Optical systems come in many shapes and sizes. Each system must perform in an environment that imposes unique constraints when combined with the operating system dynamics and other requirements. These constraints make the choice of materials a not so simple task.
An overview of the available choices in advanced materials, with an emphasis on system compatibility and dimensional stability, is presented. Materials covered include: metals, glasses and glass ceramics, composites including metal and polymer matrix materials, and plated nickel and aluminum coatings. Refractive materials have not been included, the emphasis being on mirror systems. Properties comparisons are made and fabrication methods briefly discussed. There is never a material that meets all the requirements for a particular application. This paper, together with the others in this volume, provides guidelines for selecting the most suitable material or combination of materials for almost any optical system.
Applications of materials in white-light optical imaging, remote-sensing systems are discussed. Image formation in terms of wavefronts and the influence of materials on the quality of images is given. The rationale for why some space optics structures are large is presented. Other topics are applications of adaptive spectrometers, adaptive optics, and the control of unwanted radiation. Optical materials limit the next-generation high-performance optical systems.
This paper presents an experimental database of the optical, electrical, physical, and structural properties for as-grown samples from large diameter silicon boules.1 Fourier Transform Infrared spectroscopy (FTIR) was utilized to evaluate the oxygen content and infrared absorbance in the HF laser bandwidth (2.6-3 microns). The bulk absorption coefficient over this bandwidth was quantified by performing laser absorption calorimetry. Electrical properties were obtained by performing Hall measurements at room temperature and 77 K, and the carrier concentration and impurity type were determined. The thermal conductivity was measured directly utilizing the Fourier technique, and the coefficient of thermal expansion was determined for room temperature to 600 °C via dilatometer measurements. Some of these procedures were repeated after the samples had been defect engineered, i.e., given a high temperature heat treatment, followed by a rapid quench.
First and foremost, an optical material must be able to be cost effectively made to the desired size and shape and then be polished to optical tolerances. Over the past 15 years, reaction bonded silicon carbide has been scaled from 2 inches to 1.2 meters, improved in finish from 100 Å to 10 Å and been reduced in areal density from 40 kg/m2 to 10 kg/m2. Its low thermal distortion, high stiffness, high optical quality, and its dimensional stability make CERAFORM SiC ideal for applications such as high energy laser mirrors, space-borne cryogenic mirrors, fast response scan mirrors, and high heat flux applications. In addition CERAFORM SiC products are fabricated using a cost-effective, net shape, fugitive core casting process which can be used to make complex, open or closed back, lightweight substrates. Silicon carbide is unique in that it is competitive with beryllium as a structural material, glass as an optical material, and Invar or graphite-epoxy as a metering material. This paper details the progress that has been made towards scaling facilities and optics to 2-meters and 2 Kg/m2 areal density.
The fabrication process, properties and optics applications of transparent and opaque chemical vapor deposited (CVD) (3-SiC are reviewed. CVD-SiC is produced by the pyrolysis of methyltrichlorosilane, in excess H2, in a low-pressure CVD reactor. The CVD process has been successfully scaled to produce monolithic SiC parts of diameter upto 1.5-m and thickness 2.5-cm. The characterization of CVD-SiC for important physical, optical, mechanical and thermal properties indicates that it is a superior material for optics applications. Important properties of CVD-SiC are compared with those of the other candidate mirror and window materials. The applications of CVDSiC for lightweight optics, x-ray telescopes, optical baffles, lens molds, optical standards and windows and domes are discussed in detail.
The zero expansion titania-silica binary glass ULE™ offers an optimum combination of thermal, mechanical, and optical properties which make it an ideal material for precision optical structures and lightweight telescope mirrors. Its near-zero thermal expansion over the ambient operating temperature range helps preserve optical figure; the absence of hysteresis in its thermal expansion curve ensures dimensional stability of the mirror in extreme environments; its low density affords high specific stiffness thereby reducing elastic deformation of lightweight structures; its high fatigue resistance permits higher long-term stress without compromising mechanical reliability; and its excellent optical and birefringence properties facilitate inspection and quality assurance in terms of thermal expansion homogeneity, defect level, and residual strain.
