It is a distinct privilege to speak, as a military man, to this informed space age civilian audience. I am enjoying my evening with you very much, and I would like to use 15 minutes of it to talk to you about Air Force Basic Research.
Fiber optics is a facinating field of modern optics which depends primarily on the principle of total internal reflection. Fiber optics usually refers to the technique of conveying light or images through a particular configuration of glass or plastic fibers.
The field of fiber optics has grown over the past few years, to the extent that we find fiber optics playing increasingly important roles in the advancement of current world science and technology, as well as in the shaping of future technology. The fields employing fiber optics are varied and the applications are vast. The purpose of this paper is to provide the newcomer to the field with some insight into the current technology and survey the application of fiber optics.
Considerable effort has been expended over the past 17 years in both technological and fundamental investigations pertaining to the field of fiber optics. Only recently some fiber optics devices have become an economic reality. However, there still remains considerable fertile ground to be exploited in the field of fiber optics, both from a theoretical, as well as technological, standpoint. Complex assemblies of fibers to perform numerous static scanning functions have not been fully realized as yet because of technical complexities and high cost. It is expected that light transmission, image quality, extension of spectral range, and physical ruggedness of fiber optics will see significant improvements.
This paper describes a technique for drawing continuous quartz fibers, and cladding them with a lower index material. It also describes the method used for optically evaluating bundles made from these fibers, and presents some results.
This paper reports recent work in infra-red fiber optics capable of transmitting over the spectral range 8 to 14 microns using glasses composed of As, Se, Te and S. The external transmittance of 1-inch long fiber bundles averages 40 percent in the 2 to 12 micron wavelength region. This transmission includes losses due to end reflection and packing fraction, but the end reflections may be reduced using anti-reflection coatings. It is predicted that the transmission would be increased to 70 percent in the 8 to 12 micron region. Beyond 12 microns the fiber bundles become almost opaque due to the oxide impurities in the glass. The 3 to 4 mil diameter infra-red fibers will bend over a 1/16 inch radius and will support at least 50 grams weight. Improved infrared fibers should be attained by using the new Ge-As-Se glasses and a vitreous carbon concentric crucible system.
Fiber optics devices have assumed many forms such as flexible bundles, fused plates and geometry converters. The materials used include glasses, plastics and, in nature, needle-like crystals of boron and other materials. In all cases, the basic fiber consists of a transparent core in contact with a material whose index of refraction is different, usually lower, than that of the core.
There has been a demand in recent years for fibers up to 100 feet in length having high transmittance and able to withstand severe vibrations and high temperatures. High transmittance glasses most widely used for the fiber core have been the lead flint glasses such as Schott F2. We have on occasion been able to obtain absorption coefficients as low as 0.05 per foot using this F2 glass, but the absorption increases rapidly as the fibers are heated. Furthermore, these glasses have softening points which limit their use at elevated temperatures. Several new glasses have been developed having high transmittance and able to withstand the higher temperatures. Data on the spectral transmission as a function of temperature is presented for these glasses, together with their absorption coefficients at the wavelengths of peak transmission. One of these glasses has a transmission over a 100-foot length which is five times greater than that of F2, and has an absorption coefficient of 0.027 to 0.033 per foot at the wavelength of peak transmission. During the past few years considerable advances have been made in the area of fiber optics generally and an accelerated utilization of this technology is anticipated in the future. Fused elements up to one or more inches in overall length are approaching their theoretical maximum performance, but in contrast,the performance of many flexible fiber optics elements, such as "light pipes" and imaging bundles, is very much less than that which can be ultimately achieved.
A spokesman for one of our leading optical manufacturers said, in 1963: "Fiber optics must find its place in industry within the next three years or revert to a fascinating but impractical curiosity, unobtainable for a competitive price. "
The sole purpose of a flexible incoherent fiber optic bundle is to transmit light from point A to point B; and the major utility of such a bundle lies simply in the freedom allowed the designer when choosing the path connecting A to B. Therefore, one of the primary parameters to be considered in the evaluation of such a bundle is the efficiency with which it transmits. That is, with a given input, what output may be expected for a given length of material? This efficiency determines the sophistication required of the illuminating and/or sensing components employed for a particular application, and t us substantially directs the ultimate cost of the total system.
Fiber optics is beginning to enjoy a surge of popularity in application in fields somewhat removed from physics and engineering. We are becoming familiar here with many sophisticated applications and developments. However, there are fields which very often do not have the opportunity to take advantage of these new developments to apply them to old procedures.
In the past, the task of monitoring and/or recording various fields of view, whose positions and sizes varied enough to make it impractical to perform a simple mirror scan, has been accomplished by assigning one sensor to each of the fields. These systems have been built and used successfully and are well documented in the literature. Of course, the use of this approach often resulted in significant penalties of weight, power, and costs.
Fire Detection The occurrence of a fire in a military aircraft constitutes one of the largest single causes of aircraft losses and fatalities. It is therefore important that a fire be detected in adequate time to take compensatory action. False warnings and high failure rates have plagued past detection systems, with the result that crew members have little confidence in these systems today. New systems presently under development can solve many of these problems, but as with any problem of this complexity, solutions are neither universal nor straightforward. Thus it is that high temperature quartz fiber optics bundles can be called upon to solve some of the unique problems still existing in the area of fire detection on supersonic aircraft.
The primary function of the optical combining network is to project one or more (four in this case) images onto a common image plane. The design of a rigid optical system employing conventional prisms, relay lenses and suitable combining elements would require considerable structural support and adjustable elements in each path to effect image size correction. On this application, the individual optical paths are of unequal lengths which prevent the use of identical optical elements. Some of these limitations can be eliminated or substantially reduced by using image forming fiber optics. Such a system can essentially be reduced to two lens elements and the fiber bundle. The projection path now becomes a completely flexible element wherein the orientation of the image with respect to the object can be effected by simply rotating the fiber bundle about its projection axis. Image size remains at unity between the two fiber ends independent of path length, thereby eliminating the problems associated with unequal path lengths. The optical paths are all referenced to a fixed coordinate axis in space, in this case a vidicon camera. The images are scanned in a conventional TV mode with the vidicon output being fed into a binary logic system. Each of the patterns is checked for overall geometric conformity as well as the relative position in space with respect to one another.
In order to excite only a desired mode, it is sufficient to generate an electro-magnetic field whose transverse components on the end of the fiber match those of the mode. A technique which approximately satisfies this condition is suggested. The results of experiments which involve the use of spatial filtering techniques in the pupil of a launching lens are shown. A method for determining the relative power in each of several modes at the output end of a waveguide will also be presented. It is a vectorial spatial filtering technique, based on the orthogonality of the mode fields. Experimental results confirm the applicability of the method to the one case tested, that of a large diameter optical fiber of low numerical aperture.