One of the ways that the human body maintains constant temperature is by exchange of radiant energy between the skin and objects in the immediate, environment. In 1957, Lawson observed. that cancer can alter the pattern of emission of thermal energy from the female breast.' This observation stimulated a number of studies of thermographic detection of breast cancer and a number of ,publications with contradictory conclusions regarding the effectiveness of thermographic techniques., To some degree, these contradictions probably reflect inadequate experience of persons interpreting, thermographic images, and improper ambient conditions such as asymmetrical air flow during the thermographic examination.
TOPICS: Optical communications, Electromagnetism, Physics, Information and communication technologies, Explosives, Channel projecting optics, Singular optics, Information security, Defense and security
Modern Optical Communications is a fascinating marriage of the disciplines of physics, optics and communication technology. Although the transmission of information by optical means had been practiced in rudimentary form for centuries prior to the advent of the laser, it was with the invention of that device that serious efforts got underway to exploit the potential advantages of information carriers located in the optical portion of the electromagnetic spectrum. Much of the impetus for these efforts has been provided by the explosive growth of information handling requirements generated by both the military and civilian markets, and the resultant search for ever-greater channel capacity. In this regard, the potential of a single optical channel to provide a capacity in excess of 1012 bits/second is indeed attractive. Other, less dramatic, advantages of optical links that have been cited are privacy or security, decreased susceptibility to electromagnetic inter-ference, relatively low power consumption, compactness, and comparatively low cost.
A short history of laser communication developments devoted to space communications is presented, followed by a description of present high data rate (up to 1 Gbps) systems under development, and discussion of the most promising space laser commuication techniques. Critical. components such as the laser, modulator, detector, high speed electronics, and acquisition and tracking system will be discussed.
A He-Ne laser communication system with video bandwidth capability is described. The system is suitable for the transmission of video signals generally compatible with U. S. commercial practice and the transmission of digital data at rates of approximately 10 to 1000 Kb/s, over ranges extending from a few hundred feet to several miles. The system utilizes an externally modulated He-Ne laser, direct detection with a photomultiplier tube and a frequency modulated subcarrier method for coping with the dynamic fade due to atmospheric turbulence. Use experience indicates that such systems perform according to specification in a variety of applications and over a wide range of weather conditions.
Recent advances in the semiconductor laser and photodetector state of the art has led to the development of small low cost, reliable laser transmitter and receiver modules (see Fig. 1). These modules are now being designed into a number of practical communication systems used for a wide variety of applications (see Figs. 2 and 3). These systems have been designed to transmit voice, computer data, and video sivals and offer a number of advantages over previous transmission methods including cost, security, simplicity, bandwidth and licensing requirements. Where atmospheric limitations are either absent or surmountable these optical communication systems offer the communication engineer a viable alternative.
The paper describes the design of an infrared transmitter and receiver for communication over 1000 feet typical distance. The purpo.se of the system was to circumvent obstacles in the construction of the closed circuit television systems, and it has been successfully used for that purpose in a number of applications. The authors show calculation of path characteristics which determine fade margin requirements and the level of path dependent noise. They describe experimental verification of the calculations on a 1021-foot test path. Part of the paper describes the pertinent characteristics of the Gallium Arsenide Transmitter diode and the receiver photodiode, especially with respect to bandwidth and linearity. The optical system design is described in detail, including tolerances on the aiming system and the mechanical support of the diodes. Some of the measurement_ techniques developed to insure performance of manufactured systems are described, including techniques for power measurement and alignment.
The replacement of currently employed 26-pair and 27-pair cables within tactical communications Command Posts and data processing facilities has long been a goal of Army communications personnel. The use of optical atmospheric and fiber optic links offers advantages in cost, mobility, size, and security over current cable systems. The AN/TVC-4 is an advanced development model of a tactical optical communications set. It utilizes a GaAs laser diode source and an avalanche photodiode detector to provide full duplex data and voice transmission. Maximum data rate is 200 kilobits/sec with a close-confinement, single heterostructure laser diode and 5 megabits/sec with a Large Optical Cavity (LOC) laser diode. The system is designed to maintain a 10-6 probability of error (Pe) at a 3-km range under conditions of 10 dB/km atmospheric attenuation. The complete transceiver, less battery pack, is housed in a rugged 7 X 7 X 7.5 in. package which has undergone temperature, shock, vibration and humidity qualification testing. System design, component selection and performance analysis are described in detail and the results of field tests are presented.
This report describes an availability model for short-haul optical communication links operating through the atmosphere. To check this model, to assess equipment reliability, and to determine problems associated with fog, rain, snow, and atmospheric turbulence, a commercial link was operated for six months on a 3000-ft path near Boulder, Colorado.
A general review is presented of heterojunction laser diodes suitable for efficient room temperature operation. A variety of structures are described which allow either pulsed or CW operation under operating conditions useful in optical communication systems, particularly those using glass fibers.
