of this study was to determine the displacement and normal strain induced in an optical fiber as a result of moisture absorption within the optical adhesive of the module. The displacement and the strain of single mode fiber-adhesive joint on silicon optical bench (SiOB) were measured by Micro Moire interferometry (MMI). The experiments were performed on a module consisting of SiOB and single-mode-fiber attached in a V-groove with the help of a UV-curable adhesive. Moisture saturation of the optical adhesive within the optical package was achieved via the devised moisture uptake by the capillary effect setup and specimens were placed in the set up for one week. Strain in the adhesive was measured by MMI during the moisture desorption, at 60oC. The maximum V and U field displacement demonstrated by the fiber was 0.66μm and 0.59μm respectively. The V and U field induced recovery strain of 0.00488 and 0.00438 respectively as a result of the saturation process grounds the postulation that moisture does indeed affect the optical fiber's relative position within the optical module.
Optical Testing I: Absolute Calibration and Algorithms
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Construction and testing of wavefront reference sources for interferometry of ultra-precise imaging systems
We have built and calibrated a set of 532-nm wavelength wavefront reference sources that fill a numerical aperture of 0.3. Early data show that they have a measured departure from sphericity of less than 0.2 nm RMS (0.4 milliwaves) and a reproducibility of better than 0.05 nm rms. These devices are compact, portable, fiber-fed, and are intended as sources of measurement and reference waves in wavefront measuring interferometers used for metrology of EUVL optical elements and systems. Keys to wave front accuracy include fabrication of an 800-nm pinhole in a smooth reflecting surface as well as a calibration procedure capable of measuring axisymmetric and non-axisymmetric errors.
Absolute accuracy evaluation of aspherical null testing for EUVL mirrors
Yuichi Takigawa, Shigeru Nakayama, Takahiro Yamamoto, et al.
Interferometric null metrology can produce highly precise figure of aspherical surfaces. However, because the measurement is a direct comparison of the tested surface with the reference wavefront of the null optics, measurement accuracy is equivalent to the quality of the reference null wavefront. Although the asymmetric aberration of the reference null wavefront can be calibrated by rotating the tested surface, it is more difficult to calibrate the rotationally symmetric errors. Especially, the aspherical surface of EUVL mirrors must to be measured with higher accuracy, 0.2~0.3nmRMS. We have developed an aspherical null testing system using a null lens for EUVL mirrors. We analyzed the uncertainty of null lens in each process and estimated the measurement accuracy of aspherical null testing using null lens. If the compensator lens contains only one piece of lens, the measurement accuracy is estimated to be 0.20nmRMS. If the compensator contains two pieces, the measurement accuracy becomes 0.24nmRMS. To verify our estimation, we evaluate a sample lens with aplanatic surfaces that make no spherical aberrations. In this case, we can evaluate the quality of the transmitted wavefront absolutely. The difference between the calculated and the experimental wavefronts is much smaller than our estimation. To this extent, our aspherical testing technique using null lens has been verified to be able to meet the high demanding for EUVL mirror testing.
Calibration of symmetric and non-symmetric errors for interferometry of ultra-precise imaging systems
The azimuthal Zernike coefficients for shells of Zernike functions with shell numbers n<N may be determined by making measurements at N equally spaced rotational positions. However, these measurements do not determine the coefficients of any of the purely radial Zernike functions. Label the circle that the azimuthal Zernikes are measured in as circle A. Suppose that the azimuthal Zernike coefficients for n<N are also measured in a smaller circle B which is inside circle A but offset so that it is tangent to circle A and so that it has the center of circle A just inside its circular boundary. The diameter of circle B is thus only slightly larger than half the diameter of circle A. From these two sets of measurements, all the Zernike coefficients may be determined for n<N. However, there are usually unknown small rigid body motions of the optic between measurements. Then all the Zernike coefficients for n<N except for piston, tilts, and focus may be determined. We describe the exact mathematical algorithm that does this and describe an interferometer which measures the complete wavefront from pinholes in pinhole aligners. These pinhole aligners are self-contained units which include a fiber optic, focusing optics, and a "pinhole mirror". These pinhole aligners can then be used in another interferometer so that its errors would then be known. Physically, the measurements in circles A and B are accomplished by rotating each pinhole aligner about an aligned axis, then about an oblique axis. Absolute measurement accuracies better than 0.2 nm were achieved.
