Proceedings Volume 4451

Optical Manufacturing and Testing IV

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Proceedings Volume 4451

Optical Manufacturing and Testing IV

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Volume Details

Date Published: 27 December 2001
Contents: 10 Sessions, 53 Papers, 0 Presentations
Conference: International Symposium on Optical Science and Technology 2001
Volume Number: 4451

Table of Contents

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Table of Contents

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  • Projects I: NGST
  • Projects II: Gossamer
  • Projects III: Large Optics
  • Fabrication I: Grinding
  • Fabrication II: Polishing
  • Fabrication II: MRF and Micro-Optics
  • Testing I: Profilometry
  • Testing II: Stitching and Mid-Spatial Frequency
  • Testing III: Interferometry
  • Joint Session
  • Fabrication I: Grinding
  • Testing I: Profilometry
  • Testing II: Stitching and Mid-Spatial Frequency
  • Fabrication II: Polishing
  • Joint Session
Projects I: NGST
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Development of lightweight mirror technology for the next generation space telescope
In 2009, NASA plans to launch the Next Generation Space Telescope (NGST) to the L2 point, 1.5 million kilometers from Earth. With a 6-meter diameter mirror, NGST is a successor to the Hubble Space Telescope with 5 times the collecting aperture. As part of NASA's Origins Program, the ten-year observing mission will search for the first light of the universe. NGST will provide astronomers with unparalleled light collection, mid-infrared sensitivity, spatial resolution and field of view. Mirror technology is critical to the system's success. The hard part is solving the problem of how to launch a 6-m, 600-kg, mirror into space on a 4-meter diameter rocket. Additionally, high performance is expected at operating temperatures of 50K. This paper reviews the mirror requirements and development efforts.
Final results of the Subscale Beryllium Mirror Demonstrator (SBMD)program
Timothy Reed, Stephen E. Kendrick, Robert J. Brown, et al.
The Subscale Beryllium Mirror Demonstrator (SBMD) has been fabricated and tested, successfully demonstrating some of the necessary enabling technologies for the Next Generation Space Telescope (NGST) and other lightweight cryogenic space mirror applications. The SBMD is a 0.532-meter diameter concave spherical mirror with a 20-meter radius of curvature fabricated from a single billet of consolidated spherical powder beryllium. The mirror is lightweighted by 90% through the use of open back triangular cells and a thin facesheet. The mirror is mounted to a rigid backplane with titanium bipod flexures. Surface figure requirements at 35K of 1/4 wave p-v (full aperture) and 1/10 wave p-v (1-10 cm spatial frequency) required initial vacuum cryogenic characterization of the mirror. Cryogenic deformation and repeatability were characterized using the Optical Testing System (OTS) at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The mirror underwent cryofiguring to optimize performance and was subsequently tested to verify final performance requirements of surface figure, radius of curvature, and microroughness. Presented here are the final results of the SBMD program, showing that all requirements have been met.
Cryogenic optical testing results for the Subscale Beryllium Mirror Demonstrator (SBMD)
An Optical Testing System (OTS) has been developed to measure the figure and radius of curvature of Next Generation Space Telescope (NGST) developmental mirrors in a vacuum, cryogenic environment using the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The OTS consists of a WaveScope Shack-Hartmann sensor from Adaptive Optics Associates as the main instrument and a Leica Disto Pro distance measurement instrument. Testing is done at the center of curvature of the test mirror and at a wavelength of 632.8 nm. The error in the figure measurement is <EQ(lambda) /13 peak-to-valley (PV). The error in radius of curvature is less than 5 mm. The OTS has been used to test the Subscale Beryllium Mirror Demonstrator (SBMD), a 0.532-m diameter spherical mirror with a radius of curvature of 20 m. SBMD characterization consisted of three separate cryogenic tests at or near 35 K. The first two determined the cryogenic changes in the mirror surface and their repeatability. The last followed cryo-figuring of the mirror. This paper will describe the results of these tests. Figure results will include full aperture results as well as an analysis of the mid-spatial frequency error results. The results indicate that the SBMD performed well in these tests with respect to the requirements of (lambda) /4 PV (full aperture), (lambda) /10 PV (mid-spatial, 1-10 cm), and +/- 0.1 m for radius of curvature after cryo-figuring.
NGST mirror system demonstrator from the University of Arizona
James H. Burge, Scott T. DeRigne, James Roger P. Angel, et al.
Future space telescopes require primary mirrors that are much lighter than those currently being manufactured. They also must maintain optical tolerances while operating at cryogenic temperatures. We present a Mirror System Demonstrator for the Next Generation Space Telescope (NGST) that uses a thin glass facesheet with active control to achieve low mass and high surface quality. A 2-mm thick glass facesheet is controlled by miniature actuators and held together by a rigid carbon fiber frame. The 2-m diameter mirror system weighs only 13 kg/m2, including the glass, supports, actuators, support structure, and cabling. We present the status of the development and testing of this revolutionary mirror.
AMSD test error budget sensitivity analysis
The successful augmentation of NASA's X-Ray Cryogenic Facility (XRCF) at the Marshall Space Flight Center (MSFC) to an optical metrology testing facility for the Sub-scale Beryllium Mirror Development (SBMD) and NGST Mirror Sub-scale Development (NMSD) programs required significant modifications and enhancements to achieve useful and meaningful data. In addition to building and integrating both a helium shroud and a rugged and stable platform to support a custom sensor suite, the sensor suite was assembled and integrated to meet the performance requirements for the program. The subsequent evolution from NMSD and SBMD testing to the Advanced Mirror System Demonstrator (AMSD) program is less dramatic in some ways, such as the reutilization of the existing helium shroud and sensor support structure. However, significant modifications were required to meet the AMSD program's more stringent test requirements and conditions resulting in a substantial overhaul of the sensor suite and test plan. This overview paper will discuss the instrumentation changes made for AMSD, including the interferometer selection, null optics, and radius of curvature measurement method. The error budgeting process will be presented, and the overall test plan developed to successfully carry out the tests will be discussed.
