An overview of refractometry at The National Institute of Standards and Technology (NIST) from the turn of the century until 1980 is presented as an introduction and comparison to the needs for optical-material characterization in the 1990s. The current needs, as expressed by the developers and manufacturers of transmissive-optical materials and designers of high- resolution optical systems, are reviewed. The feasibility of meeting these needs is discussed.
The development of advanced infrared optical materials has necessitated a growing requirement for refined optical and mechanical evaluation methodologies, as well as a need for an accurate, updated, optical and mechanical properties data based. Driving this need are new material fabrication and finishing techniques--often substantially different from past methods-- that can lead to optical and material properties data that is significantly at variance with data contained in the current infrared materials data based. In addition, both new and well established measurement methodologies are being applied to measurement problems that are qualitatively different from that for which they were originally developed. An overview of an optical and mechanical reliability evaluation methodology, along with representative data, is discussed.
Large sapphire single crystals with the geometry of near net shape domes are grown by the gradient solidification method (GSM). The growth is carried out in double-wall molybdenum crucibles, and perfect crystals, free of grain boundaries and scattering centers, are produced. The process is used to grow sapphire domes with different crystallographic orientations, including the optical axis. This process significantly shortens the fabrication process of the dome, thus leading to a cheaper, high-quality, product.
During recent years there has been an increasing demand for large homogeneous Ge single crystals to be used as optical components in high resolution thermal imaging systems. Thus, the authors' research focused on understanding the roles of dopant and stress distribution in large Ge crystals and their influence on the optical performance in the IR region, 8-12 micrometers . More recently, a new application for heavily dope, n-type Ge crystals with low resistivity (~0.1 Ω.cm) and high crystalline perfection (EPD ~5 X 103cm-2) has been reported. This paper presents the growth and characterization of large homogeneous Ge single crystals with diameters up to 240 mm for IR optics. Preliminary results on the growth of 75 mm diameter Ge single crystals for substrates preparation are given. These substrates can be used in GaAs solar cells for space applications.
The few coupling materials usable as optical bonding agents in the UV and IR are reviewed. The need for suitable optical cements for the wavelength regions below 250 nm and in the window bands above 3 micrometers still exists.
Nature was the original producer of optical coupling agents. Canada Balsam, the sap from a balsam tree, when distilled and filtered, yielded an extremely viscous bonding agent for optical elements. At moderate temperatures, Canada Balsam was an almost ideal bonding agent. It had a refractive index of approximately 1.5, transmitted light in wave lengths from 350 millimicrons to over 1 micron in very high per— centages and could fill minor differences between the surfaces of a crown and flint element. However, "moderate temperatures" was the problem area. In microscopes or telescopes that would be protected from severe temperatures, Canada Balsam performed just fine. But in fire control systems in tanks during desert warfare or optical systems in high altitude aircraft, Balsam thinned in high temperatures and crazed in low. World War II drove these facts home very clearly.
Optical properties, especially refractive index, dispersion, and transmission, are discussed by glass family. The uniqueness of these properties is determined by the composition; an understanding of which makes evident why there often must be a compromise between the desired optical properties and undesirable secondary properties such as size limitations, glass quality, workability, and chemical resistance.
The selection of optical glasses by the lens designer should, by all rights, be a scientifically sound and straightforward task. Unfortunately, this is not the case. The designer is faced with a myriad of potential glass types to use, and in reality, the task is both a science as well as an art. Part of the problem lies in the fact that while the basic optical parameters, namely the refractive index and the Abbe number or V-number are readily accessible in the catalogue or on the standard glass map, many other attributes regarding the specific glasses such as physical properties, resistance to stain or bubbles, and many other parameters, are somewhat buried away on the individual data sheets for each glass. The designer is therefore faced quite often with the dilemma of which glass to select. This becomes particularly challenging when the designer has varied the refractive index and the Abbe number in his or her optimization, and the glass has settled into a moderately unpopulated region of the glass map. And to further complicate the task, the designer simply does not have the time or the patience to labor over hundreds of glass data sheets. Of the three major glass manufacturers (Schott, Ohara, and Hoya) Schott's glass map presents the user with glasses printed in red and glasses printed in black. The red glasses are "preferred," which means that the glasses are more readily available. It does not mean that the glasses are lower in cost, have better stain or bubble characteristics, or are easier to work. It simply means that they are more likely to be on the shelf. This may not be an optimum criteria from a producibility or a cost standpoint. The Hoya glass map presents the designer with glasses that are shown as either big red dots, big yellow dots, smaller green dots, very small purple dots, and lastly, very small blue dots. The large red dot glasses are "melted monthly in a large mass," the large yellow dot glasses are "melted monthly in a medium mass," and eventually the very small blue dot glasses are "melted rarely and may be discontinued in the future." In addition, the Hoya glass map adds the cost relative to that of BK7 in any of the large dot glasses. This is a very useful form of glass map; however, it still does not contain any information whatsoever regarding stain, bubbles, physical characteristics, or others. We decided it was time to come up with a comprehensive visually-based data base of optical glasses, and this paper will describe the GlassView program.
