After a review of management strategies for small high-technology business in the last issue, we will consider a subject of interest to those developing and putting new products on the market. There are various strategies for patenting new developments, but to help bring us the facts about our need for retaining patent attorneys, the fbllowing article is presented. This paper was given in its entirety at the SPIE seminar on the Business Side of the Optical Industry IV: Managing Engineering & Research, August 1978, San Diego; the complete version appears in SPIE Proceedings Volume 151.
Stray light presents a continuing problem to the optical engineer. It reduces image contrast, introduces spurious signals, and in cases where a strong extraneous source is present, can completely mask the light from a weak object. Attempts to reduce or eliminate scattered light are often empirical and usually not very successful. Recently, however, powerful analytical techniques such as GUE RAP and APART have been developed to aid in system design. Using these techniques, which were described in a SPIE seminar on 'Stray Light Problems in Optical Systems," and are now published in SPIE Proceedings Volume 107 (1977), significant advances have been made in optimizing baffle design and improving system performance. The computer models, however, are only as good as the data fed into them, and there is no substitute for accurate knowledge of the scattering behavior of optical surfaces and the performance of optical blacks.
A scalar theory for scattering from multilayer coatings due to surface roughness is presented. A new model is considered which assumes that the roughness of the top surface of a given layer is due to the roughness introduced by the previously deposited layers and to the variations in thickness of the layer itself. The variations in the layer thickness are assumed to be uncorrelated from one layer to the next, but the roughnesses of the various surfaces are partially correlated. This model is compared to two existing models: one which assumes that the surfaces of the layers are completely uncorrelated and another which assumes that all of the surfaces are identical to that of the substrate. A fourth model is also introduced which assumes that the scattering is due to variations of refractive index within each layer. A matrix formulation for calculating the total integrated scattering and the change in specular reflectance and transmittance is presented. Predictions of the four different models are compared for several multilayer designs.
Vector theories dealing with the interaction of light with surface roughness are reviewed. Light scattering from microirregularities whose heights are much smaller and much larger than the wavelength of the incident light are discussed. The theories apply to metal-coated surfaces and, in addition, surfaces covered with multilayer dielectric stacks. Surface plasmon effects, which are also discussed, include the decrease in the specular reflectance caused by conversion of photon energy into surface plasmons (i.e., additional absorption due to surface plasmons), as well as the radiative decay of surface plasmons into photons (i.e., surface plasmon "scattering"). There is a brief discussion of how to use experimental surface height data in the theoretical formulae. Finally, published experimental data on scattering and surface plasmon effects are compared to theoretical predictions.
In the smooth-surface limit, the angular distribution of the light scattered from a surface maps the power spectral density of its residual surface roughness. This result is essentially independent of the scattering theory used and the statistical properties of the surface roughness. The power spectral densities of engineering surfaces are generally broad and increase with increasing spatial wavelength. As a result, practical surface finish parameters are not intrinsic properties of the surface, but depend, with varying degrees of sensitivity, on the bandwidth limits inherent in their measurement or dictated by their application. These issues are discussed with reference to two classes of finish parameters: those related to the central moments of the scattering spectrum, and those related to the coefficients in the expansion of the shape of the spectrum in inverse powers of the scattering angle. The significance of "1/02" scattering in this context is emphasized. A shot model of surface roughness is then introduced to gain further insight into the relationship between scattering and surface features. In this model inverse power terms are related to "edge" scattering effects from critical points in various types of elemental microdefects. The relationship between this view and electronic noise is pointed out; in particular, the correspondence between "1/02" scattering and "1/f" or flicker-noise phenomena.
In recent years there have been many optical system designs requiring high optical quality (sharp point spread function) as well as low scatter performance on aspheric surfaces. Because of the fabrication differences between the processes of figuring and smoothing on aspheric surfaces, it is important that the optical engineer understand how to trade off these processes during the manufacturing cycle. To describe these processes, the residual wavefront error (departure from the Gaussian reference sphere) is evaluated over a very wide spatial frequency band. From a physical point of view, the spatial frequencies of interest are grouped into three general categories: (1) the classical aberrations, which are low frequency in character and describe what might be called the figuring errors; (2) surface ripple or residual tooling errors, which arise in the fabrication of the optical elements; and (3) microroughness, the very high frequency structure on the surface, generally responsible for surface scattering. In this paper, the methods pioneered by Hopkins for aberration tolerancing and Barakat for random wavefront errors are extended to the discussion of mid- and high-spatial frequency surface errors. Some examples will be given to illustrate some general system properties such as the nature and role of the surface autocorrelation function.
