1 Historical Background

Communications may be broadly defined as the transfer of information from one point to another. In optical fibre communications this transfer is achieved by using light as the information carrier. The use of optical carrier waves for communications is a very old one. As early as 1880 Alexander Graham Bell invented the photophone (see Figure 1) which demonstrated the transmission of speech using light. Modern lightwave communications had its birth in the 1960s. The first demonstration of the ruby laser in 1960 followed by a demonstration of laser operation in semiconductor devices in 1962 were the early stepping stones which led to the continuous operation of room temperature long-lifetime laser diodes that are in common use today. The laser made available a coherent optical carrier of extremely high frequency (typically 3 x 1014 Hz). The bandwidth of a communication channel (a link between two remotely spaced users) defines the range of frequencies which can safely be conveyed by the channel. The higher the bandwidth of a communication channel the greater its capacity for information transfer. A communication system may contain many channels. An optical communication system having a bandwidth only a fraction of the optical carrier frequency thereby possesses an enormous communication capacity.

Figure 1: The Photophone: Invented by Alexander Graham Bell¹

Light waves may be carried by an optical fibre which is simply a cylindrical glass waveguide consisting of core and cladding regions (see Figure 2). The core refractive index is slightly greater than the cladding. Light travelling in the core is confined by means of total internal reflection at the core-cladding interface. An important fibre parameter is the loss which is usually given in dB/km. For example if a fibre has a loss of 3 dB/km then a light beam entering a fibre will lose half its power after travelling one kilometer. In 1966 researchers at Standard Telephone Laboratories speculated that losses as low as 20dB/km should be achievable. Since that time technology has progressed to the point of enabling production of optical fibre with losses less than 0.3 dB/km. If sea water had losses this low then it would be possible using sunlight to see the ocean floor from sea level! Commercial optical fibre uses sources and detectors operating in the 1.3 mm and 1.55 mm wavelength region. In the latter region the fibre bandwidth (region of low losses) is greater than 25,000 GHz.

Figure 2(a): Schematic diagram of an optical fibre²

Figure 2(b): Optical fibre with visible laser output

2 Optical Communication System Basics

A schematic diagram of a basic optical communication system is shown in Figure 3. The object of the system is to transmit information using an optical carrier wave from a transmit station to a receive station over optical fibre. Electrical data, usually represented as a series of '0's and '1's, modulates a semiconductor laser. The laser output is a series of light pulses representing the '0's and '1's. The modulated laser light is then sent down an optical fibre. At appropriate points in the transmission link, the light signal is either optically amplified or completely regenerated. Optical amplification is required to overcome the fibre loss. Regeneration means that the light signal is detected, reshaped, retimed and retransmitted. It is required when the light signal becomes distorted by the fibre (this effect is called dispersion) or when the signal picks up too much noise. At the receiver the light signal is detected, amplified and sent to a decision circuit. The decision circuit decides if a '0' or '1' bit has been received. With today's technology it is possible to modulate a semiconductor laser at speeds of 10 Gbit/s (That's 10,000,000,000 bits per second!) and beyond. At this speed it would take less than half a second to transmit the contents of the Encyclopedia Britannica or it would be possible to carry over one hundred and fifty thousand simultaneous telephone calls! However there are limitations as to how fast a laser can be modulated. A speed of 10 Gbit/s, while fast, is still only a small fraction of the intrinsic optical fibre capacity.

Figure 3: Basic optical communication system

3 Current developments in optical networks

Optical fibre is being installed in the ground at an ever increasing pace. Initially fibre was installed for use in high capacity links between countries and metropolitan areas. Increasingly it is being installed for use in Local Area Networks servicing the business community. Eventually it is envisaged that optical fibre will penetrate the home opening up an enormous communication capacity to the domestic customer. The development of new communication networks based on optical fibre is proceeding at a rapid pace. The most promising networks being investigated are based on wavelength division multiplexing (WDM ) technology. In WDM it is possible to independently transmit many light signals at different wavelengths down the same optical fibre. This leads to a much greater utilisation of the fibre bandwidth. A diagram of a basic WDM system is shown in Figure 4 in which a number of transmitter lasers at different wavelengths are combined together by a multiplexer and transmitted down a single optical fibre. The transmitter outputs are routed through an optical network to their appropriate destinations. Receivers attached to the network contain a tunable optical filter to select the required wavelength prior to conventional reception. The operation of a WDM network is complex and requires complex protocols (rules of information flow). Important issues which need to be addressed are how data is routed and how wavelengths are allocated to users. WDM networks also require the development of new optoelectronic devices which may be electronically controlled but where the optical data is itself not interfered with. These devices include optical amplifiers capable of amplifying many signals simultaneously, wavelength converters for mapping data from one light signal to another, fast tunable optical filters and tunable lasers. The development of protocols and devices is a very intense area of development.

Figure 4: Basic WDM system

4 Conclusion

The ideal communication network is one which offers the customer a wide variety of services with fast and cheap access. The rapid development and employment of optical fibre communications will accelerate progress towards this goal. Optical fibre will indeed become the Communications Highway for the 21st Century.

² Only light rays entering the fibre at angles less than the fibre acceptance angle are able to propagate down the fibre core by means of total internal reflection at the core cladding interface.

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