The traditional approach to designing a high-performance, apochromatic, telescope objective has always required the incorporation of an abnormal dispersion glass or crystal. These materials are notorious for their high cost and poor mechanical properties. Recently, some optical liquids have been found that have dispersion properties unequaled by any known glass or crystal. This has opened the way to lower-cost apochromatic objectives. While many of the problems associated with the use of traditional abnormal dispersion materials can be avoided, the incorporation of an abnormal dispersion liquid 'element' in combination with common (normal dispersion) glasses in a telescope objective design creates some unique design considerations. In this paper, a comparison of the dispersion characteristics of glasses and liquids are presented. Also presented are specific design examples that illustrate the performance improvements possible with glass-liquid combinations. The problems associated with obtaining high precision refractive index measurements on liquids and a comparison of liquid catalog and measured indices are discussed. Finally, some design issues unique to liquid lenses such as athermalization, sealing, volumetric expansion accommodation, freezing, and long-term stability are addressed.
The Verdet constant of the Faraday rotation has been determined experimentally as a function of wavelength for optical glasses at 20 degree(s)C in the visible spectral range. Data have been analyzed for 40 different optical glasses. Very large values of the Verdet constant have been measured for the heavy flint glasses such as SF 59, SF 58, SF 57, and SF 6. On the other hand, very low values (within the experimental error close to zero) have been measured for SF L56 and SF L6. In order to fit the experimental data, a new dispersion formula has been derived on the basis of a simple oscillator model. It is shown that the experimental data of the Verdet constant as a function of wavelength can be fitted reasonably well by this new formula.
The Large Active Mirrors in Aluminum Project (LAMA) is intended as a metallic alternative to the conventional glass mirrors. This alternative is to bring about definite improvements in terms of lower cost, shorter manufacturing, and reduced brittleness. Combined in a system approach that integrates design, development, and manufacturing of both the aluminum meniscus and its active support, the LAMA project is a technologically consistent product for astronomical and laser telescopes. Large size mirrors can be delivered, up to 8 m diameter. Recent progress in active optics makes possible control, as well as real-time adjustment, of a metallic mirror's deformations, especially those induced by temperature variations and/or aging. It also enables correction of whatever low-frequency surface waves escaped polishing. Besides, the manufacturing process to produce the aluminum segments together with the electron welding technique ensure the material's homogeneity. Quality of the surface condition will result from optimized implementation of the specific aluminum machining and polishing techniques. This paper highlights the existing aluminum realizations compared to glass mirrors, and gives the main results obtained during a feasibility demonstration phase, based on 8 m mirror requirements.
The Zerodur glass ceramic is an extraordinary material possessing a very low thermal expansion over a broad temperature range. Depending on the size of the castings, a continuous or a discontinuous melting technology is used in the manufacture of Zerodur. Continuous melters are being operated regularly in Germany and recently in the United States. The latest generation of discontinuous melters started production of 8.2 m spincast telescope blanks in early 1991. Zerodur fusion is a special process to generate lightweighted mirror blanks and special components. It requires glassy material and generates bonds that are as strong as the bulk material. Further process development such as waterjet cutting and insertion technologies are underway and may generate components of unique shapes and performance.
The U.S. Army requires very light-weight collimators for use in man-portable photonic test sets. An investigation was performed to assess the performance and producibility of optical collimators fabricated from various materials. Due to the systems requirements, a large aperture collimator is needed. A full-scale rendering has been constructed with a light- weighted aluminum primary mirror with an electroless nickel overcoat and a SXA secondary mirror and support structure. Since the coefficient of thermal expansion (CTE) is quite different between aluminum and electroless nickel, the useful temperature range of the collimator is unacceptable. The SXA material CTE is almost the same as electroless nickel. Because of this characteristic and the unknown producibility of large-size off-axis aspheric mirrors being made of SXA, it was decided to fabricate a one-third scale rendering made entirely of SXA. Design and fabrication considerations are presented along with a weight comparison for equivalent collimators made of different materials. One observation is that a collimator made of beryllium will be less than 30% with weight of the others, although consideration of other factors must be given as well.