Semi-rigid active mirror technology advancements
Gary Matthews, Frank A. Carbone, Patrick Clark, et al.
This paper will discuss the technology demonstrated to date in the development of the Advanced Mirror System Demonstrator (AMSD) program sponsored by NASA, NRO, and the AFRL. Kodak's 8-kg/m2 semi-rigid mirror is pushing the state-of-the-art in the fabrication of ultra-lightweight cored mirrors. Combining the mirror with actuators and a reaction structure will create a 15-kg/m2 primary mirror system. Future plans and milestones for AMSD will also be discussed.
In-process status of the 1.4-m beryllium semi-rigid Advanced Mirror System Demonstrator (AMSD)
Ball Aerospace & Technologies Corp. is currently under contract to design, build, and test a state-of-the-art lightweight beryllium mirror for cryogenic space applications. This Advanced Mirror System Demonstrator (AMSD) has been designed for lightweight, deployable, spaceborne mirror applications. The major components are currently being fabricated and will comprise a lightweighted mirror assembly including a composite reaction structure. The 1.4-m point-to-point hexagon, semi-rigid beryllium mirror will be integrated with the reaction structure, actuators, and flexures to achieve a mirror system capable of ambient and cryogenic (20 to 55K) operation. The mirror prescription is an off-axis asphere of a parent with a 10-m radius of curvature. Presented here is the current status and a summary of the planned optical fabrication and testing. This work is being performed under a contract to Marshall Space Flight Center (MSFC) in Huntsville, AL and is co-sponsored by the USAF and the NRO.
Projects II: Gossamer
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Results for silicon lightweight mirrors (SLMS)
Schafer Corporation is manufacturing silicon lightweight mirrors (SLMS) for both cryogenic and high-energy laser applications. SLMS are quickly and inexpensively super- polishable, stiff and lightweight, have superior properties at cryogenic temperatures, and do not out-gas. This paper presents results of the cryogenic testing of a 6-inch diameter flat SLM at NASA GSFC. Testing was done initially with the SLM integrated into a Schafer designed, Industrieanlagen-Betriebsgesellschaft (IABG) produced carbon fiber reinforced silicon carbide (C/SiC) optical mount and then later, with the SLM attached directly to the cryostat cold finger. The advantage in using a C/SiC optical mount is that the so-called A-3 formulation has a near-perfect CTE match with silicon.
Design manufacture and test of a cryo-stable Offner relay using aluminum foam core optics
Aluminum foam core optics have the desirable characteristics of being lightweight, cryo-stable, and low cost. The availability of high quality aluminum foam and a bare aluminum super-polishing process have allowed high performance foam core optics made entirely of aluminum to be produced. Mirrors with integral mounts were designed for minimum surface error induced by self-weight deflection, thermal gradients, and mounting stresses. The design of the optics was extensively optimized using Finite Element Analysis (FEA) and Geometric Element Analysis (GEA) to determine the effects of design parameters on mirror performance under the anticipated operating environments. A unique manufacturing process was developed to accommodate the aluminum brazing process used to install the aluminum foam while maintaining dimensional stability. Aluminum foam core optics have the additional advantage of being fabricated from a common aerospace structural material. An Offner relay using all aluminum optics and structure will be manufactured and tested with the goal of demonstrating that an all aluminum optical system can be aligned at room temperature and maintain alignment at cryogenic temperatures due to near zero CTE mismatch between all system components. If successful, an all aluminum Offner relay has potential uses for NGST, specifically in the testing of micro-mirror arrays.
Demonstration of a 0.5-m ultralightweight mirror for use at geosynchronous orbit
Dave Baiocchi, James H. Burge, Brian Cuerden
Future space telescopes will require apertures that are larger than the current state of the art, yet fit within the exiting launch restrictions on size and mass. The mass can be reduced by using a thin flexible substrate for the optical surface and a rigid, lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. We have designed, built and tested a 0.5-m demonstration mirror for use in geosynchronous Earth-imaging systems. The mirror has an areal density of 5 kg/m2 and is the lightest mirror we have made using the thin substrate design. This paper discusses the design, fabrication and performance of the 0.5-m mirror.
Manufacturing and evaluation of membrane optical elements for ultralightweight optics
Brian G. Patrick, James D. Moore Jr., Whitt Brantley, et al.
SRS Technologies has made significant strides in the research and development of ultra-lightweight membrane optics for future imaging applications while conducting work with NASA Marshall Space Flight Center and the Air Force Research Lab. Thin film mirrors have been manufactured using surface replication casting of CP1, a polyimide material developed specifically for space applications. In the course of such efforts processing and manufacturing techniques have been developed to produce polyimide membranes with surface roughness below 1.5 nanometers rms and sub-wavelength thickness variation for both curved and flat membranes. This has led to the production of membranes optically flat to (lambda) /13 ((lambda) equals 633 nm) and curved membranes with figure error on the order of microns over half-meter diameters. This accuracy places such membranes within the demonstrated correctable range of several advanced wavefront correction technologies.
Stretched membrane with electrostatic curvature (SMEC) mirrors for extremely large space telescopes
Brian L. Stamper, James Roger P. Angel, James H. Burge, et al.
Very lightweight mirrors can be constructed by stretching a membrane to form a flat surface. Adding tension to the membrane, making it flat, can be done by discrete attachment points, or by using a continuous boundary. Such lightweight mirrors are very attractive for space telescopes where a 100-m aperture can be made up of smaller mirror segments. Adding a slight curvature to each segment simplifies the optical train. This article looks at the making of a curved membrane mirror, and demonstrates its use. Measurements of the flat membrane, and the curved figure will be shown.
Projects III: Large Optics
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Large aspheric optics for high-power, high-energy laser
SAGEM, within its REOSC high performance optics product line, has developed through the years a specific knowledge in large plano, spherical and aspherical optics for high energy or high power laser. This paper is aimed to illustrate the application of aspheric optics for such laser application with several examples of increasing optical surface complexity.
Manufacturing and testing of two off-axis aspherical mirrors
Xuejun Zhang, Zhicheng Weng, Zhongyu Zhang, et al.
