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Fiber optics in military communications: the dawn of a new era.

Fiber Optics in Military Communications

The Dawn of a New Era

Almost since the early days of cable and wireless communications, it became quite clear that the higher the carrier frequency, the wider the bandwidth and hence the greater the volume of messages that could be carried. Today, the utilization of optical energy in communications provides frequencies in the range of [10.sup.13] to [10.sup.14] Hertz which allows engineers to iron out a vast number of transmission capacity problems.

The properties of light as an information carrier had already been researched some 50 years ago. However, there were two problems standing in the way of its practical application. The first was the lack of a light source which could be easily modulated and emit light on a single frequency. The second was the trouble-free propagation of the modulated signals. The projection of a light beam through the air was subjected to atmospheric conditions, under which absorption, scattering, diffraction, etc. - commonly called attenuation - occur in unpredictable ways. This meant that an optical communications system had to be powerful enough to overcome natural attenuation, otherwise it would be useful only under very benign atmospheric conditions. It thus became necessary to find a medium - a waveguide - which would produce a minimum of attenuation and also "guide" the waves around bends and corners. This had to be cheap and easy to install and maintain. The first requirement was to develop a powerful yet small light source which could be modulated. The breakthrough came in 1960 with the invention of the laser at the Hughes Laboratories, and the development of light-emitting diodes (LED), both of which can generate well collimated light beams of high intensity. Both devices can be simply controlled and modulated with low-voltage signals. This placed the dream of communicators within reach - an almost unlimited volume of simultaneously carried light signals.

Birth of the Fiber Optic Cable

Two approaches were initially pursued for the development of a practical waveguide. The first was the use of gas-filled tubes in which temperature was controlled at selected points for steering the refractive index of the gas. The second was the use of glass/quartz fibers in which the required refractive index was varied across the radius of the fiber during manufacture. This second approach achieved the desired goal in the late sixties when a light attenuation factor of only 20dB/km could be demonstrated.

The modern optical fiber cable was born. It has since seen a rapid progress. By 1980 attenuation had been lowered to less than 1 dB/km and modulation rates of several Giga-Hertz (GHz) became possible. In the mid-eighties, when fibers with losses of 0.5 dB/km could already be fabricated, the light intensity was approximately halved every six kilometers. This low attenuation loss was achieved at the relatively long wave-length of approximately 1.3 meters, located in the infrared region. Marconi has recently demonstrated an optical fiber system offering 0.2 dB/km attenuation with a modulation rate of up to 20 GHz. By way of illustration, this bandwidth is capable of carrying about six million telephone conversations or 3 000 TV programs simultaneously in a mono-mode glass fiber of only one-eighth of a millimeter in diameter. The very latest fiber cables operate on wavelengths in the 1.55 meters range, which has by now become a universally accepted standard, and with 0.18 dB/km come very close to the theoretical attenuation limit of 0.14 dB/km.

Lasers vs. LEDs

Compact and powerful semiconductor diode laser chips were being developed in parallel. The devices have active layers of gallium indium arsenide phosphide grown on indium phosphide substrates. However lasers, in particular those for the longer wavelengths, are difficult to manufacture, are expensive and require special precautions when used in optical fiber transmitters. For these reasons there was at first a great interest in LED transmitters, which are based on the same materials as the lasers but have simpler structures and are easier to handle in optical fiber systems. But there is a penalty: the LEDs' optical power is lower and they have a much wider light spectrum. Which type, LED or laser, is finally used depends on the system's requirements. The laser appears now to be emerging as the winner because types have been developed lately which emit on the single precise wavelength of 1.55 meters, exactly where the lowest attenuation rate in the light spectrum is found. Considerable pioneering work in this field has been performed by firms like Plessey, Hughes, Siemens, SEL, Thomson and numerous other laser specialists.

The Role of the Repeater

An essential part of any optical fiber system designed for long-range communications is the repeater (also called pulse restorer or electro-optic regenerator). Light signals can be transmitted through an optical fiber either digitally, by pulsing the light, or in analog form, by varying its brightness. The usable bandwidth results from the maximum rate at which the pulses comprising the signals can be sent and received without error. A form of attenuation called dispersion sets a practical limit to that rate. The typical speed of a digital on/off pulse is one billionth of a second; here dispersion means that a pulse is spread out in time and part of it arrives too late to be interpreted correctly at the receiving end. To "boost" the pulse one uses semiconductors incorporating a receiving system (usually an avalanche photo diode), amplifiers and diode lasers or LEDs as transmitters. This assembly, usually mounted on chips, receives and reforms the light beams and re-injects them further down the optical fiber. The power required for the operation of the repeater - a mere 1/1000th of a Watt -, is almost negligible. Technically, the repeater's design is not unlike that of the transmitter and receiver at the terminal ends of the communication line, where the digital signals are demultiplexed, amplified and forwarded digitally, or demodulated to analog format.

