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infrared Physics, 1968.Vol. 8, pp. 123429.Pergamon Press.Printedin GreatBritain INFRA-RED OPTICAL COMMUNICATION J. LYTOLLIS, C. K. KAO Standard Telecommunication Laboratories and G. I. SYSTEMS* TURNER Limited, London Road, Harlow, Essex (Received 13 July 1967) Abstract-This paper presents a survey of the present position of the technology of optical communication systems. The gallium arsenide laser and gas lasers are compared as sources. The Pockel effect, acoustic and Faraday effect modulators are discussed. The advantages and disadvantages of various transmission systems are also discussed. Brief comments are made about various types of detectors. It is concluded that the information capacity is limited by the bandwidth of the source, modulator or detector. No satisfactory transmission system for long distance telecommunications use has yet been perfected, but guided systems of the type illustrated by a dielectric fibre guide, seem to offer the most promise. Despite the advances made in sources and modulators, many problems still remain before optical frequencies can be used for long distance telecommunication systems. Short range and outer space systems are feasible with the existing techniques. INTRODUCTION OPTICAL communication systems using infra-red radiation are by no means a new idea. Many countries in the Second World War developed optical communication systems for use on the battlefield using beams of i.r. radiation modulated at audio frequencies. The advantages and disadvantages of infra-red communication systems are shown in Table I. TABLE 1. INFRA-RED COMMUNICATION SYSTEMS Disadvantages Advantages Atmospheric Secure Cannot be jammed Causes no interference Equipment smaller, lighter and cheaper than radio equipment Wide bandwidth available and weather conditions Line of sight affect range World War II military systems fulfilled the purpose for which they were designed, namely short range voice channels, but they did not use the wide-band capabilities of the high optical frequencies. Most of the systems relied on mechanical methods of modulation, and such systems are necessarily slow. However, as Table 2 illustrates, the wide bandwidth capability is there waiting to be used if only the right techniques of transmission, modulation and detection can be developed. It has been estimated that all the telephone systems in existence in the world could be impressed on one optical communication channel. This paper presents a survey of the present position of the technology of optical communication systems. The aspects to be considered are sources, modulation techniques, transmission, detection and information capacity. *@ 1967 Standard Telecommunication Laboratories Limited. 123 124 J.LYTOLLIS,C. K. KAO and G. ~TURNER TABLE 2. CHANNEL CAPACITY OF VARIOUS SYSTEMS Communication type Radio Microwave Optical Frequency (Hz) 4.2 ir IO* 4.2 x 10” 4.2 v lOI1 Usable bandwidth (%I IO I 0.1 Bandwidth (Hz) Channel bandwidth (Hz) 4.2 x lo3 4.2 x 10s 4.2 x 101’ 4.2 x lo3 4.2 ;’ to” 4.2 x 10” Number of channels 1 10000 I oo,ooo,ooo SOURCES The invention of the laser was the beginning of renewed interest in the use of optical frequencies for communication purposes. Here was a source which was extremely bright and which emitted light that was coherent, just as radio waves are coherent. What one could do at radio frequencies was now possible at optical frequencies, at least in theory. The emitted beam was well collimated at source and so all the power output of the source could be collected into a simple optical system and projected as a parallel beam of high power density. A ruby laser was used by American scientists to project a beam of light from Earth to Moon(l) using a 30 cm telescope in the transmitter and, despite the diffusing nature of the Moon’s surface, the return signal was successfully picked up and detected using a 120 cm telescope in the receiver. There are three classes of laser. (1) Solid state crystal lasers, of which ruby and neodymium doped lasers are the best known examples; (2) gas lasers, of which neon-helium is the most used, but the new powerful carbon dioxide laser is now attracting great attention; (3) semi-conductor lasers, of which GaAs is the best known and most highly developed. The GaAs laser is operated by passing a current through the p-n junction in the forward direction. The output of the laser can be modulated by simply modulating this current. It therefore looks attractive as a source for optical communications. Its advantages and disadvantages are listed in Table 3. The GaAs laser is a good source for communication systems in so far as it is an efficient, intensely bright source capable of being modulated at TABLE 3. GAAS LASER AS A SOURCE FOR OPTICAI. COMMUNICATIONS Disadvantages Advantages I. 2. 3. 4. 5. 6. Rugged, small device. Simple pumping process [d.c. current at 1 i volts] Short time constant [ < 1O-g set] ... Easily modulated to 1000 MHz bandwidth High efficiency [lo0 % internal, 40 % external] High brightness [lMW/cmz] High average power [watts at 77”K] 1. Refrigeration 2. 