In this paper, the manufacturing and testing procedures to make large off-axis aspherical mirrors are presented. The difficulties in polishing and testing for both circular aperture and rectangular aperture mirrors are previewed, and a possible solution is given. The two mirrors have been polished by means of CCOS, the final accuracy is 25-nm rms for 770-mm x 210-mm rectangular mirror and 20-nm rms for (phi) 600-mm circular mirror. These results just meet the optical tolerances specified by the designer, and the manufacturing and testing procedures presented here show good ability to make large off-axis aspherical mirrors.
SOFIA primary mirror fabrication and testing
Roland Geyl, Michel Tarreau, Patrick Plainchamp
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint American-German project dedicated to performing IR astronomy on board a Boeing Aircraft, in near space condition. First flight of the Observatory is planned for 2003. The REOSC Products Unit of SAGEM SA (France) has been contracted by Kayser Threde (Germany) for the design and fabrication of the 2.7-meter diameter, F/1.19 parabolic lightweight SOFIA primary mirror as well as the M3 dichroic and folding mirror assembly. This paper will report the design, fabrication and test of the lightweight primary mirror. The mirror structure has been obtained by machining it out from a solid Zerodur blank. It is the world's largest of this type today. Axial and lateral mirror support system has been conceptually designed and engineered by SAGEM-REOSC engineers in relation with Kayser Threde team. The optical surface is an F/1.19 parabola polished to a high level of quality.
Manufacturing of primary mirrors from Sitall CO-115M for European projects TTL, NOA, and VST
Magomed A. Abdulkadyrov, Sergey P. Belousov, Alexandr N. Ignatov, et al.
JSC LZOS under the contract with Carl Zeiss Jena, Germany produced three 2050 mm (F/3) primary hyperbolic mirrors for TTL project (Telescope Technologies Limited, Great Britain) during 1997-2001. The asphericity is approximately 20 micrometers from the nearest sphere. The telescope field of view is approximately 40 arcmin. 80% encircle energy within less than 0.2 arcsec was achieved from all mirrors. The surface error RMS is less than 9 nm. 2280 mm (F/2.3) primary mirror for NOA project (Astronomical Institute - National Observatory of Athens, Greece) was produced. The asphericity is approximately 40 micrometers . The telescope field of view with corrector is approximately 1.04 degrees. The primary mirror is classical one with 300 mm thickness and mirror diameter to mirror thickness ratio (aspect ratio) of 7.6:1. The primary mirror has 80% encircle energy within less than 0.2 arcsec and surface error RMS less than 9 nm. 2650 mm (F/1.8) primary mirror for VST project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. The asphericity is approximately 100 micrometers . 1.5 degrees telescope field of view with corrector will be achieved. VST primary adaptive mirror is 140 mm meniscus. The aspect ratio is 19:1.
Manufacturing of secondary mirrors from Sitall CO-115M for European projects TTL, NOA, and VST
Alexandr P. Semenov, Magomed A. Abdulkadyrov, Sergey P. Belousov, et al.
JSC LZOS under the contract with Carl Zeiss Jena, Germany produced 645 mm (F/2.5) secondary mirror for TTL project (Telescope Technologies Limited, Great Britain) during 1999- 2001. The asphericity is approximately 12 micrometers from the nearest sphere. The system of primary and secondary mirrors has 80% encircle energy within less than 0.2 arcsec. The surface error RMS is about 9 nm. 753 mm (F/2) secondary mirror for NOA project (Astronomical Institute - National Observatory of Athens, Greece) was produced. The asphericity is approximately 26 micrometers . The surface area RMS is about 12 nm. The telescope field of view is approximately 1.04 degrees. The system of primary and secondary mirrors has 80% encircle energy within less that 0.3 arcsec. 938 mm (F/2.3) secondary mirror for VST project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. The asphericity is approximately 100 micrometers . The telescope field of view with corrector will be 1.5 degrees. Three Hindle sphere 1610 mm, 1640 mm and 1985 mm in diameter with radii of 6300 mm, 3995 mm and 2708 mm were used to test three secondary mirrors. Each convex hyperbolic mirror was tested by using two Hindle spheres. The wavefront of tested mirror was determined by wavefront superposition method.
Fabrication I: Grinding
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Modeling of tool shape evolution in conformal (raster) grinding
Rattaporn Thonggoom, Paul D. Funkenbusch, Sheryl M. Gracewski, et al.
CNC grinding relies on accurate control of the tool shape and position relative to the workpiece. However, tool wear can significantly alter tool shape, potentially producing figure errors. This problem can be particularly important in conformal grinding applications which require the grinding of large areas to optical tolerances and/or the use of relatively small tools (e.g. to grind deep complex shapes). In this study the wear of grinding tools during raster grinding of a conformal component is modeled. The goal is to predict the errors resulting from tool wear and, ultimately, to allow the development of simplified models that can be used to reduce the effects of wear via tool path compensation. In the modeling, wear at each point on the tool is assumed to be proportional to the matching workpiece volumetric removal at that point and, thus, is dependent on the workpiece surface left by the previous raster. An iterative technique is used to predict the tool shape and workpiece surface profile as removal progresses. The effects of process parameters (e.g. raster spacing and tool tilt) are examined. The results are also used in the evaluation and development of a simplified model, which approximates the worn tool shape as a flat bevel.
Development of optimal grinding and polishing tools for aspheric surfaces
James H. Burge, Bill Anderson, Scott Benjamin, et al.
The ability to grind and polish steep aspheric surfaces to high quality is limited by the tools used for working the surface. The optician prefers to use large, stiff tools to get good natural smoothing, avoiding small scale surface errors. This is difficult for steep aspheres because the tools must have sufficient compliance to fit the aspheric surface, yet we wish the tools to be stiff so they wear down high regions on the surface. This paper presents a toolkit for designing optimal tools that provide large scale compliance to fit the aspheric surface, yet maintain small scale stiffness for efficient polishing.
Surface features and residual strains in AlON grinding
Stephen J. Burns, Paul D. Funkenbusch, Sheryl M. Gracewski, et al.