This paper reviews the key physical properties of ULE glass and the various fabrication techniques available for making lightweight mirrors. Special emphasis is given to ULE's mechanical behavior which controls the long-term reliability of mirror blanks during fabrication, shipping, installation, and operation. It describes the 8-meter class mirror blank manufacturing process and the stress/time histories to which such blanks are exposed. These parameters, together with strength and fatigue data, were used to optimize surface finish for adequate strength and to evaluate an 8-meter blank support system to ensure long-term reliability of the blank during transportation. The paper concludes with recent advances and development programs in support of the manufacture of precision optical mirror blanks.
Sapphire has been used for many optical applications. However, smaller sizes have been used, even though the Heat Exchanger Method (HEM) has produced 20 cm diameter crystals. New generation systems require outstanding optical properties, high strength, and abrasion and thermal shock resistance. Therefore, the choice is limited to sapphire. Crystals up to 34 cm diameter, 65 kg have been grown by HEM, and it is planned to scale up the size to 50 cm diameter. In addition to larger size, the optical quality has been improved to cover the vacuum ultraviolet (VUV) and the near infrared wavelengths. Fabrication technology was advanced to fabricate larger size, higher precision optics cost effectively. Improved transmitted wavefronts and higher quality surfaces have been produced to address current applications.
The objective of this paper is to provide an overview of ultralightweight composite mirror technology. The overview includes a description of the technology, differences between traditional and composite designs, significant industry-wide, demonstrations of the technology based on available literature 1-18, and a projection for future applications. The emergence of composite designs provides exciting potential for nontraditional, accurate, lightweight, stable, stiff, and high strength composite mirrors, such as those shown in Figure 1. This evolving technology promises significant improvement in reducing weight, cost and cycle time for future infrared, visible, and x-ray systems. Customers currently embracing composite mirror technology for radiometric use are already reaping substantial system performance benefits. Other customers interested in lidar, IR, visible, and grazing incidence x-ray applications are eagerly awaiting successful completion of current technology development and demonstration efforts.
Electroless nickel (EN) is a nickel-phosphorus alloy which is currently used in the precision optics industry as a polishable surface layer on difficult-to-finish mirror substrate materials, such as beryllium. High performance applications for such reflective components range from commercial laboratory instruments to large spacebome telescopes to cryogenically cooled optical sensor systems. This paper presents a discussion of the fundamental processing parameters and control methodologies for the electrochemical growth process as well as the performance of the resulting EN coating. The coating performance evaluation will focus specifically on the influence of processing parameters on the EN composition and microstructure and the correlation between composition and critical material properties, such as thermal expansion, elastic modulus, density and hardness.
This paper reports on current manufacturing and polishing characteristics of sputtered and optical-grade bare beryllium. The Ball Aerospace and Technologies Corp. has evaluated flight optics and samples which have determined some of the current characteristics and capabilities of these beryllium materials. The capability of some optical component manufacturing companies to produce optics from bare, sputtered, and electroless nickel-plated beryllium has also been evaluated. A variety of optics and test samples were made, including polished bare beryllium, beryllium-sputtered, and electroless nickel-plated beryllium components. Two types of sputtered beryllium were evaluated, and several component manufacturing houses were used to polish the sputtered beryllium. The results from new technologies for polishing bare beryllium have been assessed.
The paper will provide a general overview of success and limitations of the manufacturing, polishing, and figuring of the above materials. The current status of spherical powder beryllium will also be discussed. The manufacturing techniques for creating high quality bare beryllium, sputtered beryllium and electroless nickel-plated beryllium optics will be presented. New testing methods for evaluating a test sample will also be discussed.
A newly developed family of Aluminum Beryllium (AlBeMet®) metal matrix composite materials has been developed for use in. satellite, structures to address the needs of the designer for lightweight, stiff, thermally stable structures. This paper will present a overview of the development of these metal matrix composites materials and their use in satellite structures.
Lightweight and high modulus Aluminum - Beryllium composites offer significant performance advantages over traditional aluminum and organic composite materials. Aluminum- Beryllium composites also can be fabricated using conventional aluminum machining, joining, and coating technologies thereby reducing the cost of the final assembly and eliminating any special tooling or non destructive testing(NDT) that is sometimes required when designing and fabricating structures out of fiber reinforced composites.
This paper will present the thermal, physical, and mechanical properties of these composites, as well as providing structural test data for satellite components that have utilized aluminum - Beryllium materials, such as the ORBCOMMsm 1 satellites.