A simple ball averager for reference sphere calibrations
Ulf Griesmann, Quandou Wang, Johannes Soons, et al.
When measuring the form errors of precision optics with an interferometer, calibration of the reference wavefront is of central importance. In recent years, ball averaging, or random ball testing, has emerged as a robust method for calibrating spherical reference wavefronts in converging beams. We describe a simple instrument, consisting of an air bearing and two electric motors, that can rotate the test ball around three axes as required for a ball averaging test. The performance of the instrument is demonstrated by using it to calibrate a concave transmission sphere. Further we discuss the effects of image sampling at random locations or on uniform grids, and the effect of correlated measurements. Finally, we describe a method to determine the number of measurements which are sufficient for a ball averaging calibration.
New methods for calibrating systematic errors in interferometric measurements
S. O'Donohue, G. Devries, P. Murphy, et al.
Interferometers are often used to measure optical surfaces and systems. The accuracy of such measurements is often limited by the ability to calibrate systematic errors such as reference wave and image distortion. Standard techniques for calibrating reference wave include the two-sphere and random-ball test. QED Technologies® (QED) recently introduced a Subaperture Stitching Interferometer (SSI®) that has the integrated ability to perform reference wave calibration. By measuring an optical surface in multiple locations, the stitching algorithm has the ability to compensate for reference wave and imaging distortion. Each of the three reference wave calibration methods has its own limitations that ultimately affect the accuracy of the measurement. The merits of each technique for reference wave calibration are reviewed and analyzed. By using the SSI-computed estimate and the random-ball test in tandem, a composite method for calibrating reference wave error is shown to combine the benefits of both individual techniques. The stitching process also calibrates for distortion, and plots are shown for different transmission optics. Measurements with and without distortion compensation are shown, and the residual difference is compared to theoretical predictions.
An overview of power spectral density (PSD) calculations
Richard N. Youngworth, Benjamin B. Gallagher, Brian L. Stamper
Specifications for optical surfaces have traditionally been given in terms of low frequency and high frequency components, often with a separate classification for surface slope. Low spatial frequency components are commonly referred to as figure errors and can be described by the standard 37-term Zernike polynomial set. High spatial frequency errors are commonly referred to as finish and are quantified using rms roughness. Specification with the qualitative scratch and dig classification is done usually for cosmetic or aesthetic purposes. Mid-spatial frequency errors such as waviness, ripple, and quilting can be important and are not explicitly covered by such traditional figure and finish specifications. In order to bridge the gap to cover mid-spatial frequencies, in terms of quantifying surface characteristics, Power Spectral Density (PSD) can be utilized. For such usage, it is important for the greater optics community to understand the metric, how to calculate it, and how to use it. The purpose of this paper is to provide an overview of PSD, its application in optics, and an outline of calculations needed to effectively apply it to specify optical surfaces.
Optical Testing II: Error Analysis and Instrumentation
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Retrace error evaluation on a figure-measuring interferometer
Neil Gardner, Angela Davies
A micro-refractive lens figure error measurement is performed at the confocal position with the interferometer in reflection mode. The wavefront in the interferometer reflecting from the test surface inherently has aberrations at some level, and reflection from an imperfect test surface further deviates the wavefront and adds to the interferometer aberrations. The interferometer aberration causes each ray of light to reflect off the test lens and back into the interferometer at a different angle. Consequently, the ray takes a different path back through the interferometer and therefore accumulates a different aberration. The result is a re-trace error which increases with the test lens surface curvature and becomes significant in the micro-optic range. The dependence of test part radius on micro-lens figure-error-measuring interferometer wavefront bias data was confirmed both experimentally and by software simulation. Results clearly indicate that the re-trace error increases with test lens surface curvature. The fact that re-trace errors depend on the radius of the test part implies that when calibrating the instrument even with a perfect artifact, the calibration is nominally valid only when measuring parts with the same approximate radius as the calibration artifact. A compact micro-interferometer useful for measuring several properties of micro-lenses including figure error, was developed to verify this phenomenon. The instrument has the capability of measuring micro-lenses with radii of curvature between 150 μm and 3 mm.
