With a steady growth in technology, we are confronted increasingly with adverse environments for both human and human-made systems. Such challenges can be met in two ways: 1) the environment can be changed or controlled so that it becomes or remains conducive to the proper functioning of the system, or 2) the system can be designed and built to withstand the hostile influences. In the case of the human system, design changes within a reasonable time frame are not possible; consequently, its proper functioning is dependent on the creation and maintenance of a suitable environment (e.g. the space suit). But for the human-made system, it is mostly the opposite. To a large extent, it would not be possible to achieve the technological advances (e.g. laser fusion) we now seek without the creation of systems which function properly in the midst of hostile environments. In certain cases, both alternatives are available, but the creation of a fully benign environment would not be cost effective (e.g. spaceborne fiber optic waveguide communication system).
A review of the behavior of state-of-the-art low loss optical fiber waveguides in both pulsed and steady state radiation environments is given. The influence on radiation-induced transmission loss due to experimental parameters such as total dose, dose rate, time after irradiation, temperature, wavelength, injection conditions, light intensity and radiation history, and materials parameters such as OH and impurity content and dopant type and concentration is described. Data are reported for both step and graded index doped silica core fibers and glass clad and polymer clad silica core fibers. Candidate fibers for deployment in certain specific, limited nuclear environments are identified.
The proposed use of lasers in satellite ranging and optical communications requires an analysis of the effects of natural and man-made radiation upon the performance of these systems. Spaceborne laser systems will be exposed to high energy protons and electrons in the Van Allen belts, solar flares, and galactic cosmic rays. The use of on-board radioisotope thermoelectric generators will expose system components to neutrons and gamma rays. This paper reviews the effects of this radiation upon two types of laser range finders: Nd:YAG (1.06 pm) and CO2 (10.6 gm). The radiation-induced degradation in component response is translated into reductions in system performance. We conclude with recommended nuclear hardening design guidelines.
The introduction of low-loss optical fibers (<20 db/km) has led to their increased use in optical communications and data acquisition systems. Optical fibers have intrinsic advantages over wire bundles. However, the application of ionizing radiation (gamma rays, X-rays, and electrons) causes increased attenuation (loss of transmission) in the fibers. This paper summarizes the various ex-perimental facilities and optical systems used by investigators to study the effects of ionizing radiation on optical fiber waveguides. The applicable equations used by investigators to express optical fiber transmission loss and absorption are summarized. Recent results from irradiating silica optical fibers doped with phosphorus, boron, and/or germanium are presented. The effect of temperature on transmission is discussed. Transient recovery mechanisms such as geminate recombination and tunneling, as applied to irradiated fiber optic waveguides, are reviewed. Areas meriting further investigation are outlined.
The fusion experimental lasers at Lawrence Livermore Laboratory, called Argus and Shiva, operate with very high fluence on the systems optics. The optical surfaces may be damaged by the laser beam both because of the limitations inherent in currently available bulk materials and coatings, and because debris contaminating the surfaces initiates damage. The damage thresholds which we have experienced on our operating systems are presented. A unique solution for recycling debris shields exposed to target microexplosions is also discussed. Comments on the development of improved damage thresholds are included.
We review the magnitudes of the damage thresholds of practical optical materials, surfaces, and coatings found useful in short-pulse CO2 fusion lasers and discuss the materials properties and laser parameters that affect the thresholds. The optics used in the eight-beam, 10-kilojoule, 20-terawatt, Helios system at Los Alamos are used as specific examples. Among the optical components that characterize the Helios system are NaCl windows up to 41 cm in diameter, diamond-turned copper mirrors of similar size, CdTe Pockels cell crystals, germanium saturable absorbers and polarizer plates and multilayer-coated ZnSe partial reflectors.
Efficient T1I/KC1/T1I antireflection (AR) coatings (< 0.1% reflectance per surface), which are highly resistant to laser radiation damage (< 0.04 % absorption per AR coating at A = 10.6 um) have been fabricated. The three-layer AR coatings were designed by replacing one of the layers of a two-layer AR coating with a Herpin equivalent structure. Thickness of the layers of the resultant three-layer AR coatings can be varied continuously over ranges defined by the indices of the coating materials and the substrate. The ability to vary the thickness of the layers enables the optimization of the coating design with respect to one or more requirements, for example, low absorption, environmental durability or ease of fabrication. The TlI films deposit with an orthorhombic structure, the orientation of which is dependent on the KC1 substrate orientation. Therefore, the refractive index of the TlI films depends on the substrate orientation. The influence of the substrate orientation on the TlI refractive index can be eliminated by coating the KC1 substrate with a thin layer (~200 A) of SrF2 prior to depositing the TlI film.