8. 4. required for high duty cycles Poor spatial coherence [beam > 10” wide] Multimode . Heterodyne detection is not possible Low impedance [ < 191 ... Impedance matching difficult GHz bandwidths because of its short time constant. It is small, robust, has a long life, and can be produced cheaply in quantity. Its main disadvantages are that it requires cooling to liquid nitrogen temperature if continuous operation is required, and that it can only be operated at its full bandwidth capability if overall efficiency is sacrificed to achieve a good impedance match. Subsidiary disadvantages are that it does not have the good beam collimation normally associated with lasers, and that multimoding increases the linewidth. Infra-red optical communication systems 125 Improved and new fabrication techniques have steadily improved the characteristics of the laser at room temperature, but still only pulsed operation at low duty cycles is possible. The best reported performance is 105 pulses/set with 3 W peak power output using drive pulses of 50A, 70 nsec wide, However, continuous operation at room temperature is the real requirement. This can be obtained from the GaAs diode in its non-lasing form. Such a diode is a less efficient source than the laser, has no coherence, and a smaller bandwidth. However, continuous output powers of 0.1 W at room temperature and 1 W at liquid nitrogen temperature have been achieved.@ The bandwidth obtainable is up to several tens of megahertz. We have successfully transmitted television pictures using GaAs diodes. A pulse-width-modulated data transmission link with a 3 MHz bandwidth has been built and demonstrated. Figure 1 shows the transmitter in schematic form. Figure 2 shows the receiver layout. We have also built intruder alarms using GaAs diodes as sources and silicon diodes as detectors. Figure 3 shows a diagram of the system. The beam is modulated diode r-l 7 MC/S sowtooth generotor Optical system FIG. I. STL optical link transmitter. Frc. 2. STL optical link receiver. up to 300 m FIG. 3. STL intruder detector. electrically at a fixed frequency, and a narrow band amplifier is used in the detector circuit. Any interruption of the beam can be used to sound an alarm. The range is up to 300 metres in clear weather. 126 J. LYTOLLIS,C. K. KAO and G. I. TURNER MODULATION TECHNIQUES There was a time when only the GaAs laser could be modulated at high frequencies with a wide bandwidth. However, great progress has been made in the last few years in producing modulation methods which enable any class of laser to be so modulated. The gas laser was always attractive as a source for communication purposes because of its room temperature operation, good stability and high coherence but, until recently, it lacked efficiency and high power. Now, however, tens of watts power can be produced by gas lasers, and carbon dioxide lasers can operate with 10% efficiency to produce up to + kW of power.c3) A gas laser combined with one of the new modulation methods therefore now looks more attractive than the GaAs laser. The Pockel effect in certain electro-optic crystals such as potassium di-hydrogen phosphate, ammonium di-hydrogen phosphate and potassium tantalate niobate is the most widely used modulation method. These materials are commonly known as KDP, ADP and KTN. The Pockel effect is the solid state equivalent of the Kerr effect in liquids. Table 4 shows the performance of a cavity modulator and of a travelling wave modulator.(4) The latter gives the better performance. Bandwidths of GHz are seen to be possible at low power levels. More recently, lithium tantalate has been used, and this requires only l/20 power of KDP.@, TABLE 4. POCKEL EFFECT MODULATORS Type Cavity Travelling wave Material Modulation frequency Bandwidth KDP KDP 3GHz Baseband at microwave frequencies 4MHz 5GHz Modulation power WI 1.5 12 Modulation index 0.13 -1 - Another modulation method is based on electro-acoustic effects in piezo-electric crystals such as quartz or barium titanate, or electro-magnetic effects in electro-optic crystals of high refractive index. High frequency signals are applied to transducers attached to the crystals, and a standing wave field pattern established within the crystal. This periodically varies the refractive index of the crystal to establish a grating effect, so that the emergent light forms a diffraction pattern. Suitable positioning of a slit and detector enables modulation of the light to be detected. Microwave carrier frequencies can be used, and only low voltage drive is required. The bandwidth is 10% of the carrier frequency, and depths of modulation of 100 o/0can be approached.