Although all the early telescope mirrors were made of speculum metal, once the chemical process for metallizing a glass surface became known in about 1850, glass replaced metal as the most common substrate material. However, there are now new applications, such as in the high energy laser field, where the high damage threshold and potential lower fabrication costs make the use of metal mirror substrates appealing. These new applications, coupled with recently developed diamond-turning fabrication techniques, have, during the past five years, made the field of metal optics extremely interesting, and the potential developments for the next five or ten years are even more exciting. The purpose of this special issue on "Metal Optics" is to summarize the reasons for the revival of the use of metal substrates. The issue describes the basic properties of metal mirrors and the most modern fabrication and testing techniques, as well as some of the applications.
The mirror of the first reflecting telescope was polished by Newton in a speculum metal substrate. Subsequent inventions of practical methods of "silvering" glass diverted attention from metal substrates for many decades, but new applications, new environments, and improvements in manufacturing technology have forced a relatively recent (10-15 year time scale) return to more extensive use of metal mirrors. This paper surveys the basic properties of metals of practical utility for mirror substrates. Desirable combinations of property values, or figures of merit, for different environmental applications are discussed. The extrinsic properties, i.e. those which may be peculiar to a particular manufacturing process for a given metal, or combinations of metals, may also affect the choice of material for a particular application and some of these are briefly noted.
Diamond machining of materials for optical applications is becoming an important fabrication process. This report describes current development work in material-removal technology to better understand the mechanics of the diamond-turning process, its limitations, and applications. The technique is presently limited to a select group of metals, most of which are of a face-center-cubic crystal structure. Machinability studies were done which were designed to better understand diamond compatibility and thus expand the range of applicable materials. Nonconventional methods such as ultrasonic tool stimulation were investigated. Work done to determine the machinability of infrared window materials indicates that this is a viable fabrication technique for many materials, although additional effort is needed to optimize the process for particular materials.
We present some observations on traditional (nonchemical) methods of lapping and polishing metals. Both relatively hard, nickel and hardened stainless steel, and relatively soft, copper and aluminum, materials will be discussed. Lapping and polishing techniques for the more easily polished hard materials yield poor to disastrous results for the softer metals. We further observe that metals require techniques different from glass due to the generally crystalline structure of metals. Glass being amorphous has a uniformly hard surface and responds uniformly to a variety of (sometimes rather brutal) polishing techniques. Metals on the other hand vary in hardness on the microscopic level due to crystal orientation and grain boundaries. Worse yet, they may also be stressed or work hardened from previous machining operations. Finally, the apparent hardness is affected by abrasives being embedded in the surface during lapping and initial polishing processes. Prior to obtaining a satisfactory finish, all these surface defect areas must be polished out using a lap and polishing compound designed to remove the harder material as easily as the softer. These considerations lead to diamond being considered a natural choice for polishing compound.
Various combinations of ion polishing and vacuum annealing were used in an attempt to reduce the optical absorptance and to smooth the surfaces of polycrystalline metal mirrors for high power lasers. The samples included micromachined copper and conventionally polished copper and molybdenum mirrors. It was found that ion polishing and vacuum annealing resulted in a large decrease, 40 to 50%, in the absorptance at 10.6 pm for conventionally polished copper samples and a small decrease, 5 to 10%, in the absorptance of micromachined copper and conventionally polished molybdenum mirrors. After ion polishing and annealing, the absorptance of the conventionally polished copper mirrors was as low as that of the micromachined mirrors. Ion polishing proved beneficial in removing surface defects, such as machining marks, scratches, and embedded polishing grit, and oxide layers; however, for large amounts of material removed, differential sputtering of the various crystallites led to grain boundary delineation and surface roughening. Thus, there exists an optimum amount of material to be removed by ion polishing which results in both maximum removal of surface defects and minimum roughening of the surface. The optimum amounts were found to be: 2500 to 5000 A for micromachined copper, 5000 to 7000 A for conventionally polished copper, and less than 1500 A for molybdenum.