Aluminum Oxynitride (AlON) is a material of interest to the military for a variety of optical applications, including conformal optics and transparent armor. However, its high hardness and large grain size (on the order of 100-200 micrometers s) produced by current powder metallurgy processes present challenges to deterministic microgrinding. For example, typical contact areas between the tool and work surface for contour grinding are on the order of the AlON grain size. Therefore, individual grains often appear in surface relief (orange peel effect) following contour grinding. In addition, small pits, on the order of 10 micrometers diameter and up to a few microns deep have been observed throughout the grain structure after fine grinding with a 2-4 micrometers diamond tool. In this paper, an overview is given of our experience micro-grinding AlON. First, some of the features observed in fine ground AlON surfaces are described in detail. A theory, based on micro-indentation, is presented to explain the generation of the surface pits. Finally, an estimate of the residual surface stresses after grinding, using x-ray diffraction techniques to measure the strains, is presented.
Suitability of igneous rock for precision tooling
Martin J. Valente, James H. Burge, R. DiCenso, et al.
Surface plates and blocking tools are commonly made of granite because of its good stability. But how stable is the granite, and which type of material is optimal? We have explored several materials and manufacturing processes for a 4-m aspheric reference surface that would serve as a tool for laying up composite optics. In this paper, we discuss the materials selection, stability to thermal and moisture effects, and parameters for processing the surface to give sub-micron accuracy and stability.
Manufacturing-induced residual stresses in optical glasses and crystals: Example of residual stress relief by magnetorheological finishing (MRF) in commercial silicon wafers
John C. Lambropoulos, Steven R. Arrasmith, Stephen D. Jacobs, et al.
The Twyman effect refers to the generation by grinding of residual compressive stresses on the surface of a brittle material. Two issues are the correlation of stress-induced deformation (bowing) with the compressive force, and the depth over which the compressive stress acts. We summarize results from optical glasses, and we then apply a new analysis to optical crystals, specifically silicon. The issue of grinding-induced stress depth is addressed by demonstrating the relief of lapping-induced stresses in commercial Si wafers by magnetorheological finishing (MRF). MRF is a novel process that is effective for fine figuring and polishing of a variety of optical glasses and crystals. We demonstrate that MRF is suitable for finishing the surfaces of electronic grade single crystal silicon wafers. Four inch diameter <111> silicon wafers were loose abrasive lapped with various sized abrasives. The lapping- induced stress in the wafer surface was extracted by interferometrically measuring the curvature of the wafer due to the Twyman effect. Subsequent polishing by MRF was found to be effective in removing the associated residual stress generated in the wafer surface during loose abrasive lapping. The lapping-induced residual stresses are largest near the lapped surface, decaying with a characteristic length of 0.4-0.5 micrometers into the lapped surface.
Fabrication II: Polishing
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Quantitative characterization of optical polishing pitch
Jessica E. DeGroote, Stephen D. Jacobs, Leslie L. Gregg, et al.
Optical polishing pitch has properties that can be quantitatively examined. These properties may be used to check batches of pitch for consistency, or to evaluate products for differences. In this work we explore the hardness, softening point and viscosity of pitch. The testing methods involved require little preparation, have quick turnaround time, use less than 200 g of pitch, and produce statistically significant results. The tests include: Shore A Durometer hardness test, ASTM Mettler softening point method, and dynamic viscosity measurements via a novel falling needle viscometer. Results from all three tests are given as averages with standard deviations for a variety of wood-based and petroleum-based products.
Nanometer accurate shaping with fluid jet polishing
Silvia M. Booij, Hedser H. van Brug, Mandeep Singh, et al.
This article describes the Fluid Jet Polishing process. An overview of the theoretical dependence of various important parameters is given. We discuss some results obtained with FJP, including typical material removal rates and roughness values. Some recent experiments are described that show that it is also possible to obtain removal rates as small as one nanometer per minute for glass surfaces. Specific surface profiles are created, both with and without the use of surface protecting masks.
Constellation-X cylinder figuring and polishing studies
Eugene Waluschka, Petar Arsenovic, David A. Content, et al.
In support of Goddard Space Flight Center's (GSFC) Constellation-X mandrel manufacturing effort, a series of fabrication experiments are being performed to determine a best approach, and to supply the project with precision mandrels. Currently, polishing immediately after diamond turning produces a RMS surface roughness of 0.3 nm, on an electroless nickel-plated aluminum mandrel. Studies currently under way will incorporate an abrasive-figuring step to be followed by this polishing operation. The current diamond turning, figuring and polishing procedures will be described and the results presented.
Precision optical asphere fabrication by plasma jet chemical etching (PJCE) and ion beam figuring
Axel Schindler, Georg Boehm, Thomas Haensel, et al.
We develop a Plasma Jet Chemical Etching (PJCE) technique for high rate precision machining of optical materials aiming in a technology mature for precision asphere and free-form surface topology fabrication. The present contribution summarizes the achievements after about twelve months experience with a prototype production tool facility. PJCE is performed with the help of a microwave driven reactive plasma-jet working in a broad pressure range (10-600 mbar). We developed a moveable lightweight microwave plasma jet source for dwell time techniques performed in a roughly pumped process chamber equipped with a six axis system for precision workpiece and plasma source movement. Volume etch rates of some 10 mm3/min have been achieved for fused silica and silicon, respectively, using reactive (CF4,SF6,O2) and inert (Ar,He) gas mixtures and applying a microwave (2.45 GHz) power in the 100-200 W range. Large quartz plates (80-160 mm) have been figured using dwell time methods to achieve aspheric deformations of some 10 micrometers . The figured surfaces show shape errors of 1-2 micrometers and a microroughness of 50-100 nm RMS but no sub-surface damage enabling a small tool shape conserving post polishing up to the sub-nanometer roughness level. Thus, surface shaping to the nanometer error range can be done by ion beam finishing.
Wet-etch figuring: optical surfacing by controlled application of etchant solution using the Marangoni effect
Michael C. Rushford, Jerald A. Britten, Curly R. Hoaglan, et al.