Advantages of Optical Fibers

The primary impetus for the development of optical fibers originated in the commercial and not the military field. The communications industry was beset with two major problems; first the laying and maintenance of costly conventional undersea cables, whose capacity by the late sixties was becoming insufficient; and the high-risk ventures of operating satellite networks, which have an average life expectancy of only seven years. It had therefore been looking for a long time for a suitable way of extending these established means of communication.

ATT-Bell, Japan's NTT, France Telecom, British Telecom, German Telecom, Northern Telecom, plus communications equipment producers such as CIT Alcatel, Philips, AEG Kabel, Siemens, GTE, Plessey, Marconi, Italtel. RCA, Siecor (a joint venture between Siemens and Corning Glass in the USA) or Raytheon, just to name a few, have over the past 30 years invested billions in optical fiber research. Today optical fiber undersea cables, no thicker than a garden hose but capable of carrying up to 80 000 signals simultaneously, link Europe, America and Asia. German Telecom has already close to 800 000 km of cables in operation. This is only the beginning, since fiber optics, next to microelectronics and software, is regarded today as one of the three key technologies of the future. It is surprising that the military and the defense industry embarked rather late on utilizing optical fibers in spite of their obvious military potential, as the following amply demonstrates.

Apart from their large message-carrying capacity the greatest value of optical fiber systems is their very high data transmission rates: today these reach several hundred megabits per second (Mb/sec). This permits the design of networks with an inherent multiple redundancy in cabling which serves to increase overall survivability. The electric power needed to run the system is minimal because the optical elements, such as the laser or LED diodes, need virtually no amperage.

Of even greater attraction to the military is that optical fiber systems are immune from all types of electro-magnetic interference. They are thus the most secure means of communication since intercept is virtually impossible. In the first place the sheer mass of data passing through the cable will make it extremely difficult to extract a specific signal. Secondly, the lines cannot be tapped without alerting the users. In addition glass cables are ECM-and EMP-proof; they feature complete electric isolation, as no part of the system has to be grounded; and they generate no electric field and no electrical hazards. If the lines are well camouflaged they are difficult to detect because they need not be much thicker than a pencil, even for mass communications, and for simple temporary communication tasks or the guidance of weapons of ground-, sea- and air-launched weapons they can be hair-thin. Even heavy duty optical fiber cables weigh up to 10-20 times less than a copper cable of equivalent capacity. A further advantage in their manufacture is that in contrast to copper, which is in short supply, cheap silica and quartz needed for glass production are available in abundance. A properly designed and manufactured cable is rugged and durable, it will withstand temperature extremes and features an inherent material resistance to damage far superior to that of any multi-wire, coaxial copper cable. Standard optical fiber cables produced for the communications industry withstand proof stresses of 3 500 kg/[cm.sup.2], those made for mobile military purposes almost ten times as much. A tank can roll over a fiber cable without causing damage.

The Cost Factor and Other Draw-backs

Optical fiber-based systems do have disadvantages, however. They are still about three times as expensive as conventional systems. Although the cable as such costs considerably less than its copper equivalent, it is the complexity of the laser and/or LED diodes which must be incorporated in special highly integrated chips that raises the cost. In addition, customized interfaces to link old and new communication systems have to be developed, and new man-machine interfaces have to be designed.

A German Army policy paper issued in 1985 stated that although implementation of fiber optics was desirable, it was impractical for mobile warfare where radio and data link traffic was preferable. For stationary peacetime use the networks of the public telecom services were to be utilized. This may be a reason why in case of a conflict NATO is thinking of tying special optical fiber equipment modules into the Telecom-owned ISDN (Integrated Services Digital Network) optical fiber-based networks of the future, instead of setting up duplicate tactical or strategic [C.sup.3]I optical fiber networks. However, this attitude may change when the old generation of [C.sup.3]I systems is due for replacement by the turn of the next century.

A compromise solution, not unlike that adopted by the communications industry, is the gradual introduction of optical fiber systems at those points in existing military systems faced with signal traffic density and other critical operational problems. In the military such problem areas are the larger headquarters and some important [C.sup.3]I network nodes. For meeting the requirements of the German Army, SEL Alcatel has developed its TACLAN (TACtical Local Area Network), which is currently being put into service. It provides integrated communications services for brigade, division and corps headquarters.