The possibility for forming fused quartz layer on the substrate of carbon fiber reinforced fused quartz (CFRFQ) was investigated by melting fused quartz powder with an oxygen-hydrogen flame, chemical vapor deposition, and sol-gel method. Many kinds of bonding methods between fused quartz mirror surfaces and CFRFQ substrates were investigated, including hot- pressing directly with the mixture of fused quartz power and carbon fibers and hot-pressing secondly by adding powder between the mirror surface and the substrate. The fused quartz block could be bonded firmly under conditions of 20kg/cm2, 1240 degree(s)C, and 5 minutes, when silica powder prepared by sol-gel method was added in the midst. There is no crystallization in the middle silica layer, and the tensile strength is large. Three specimens with a diameter of 100mm and some small ones have been fabricated by this method, and the thermal deformation stability of this type of composite reflector is better than fused quartz mirror.
Cr-Alexandrite or Ti-Sapphire crystals are well known as tunable laser material. How about in glass? Is it possible to get tunable lasers from glass? Generally, glasses have lower crystalline field for Cr3+ ions. But, what happens in glass with a higher crystalline field? And what happen in Cr-doped aluminate glass or gallate glass in which the network former are the same as Alexandrite and GSGG? This paper examines the possibilities of getting tunable laser glass.
Christiansen filters (CFs) are scattering filters composed of two materials with dispersion curves (for refractive index) that intersect at one wavelength. These filters have been fabricated with solid host matrices by mixing finely ground optical glass powders with pellets of optical grade resins and injection molding the combination into planar filters of various sizes. These filters transmit unscattered light of the intersection wavelength and incoherently scatter light of other wavelengths. Unlike filters based on optical interference, the transmitted wavelength does not depend on the angle of incidence on the filter. This paper will review the concept of CFs, describe fabrication procedures for composite materials, and display the spectral transmission measurements of the most promising filters.
A chalcogenide base-glass of the Ge-Se-Te system was nucleated by various amounts (0.1- 0.50 mol%) of either ZrSe2 or ZrTe2. The aim of this study was to produce chalcogenide glass-ceramic materials with extended transmission in the infrared, up to 15 micrometers . The thermal, structural, and optical properties of such materials were investigated using DSC, SEM, EDAX, and FT-IR techniques. It was found that even small amounts of impurities, such as oxygen, cause uncontrolled nucleation and crystallization. The measurements revealed that in such cases, the crystalline particles formed in 'as quenched' glasses were those of the oxidized raw materials: SeO2; GeO2; or ZrO2, and the transmission was greatly reduced even without further heat-treatment. In case of glasses prepared from purified starting materials, the nuclei formed were too small to be detected by x-ray analysis. The crystals formed after heat-treatment were GeSe2 or GeTe2. No apparent difference was found in the effect of either ZrSe2 or ZrTe2 on the thermal and optical properties of the prepared materials.
Apochromatic designs of lens systems utilizing two and three optical materials are described. The performance evaluations of these designs are shown. The chromatic correction of some of these apochromats is investigated using measured values of refractive indices of glass melts.
A model based upon Mie theory was developed to calculate the level of scattering caused by the incorporation of a second-phase. Various transmission curves were calculated in order to study the microstructural scattering effects of pores and ZnGa2S4 particles. The model was able to reproduce the spectral transmission curve (2.5-10 micrometers ) of a hot-pressed ZnS sample containing a porous scatterer only when a size distribution effect was included in the model. A bimodal distribution of pores was found to be responsible for the detected scattering.
Prolonged exposure of chemical vapor deposited, polycrystalline ZnSe to high ac voltages in a 1 molal NaCl solution induces severe mechanical damage. Exposure produces defects that originate at the surface and grow intergranularly into the bulk with a bush-like morphology. Single crystal KRS-5 exposed to the same environmental conditions does not display the same mechanical damage. Possible causes for the degradation are discussed.
A sample of cadmium telluride coating was formed onto a germanium substrate by thermal evaporation. The optical constants were measured by a modification of the envelope method in which only data from transmission spectrum are used and the procedure is simple. Accuracies are about one part in the third decimal place with the technique. The sample was mounted in a vacuum chamber and its optical constants were measured as a function of wavelength, from 2.5 - 20micrometers , at four fixed temperatures. Results in the form of refractive index versus temperature and wavelength are presented, as well as values of dn/dT as a function of wavelength and temperature.
Recently, a group of glasses has been investigated that have high refractive index and low dispersion. These glasses are known as reluctant glass formers and lie in the "forbidden" region of the conventional glass map. Research is continuing to develop means and techniques to produce usable sizes and optical characteristics. Since these glasses have unusual optical properties, a study was performed to gain an understanding of the impact such glasses could have on optical design and to establish desired baseline optical properties as objectives for the material scientists. Several generic optical configurations were explored to determine if a meaningful improvement could be realized. Study results are presented and discussed to illustrate that a notable enhancement in optical performance is attainable.