Wet-etch figuring (WEF), a computer-controlled method for generating arbitrarily shaped optical surfaces using wet chemical etching, has been developed. This method uses applicator geometry and surface tension gradients (the Marangoni Effect) to define and confine the footprint of a wetted etchant zone on the surface. Capillary forces attach the flowing etchant solution to the underside of the optic being figured. No mechanical or thermal stresses or residues are applied to the optic by this process. This enables interferometric measurement of the glass thickness while surfacing, which then controls the placement and dwell time of the wetted zone. The result is a truly deterministic, closed-loop figuring process with a high degree of optical precision. This process can figure submillimeter thickness, large-aperture plates or sheets that are very difficult to finish by conventional methods. Automated linear and circular spot etching tools were used to demonstrate surfacing on 380 micron-thick glass sheets, to Strehl better than 0.8, as specified by data array or Zernike polynomials.
Research on the microstructure of ceramic glass and its polished surface quality
Feihu Zhang, Qingliang Zhao, Dagang Xie, et al.
Ceramic glass has been increasingly available in the field of optics and others due to its excellent performance. So the demands of precisely processed surface quality has constantly increased. There is difference in its processing performance with the homogeneous materials because it consists of crystallization phases and glass phases. In this paper the internal structure of ceramic glass is considered. The crystallization phases is polycrystalline structure, the crystal grain is very small, and its main crystallization phases is (beta) -solid solution of quartz. There is remaining glass phases distribution among crystal grains. Tested by X-ray diffraction, the size of crystallite measured is between 300 and 400 angstroms and there is about 20 percent of glass phases. The surface appearance and surface defect of ceramic glass are also presented after cryogenic polishing. Because ceramic glass is constituted with crystallite, which is free-orientation, it is not compact compared with K9 glass and fused quartz. So the super-smooth surface of the ceramic glass is not as good as that of the compact material.
Fabrication II: MRF and Micro-Optics
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Exploring anisotropy in removal rate for single crystal sapphire using MRF
Irina A. Kozhinova, Steven R. Arrasmith, John C. Lambropoulos, et al.
Magnetorheological (MR) fluids with two types of abrasives (diamond and alumina/spinel) were used to study anisotropy in removal rate for C-cut single crystal sapphire. Interferometrically flat, basal plates (0001) employed in the experiments were characterized by different degree of C- axis (small) tilt from normal. The removal rate anisotropy depends on the type of abrasive, with anisotropy being more pronounced for the alumina/spinel abrasive. The anisotropy exhibited 2-fold symmetry, with the C-axis lying in the plane of symmetry. Roughness was found to depend on the basal plate orientation and the type of abrasive used. Diamonds improved the initial surface roughness of a polished plate regardless of orientation, while alumina/spinel abrasives increased the roughness, especially in the down-the-steps direction of fluid flow with respect to basal plane inclination. The results of this polishing experiment are in agreement with earlier studies of anisotropy observed in wear experiments on spherical surfaces of single crystal sapphire along different crystallographic orientations.
Use of magnetorheological finishing (MRF) to relieve residual stress and subsurface damage on lapped semiconductor silicon wafers
Steven R. Arrasmith, Stephen D. Jacobs, John C. Lambropoulos, et al.
Magnetorheological finishing (MRF) is a novel process demonstrated to be effective for fine figure control and polishing of a variety of optical glasses and crystals. This paper discusses the use of MRF to stress relieve the surfaces of single crystal silicon wafers, of the type used in the semiconductor industry to fabricate integrated circuits. One hundred-mm diameter silicon wafers with a <111> crystallographic orientation were loose abrasive lapped with three different sizes of alumina abrasive to introduce compressive surface stress. The stress generated in the wafer surface was characterized by interferometrically monitoring the bending of the wafer due to the Twyman effect. The thickness of the subsurface damage (SSD) layer was characterized using a dimpling method with a fixture developed at COM. Subsequent polishing by MRF was found to be effective in removing the subsurface damage and associated residual stress generated in the wafer surface during loose abrasive lapping.
Fabrication of integrated diffractive micro-optics for MEMS applications
Larry R. Senesac, R. H. Farahi, James L. Corbeil, et al.
We investigated the fabrication of integrated diffractive micro-optical features on MEMS structures for the purpose of motion detection. The process of producing the diffractive features and the MEMS structures by focused ion beam milling is described in detail, as is the ion beam sputtering process used to produce coatings on these structures. The diffractive features of the circular Fresnel zone plate (FZP) and spiral FZP were fabricated on MEMS structures and the relevant diffraction theory is discussed. The spiral FZP diffractive features produced well defined foci whose intensity varies with distance from the FZP. Observation of these intensity variations enable us to detect the motion of the MEMS structure, and the resulting device was used to scan an IR image of a hot object.
Scanning electron and atomic force microscopy measurements of ion beam etched InP samples using Ar/H2 chemistry
Bulent Cakmak, Richard V. Penty, Ian H. White
This paper reports scanning electron (SEM) and atomic force microscopy (AFM) results of ion beam (IBE) and chemically assisted ion-beam etched (CAIBE) InP wafers. While Argon (Ar) alone is used for the IBE process, the CAIBE is carried out by using Ar/H2/CH4 or Ar/H2 gases only. The evolution of the surface roughness and morphology is presented comparatively by varying acceleration voltage (Vacc), discharge current (Idis) and ion incidence angle. A drastic improvement of the surface roughness is obtained for the CAIBE using Ar/H2 chemistry and verified by atomic force microscopy measurements. The anisotropy of InP samples is also presented for two different masks; Al2O3 and Titanium (Ti) in the case of CAIBE mode. The most anisotropic structure of 83 degrees is performed by using the Ti mask. Finally, by using atomic force microscopy technique the lowest rms roughness of 4.3 is found in the case of using only Ar/H2 gasses.