High-energy laser (HEL) systems involve an optical train consisting of mirrors and windows, which may compromise the system's operation because of unavoidable irradiance-mapping aberrations resulting from the absorption of some fraction of the incident laser-beam energy. This paper describes an analytical investigation of relevant processes and discusses how laser-driven mirror/window distortions may affect the performance of HEL systems with regard to focal intensities and on-target fluences. An approximate expression for the brightness at the Gaussian focus is derived on the basis of a far-field degradation model and shown to be simple enough to allow HEL system designers to assess the capability of a contemplated optical train and to evaluate its behavior as a function of beam-power level and laser run-time. Two figures of merit are introduced for the purpose of characterizing the response of power-optics mirrors and windows in terms of thermally induced wavefront errors. For cooled mirrors, it is proposed to consider a figure of merit FoMm = K'eff/ (Am), where K'eff relates to the thermal conductivity, the heat-transfer coefficient, and the thickness of the faceplate, whereas AM measures the mirror absorptance, and E is the distortion coefficient. Similarly, the window figure of merit is FoMw = Cp/ (AW'x), where C'P represents the specific heat per unit volume, AW measures the window absorptance, and x is a distortion coefficient as defined in earlier investigations. On using these figures of merit, an important result emerges: if the window material has a positive distortion coefficient, window lensing suppresses the steady-state mirror-induced aberration at time tc = N(D/d)2FoMw/FoMm, where N is the number of relay mirrors and D/d is the telescope magnification. Under normal circumstances, the far-field irradiance exhibits strongly nonlinear features. Refocusing can eliminate the effect of mirror distortion but, ultimately, window distortion dominates and
Alkali halides and alkaline earth fluorides have many attractive optical properties which are of interest for component applications in several optical systems operating in adverse environments. The fact that many of these materials have low yield strengths and fracture energies in single-crystal form often restricts their applications. The yield strengths and fracture energies of single crystals of many alkali halides can be increased significantly, in many cases by an order of magnitude or more, by deformation processing, such as press forging. The slow compressive deformation and high temperatures to which materials are subjected during press forging introduce low- and high-angle grain boundaries which are responsible for the observed improvement in mechanical properties. This paper describes a forging technique which repeatedly produces crack-free, fine-grained forgings and discusses the optical and mechanical properties of press forged LiF and CaF2 in relation to those of starting single crystals.
The general erosion behavior of materials used for radomes and irdomes exposed to both rain and solid particle environments is described. Differences in the loading conditions, nature of the fractures produced, and modes of material removal due to impacting water drops and solid particles are explained. It is concluded that there is a general need for data pertaining to transmittance degradation in sensor window materials as a function of the exposure time in a well-characterized erosive environment. The experimental data can be used with a model of the operational environment in conjunction with a proposed model to determine the amount and nature of the particulate matter impacting the sensor window to establish the potential limitations on its operation.
This paper is a survey of several cryogenic effects on optical components and the problems associated with operation of optical systems at cryogenic temperatures. These potential problem areas evolved, in most cases, from operational difficulties in large cryogenically cooled space test chambers and were studied experimentally in smaller research laboratory chambers. The specific areas discussed are the temperature sensitivity of the complex refractive index, the usage of polystyrene as a wavelength calibrator at 20 K, the transmittance and reflectance changes with temperature, the infrared spectra of typical condensed gases on a cryogenic window, and the use and calibration of blackbodies at temperatures below 400 K.
This paper discusses the development of a three-channel high-resolution image-generation system consisting of a matrix of three television cameras capable of operating with the output of a single wide-angle (140 FOV) optical probe. The television cameras are capable of correcting the geometric errors of the optical probe, the television display, and the display optics as well as the internal scanning errors of the cameras themselves from an external con-trol panel. The cameras achieved an overall geometric error of less than 0.25 and exhibited a stability of 0.1 %, for a 24-hour period. The three video images, in conjunction with a suitable matrix of displays, are capable of presenting a continuous visual scene 140 deg. wide and 45 deg. high, with a resolution of 2400 television lines per picture width. This program also included a three-channel special-effects generator capable of electronic generation of fog, haze, clouds, and electronic sky for the upper portion of the scene above the model board and operation under day, dusk, and night conditions. The unit also has the capability to enhance cultural lighting which can be accomplished without any change in the model lighting or camera operation. The unit allows for dynamic flight characteristics such as roll, pitch, yaw, and flight above and below clouds across the entire matrix of video scenes displayed.
This paper describes the development of a high-resolution 25-inch color display for simulator visual systems. One of the unique features of the display is the incorporation of a microprocessor-based digital convergence system capable of achieving absolute convergence at 256 points. This system permits the operator to adjust convergence, geometry, size, position, brightness, and contrast of the display through the simulator optics system by means of a remote-control unit. This feature can be used to correct for color aberrations of the optics as well as the normal misregistration (misconvergence) associated with color cathode ray tubes (CRTs). Linear feedback amplifiers are used in the display to achieve superior long-term convergence stability. The control functions available include not only the normal red, green, and blue adjustments but also dynamic blue-lateral control. These unique design features greatly simplify the convergence- and geometry-correction process; and, as a result, a relatively unskilled operator can accomplish precision registration of a simulator visual system, even with a matrix of color displays.
