Plots of intrinsic scattering and absorption coefficients, the so-called V curves, are compiled for 18 crystalline materials. Extrinsic scattering coefficients are evaluated for voids, inclusions, surface imperfections, dislocations, strains and anisotropic grains. Reduction of this extrinsic scattering is a major problem that must be solved in order to attain low-attenuation (10-2dB/km) fibers.
Since the development of the first polycrystalline infrared (IR) fiber waveguides by Hughes Research Laboratories, there has been an increasing effort to improve losses in waveguides fabricated from IR transmissive crystalline materials. The current fiber losses of 400 dB/km for KRS-5 (thallium bromoidide) fiber are three orders of magnitude above the intrinsic limit for this material at 10 pm and far above the ultimate projected loss near 10-3 dB/km for this and many other IR crystalline solids. Therefore, applications of present IR fibers in IR sensor systems are limited to lengths less than 2 to 3 m while future long-distance communication links await the development of the ultimate low-loss potential of these materials.
Fiber optics operating in the mid-infrared offer the potential for lower losses and better tolerance to nuclear radiation than current silicate-based fibers. Moreover, mid-IR fibers may be useful for a variety of shorter-distance applications such as laser surgery, spot welding and infrared integrated optics. Although a wide array of potentially highly-transmissive mid-IR materials are available in bulk form, most are not suitable for fiber fabrication. Recently, however, a variety of new multicomponent glasses based on the fluorides of heavy metals have been developed, which may offer the best prospects to date for high performance mid-IR fibers. A critical comparison of the advantages as well as the problems associated with various prospective materials and fiber fabrication techniques is given.
In this paper I will discuss both the potential and application of infrared fibers, specifically for the wavelength range covering 0.8- to 12 μm. Much has been said about the potential of IR fibers, what can be done and what is being done, so it will not be necessary for me to emphasize that aspect. However, any good talk is normally organized in terms of what, how, and why. Since in some sense I am going to speak for the users of infrared fibers and their accompanying technology, I will concentrate on why, rather than how. I will try to provide the necessary justification for the need for low loss fibers in the IR region.
Both multimode and single mode optical fiber transmission systems can provide long distance transmission at moderate to high data rates. Multimode graded index optical fibers are clearly adequate up to about 20 Mb/s at any wavelength between 0.8-1.6 μm. On the other hand, the lower dispersion of single mode fibers is advantageous at data rates above 100 Mb/s. Between 20 Mb/s and a few hundred megabits per second, there are tradeoffs between the choice of multimode and single mode optical fiber transmission. This paper addresses these tradeoffs and the practical limit for transmission between 0.8-1.6 μm. Both attenuation and dispersion limits to transmission distance are reviewed. The limitations due to the small diameter of the single mode fiber are also highlighted.
This paper reviews recent progress in the development of InGaAsP optoelectronic devices for fiber optic communications in the 1.0-1.7 μm wavelength bands. With this material system high performance optical sources (both lasers and LEDs) and photodetectors may be prepared. Both vapor phase epitaxy (VPE) and liquid phase epitaxy (LPE) have been used to construct devices. Edge emitting LEDs with spectral widths of only 60 nm and modulation bandwidths exceeding 200 MHz can couple over 100 μW of optical power into 0.55 μm core graded index fiber. Single mode lasers with threshold currents below 100 mA couple more than 1 mW of optical power for long range communication applications. High speed PIN photodetectors with rise times less than 0.5 ns complete the picture. For the first time a complete optoelectronic technology can be developed around a single material system (InGaAsP) for the entire 1.0-1.7 μm spectral range.
The operating characteristics and the state-of-the-art of PIN and avalanche photodiodes for fiber optical communications over the 0.8 - 1.6 μm range will be reviewed. The joint influence of detector and receiver circuit element parameters on digital and analog receiver sensitivity will be discussed.