Testing I: Profilometry
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Self-referencing, motion-insensitive approach for absolute aspheric profiling of large optics to the nanometer level and beyond
We describe a new noncontacting approach for obtaining the full aperture, absolute aspheric profile of large optical surfaces. The metrology instrument is placed in close proximity to the test piece instead of at the center of curvature, and is thus equally useful for measuring concave, flat, and convex optics - even fast (low f-number) optics. It combines the data from multiple probes in a manner that makes the measurement completely self-referencing, and completely insensitive to any small relative rigid body motions between the instrument and the test piece. The relative compactness of the instrument combined with its inherent rigid body insensitivity make it suitable ultimately for in situ measurements. Furthermore, replacement of the noncontacting optical probes with contacting mechanical probes would make the instrument suitable for profiling ground surfaces to a very small fraction of a micron. We have built a prototype instrument to prove the concept, and have demonstrated sub-nanometer capabilities for the optical probes, with full surface figure accuracy capabilities of a few nanometers in an uncontrolled thermal environment. The full surface figure accuracy is improving as we implement modest environmental controls. In this paper, we first describe the underlying theory of the measurement approach, and then describe the prototype instrument. Finally, we summarize the measurements made to date, and discuss likely future applications and projected accuracies.
Analysis of the uncertainty of the ultraprecise large-area curvature scanning technique for measuring steep aspheres and complex surfaces
Ingolf Weingaertner, Michael Schulz
Steep aspheres and general complex optical surfaces are of great importance for many optical technologies and necessary for many high-technology applications where nanometer and sub-nanometer accuracy is indispensable. An example of such an application is lithography in the DUV, VUV or EUV region of the electromagnetic spectrum. The problem of measuring the topography with this high accuracy has not been solved in general. Recently, a particular measurement principle has been developed and investigated. It is referred to as high- resolution large-area curvature scanning, and the topography is determined by mathematical calculations on the basis of the information available about the curvature. It focuses on the principles of the traceability and the avoidance of error influences and is intended for determining the figure of steep aspheres and complex surfaces with ultra-precision. An uncertainty budget will be presented for the method and the facility. Special emphasis will be put on the different principles of intrinsic two-dimensional methods, in contrast to scanning methods, external references and their influences, errors of scanning stages and their influences, whole-body movements of artifact and their influences, the properties of the measurement signal, lateral and vertical resolution of the detector, long-term stability of the facility, etc.
Applicability of iTIRM for roughness reduction monitoring
Robert-Jaap van der Bijl, Hedser H. van Brug, Oliver W. Faehnle, et al.
The results obtained with iTIRM during polishing are presented. It is shown that iTIRM unites the working ranges of several other techniques where iTIRM can be used during production where the others cannot. The applicable range of iTIRM is shown to be at least 1 micrometers down to 0.1 nm rms.
New approach for testing of aspherical micro-optics with high numerical aperture
Volker Kebbel, Hans-Juergen Hartmann, Werner P. O. Jueptner
The collimation of strongly diverging laser beams emitted by diode lasers is performed with aspherical micro-optical components. In order to obtain a good beam profile high- quality micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is not suitable, costly or inaccurate with respect to the required accuracy of the lens shape. In this paper Digital Holography as a measurement tool for the characterization of micro-optical components is presented, which has some advantageous properties with respect to other interferometric techniques. Results of the characterization of single cylindrical microlenses as well as microlens-arrays used for laser beam collimation and shaping are presented.
Fast surface profiler by white-light interferometry using a new algorithm, the SEST algorithm
We devise a fast algorithm for surface profiling by white- light interferometry. It is named the SEST algorithm after Square Envelope function estimation by Sampling Theory. Conventional methods for surface profiling by white-light interferometry based their foundation on digital signal processing technique, which is used as an approximation of continuous signal processing. Hence, these methods require narrow sampling intervals to achieve good approximation accuracy. In this paper, we introduce a totally novel approach using sampling theory. That is, we provide a generalized sampling theorem that reconstructs a square envelope function of a white-light interference fringe from sampled values of the interference fringe. A sampling interval in the SEST algorithm is 6-14 times wider than those of conventional methods when an optical filter of the center wavelength 600 nm and the bandwidth 60 nm is used. The SEST algorithm has been installed in a commercial system which achieved the world's fastest scanning speed of 42.75 micrometers /s. The height resolution of the system lies in the order of 10 nm for a measurement range of greater than 100 micrometers .
Testing II: Stitching and Mid-Spatial Frequency
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Stitching interferometry and absolute surface shape metrology: similarities
Stitching interferometry is a method of analysing large optical components using a standard small interferometer. This result is obtained by taking multiple overlapping images of the large component, and numerically stitching these sub-apertures together by computing a correcting Tip- Tilt-Piston correction for each sub-aperture. All real-life measurement techniques require a calibration phase. By definition, a perfect surface does not exist. Methods abound for the accurate measurement of diameters (viz., the Three Flat Test). However, we need total surface knowledge of the reference surface, because the stitched overlap areas will suffer from the slightest deformation. One must not be induced into thinking that Stitching is the cause of this error: it simply highlights the lack of absolute knowledge of the reference surface, or the lack of adequate thermal control, issues which are often sidetracked... The goal of this paper is to highlight the above-mentioned calibration problems in interferometry in general, and in stitching interferometry in particular, and show how stitching hardware and software can be conveniently used to provide the required absolute surface shape metrology. Some measurement figures will illustrate this article.
Correlation of test data from some NIF small optical components
Robert Chow, Michael S. McBurney, William K. Eickelberg, et al.
The NIF injection laser system requires over 8000 precision optical components. Two special requirements for such optics are wavefront and laser damage threshold. Wavefront gradient is an important specification on the NIF ILS optics. The gradient affects the spot size and, in the second order, the contrast ratio of the laser beam. Wavefront errors are specified in terms of peak-to-valley, rms, and rms gradient, with filtering requirements. Typical values are lambda/8 PV, lambda/30 rms, and lambda/30/cm rms gradient determined after filtering for spatial periods greater than 2 mm. One objective of this study is to determine whether commercial software supplied with common phase measuring interferometers can filter, perform the gradient analysis, and produce numbers comparable to that by CVOS, the LLNL wavefront analysis application. Laser survivability of optics is another important specification for the operational longevity of the laser system. Another objective of this study is to find alternate laser damage test facilities.