Designed for operation on the purely local headquarters level, TACLAN consists essentially of two common basic component types - field-deployable fiber cables and so-called network access units (NAU). The cables and NAUs constitute a distributed exchange, each access unit providing access to the system for eight voice connections and four digital accesses for computers, data banks, work stations and other [C.sup.3]I elements. Voice and data traffic is handled simultaneously as the optical fiber bus transmission rate of 10 Mb/sec is easily capable of handling the requirements of up to 60 subscribers. If needed, the network access unit's operation and accessibility can be regulated by a commercial micro-computer. The optical fiber cabling serves to link the various vehicle-mounted shelters of the command post, which can thus be dispersed over a large area for better camouflage and protection.

Another typical product for meeting the almost identical requirement is the Swiss Siemens-Albis ATICOS (Albis Tactical Integrated Communication System). Essentially the equipment is a flexible, mobile, militarized version of a commercial ISDN node. It can thus provide fast, secure and reliable communications for voice, data, fax and video and can interface with new optical fiber systems or existing conventional networks. All the required options to interconnect with fiber, copper cable or radio nets are included as standard features. ATICOS' user ports can be configured by the software to support either analog or digital terminal equipment of a large variety of interface standards. This means that virtually any existing communications network, be it commercial, private or military, can be accessed and utilized.


Both the above systems have been designed to operate with conventional long-range communications such as data-link or radio. For the US Army's Fiber Optic Transmission System (Long Haul) or FOTS (LH) program, ITT's Defense Communication Division is developing equipment suitable for mobile field use. The equipment available hitherto meets the goals of mobility, easy deployment and survivability required for tactical cabled communications networks. The target of the program is the eventual replacement of all twin, metallic, coaxial [C.sup.3]I cables. Apart from its primary long haul function the new system will provide radio remoting and inter-shelter connections at headquarters. In the long haul application the system transmits optical signals over distances of up to 64 km at the rate of 20 Mb/sec. Small battery-powered repeaters are needed at every six kilometers. The 5.5 mm-diameter cable, named in military terminology FOCA (Fiber Optic Cable Assembly), is available in standard lengths of one kilometer weighing 29 kg, and is fitted with standard connectors for easy coupling. These connectors are highly critical elements if numerous FOCAs are joined for long haul transmissions. A connector interrupts the smooth flow of light waves in the cable's optical core. The task was to produce a sturdy connector joining two cable ends so tightly that only minimal losses occur. This means that each connector must contain optically polished and physically protected cable ends. Although such connectors are high-precision optical instruments they must be able to withstand rough use in military service. STC Defence Systems of the United Kingdom is a specialist in this field and collaborates with ITT, GTE, Plessey, Siemens and the research establishments of the British and US armies.


The STC-produced FOCA connectors and cables can be immersed in water, are resistant to sand, dust, salt spray, fuels, fungi and oils and can be mated 2 500 times before showing any loss of performance. The cable as such will withstand 100 impacts of 1.5 kg and can be bent sharply at the same place more than 2 000 times before suffering any attenuation. In terms of normal use the FOCA is pretty well indestructible. The signal attenuation of the standard one-kilometer cable and its two connectors is about four decibel.

The implementation of such FOTS (LH) systems for the US Army is going ahead. GTE has already received the contract for providing 18 TGCR (Tactical Generic Cable Replacement) terminals. Each terminal will replace a 26-pair copper cable, leading to major reductions in weight and volume in addition to high security and almost total ECM and EMP-protection. GTE claims that TGCR, for which Plessey is under contract to provide the LED diodes, will under identical conditions carry signals further than copper cable systems.

Long Haul Cable Networks

Apart from serving command and control functions, such long haul cable networks will eventually offer surprising benefits in the real-time combat intelligence field. Each terminal of an optical fiber-based [C.sup.3] network offers direct access to any subscriber, who can rapidly and directly insert an enormous quantity of information without overloading the system. It is therefore possible to feed into the network seemingly irrelevant information which can be filtered and evaluated by computers, operated with neutral/artificial intelligence software (see Armada 1/90), for previously established target parameters.