Three f/2 paraboloids, 12 inches in diameter, were given short final polishing runs using colloidal silica and full-sized flexible laps. No degradation of surface figure was detected, and we obtained a reduction in surface scatter to levels indicating a surface microfinish of 15 A rms or less.
Most infrared (IR) optical systems use blackened surfaces to reduce stray light. This blackening may be in the form of paint, chemical blacks, deposited metal blacks, or anodization either dyed or undyed. One method of determining the optical absorption of these surfaces is by measuring their spectral emittance. We have made such measurements on a number of samples in the range from 3 to 22 micrometers and on a few samples as far as 125 micrometers. Since the absorption properties may be temperature dependent, we have made the measurements at or near the ex-pected operating temperatures of the samples. Data are presented on a number of samples at temperatures from 4.2 K to 373 K.
Some optical systems can tolerate only a small amount of light scattering from mirror surfaces within the system. If low scatter mirrors are used in such an optical system, the mirrors must be monitored by some means to be sure that contamination does not increase the intrinsic scatter of the mirrors. In this paper the measurement technique of Bidirectional Reflectance-Distribution Function (BRDF) is presented as a method to monitor the scatter of mirror surfaces. A description of the test apparatus needed to measure BRDF is discussed. Curves are given which show how such contaminants as dust, hydrocarbon oil, acrylic, and peelable coating residue affect BRDF. It is shown that BRDF measurement provides a very practical and accurate method of measuring the scatter degradation of a low scatter mirror surface that has become contaminated.
Variation in coupling of unstable resonators is analyzed for the case of low order medium inhomogeneities. The analysis examines one-dimensional confocal unstable resonators with linear or quadratic index profiles in the resonator medium. These types of effects can arise from the bulk and wall heating present in gas lasers. The analysis shows that both the effective Fresnel number and the geometric magnification influence the local stability of the design point coupling. Choosing the Fresnel number to maximize mode loss separation may not be consistent with the choice of Fresnel number to minimize the influence of medium variations.
Methods for applying holographic interferometry in the study of turbulence are described. Examples of the effect of various types of turbulent cells on an optical wavefront are presented. Finally, a relatively new technique in which phase control of the two mixed reconstructed waves is discussed. It is concluded that the technique can ultimately provide an extremely high sensitivity for the viewing of phase objects and can provide the basis of automated data reduction based on heterodyne interferometry.
This overview addresses three general areas: spectroscopy; molecular interactions and reaction dynamics; interrelation of chemistry and laser development. The various topics and specific chemical examples contained in the overview are by no means exhaustive, but are chosen simply to give the reader a flavor of the role of lasers in chemistry.
A laboratory photographic system employing a giant pulsed ruby laser as a light source was adapted to field use in recording erosion and rain impact phenomena on controlled rain-erosion testing at the Holloman High Speed Test Track facility. Techniques using the short-time duration of 20 nanoseconds to effectively stop the sled motion of specimens traversing a rain environment at speeds in excess of 1.8 km/sec are described. A front/back light method for recording erosion detail, flow visualization and shadowgraph information with a single laser as the light source for multiple cameras is presented. Photographs of rain drops impacting on sled specimens are shown along with a comparison with conventional photographic techniques. The advantages of employing the single laser with multiple cameras discussed in this paper demonstrate its applicability to the diverse problems of recording erosion, abla-tion and aerodynamic data of test specimens traveling at hypersonic speeds.
This paper describes a simple method of measuring refractive indices of liquids. The basic element is a thin wedge cell and the source of light is a low power HeNe laser. Conditions are derived, to obtain the best possible accuracy using this setup. Some applications of this method are suggested.
A review of the photochemical methods that are applicable to purification of the rare earths in the liquid state is presented. Of the types of processes possible in solution, photoredox has been applied to the isolation of both europium (with separation factors exceeding 1000) and cerium from mixtures of lanthanides. Photosubstitution is the method of choice for those lanthanides displaying only one stable oxidation state in solution and has been demonstrated using chelates of europium.