Use of beam parameters in optical component testing
Daniel R. Neal, James K. Gruetzner, James P. Roller
We are investigating the use of a Shack-Hartmann wavefront sensor for measuring optical component quality during manufacture and testing. In a variety of fields, an optical component is designed to pass an optical signal with minimal distortion. Quality control during the manufacturing and production process is a significant concern. Changes in beam parameters, such as RMS wavefront deviation, or the beam quality parameter M2, have been considered as indications of optical component quality. These characteristics can often be quickly determined using relatively simple algorithms and system layouts. A laboratory system has been prepared to investigate the use of a wavefront sensor to measure the quality of an optical component. The instrument provides a simultaneous measure of changes in M2 and induced RMS wavefront error. The results of the investigation are presented.
Testing III: Interferometry
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Interferometric measurement of refractive index inhomogeneity on polished sapphire substrates: application to LIGO-II
Bozenko F. Oreb, Achim J. Leistner, GariLynn Billingsley, et al.
In order to improve the detection sensitivity of the Laser Interferometer Gravitational-wave Observatory (LIGO) the use of 40-kg sapphire test masses is being considered for the next instrument upgrade. Currently, sapphire material of adequate size is only available with the optical axis aligned with the m axis of the crystal. To determine the material's suitability it is necessary to characterize the refractive index inhomogeneity of the sapphire substrates for two orthogonal directions of polarisation, to a fraction of a part per million (ppm). We report on a method used to measure the refractive index inhomogeneity which requires three separate measurements of the polished sapphire blank in a Fizeau interferometer. These measurements are of the surface shapes or figures of the two polished sides of the blank and that of the wavefront entering side one propagating through the blank, reflected off side two and exiting through side one. The phase maps corresponding to these three measurements are combined to obtain the refractive index inhomogeneity map distribution. Measurements were carried out on two sapphire substrates (m axis) produced by the heat exchange method. The inhomogeneity maps show features which depend on polarisation direction. The physical origin of the inhomogeneities is discussed as well as the probable impact on the detection of a gravitational wave signal.
Multiple-surface phase-shifting interferometry
I show how phase shifting interferometry can be extended to account for multiple interference effects using a Fourier based analysis technique combined with wavelength tuning and a particular four-surface interferometer geometry. The technique is demonstrated by simultaneously measuring both surface profiles, the optical thickness variation and index homogeneity of a parallel plate. In addition, unlike traditional phase shifting techniques, the linear component of the homogeneity can be measured with high precision. It is shown that a significant linear component exists in a commercially supplied flat.
Uncertainties in interferometric measurements of radius of curvature
The radius of curvature of spherical surfaces may be determined using the well-known radius, or optical, bench. In this method, a figure measuring interferometer is employed to identify the null positions at the center of curvature (confocal) and surface (cat's eye) of the test optic. A linear slide provides motion between these positions and one or more displacement transducers is used to record the displacement between the cat's eye and confocal positions and, hence, the radius of curvature. Measurements of a polished Zerodur sphere have been completed on the X-ray Optics Calibration Interferometer (XCALIBIR) using both Twyman-Green and Fizeau configurations. Mechanical measurements of the spherical artifact have also been completed using a coordinate measuring machine (CMM). Recorded disagreement between the individual transmission sphere measurements and CMM measurements under well-controlled environmental conditions is larger than the limits predicted from a traditional uncertainty analysis based on a geometric measurement model. Additional uncertainty sources for the geometric model, as well as a physical optics model of the propagation of light, are therefore suggested. The expanded uncertainty analysis is described.
High-precision 2D angle measurement using fringe analysis techniques
Zongtao Ge, Takayuki Saito, Shinichi Matsuda, et al.
In this paper, a novel angle measurement method using the fringe analysis techniques is proposed. In this method, a 2D surface profile including a tilt is obtained by fringe analysis, and the tilt in the 2D surface profile is determined by fitting the obtained surface profile with a plane. From the fitted plane, 2D angles can be easily obtained. Simulations using the Fourier transform and phase- shift methods are performed, and the results are compared with each other. Simulations show that 2D angle measurement using the fringe analysis techniques can achieve simultaneously a wide measurement range and high precision.
Joint Session
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Thermo-mechanical performance of precision C/SiC mounts
William A. Goodman, Claus E. Mueller, Marc T. Jacoby, et al.
For complex shaped, lightweight, high precision opto- mechanical structures that must operate in adverse environments and over wide ranges of temperature, we consider IABG's optical grade silicon carbide composite ceramic (C/SiC) as the material of choice. C/SiC employs conventional NC machining/milling equipment to rapidly fabricate near-net shape parts, providing substantial schedule, cost, and risk savings for high precision components. Unlike powder based SiC ceramics, C/SiC does not experience significant shrinkage during processing, nor does it suffer from incomplete densification. If required, e.g. for large-size components, a fully-monolithic ceramic joining technique can be applied. Generally, the thermal and mechanical properties of C/SiC are tunable in certain ranges by modifying certain process steps. This paper focuses on the thermo-mechanical performance of new, high precision mounts designed by Schafer Corporation and manufactured by IABG. The mounts were manufactured using standard optical grade C/SiC (formulation internally called A-3). The A-3 formulation has a near-perfect CTE match with silicon, making it the ideal material to athermally support Schafer produced Silicon Lightweight Mirrors (SLMs) that will operate in a cryogenic environment. Corresponding thermo- mechanical testing and analysis is presented in this manuscript.
Measurement of the dynamic deformation of a high-frequency scanning mirror using a Shack-Hartmann wavefront sensor
Margaret K. Brown, Timothy J. Gong, Daniel R. Neal, et al.
Scanning mirrors for micro-display systems typically require operation at frequencies over 15 kHz. These mirrors undergo large dynamic stresses and inertia related deformations. We report here on the measurement of these dynamic deformations using a commercially available Shack-Hartmann wavefront sensor with data reduction software. The measured deformations using the Shack-Hartmann wavefront sensor are shown to agree with measurements obtained using a stroboscopic interferometer. Advantages of the Shack- Hartmann wavefront sensor are discussed.