Precision metrology of dihedral angle error in prisms and corner cubes for the Space Interferometry Mission
Recent work done at CSIRO's Australian Centre for Precision Optics has pushed the fabrication limits on optical retroreflectors. To achieve dihedral angle errors well below an arcsecond, the measurement accuracy was required to be 0.1 arcsec or better. On this level, the error introduced by the interferometer's instrument function is not negligible. So-called "collimation errors", or more generally, wavefront aberrations, can significantly falsify the angle measurements. This paper describes and demonstrates the basic concepts for interferometric measurement of dihedral angle errors in retroreflectors from a practical point of view. We discuss obvious and subtle stumbling blocks, summarize practical experiences, and show some results of the recent joint work between JPL and CSIRO.
Effects of birefringence on Fizeau interferometry that uses polarization technique
Chunyu Zhao, Dongyel Kang, James H. Burge
Interferometers that use different states of polarization for the reference and test beams can modulate the relative phase shift using polarization optics in the imaging system. This allows the interferometer to capture simultaneous images that have a fixed phase shift, which can be used for phase shifting interferometry. Since all measurements are made simultaneously, the interferometer is not sensitive to vibration. Fizeau interferometers of this type have advantage over Twyman-Green type systems because the optics are in the common path of both the reference and test wavefronts, therefore errors in these optics affect both wavefronts equally and do not limit the system accuracy. However, this is not strictly true for the polarization interferometer when both wavefronts are transmitted an optic that suffers from birefringence. If some of the components in the common path of the reference and test beams have residual birefringence, the two beams see different phases. Therefore, the interferometer is not strictly common path. As a result, an error can be introduced in the measurement. In this paper, we study the effect of birefringence on measurement accuracy when different polarization techniques are used in Fizeau interferometers. We demonstrate that measurement error is reduced dramatically for small amount of birefringence if the reference and test beams are circularly polarized rather than linearly polarized.
Sub-pixel spatial resolution interferometry with interlaced stitching
In this paper we describe a growing need for an increase in the spatial resolution of interferometric surface measurements of precision optics and present a new method to address this need. An increase in the spatial resolution of interferometric surface measurements arises from evolving surface figure and micro-roughness specifications for higher quality optics, demand for larger optics, and recent advancements in deterministic polishing. These three topics will be discussed and their relationship to increased spatial resolution will be described. A solution to increase the spatial resolution using a process called "sub-pixel spatial resolution interferometry" will be presented. In this process, multiple interferometric measurements are made as the optic under test (or the CCD array) is shifted at sub-pixel increments. The measurements are then combined to construct a measurement with higher spatial resolution than the original measurements. Initial results obtained using this process with a commercially available Fizeau interferometer will be presented.
Optical Testing III: Test Setups and Shack-Hartmann
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Sub-aperture interferometric testing of a large-scale elliptical mirror under thermal-vacuum conditions
James N. Howell, Richard T. Summers, Michael G. Dittman, et al.
Interferometric testing of large-sized optics in a thermal vacuum environment poses challenges not normally found in an optical metrology lab. Unless the test equipment is thermal-vacuum compatible, it must be installed in an ambient environment with the test item viewed through a window in the thermal-vacuum chamber. Limitations in chamber port size preclude normal-incidence viewing of the full aperture of large-sized optical elements. This necessitates the use of a mechanical translation of the test item to acquire multiple overlying interferograms. The interferograms are then concatenated in order to produce a full-aperture surface map of the test item. This is then used to confirm surface deformation of the entire test mirror. This paper will discuss the challenges, solutions, and results of a series of thermalvacuum tests performed on a large-scale (>40cm) silicon carbide mirror at ambient temperatures.