Optical waveguide operation in the mid-i.r. waveband requires the use of alternative materials to those conventionally based upon silica, this being a necessary consequence of fundamental absorption mechanisms; in selecting such materials the anionic component is the primary consideration. For high performance waveguides it is necessary to have as nearly coincident zero material dispersion and minimum loss wavelengths as possible in order to utilise the i.r. low loss potential and also to maximise bandwidths for monomode operation. Short distance power or image transfer applications are dependent solely upon the material minimum loss wavelength.
The growth of single crystal AgBr optical fibers is described. A form of the Czochralski process is used and lengths of fiber up to 10 m have been made. Transparency of AgBr extends from 0.5 to 25 μm and measured attenuation of fiber and bulk material agree at ~ 2x10-2cm-1. Crystal growth occurs consistently with the fiber axis in the  direction. The absence of grain boundaries should result in low optical scattering. It should be possible to grow optical fibers from many other crystals by this method. Numerous applications in infrared transmission and in active laser devices exist.
Crystals which transmit radiation in the 3 to 35 micrometer range are candidates for optical fibers. Some optical and mechanical properties of alkali halides, thallium halides, and alkaline earth fluorides are presented, with emphasis on the effects of impurities. Crystal growth and fiber production are discussed briefly.
A new family of glasses derived from hafnium fluoride (HfF4), barium fluoride (BaF2), and various rare earth fluorides has been studied. The spectral and thermal properties are given, as well as hardness, rupture strength, and coefficients of expansion. Reactive atmosphere processing (RAP) of the individual components and the molten glass with anhydrous CF4, BF3, HF and CC14 are described. RAP eliminates anionic impurities such as 0H- and 0= which enter the condensed phase through hydrolysis during the melting and mixing stages of preparation. Elimination of these impurities through RAP maximizes IR transparency and mechanical strength. The glasses are resistant to room temperature hydrolysis, are hard and strong, and are continuously transparent from 0.2 μm to 9 μm.
This paper will describe a proposed optical undersea communication system targeted for transatlantic service in 1988. The system uses multiple single-mode optical fibers carrying 4,000 voice circuits each way on digital streams at 274 Mb/s or higher. With the aid of a digital time assignment interpolation system (TASI), the capacity per fiber could be increased to 12,000 voice circuits. The system will contain up to six fiber pairs for a maximum capacity of 36,000 two-way circuits. The 1.3 μm transmission wavelength will permit repeater spacings in the order of 35 km. The discussion will cover system para-meters, regenerative repeaters and the fiber cable.
In 1978, Hughes Laboratories reported development of fiber optics that were capable of transmitting CO2 laser energy. These fibers are now being tested for medical applications. Wide ranging medical investigation with CO2 lasers has occurred during the twelve years since the first observations of laser hemostasis. Specialists in ophthalmology, neurosurgery, urology, gynecology, otolaryngology, maxillo-facial/plastic surgery, dermatology, and oncology among others, have explored its use. In principle, all these specialists use CO2 laser radiation at 10.6 microns to thermally destroy diseased tissues. As such, CO2 lasers compare and compete with electrosurgical devices. The fundamental difference between these modalities lies in how they generate heat in treated tissue.
A new class of optical fibers operating in the 2 μm to 6 μm region and at longer wave-lengths is presently under development. These fibers offer the enticing possibility of losses far below those currently or potentially achievable with conventional silica fibers. They may be useful in applications requiring short links, such as those found in a variety of instrumentation systems. Ultimately, when their development reaches the predicted performance, their use in repeaterless communication systems, over hundreds to thousands of kilometers in length, may become a reality. A very important aspect of the systems problems that must be considered is the availability of sources and detectors. This paper briefly reviews the state-of-the-art in laser sources and photodetectors that are appropriate candidates to be used in conjunction with these fibers. The characteristics of presently available Pb-salt and other lasers are reviewed; the performance parameters attained with HgCdTe and other detectors are summarized.