Optical fabrication and testing in the United States have been significantly brought together by a series of Optical Fabrication and Testing Workshops in the 1974-1977 time frame. This issue of Optical Engineering features optical fabrication and is intended to be another step in the process of bringing the optical fabrication community together. Our goal is to begin to overcome the natural tendency of the industry to withhold details regarding fundamental principles and important techniques. It is our opinion that the optics industry in the United States would benefit if this policy were universally accepted.
The optical fabrication industry is troubled on one hand by a lack of standardized specifications and on the other hand by the overzealous application of the few formal specifications which it does have. The article comments on the various philosophies of tolerancing, the difficulties in communicating tolerances, and the pitfalls to the implementation of specifications. The nature, use and abuse of the MIL-0-13830 scratch and dig standards are also considered.
The structure of a surface layer of ground glass consists of relief layer and crack layer. Their dimensional characteristics influence directly the subsequent polishing process. The understanding of the role of both layers in the polishing process is the key factor for optimizing the efficiency of optical surfacing by selecting proper techniques for both grinding and polishing operation. In this paper, the role of the microstructure of the grind surface layer in polishing process will be discussed; methods of its evaluation presented, and several examples of surface structure for some representative grinding methods evaluated.
Plummer Precision is dedicated to the proposition that closely controlled polished spherical surfaces are best accomplished by closely controlling sphericity, and depth of fracture in the initial diamond generation, and further, that the subsequent operations of fine grinding and polishing are merely enhance-ment of the generated surface. To this end we have designed and manufactured very rigid generators in which we employ direct driven spindles running on close tolerance bearings. A description of the basics of these machines will be revealed as well as an analysis of the glass surfaces which are produced by them.
From the optician's viewpoint, aspheric optical surfaces are usually the most difficult surfaces to produce accurately. For this reason, the cost of aspheric elements is high and production forecasting is often not valid. In this paper, a simple numerical method is presented which accurately predicts material removal rates for a given set of optical polishing machine and surfacing tool parameters. The material removal curves are then applied to specific surface aspherization problems. Examples are shown which demonstrate the validity of the method.
In recent years, the continuously dressed flat annular machines widely used for flat grinding have been adapted to polishing ultra-flat optical surfaces for interferometry and laser applications. This paper describes these machines and the theory and practice of their operation to achieve peak to valley flatness better than 1/10 wavelength in the visible.
The engineering requirements for a high speed production line for optical elements have been investigated. The studies were based on a three-step process-diamond tubular tool generation, diamond pellet lapping and polishing with polyurethane bonded cerium oxide. Emphasis was placed on precision ground surface generation based on an analytical model. The generation process resulted in surfaces with a precision of one interference ring and an accuracy of three interference rings with respect to the desired radius of curvature. The lapping and polishing stages were designed to reduce the microstructure of the work while leaving the surface shape unaffected. At each stage of the process the surface figure was measured. A new testing technique was used to study and to quantify the microstructure of the lapped and polished surfaces. The microstructure was measured to 20 Angstroms peak-to-peak using a Scanning Fizeau Interferometer.
This paper presents a method for predicting wear patterns for aspheric axisymmetric lens figuring employing simple graphic and algebraic computation. It is written for the small shop optician rather than the academician.
The mounting of lenses is a major consideration in the manufacture of lenses. The design of the mount, the material used and the manufacturing methods can all significantly affect the performance, cost and profit margin in lens making. Various methods of mounting will be described and some ideas for improvement will be presented. The suggested improvements are based on the observation that the optical shop is a hostile environment for precision instruments and machines. The grinding materials and liquids are ruinous to precision tools. The one process in lens making that is unique to lens making is the degree of concentration on polishing spherical surfaces. To optimize the manufacturing process one should consider reducing precision requirements as much as possible in all the steps except for the polishing. This thinking leads to looking at new methods of assembly which can be done in clean room conditions. Modern epoxy cements, inch worms, interferometers, and minicomputer control units can be used in assembly to short cut the need for precision lens shaping and tight mounting tolerances.
Recent developments in diamond turning of optics are reviewed. Improved surface figure and surface finish have been achieved as well as metrology of the machined part. Reflectivities of diamond turned metals at various wavelengths are summarized. Application of diamond turned optics include laser resonator mirrors, x-ray microscopes, x-ray telescopes, missile optics, and scanner mirrors. The technology looks especially promising for present infrared requirements since both reflective, refractive, and transmittive components can be fabricated. Diamond turning of optics can be defined as the use of a diamond tool on a precision lathe under very precisely controlled machine and environmental conditions to fabricate a finished optical component. The specific application of precision machining principles to diamond turning has been led by the Lawrence Livermore Laboratory (LLL), Livermore, California and Union Carbide Y-12 Plant (Y-12), Oak Ridge, Tennessee.
The technique of fabrication of optical surfaces by epoxy replication, historically used for the production of quality diffraction gratings, is presently being applied to producing a widening variety of optical components. Materials used in the optical replication process are reviewed along with the techniques for using them. Limitations in the process caused by materials and their impact on the optical product are presented. Other limitations caused by the operating environment are discussed and evaluated. Applications to both non-conventional, traditional and optical components, designed specifically to be fabricated by replication are presented.