Testing an off-axis parabola with a CGH and a spherical mirror as null lens
Chunyu Zhao, Rene Zehnder, James H. Burge, et al.
Steward Observatory Mirror Lab is currently polishing an off-axis parabola which will be the primary mirror of the New Solar Telescope. To test this mirror, we built a test equipment to combine a spherical mirror and a Computer Generated Hologram (CGH) as null lens. The spherical mirror is tilted to compensate much of the astigmatism and some coma. And the CGH compensates rest of aberrations. The combination of a spherical mirror and a CGH makes the test system compact. The technology developed here will be used to test the Giant Magellan Telescope's primary mirror segment--a five times larger off-axis parabola.
Alignment and testing of piston and aberrations of a segmented mirror
Qian Gong, William Eichhorn
This paper describes measuring and aligning segmented mirrors with Point Diffraction Interferometer (PDI). It discusses how to use the different coherent lengths of the selected light sources to simplify the piston error adjustment process. It addresses the theory behind the experiment, models the experimental setup, analyzes the data, and discusses the result. The procedure of using PDI to adjust the piston error is provided.
Development of Cassegrain type 0.9-m collimator
Ho-Soon Yang, Myung-Seok Kang, Sung-Hee Kim, et al.
Collimator is essential to evaluate and assemble the other telescopes. Its diameter should be larger than that of the target telescope for the correct use. We are currently developing the Cassegrain type collimator of which diameter is 0.9 m. The primary mirror is light-weighted so that its weight is only 70 kg. Due to its structure, the primary mirror can be supported only at the backside of the mirror. This mirror is tested with the combination of null Hartmann test and interferometer. The secondary mirror is tested with a Hindle method. This method requires 600 mm high quality spherical mirror. The distance between the primary and secondary mirror is maintained by the Carbon composite material. The assembly of two mirrors is carried out by the computer aided alignment method. The whole structure is designed to maintain the performance of the collimator under +/-5 degrees of temperature variation.
Reverse optimization of a Shack-Hartmann wavefront sensor
Reverse optimization has been used in other metrology instruments to obtain good calibration results. This paper describes three approaches for using reverse optimization with a Shack-Hartmann wavefront sensor. Two of the approaches give some insight into problems encountered in reverse optimization and a third approach, involving an aberration function model of the Shack-Hartmann wavefront sensor, resolves these issues. Simulated results are given and the approach is shown to produce calibrated wavefront measurements accurate to one part in one thousand, in the presence of significant centroid noise.
Real-time measurement of the small aspheric surface
Most aspheric surfaces have been measured by null lens test or computer generated hologram (CGH) method. This approach, however, often fails when there are many aspherical terms or target surface is very small because it is not easy to design the conventional null lens or CGH. Hartmann test is a good choice for this case because it has a larger dynamic range than the general interferometer. This means that the surface can be measured with a Hartmann test without null correctors or with incomplete null correctors. In this paper, we apply the Hartmann test to the measurement of convex aspheric surface of which diameter is about 16 mm and has 4 additional aspheric terms. In order to measure the surface in real-time, we developed some CGH target that simulates the ideal target surface and the simple optical system. The measurement result can be served as a reference so that the form error of the target surface can be obtained by the subtraction of this reference, to achieve highly accurate measurement in real-time.
Differential wavefront curvature sensor
In this paper, a wavefront curvature sensor is presented. This sensor is based on the measurements of the differentials of wavefront slopes, where the wavefront slope measurements can be achieved by a Shack-Hartmann sensor. A Shack-Hartmann sensor with three output collimated beams will be introduced with a lenslet array installed in each beam. By shifting two of the Shack-Hartmann grids in the x- and in the y- directions independently compared to the third grid, we can measure the slope differences and obtain the wavefront curvatures at each grid point. A wavefront reconstruction can be performed with the wavefront curvature measurements.