The Laser Wavefront Analyzer (LWA) determines the intensity and phase of a CO2 laser operating over a spectral range from 9.1 pm to 10.7 pm. The LWA is a sliding reference interferometer and measures differential optical phase over a 32 x 32 element format of the input beam. A resolution element is scanned every 10 ptsec. The theory and general design aspects of the LWA are discussed in this paper. Phase and intensity data outputs obtained with the instrument during calibration tests performed by the Air Force Weapons Laboratory at Kirtland Air Force Base are presented. These calibration data were obtained by utilizing low power CO2 gas lasers with appropriate optical components to magnify and alter the phase front of the beam entering the analyzer sensor aperture. Phase accuracy and precision data from these calibration tests are also presented.
A method for real-time matrix multiplication is presented. This paper describes the geometrical interpretation of the mathematical manipulations between the two matrices. Three coherent optical astigmatic systems are developed based on the analysis. Each system is essentially composed of two subsystems that are connected in series. The first one performs multiplications between the corresponding elements of the matrices coded in the amplitude transmittance of the transparencies. The results are received by the second subsystem that performs the necessary summation operations to give the calculated rise to each element in the final result, the product of the two matrices. In these processes, no preparation of a hologram or intermediate memory is required. The operations are done in parallel. The multiplication between an N x N matrix and an N x 1 vector is discussed in detail. Multiplication between N x N and N x N matrices is also presented.
Angle-scanned interferometers are a class of ac interferometers in which the optical path difference is varied as a function of time by varying the incidence angle. The resulting interferogram has the properties of a hologram of a point source and may be used to determine the input energy wavelength and incidence angle. A new category of interferometer type is suggested, beam symmetry, which determines the output fringe pattern behavior as a function of input angle. An interference circle diagram is presented as an aid to understanding the characteristic response of symmetrical interferometers and angle-scanned interferometers. A number of angle-scanned interferometer examples are described in terms of the interference circle diagram.
The axial coherence length of laser radiation is both specific and unique to lasers. An angle-scanned interferometer is described as a coherent energy modulator in a coherence-discriminating laser receiver. A symmetrical solid-spaced Fabry-Perot interferometer etalon is used. Etalon pairs are used in a logic arrangement that passes coherent laser signals and rejects incoherent background signals. Two pairs of stepped etalons are used to detect a single laser pulse. The fundamental limits of performance of this type of laser receiver are listed.
The military as well as some commercial users of technical manuals are moving toward a universal requirement of microfilmability for these documents. The halftone photographic process presently utilized for illustration purposes is not suitable for microfilming. The employed solution to this problem is the production of a line-art illustration by manually tracing the various objects of interest in the photographs. This technique results in substantially increased costs for technical manual development meeting the microfilmability requirements. An approach to simplifying the conversion of continuous-tone or halftone photographs to line art has been developed utilizing optical processing techniques. This paper describes the approach, including development of an optimal enhancement filter, the implementation and results of a cost/benefit study.
A noncontacting optical transducer, which measures the profile of machined surfaces, was constructed and tested. The profile transducer consists of a microscope objective, a point-source illuminator, and two photodetectors. The dynamic range of the instrument is typically 500 microinches with an accuracy of better than 10 microinches. The output of this transducer is a profile signal along a line on the surface under examination. The profile of a machined surface obtained by the optical-transducer correlates very well with that obtained by a mechanical Proficorder which uses a diamond stylus. Principles of operation, results of various performance tests, and potential applications of the optical-transducer are discussed.
This paper describes an unequal-path interferometer that requires only one precision component and that can be constructed as a compact and inexpensive instrument. Principles of operation are outlined, and requirements for optical components and alignment are investigated. An actual instrument and considerations for its design are described.
The design of a power meter for measuring continuous wave (CW) outputs is presented. The principle of the operation is to measure the temperature rise at the surface of an aluminum slab that absorbs the energy of the incident 10.6 micron radiation. In order to make effective absorption, the surface is covered with a layer of Al203 of a few microns in thickness. Linear response to the incident power up to 200 W and allowable power density of more than 500 W/cm2 are attained.