Fabrication I: Grinding
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Ultraprecision grinding machining of optical aspheric surface in ductile mode
Mingjun Chen, Feihu Zhang, Qingliang Zhao, et al.
In this paper, in order to grind optical aspheric surfaces with high quality and high precision, some factors that influence the roughness and profile accuracy of machined surfaces were theoretically analyzed. At the same time, all kinds of parameters of ultra-precision grinding optical aspheric surface in ductile mode were optimized. Afterwards author developed the ultra-precision aspheric grinding system. Its principal axis of the workpiece, traverse guide, longitudinal guide and principal axis of the grinder were aero-static bearing form. Turning accuracy of principal axis of the workpiece was 0.05 micrometers . The highest rotate speed of the grinder was 80000 rev/min. Its turning accuracy was 0.1 micrometers . The resolution of linear displacement of the traverse and longitudinal guide was 4.9 nm. Micro-adjusting accuracy of the center high micro-adjusting machine of the grinder was 0.1 micrometers . Finally, we performed grinding aspheric surface experiments on this grinding system. The results show that to obtain high accuracy and high quality aspheric surface, the mean size of grains of diamond wheels should be smaller than 10 micrometers , and also the high speed of the wheel and small feed rate are needed. After optimizing these grinding parameters, the final machined aspheric profile accuracy can reach 0.4 micrometers and surface roughness can be less than 0.01 micrometers .
Effects of diamond cutting tool's rake angle and edge radius on the diamond turned surface quality
Qingliang Zhao, Shen Dong, Ying-Chun Liang, et al.
For the purpose of realizing the brittle material's turning in ductile mode on the basis of optimizing the diamond cutting tool's geometrical parameters, the Linear Elastic Fracture Mechanics and Finite Element Method are applied to stimulate the stress distribution and micro-cracks' propagation in the cutting region generated under different rake angles and edge radius. The cutting experiments on single crystal silicon surface are then conducted to verify the stimulation results, which show that the propagation of micro-cracks can be restrained from atomic-size cracks when utilizing the diamond cutting tools with -15 degree to -25 degree rake angles. As a result of increasing the critical depth of cut value of brittle-ductile transition, the goal of ductile-mode turning can be achieved. In addition, with a smaller cutting edge radius, the turning of brittle material in ductile-mode can be easily realized, resulting in good diamond turned surface quality.
Testing I: Profilometry
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Calibration of a vertical-scan long trace profiler at MSFC
The long trace profiler (LTP) is the instrument of choice for the surface figure measurement of grazing incidence mirrors. The modification of conventional LTP, the vertical- scan LTP, capable of measuring the surface figure of replicated shell mirrors is now in operation at Marshall Space Flight Center. A few sources of systematic error for vertical-scan LTP are discussed. Status of systematic error reduction is reported.
Design and application of Dall compensator for null testing of large-aperture aspherical surface and convex lens
Fan Wu, Jianguan Tang
According to the third order aberration theory, the calculating formulae of original geometrical structural parameters of Dall compensator for null testing of large aperture aspherical surface and convex surface of lens are derived. The computer optimum design method is discussed, and some design examples are given.
Testing II: Stitching and Mid-Spatial Frequency
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Analysis of Hartmann testing techniques for large-sized optics
The quality control of large-sized astronomical optics frequently is produced by a Hartmann technique. For check of accuracy and efficiency of the various schemes of this method it is necessary to create the mathematical model. In this paper, the results of microlens array based sensor simulation and comparison of it with other modifications of the testing schemes are presented.
Fabrication II: Polishing
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First aspheric form and texture results from a production machine embodying the precession process
David D. Walker, David Brooks, Richard Freeman, et al.
We report on progress developing the Precession Process, that has recently been embodied for the first time in a fully-productionised aspheric polishing machine. We describe how the process uses inflated polishing tools of continuously-variable size and hardness. Despite the rapid tool rotation needed to give high removal rates, the method produce well-behaved and near-Gaussian tool influence functions, by virtue of the precession of the spin axis. We then describe how form errors are controlled. The method takes influence-function data and an error map as input, together with, uniquely, weighting factors for height and slope residuals and process time. A numerical optimisation of the cost function with variable dwell time, tool path and tool size is then performed. The advantages of this new technique are contrasted with conventional deconvolution methods. Results of form control on aspheric surfaces are presented, with an interpretation in terms of spatial frequencies. We draw particular attention to control of form at the centre and periphery of a workpiece. Finally, we describe how Precession processing gives multi- directional rubbing of surfaces, and we present the superb texture achieved on samples.
Joint Session
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Silicon carbide high performance optics: a cost-effective, flexible fabrication process
John M. Casstevens, Abuagela Rashed, Ronald Plummer, et al.
Silicon carbide may well be the best known material for the manufacture of high performance optical components. This material offers many advantages over glasses and metals that have historically been used in high performance optical systems. A combination of extremely high specific stiffness (E/r), high thermal conductivity and outstanding dimensional stability make silicon carbide superior overall to beryllium and low-expansion glass ceramics. A major impediment to wide use of silicon carbide in optical systems has been the cost associated with preliminary shaping and final finishing of silicon carbide. Because silicon carbide is an extremely hard and strong material, precision machining can only be done with expensive diamond tooling on very stiff high quality machine tools. Near-net-shape slip casting of silicon carbide can greatly reduce the cost of silicon carbide mirror substrates but this process still requires significant diamond grinding of the cast components. The process described here begins by machining the component from all special type of graphite. This graphite can rapidly be machined with conventional multi-axis CNC machine tools to achieve any level of complexity and lightweighting required. The graphite is then directly converted completely to silicon carbide with very small and very predictable dimensional change. After conversion to silicon carbide the optical surface is coated with very fine grain CVD silicon carbide which is easily polished to extreme smoothness. Details of the fabrication process are described and photos and performance specifications of an eight-inch elliptical demonstration mirror are provided.