Optical Testing IV: Profilers and Coarse Metrology
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Design of a machine for the universal non-contact measurement of large free-form optics with 30 nm uncertainty
Rens Henselmans, Nick Rosielle, Maarten Steinbuch, et al.
A new universal non-contact measurement machine design for measuring free-form optics with 30 nm expanded uncertainty is presented. In the cylindrical machine concept, an optical probe with 5 mm range is positioned over the surface by a motion system. Due to a 2nd order error effect when measuring smoothly curved surfaces, only 6 position measurement errors are critical (nanometer level). A separate metrology system directly measures these critical errors of the probe and the product relative to a metrology frame, circumventing most stage errors. An uncertainty estimation has been performed for the presented design, including a calibration uncertainty estimation and a dynamic analysis. Machine dynamics certainly cause relative motion between probe and product, but due to the non-contact nature of the measurement and the short metrology loop, these motions do not cause significant measurement errors. The resulting shape measurement error for aspheres up to medium free-forms is between 24 and 37 nm, and 30 - 85 nm for medium to heavily free-form surfaces. The suitability of the proposed design is herewith confirmed. A detailed design and a prototype of the machine are currently being developed.
Measurement of a 2-meter flat using a pentaprism scanning system
A scanning pentaprism system may be used as an absolute test for an optical flat. Such a system was built and used to test a 2-meter flat mirror. This system uses light from an autocollimator that is reflected from 2 pentaprisms to project reference beams of light onto the flat mirror. The light reflected from the mirror back through the pentaprisms provides information on low order optical aberrations in the flat mirror. We report results of the test on a 2-meter flat.
Dynamic phase-shifting electronic speckle pattern interferometer
The technique for measuring changes in diffuse surfaces using Electronic Speckle Pattern Interferometry (ESPI) is well known. We present a new electronic speckle pattern interferometer that takes advantage of a single-frame spatial phase-shifting technique to significantly reduce sensitivity to vibration and enable complete data acquisition in a single laser pulse. The interferometer was specifically designed to measure the stability of the James Webb Space Telescope (JWST) backplane. During each measurement the laser is pulsed once and four phase-shifted interferograms are captured in a single image. The signal is integrated over the 9ns pulse which is over six orders of magnitude shorter than the acquisition time for conventional interferometers. Consequently, the measurements do not suffer from the fringe contrast reduction and measurement errors that plague temporal phase-shifting interferometers in the presence of vibration. This paper will discuss the basic operating principle of the interferometer, analyze its performance and show some interesting measurements.
Surface shape measurements using digital holography with a fringe projection system and partially coherent illumination
In recent years several methods based on digital holography to combine shape and deformation analysis in the same setup have been proposed. For measuring the object deformation in the most cases two holograms with a certain wavelength have to be recorded for different object states. For shape measurement the object has to remain unchanged while two holograms with slightly different wavelengths or slightly different illumination points are recorded. In the paper a different, simple and inexpensive method for shape and deformation measurements of rough surfaces is proposed. The method uses fringe projection combined with digital holography. First complex amplitude from surface under test is registered using digital holography without any optical system. Next, the fringe image is reconstructed from the hologram as an intensity distribution what allows determination of the surface shape. Small displacements are measured using reconstructed object phase distribution and digital holography interferometry, larger ones by comparing the shapes before and after deformation. Because only one hologram for the shape and two for the displacement investigation are necessary, quite fast measurements are possible. Light detector used in the arrangement mainly restricts measurements time and methods precision. Unfortunately an application of coherent radiation introduces high contrast coherent noise into the holographic imaging. To reduce it the partially coherent illumination is used instead. The coherence degree of illumination beam is chosen to limit spatial bandwidth of holographic fringes according to used detector.