Nonconventional-shaped optics are being machined for use in laser optical systems. The fabrication processes incorporate special-quality diamond tools and specially constructed turning machines. The shapes produced include axicons (conical-shaped mirrors), waxicons (a compound axicon with a "W" cross section), torics, and multifacet mirrors. Whereas conventional-shaped optics are readily producible by the lapping process, these nonconventional-shaped optics are very impractical to lap. The axicons and waxicons produced were estimated to have surface straightness as good as 5 pin (125 nm), over 3 inches (76 mm) of length, and angular accuracy as good as 2 arc seconds. A toric mirror was estimated to deviate (peak to valley) from a best-fit radius by 4 pin (100 nm) over 2.25 inches (57 mm) of surface length.
This paper describes the theory, design and fabrication of a complementary pair of cone-like mirrors which transform an annular collimated laser beam into a gaussian profiled collimated beam without obscuration. The details of a simple computer algorithm are revealed which explain the numerical procedure for computing the coordinates of the mirror surfaces. Also discussed is the procedure to diamond turn the nonlinear surfaces using the development lathe at the ERDA Y 12 Plant and the metrology of the first parts produced.
This paper discusses the measurement of the finish of diamond-turned surfaces by differential light scattering. Experimental scattering data are analyzed by electromagnetic theory to give the two-dimensional power spectral density of the surface roughness. These spectral densities are direct functional measures of the surface quality, and may be characterized in terms of topographic finish parameters. These parameters can then be used to specify surface finish, to predict scattering under a variety of conditions, and to aid in studies of other functional properties of these surfaces. Scattering spectra are separated in-to three groups corresponding to three classes of surface roughness: periodic tool marks and one- and two-dimensional random roughness. Periodic tool marks give rise to discrete diffraction lines in the scattering spectrum and are characterized by their surface periods and their Fourier amplitudes. Random one- and two-dimensional roughness give rise to one- and two-dimensional continua underlying the diffraction lines and are characterized by band-limited values of the rms surface heights and slopes, and transverse length parameters. Using HeNe light, vertical roughnesses are measured from a fraction of an Angstrom to several hundred Angstroms, for transverse spatial wavelengths from a fraction of a micron to several hundred microns. We re-view experimental techniques for making these measurements with emphasis on the scatterometer developed in our laboratory, which uses a fixed source-detector geometry and a rotating sam-ple. Results are illustrated by a number of scattering spectra taken with this instrument.
This paper demonstrates that the optical testing of diamond-turned surfaces is best accomplished by interferometry and not by tests which measure wavefront slope. Certain conditions regarding the interferometer configuration must be met in order to generate meaningful and accurate interferograms. A 40 cm diameter aperture modified Mach-Zehnder interferometer mounted directly on the diamond-turning lathe to facilitate rapid testing of figure between fabrication cuts is described. Results for a spherical surface tested in a Twyman-Green interferometer and an off-axis parabola tested in the Mach-Zehnder interferometer are illustrated.
Large, short-pulsed CO2 laser systems have been developed to investigate the feasibility of laser driven fusion. Metal mirrors are an important component in these systems. This paper discusses CO2 systems and mirror considerations. Mirror material constraints and choices as well as fabrication techniques are reviewed.
Diamond turning is immediately applicable to the fabrication of infrared optical components because presently available machines can meet the reduced absolute accuracies required at 10 micrometers. An initial survey of infrared sensor programs at the Honeywell Radiation Center has been conducted to predict the near term and future demand for diamond-turned optical components. Not only does the fabrication process promise significant cost savings as compared to conventional lapping and polishing methods, but, as in the case of aspheric lenses, wider applications are also sought to reduce weight and space requirements. In addition, broader usage of diamond-turned aspherics reduces total parts count and assembly and alignment time, provided proper tools and test equipment are employed. The potential cost savings of diamond-turned vs conventionally fabricated optics are summarized for contracts at the Radiation Center. The savings were calculated by subtracting the difference in fabrication costs and multiplying by the number of items expected to be produced into the mid-1980s. Technical fallout potential of the diamond-turning process is also noted in the apparent ability of a coated sample to pass a 24-hour salt fog test.