The tolerances on optical components used in multicomponent infrared systems handling large power levels may be comparable to or even more severe than those commonly used in high quality optics for the visible spectral region. In addition, the optician may be asked to polish unconventional materials in such a way as to obtain minimum surface absorption and high laser damage resistance as well as good optical figure. Minimum microroughness and scratch density are also often required both for components used in infrared lasers and for more conventional applications where scattered light is the primary concern. Fortunately, techniques are being developed which satisfy these often conflicting requirements to a large degree.
Most modern diffraction gratings are metal coated plastic film replicas, on rigid substrates, that were taken from a master. Master gratings are made either by mechanical ruling, where uniformly spaced grooves are burnished in soft metal films by a diamond tool, or by exposing a photoresist coated blank to an interference fringe field generated with the aid of high powered lasers. Basic techniques are described for plane and concave gratings. Performance tests are concerned primarily with diffraction efficiency and with perfection of the diffracted wavefront. Imaging defects show up in reduced resolution. Groove spacing effects show up as ghosts, stray light and satellites. These will be reviewed.
The period 1970-1975 has been one of unusual activity in the field of infrared materials. Driven primarily by the need for highly transparent lens and window elements in high power laser optics, significant advances have been made in the quality, physical strength and size of a variety of materials and in particular the alkali halides, alkaline earth fluorides, zinc selenide, and for use in fiber optics, fused silica. The state-of-the-art of these materials is outlined, the basic physical processes that determine their optical and mechanical limits are reviewed, and suggestions are made for future work in electro-optic and magneto-optic materials research.
A rapid procedure is outlined that permits accurate alignment, focus, and scaling of imagery at two separated locations in an optical system. The moire pattern formed by pairs of identical gratings permits the full image pupil to be observed and adjusted. Alignment and scaling can be adjusted to an accuracy of the order of the grating's fundamental spacing.
An oblique incidence interferometer has been devised for the testing of unpolished surfaces and non-optical surfaces such as lapped metal plates, fine ground surfaces, etc. The interferometer makes use of a wedged plate as a beam splitter. Large surfaces, at one stretch, can be examined at angles of incidence around 80°. It is also possible to vary the sensitivity of the interferometer to some extent by the readjustment of the wedge plates. It is also possible to use the interferometer to test the straightness of cylindrical rods, tapered rods etc.
A Mach-Zehnder optical heterodyne interferometer with a radio-frequency phase reference is described. The system, intended for use in the study of turbulent gases and plasmas, has a high phase data rate (3kHz) and can monitor unlimited relative phase excursions. Possible extensions to higher data rates and to dual wavelength operation are discussed.
An analytical model of the synthetic aperture radar system is outlined. This model describes the radar image as a spatial two-dimensional bandpass filtering of the radar scene reflectivity density. The offset of the passband along the range frequency axis determines the major wavelength and geometrical properties of the simulated radar image. Through the use of a Fourier optical system, the photographs of radar target models and an aerial scene were bandpass filtered. The resulting images exhibit the characteristics of radar imagery, including texture (fading) and image dependence on object orientation. Use of the model and optical processing to simulate radar imagery from aerial photographs is discussed.
This paper reviews the technical aspects and status of optical fiber waveguide, cabling, light sources, and signal detection. Advances in fiber waveguide have made possible the transmission of high data rate information over very small diameter waveguide. Four fiber types are currently receiving most attention as a result of their attenuation, dispersion, and cost characteristics. Light sources for fiber optical applications are very promising due to recent developments in semiconductor injection lasers. Receiver technology is well developed and avalanche photodiodes provide high sensitivity receiver performance for most data bandwidths of interest.
The use of dichromated gelatin for holographically recording the component single frequency gratings of a high efficiency grating lateral shear interferometer is described. Interferograms obtained using a grating lateral shear interferometer simultaneously having a diffraction efficiency of 30% in each of two shearing interferograms are presented.
The initial investigations of laser performance with short interferometers at RCA were performed with an electro-optic Q-Switch. These results showed that beam divergences of one to two milliradians were easily achieved with an interferometer length of about 2.5 inches. However when the dye Q-Switch was substituted for an electro-optic switch,1 and the interferometer shortened to 2.0 inches, the beam divergence increased to six milliradians or larger. Many tests were run with the 2.0 inch interferometer because it was found that the divergence varied with the quality of the Q-Switch material. The best that was achieved was five milliradians, independent of optical density or quality of the Q-Switch used. In all these tests the interferometer consisted of a 4mm x 40mm neodynium doped Y.A.G. laser rod and a flat 100% mirror that could be moved to vary the interferometer length.
This column in the May-June 1976 issue was devoted to a review of the NBS Technical Note 910-1, Self-Study Manual on Optical Radiation Measurements. In any review I expressed surprise that although Fred Nicodemus was the editor and one of the authors of the "Self-Study Manual," the book omitted any mention of the terms "sterance," "pointance," and "areance" previously advocated by Nicodemus and endorsed by me.