(4) Significantly, a recent paper reports that tellurium has been used to modulate light from a carbon dioxide laser at 10.6 p wavelength at frequencies of 20-300 MHz with good efficiency.cG) Other possible modulation techniques are free carrier absorption effects in semiconductors, absorption edge shift by electric fields and the Faraday magneto-optical effect. The first two methods are useful at low frequencies only. The third method can be used at microwave frequencies. Recently, using gallium doped yttrium iron garnet, modulators have been produced with 200 MHz bandwith and 40% depth of modulation requiring only l/10 watt driving power. With l/3 watt driving power, a bandwidth of 400 MHz was achieved. (5) Infra-red optical comn~unication systems 127 TRANSMISSION The atmosphere is not an ideal medium for the transmission of optical signals. Selective absorption occurs due to presence of water vapour and carbon dioxide (Fig. 4). Smoke, dust, mist, haze, fog, rain and snow, all cause attenuation of the beam, which may be severe and frequency independent. However, point-to-point systems can be operated over lO20 km ranges using only milliwatts of power in clear weather. Thermal gradients cause refractive index changes within the air mass, and this deviates the beam randomly, causing scintillation effects. Wavelength, FIG. 4. Smoothed vertical transmission /A curve of the earth’s atmosphere. For long distance communication systems, transmission which does not depend on the atmosphere must be used. Several proposed systems(‘-“) rely on a hollow pipe which contains optical refracting and reflecting components, is sealed, and is filled with a dry gas under pressure. The disadvantages of such systems are their lack of physical flexibility, the close engineering tolerances with which they must comply, and the high cost of installation. We are currently exploring the possibility of using dielectric waveguides to transmit optical signals. The theory of the method has been written up in the Proceedings of the IEE(rO), and will not be dealt with further here. An optical fibre with a high refractive index core and low refractive index cladding is the guide, the core having a diameter of several times the wavelength of light, and the cladding making up a diameter of say, 50100 I”. The light is launched into this fibre, and is guided by the core, but most of the light travels in the cladding (see Fig. 5). The great advantage of such a guide is that it can be bent round sharp curves without severe attenuation, and so it could be used in existing cable ducts if necessary. We have demonstrated single and multimode transmission at optical frequencies using such fibres (Fig. 6). Light from gas lasers and from GaAs lasers has been launched into the fibres using suitable optics. Losses in such fibres are at present high because of the high absorption of even good optical quality glass due to certain impurities which are difficult to remove-ferrous and ferric ions being the chief offenders. IP-I 128 J. LYTOLLIS,C. K. KAO and G. I. TURNER Medium 3 FIG. 5. Propagation cj=l in single-mode optical fibre. However, it is possible to produce lower loss glasses with present aim is to bring losses down to 20 dB/km. day knowledge, and our DETECTORS Below 1~ wavelength, photo-emissive detectors can be used, and no real difficulty exists in detecting even microwave modulation frequencies using travelling wave photo-detectors or specially based photo-emissive diodes. For longer wavelengths, only p-n junction diodes of specially low capacity can detect at frequencies in excess of 10 MHz. Extrinsic photoconductive detectors can operate out to 40 p wavelength, but are limited to time constants of lo-’ set or more, and need refrigeration. INFORMATION CAPACITY The source, modulator and detector generally govern the information capacity of a communication system. A reasonable capacity would appear to be around 100 MHz for an infra-red system at wavelengths longer than 1 p, with somewhat higher capacity below this wavelength. Of course, many different wavelengths can be transmitted along the system simultaneously, and dispersive optics used to feed-in and feed-out each channel at either end. Using fibre optical guides, bundles of fibres could form multi-channel transmission systems with small cross-sectional areas. CONCLUSION In conclusion one can say that advances have been made towards the achievement of practical wideband communication systems at optical frequencies, but that problems still remain. Any system developed must be cost competitive with existing systems on a cost per channel basis if it is to be commercially acceptable for telecommunication purposes. Short range systems have been improved greatly by the invention of lasers and the developments which have taken place in modulators. Of course, in outer space, where there are no atmosphere problems, optical communication systems are easier to realise, and have already been used. Acknowledgement-The authors thank the management Limited for permission to publish this paper. of Standard Telecommunication Laboratories FIG. 6(a). FIG.6(b). Fucing page 128 Fro. 6(c). FIG. 6(d). FIG. 6. Photographs of light patterns produced by modes Infra-red optical communication systems REFERENCES I. BOWNESS,C., D. MISSIOand T. ROGALA, Proc. Insf. 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