Reflection grating photogrammetry: a technique for absolute shape measurement of specular free-form surfaces
Marcus Petz, Rainer Tutsch
While for non-reflecting surfaces a variety of optical techniques is available that allow the flexible geometric measurement of free-form surfaces, established approaches for the testing of specular surfaces are limited to basic geometries or slight deviations from an assumed reference geometry. As not only the intensified use of aspheric optics but also the increasing quality standards for technical surfaces call for an enhanced measurement range, the authors have developed two related techniques for the direct three-dimensional measurement of specular reflecting surfaces. These techniques are based on the observation of the mirror image of a grid-like reference structure and apply principles that are well-known from measurement systems for non-reflecting surfaces, such as photogrammetry and structured illumination, to the evaluation of specular surfaces. While the first of these approaches works with an active triangulation process based on one camera and a pseudo three-dimensional reference structure, the second one utilises a stereo-photogrammetric camera system in conjunction with a merely two-dimensional reference structure. Both systems allow the unambiguous measurement of reflecting free-form surfaces and may, by the use of multiple wavelength and photogrammetric stitching techniques, be extended to the measurement of rather complex geometries. Besides the fundamental mode of operation of this so-called reflection grating photogrammetry, the properties of a suitable reference structure will be presented in this contribution. Furthermore the photogrammetric calibration procedure and the used calibration models will be discussed. Finally the measurement uncertainty is evaluated based on both, experimental and theoretical considerations.
Testing big roughness surfaces by using shadow moiré
J. Castro-Ramos, S. Vazquez-Montiel, S. Quechol-Lopez, et al.
Working with big mirrors always is a great challenge, even more if the surfaces have great roughness. In this work we present a technique to verify the quality for surfaces around 3 meters in diameter and with roughness around 30 microns. In order to reach our goal we made an analysis of the grating pitch to avoid the roughness and we implement a common source light, which is independent of the angle of illumination of the surface under test. Also we implement the shadow moire and the phase shift method to obtain the wave front aberrations.
Fiber and Nano-Fabrication Posters
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A new hot-jointing technique for the preform fabrication of tellurite based single mode optical fibers
Xunsi Wang, Qiuhua Nie, Tiefeng Xu, et al.
The new preform fabrication technique, based on the direct-melt method and modified by the hot-jointing process, for tellurite based single mode optical fibers was developed. There are only some simple moulds needed, which are designed according to the dimension of the perform required. The different combinations of these moulds could be met the various requirements, such as the preform with high diameter ratio of cladding to core (DRCC) for single-mode optical fibers. The adoption of the hot-jointing method can prevent the preform from the fragile and inner stress comparing to the direct glass machining. This technique can also be used to fabricate other oxide and low melting point glasses. The jointing process is performed at a high temperature, which results in the core and cladding boundary melted well without the bubbles. The experimental results show that the large DRCC preforms could be made by this new technique, the preforms for drawing single mode optical fibers with the DRCC of 25 were fabricated. The single mode fibers were drawn from these preforms and the upconversion luminescence of Er3+ in the fiber with the length of 70cm could be observed.
Development of laser scissors for carbon and silicon carbide fiber cutting
Hiroshi Murotani, Hironobu Sakata, Akira Yoshikado, et al.
Laser cutting of carbon (C) and silicon carbide (SiC) fibers was performed using a second harmonic generation (SHG) sheet beam of Q-switched Nd:YAG laser with an out put energy of 4J/pulse, pulse width of 10ns and a fluence of 3.0J/cm2. The beam was irradiated respectively on monofibers of C and SiC placed on an optical glass slide in air at room temperature. A single pulse was irradiated every one second and repetition number of shot pulses N was varied from N=10 to 40 pulses. Cutting of C and SiC fibers occurred for N = 20-40 pulses. A cut portion of C fiber fixed on the substrate for one end in tension free was found to have very sharp wedge like profile. Laser irradiation for C fiber fixed with both ends on the substrate made the fiber fractured with thinner fiber diameter. Similar profile was also observed in a SiC fiber at the cut portion. Smooth flat surface of laser cut portion of C and SiC fibers suggested a laser ablation by evaporation. In the SiC fiber, sticky and viscous layer was observed at the cut surface, suggesting vitreous silica formed by oxidation of laser-irradiated fiber.