A rapid-scan spectrometer employing a silicon-target vidicon detector was used to study the time-resolved emission spectra of laser-ignited metals. Bulk specimens of Ca, Mg, Zr, Ti and several Ti alloys were ignited with a 90 W cw CO2 laser in air or under a gentle flow of oxygen. Line and band emissions observed between 300 and 1100 nm during combustion help to identify vapor phase reactants and products and their locations in the flame. Disappearance of discrete spectra during the transient combustion of Ti and Zr gives information on the accumulation of molten oxide products. Observations of the continuum radiation emitted by laser-irradiated flames indicate a laser-stimulated luminescence from condensed metal oxide particles.
Photographic film was calibrated for energy measurements of pulsed ruby lasers. The film can be used to obtain energy values accurate to about 20%. The method works under field conditions when the use of electronic instruments may be impractical.
The two-dimensional optical Fourier transform of a raster recorded input electrical signal has been shown to result in a "folded spectrum." Selected examples and applications of this optical signal processor in signal demodulation and radar signal processing are discussed.
An optical microscopic instrument is described that will allow visual observation and photoelectric detection measurement and classification of moderate concentrations of viruses in biological fluids in a few minutes. The instrument combines fluorescence correlation spectroscopy and ATR fluorescence with special electronics and a correlator-fast Fourier transform data system to measure the size of viral particles while disregarding background objects found in typical biological fluids. Further data processing allows calculation of the nucleic acid mass of individual particles and their total concentration. A novel chemical procedure is employed to differentiate various types of viral nucleic acid. The combination of these measurements is then used to classify a virion without requiring the use of a specific antibody for it. Calibration of the system against test objects and compounds, as well as measurements on several virus types will be shown.
Ionography, which is currently receiving much attention, is an x-ray image recording system in which the detector is a thin ion-ization chamber usually containing a heavy, high pressure (10 atmospheres), gas. Ions formed by X rays in the gas are swept across the chamber by an electric field onto a thin insulating film to form a charge image. In current systems the chamber is then depressurized, the film removed, and its charge image made visible using powder or liquid toner. An alternative method for making the charge images visible is herein described which overcomes the mechanical problems of film and gas handling of present techniques. An added advantage is that it produces instantaneous bright, stored, projected images which offer the potential of fluoroscopic ionography. In the new technique charges are collected onto a thin clear deformable layer of oil or soft elastomer coating a front surface mirror in the chamber. Electrostatic forces between the surface charges and the conducting mirror cause imagewise deformations which are immediately made visible on an external screen by a Schlieren optical projection system using light which enters and leaves the chamber through a high pressure glass window. Images, with resolution better and sensitivity close to film/screen methods, have been obtained.
Spatial rejection is critical when exo-atmospheric or in situ radiometric measurements of faint sources, such as air glow emissions, are made in the presence of relatively intense sources such as the sun, moon, or earth. Scattering from atmospheric molecules and aerosols during evaluation in an earth-bound laboratory makes the high-rejection baffle appear to be less effective than it actually is in the measurement situation. A Monte Carlo computer program SCAT was written to predict the effects of atmospheric scattering upon field-of-view calibration measurements of optical baffling systems. It was found that Mie scattering could be reduced in a clean room environment and Rayleigh scattering was determined to be the limiting mechanism in a special chamber designed for off-axis measurements. The background flux at 40° off axis was found to be 1 x 10-9 of the on-axis incident flux for a 5° full-angle baffle when illuminated by a 10.2 cm diameter collimated beam. A two-step process was used to measure a baffle response at 40° off axis down to 1 x 10-11 of the on-axis incident flux.
In this paper a field calibration procedure for the AGA 680 Thermovision system is described. The procedure is necessary because there are numerous uses of a thermovision system for which the standard calibration curves supplied by the manufacturer are inadequate. These uses include measurements with bandpass filtered optics, as well as measurements of radiance. In effect, the system may be calibrated so that it serves as a field radiometer.
The paths to greatness in optics are as numerous and diverse as are the roads to Rome. I wish to illustrate this point by citing some of the incidents in the life of one man, Dominique Francois Jean Arago. Although Arago is sometimes overlooked when compiling a list of the greats in optics, his influence on other investigators, his own research, and his lectures have made a profound influence on the subject. I would like to utilize this column to describe some of his training and a later column to describe a number of those many contributions.