A review is given of some of the fundamentals of fiber optical transmission systems including fiber optical waveguides, optical sources, and photodetectors. Fiber optical systems can, in principle, compete with twisted pair, coaxial cable and waveguide facilities in virtually all sectors of the communication network. These systems are attractive because fiber optical waveguides are potentially inexpensive, have miniature cross section, are strong and flexible, have light weight, and can transmit broad band signals over long distances with low distortion. Recent advances in glass materials technology and fiber fabrication procedures have resulted in fiber waveguides with losses as low as 4dB/km in the important wavelength region around 8200A corresponding_ to the output of the GaA1As laser. The range of applications of fiber optical systems extend from short and long haul communications to wiring within buildings and between computer terminals, as well as to a variety of military applications which capitalize on the unique properties of glass fibers.
Holographic techniques offer practical methods of creating automated, computer-controlled, mass storage systems for recording and retrieving cartographic and mapping information. Graphs, overlays, pictures, maps, multispectral data, etc., can be stored at high density in either digital or analog form or both on light-sensitive, nonmagnetically alterable media. Whereas conventional micrographic techniques can be used to reduce the volume of images stored using demagnification factors of 16 to 100 times or more, these techniques are limited in practice by requirements to maintain document resolution and by necessary depth of focus considerations.
A high brightness display is achieved using tungsten sources to read out phase image-plane holograms. Full color is provided by using three superimposed component holograms derived from three primary color separation object transparencies of the original subject. The holograms are contained on transparent plastic tape which is surface modulated by embossing from electroplated masters. Any frame on the tape can be retrieved rapidly by means of the superimposed frame address Fraunhofer binary holograms which use a gallium arsenide laser to reconstruct the image on a silicon diode array. The system lends itself to post-embossing annotation. Reference Fresnel holograms recorded on each frame-enable the frame to be positioned in X and Y so as to allow the map image to be driven in response to navigational tracking requirements. Because incident readout light is modulated by diffraction rather than absorption, the holograms do not rise in temperature as a result of the conversion of light to heat. This feature enables considerably more light flux to be gated by the holograms than could be gated by any other medium of the same area. The resulting image brightness is adequate for viewing in the high ambient light environments. Since the production of color involves diffraction rather than absorption by dyed emulsion, bleaching with consequent color degradation cannot occur.
Topographic mapping can be considered as the combination of processes required to produce a topographic map. Coherent optics has the potential for playing many roles in mapping systems of the future. The purpose of this paper is to indicate some of these roles. The limited discussion must be considered as an initial ad hoc attempt to define potentials in the eight areas of Photogrammetric Data Reduction, Image Processing, Optical Memories, Pattern Recognition, Performance Prediction, Holographic Terrain Displays, All-weather Mapping, and Field Applications. Other papers from the proceedings are broadly integrated along with selected references.
A surprisingly large number of papers have been written on the effects of partially coherent light in image formation (for a recent review see Thompson 1969). Thus the major effects that can occur are well documented. With imaging systems that are used with transparent objects the characteristic edge ringing and edge shifting are familiar results. In reflective systems the speckle pattern is usually the most severe effect that often masks the ringing produced by coherent object illumination. Thus this paper will be primarily concerned with systems used in transmission. It is important to note that the majority of papers in this field assume diffraction limited performance of the imaging optics; this is, of course, laudable if the system is to be used with incoherent object illumination. If, however, the system is to be used with coherent or partially coherent object illumination then other considerations become important.
This paper describes a method of measuring the amplitude and phase distribution of the optical field at and in the vicinity of the focus of any imaging system. The directional characteristic of optical heterodyne detection provides a means of obtaining extremely high spatial resolution in measurement. Experimental results include the measured amplitude distribution at the focus of an off-axis holographic zone plate. The results are extended to the case of a diffusely reflecting or transmitting object to obtain a position-sensing gauge. Analysis shows that for good resolution in position sensing the illuminated area of the object should be equal to the diffraction limited spot size of the receiver aperture. A description of the heterodyne interferometer and its possible areas of application are included.
In the development of modern linear microdensitometry, the trend of optical system design has been towards the condition of underfilling the efflux optics, with total collection of light after it passes through the sample. The system transfer function is therefore attributable to the influx optics, and the sampling aperture is the light distribution impinging on the sample, reduced from an illuminated slit or circular aperture through the influx optical system. The maximum frequency response of the system is obtained when the sample is illuminated with the impulse response of the influx optics. However, the theoretical impulse response can only be realized by imaging a delta-function and this is photometrically impossible. Similarly, because the system images an illuminated aperture onto the sample, scanning with a pure, geometrically-characterized slit or spot is not possible due to lens response and diffraction. These two problems are investigated, for both coherent and incoherent illumination. For both impulse response and slit image, the MTF is investigated, and its deviation from the ideal is calculated. The results are characterized in terms of RMS-MTF differences over the spectrum for 10, 5, 2 and 1% levels. The controlling parameter is the ratio N/R, where R is the reduction factor employed for the influx optics, and N is the number of resolution elements contained within the nominal object slit width. The study shows that there are significant differences in these values for the same RMS difference level, with coherent and incoherent illumination, and that there are compromises to be made with both kinds of illumination. The results of this study facilitate calculation of system response for any configuration of object slit and influx optics (within the linear microdensitometer system), and defines limits on slit sizes for operation with impulse response and geometrically characterized slit images for the sampling aperture. The effects expected with the linear polarization associated with laser illuminat