Pyroelectric detectors appear to be an attractive alternative to the conventional immersed bolometers which are used in horizon sensors for spinning satellites. The noise of the detector decreases with frequency thereby offering very good signal-to-noise ratio for a fast-spinning satellite. In addition, the increase of responsivity at low frequencies again provides better signal-to-noise ratio than could be achieved with immersed thermistor bolometers:
The latest results in the Naval Research Laboratory program of far-ultraviolet electrographic camera development, and application of these cameras to astrophysical and upper-atmospheric investigations, are presented. A new large electrographic Schmidt camera, of 15 cm aperture and f/2 focal ratio, has been successfully used in two sounding rocket flights, one for direct imagery in the 1230-2000 A wavelength range and the second for objective spectrography in the 950-2000 A range, of stars and nebulae in the Cygnus region of the sky. The camera has an 11° field of view and better than 30 arc sec resolution (2 A spectral resolution with 600 line/mm objective grating). A nebular spectrograph, based on a microchannel-intensified electrographic Schmidt camera, was then payload of a June 1979 rocket flight. It covers the 1050-2000 A range, and can reach emission line features as faint as 10 Rayleighs in 60 second ex-posures with 5 A spectral resolution. Electrographic cameras with mesh-based semitransparent photocathodes, capable of observations in the extreme ultraviolet below 1050 A, are being developed for a number of space science applications. A camera with microchannel intensification is the detector in an XUV airglow/auroral spectrograph used in a February 1978 rocket flight, and cameras without microchannel intensification are proposed for various solar XUV spectrographic and spectroheliographic applications.
Cost and weight data for optical and radio telescopes are analyzed to derive scaling laws appropriate to the design and costing of very large telescopes of the future. A scaling law exponent close to the 2.0 power of aperture diameter is found for telescopes of comparable sophistication ranging from a 0.4 m to 5.0 m aperture, in contrast to the often cited 2.7 exponent. Predicted characteristics for a 25-meter aperture steerable dish Next Generation Telescope (NGT) are 5 x 10 6 kg, 20 $/kg and a total cost of 1.3 x 10 8 $.
The fundamental measurement required for proof of quality of an optical system is the shape of its transmitted wavefront. A knowledge of wavefront error and the transmission characteristics of an optical system allow calculation (at the measurement wavelength) of all commonly used quality factors such as optical transfer function, point spread function, Strehi ratio, etc. This paper describes a recently developed shearing interferometer that is designed for general purpose laboratory use. Included is a description of the instrument characteristics and principle of operation as well as a comparison of measured and theoretical accuracy limits.
The performance and dosimetric characteristics of the first U.S.-installed hybrid head scanner developed commercially by CGR Medical Corporation have been evaluated. Using a translate-rotate motion with a xenon gas detector array, the scanner produces excellent low contrast resolution at low dose levels (single slice surface dose of 1 rad). Console selection of scanning parameters, viz.: scan time, kV, mA, beam filtration, slice width, and pixel size permits optimization of image quality for various exams. For optimizing either spatial or low contrast resolution, a push-button choice of reconstruction algorithms is available. Using the American Association of Physicists in Medicine (AAPM) computed tomography (CT) phantom, spatial resolution and line spread functions for both algorithms were determined from the graphical display of CT numbers. Low contrast resolution was measured at various contrast levels as a function of kV, dose, beam width, pixel size, and algorithm. Values for noise (a) were also recorded for these scanning parameters. Patient dose distributions for various operator-selectable beam widths (3, 6, 9 mm) were obtained using film and TLDs. CT number linearity and contrast scale were determined using different plastic rods.
The Modulation Transfer Function (MTF) of an x-ray focal spot has been obtained without the aid of a microdensitometer. The modified method described in this study utilizes radiographic magnification techniques to eliminate the need for a microdensitometer. In this study a radiographic magnification factor of 25X or higher has been employed to obtain the slit-camera images of x-ray focal spots. The Line Spread Function (LSF) is obtained by scanning the focal spot slit-camera image acquired with the modified method on an ordinary densitometer equipped with a 0.2 mm aperture. The MTF is subsequently calculated from the LSF and compared with the MTF that was obtained by means of the traditional method. The results show good agreement between the MTFs obtained by these two methods.
Optical Engineering has grown with our Society over the years, and today enjoys an international reputation as a journal of archival quality. The articles and papers which have been published within its covers are referenced more and more frequently in new works, attesting to the stature of Optical Engineering within the community. I am sure most readers are aware of its reference value in the daily practice of our optical engineering profession.