A potential source for such data might be optical fiber-guided, anti-tank and air defense missiles currently under development. The best known missiles are the Boeing/Hughes FOG(M) or Euromissile's Polyphemus projects. The MBB Division of Deutsche Aerospace is actively engaged in this field and six years ago flew experimental anti-tank missiles equipped with TV and IR cameras in the nose. MBB uses a very thin lightweight optical guidance fiber more than 15 km long with an optical attenuation of only 1.4 dB/km. During the missile's flight the camera can scan the battlefield, and the images can be fed simultaneously into the [C.sup.3]I net for instant evaluation. For battlefield surveillance drones can be tethered by a lightweight optical fiber to a forward ground station which can feed the recce results directly into the [C.sup.3]I net. The image resolution attained is of photographic quality. This image quality, however, is not dependent on the overall performance of the transmission medium - as is the case with microwave data-links - but depends exclusively on the resolution of the camera's optics.

Weapons Systems Applications

The options for the use of optical fiber are virtually unlimited. Cameras might also be mounted on tank or special, unmanned robot vehicles tethered by a hair-thin 20 to 30 km-long cable providing the lower echelon commanders with a first-hand view of the action. Such air and ground-based reconnaissance systems are currently under study. Tied into a FOTS (LH) type net, they can give the theater commander for the first time in history real-time recce data.

The options go far beyond that. Combat robots can be controlled from secure locations and remote operation of conventional weapons is possible because the information flow which can be carried by the almost totally ECM-proof fiber links is virtually unlimited.

The reconnaissance possibilities offered by optical fiber technology are also of great interest to naval forces. An IR or TV-camera system lifted to 8-10 km altitude by a rocket can enormously extend the visible horizon. The images can be video-taped and replayed in slow motion for thorough evaluation.

Such rocket-carried recce systems are of major interest to submarines since even under the very best sea conditions, the visual range of a periscope is very limited. The reconnaissance package can even be launched from submerged submarines.

A similar launcher could be loaded with optically-guided anti-helicopter, missiles and fired with deadly accuracy from submerged submarines at attacking ASW rotorcraft. Euromissile, utilizing MBB-developed optical fiber technology, is said to be working on such a missile for a new generation of attack submarines.


Thomson-CSF is experimenting with optical fiber-guided torpedos. Wire-guided torpedos have always suffered from the low message-carrying capacity and weight of the wire link. The capabilities of the weapon's sophisticated sonar head could be far better utilized if an improved control by a high capacity, two-way data link was available. In addition, the optical fiber would be lighter and less prone to break than the currently used metallic wire. Although modern torpedos are equipped to operate independently after loss of direct control, their efficiency is considerably reduced (they have problems in distinguishing between a decoy and the target). Optical fiber guidance reduces this danger and thereby increases the lethality of the weapon.


Numerous weapons will benefit from the large message-carrying capacity of fiber optics. For example, rockets or guided missiles could be used for laying small capacity cables from point A to point B over distances of more than 20 km. These cables, of not more than one millimeter in diameter, would be fired without connectors. Simple-to-operate equipment for polishing and splicing fiber cables already exists and could be used to prepare the bare cable ends for direct connection with the terminal's interface. A forward observer, a patrol or an advance group of armored vehicles could thus tie itself quickly into a [C.sup.3]I network without running the danger of discovery or radio intercept by the enemy.

GTE has developed and successfully tested a fiber optic cable for such purposes. It operates with a minimal attenuation and does not require repeaters. The reformation of the time-distorted signals and their amplification are performed by an optical re-shaping of the pulses within the cable. Provided a simple production process can be developed for the fiber, which consists of a specially doped glass mixture, this marks an important breakthrough. On the whole, for the military - as well as for commercial operators - optical fiber technology is about to open up a new age of perfect, secure and comprehensive communications.

PHOTO : Consisting of seven 225-fiber strands, this Siemens optical fiber cable can carry up to

PHOTO : 60 000 telephone calls.

PHOTO : Microscope photo of an SEL mono-mode fiber optic cable end. The fiber is visible in the

PHOTO : center of the sheath.

PHOTO : Siemens gate array chip for use in optical fiber systems. It performs 2 500 gate functions

PHOTO : and operates at 565 Mb/s with a clock frequency of 350 MHz.

PHOTO : The McDonnell Douglas/BAe Harrier II is the first fighter equipped with optical fiber

PHOTO : systems, chosen for their signal-carrying capacity and light weight.

PHOTO : Close-up of STC Defence Systems' Fiber Optic Cable Assembly developed for the US Army FOTS

PHOTO : (LH) system.

PHOTO : British Telecom technicians installing a repeater in a heavy duty optical fiber cable.

PHOTO : The optical fiber-guided missile of the future being developed by Hughes. It carries

PHOTO : optical sensors in the nose.
COPYRIGHT 1990 Armada International
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Copyright 1990, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Geisenheyner, Stefan
Publication:Armada International
Date:Aug 1, 1990
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