Optical polishing with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the newest process is called magnetorheological finishing (MRF). In MRF, all of the process parameters are controlled by utilizing the state of hydrodynamic flow of a magnetically stiffened magnetorheological abrasive fluid through a converging gap formed by a lens workpiece surface and a moving wall. The shear flow of “plastic” MR fluid results in the development of high stresses in the interface zone and material removal over a portion of the workpiece surface, referred to as the “polishing spot”. The polishing spot is an abrasive-charged, sub-aperture lap that automatically conforms to the local shape of the lens surface. Deterministic finishing is accomplished by mounting a lens on a rotating spindle and sweeping it through the MR fluid with a computer numerical controlled (CNC) machine. A computer program generates both a dwell time schedule for the MRF machine and an accurate prediction of finished surface shape, using a material removal function and initial surface condition information as input. In this paper, we describe the MRF process, a preliminary theory of material removal, properties of the MR fluid, machine configurations, software for finishing, and finishing experiments on a variety of surface shapes (spherical, flat, aspheres) and materials of interest to optics manufacturing. Advantages and current limitations to the process are also described.
The Infrared Technology Testbed Telescope (1T1T) is a demonstration telescope meeting the needs of the SIRTF mission. It is a Ritchey-Cretien form designed for diffraction limited performance at 6.5 pm, at 5.5 K with an 85 cm. clear aperture. The mirror and system focal ratios are f/1.2 and f/12 respectively. This paper describes the design and fabrication of the efficient, ultra-lightweight, all-beryllium telescope. The design incorporates a central metering tower and single arch primary mirror to achieve a total telescope mass of less than 30 kg. Cryogenic testing of the primary mirror demonstrates the stability of the I-70-H (special) Be and the fabrication process. No thermal hysteresis was observed after repeated cycling to 5 K, and cryo-null figuring was utilized to overcome the small thermal instability observed at that temperature.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument is the key facility instrument on board the NASA-GSFC “Mission-to-Planet Earth” EOS-AM spacecraft. This instrument is designed to study Earth system processes and includes 36 spectral bands for study of oceanographic, atmospheric, and land surface phenomenon. Launch of this 705 km high, polar orbiting platform, mounted atop an Atlas IIAS, is scheduled for June, 1998 from Vandenberg AEB, California. The MODIS Protoflight instrument has been delivered to Lockheed Martin Marietta’s Valley Forge facility for spacecraft integration and testing. The primary structure of the MODIS instrument is the “mainframe” and this paper discusses the geometrical design, material selection and processes, static and dynamic analyses and environmental testing required to ensure spaceflight reliability. Comprehensive studies of candidate materials led to the selection of beryllium made by the “Hot Isostatic Process”.
This paper relates the development undertaken to produce the first unit of the European ESO VLT Secondary Mirror in Beryllium. The specification will be presented with the historical evolution of the mirror substrate material choice. Beryllium material quality will be discussed regarding the long term stability and mirror design will be presented. An optical test set-up has been specifically designed and fabricated for the mirror performance verification. Blank fabrication and polishing is reported with the key step of external edge lip cutting to achieve coincidence between mechanical diameter and clear aperture as required for I.R astronomical observations. Mirror long term stability is assessed from data gained during fabrication.
We consider the materials choices available for making optical substrates for synchrotron radiation beam lines. We find that currently the optical surfaces can only be polished to the required finish in fused silica and other glasses, silicon, CVD silicon carbide, electroless nickel and 17-4 PH stainless steel. Substrates must therefore be made of one of these materials or of a metal that can be coated with electroless nickel. In the context of material choices for mirrors we explore the issues of dimensional stability, polishing, bending, cooling, and manufacturing strategy. We conclude that metals are best from an engineering and cost standpoint while the ceramics are best from a polishing standpoint. We then give discussions of specific materials as follows: silicon carbide, silicon, electroless nickel, Glidcop, aluminum, precipitation-hardening stainless steel, mild steel, invar and superinvar. Finally we summarize conclusions and propose ideas for further research.
Traditionally, high-energy lasers have used optics which have been actively cooled, primarily with temperature-conditioned, deionized water. The Ballistic Missile Defense Organization (BMDO) has successfully developed Very Low Absorptance (VLA) coatings for optics for Hydrogen-Fluoride (HF) lasers. These coatings have produced the next generation in high energy optics: uncooled optics. This paper addresses the successful transfer of this technology to Deuterium-Fluoride (DF) lasers. Both analytical predictions and experimental results are presented.