The Medium Is the Message

Fibre Optics

Optical communication systems date back two centuries to the “optical telegraph” that French engineer, Claude Chappe, invented in the 1790s. His system was a series of semaphores mounted on towers where human operators
relayed messages from one tower to the next. In 1880, Alexander Graham Bell patented an optical telephone system called the photophone. 

During the period of 1840s, Swiss physicist, Daniel Collodon, and French physicist, Jacques Babinet, showed that light could be guided along jets of water for fountain displays. British physicist, John Tyndall, popularized light-guiding in a demonstration he first used in 1854, guiding light in a jet of water flowing from a tank. 

Though experimental underwater cables were first laid in 1842, the first transatlantic submarine telegraph cable was laid in 1866 between US and France. Between 13 and 27 July that year the world’s largest steamship, The Great Eastern, with James Anderson as captain, laid the cable. The cable was manufactured by Telecon company and was both lighter and stronger than previous cables that had failed. In the Indian context, the first submarine telegraph cable was laid from England to India (Mumbai) in 1870. 

Optical fibres went a step further. They are essentially transparent, long, and flexible rods of glass or plastic.
During the 1920s, John Logie Baird in England and Clarence W Hansell in the US patented the idea of
using arrays of hollow pipes or transparent rods to transmit images for television or facsimile systems. However, the first person known to have demonstrated image transmission through a bundle of optical fibres was Heinrich
Lamm.

Neither Van Heel nor Hopkins and Narinder
S Kapany made bundles that could carry light far, but their reports brought the
fibre optics revolution. Van Heel made the crucial innovation. All earlier fibres were “bare” with total internal reflection at a glass-air interface. Van Heel covered a bare fibre or glass or plastic with a transparent cladding of lower refractive index. This protected the total-reflection surface from contamination, and greatly reduced cross talk between fibres. The next key step was development of glass-clad fibres, by Lawrence
Curtiss.

The cutting edge of communications research were millimeter-wave systems, in which hollow pipes served as wave-guides to circumvent poor atmospheric transmission at tens of GHz, where wavelengths were in the millimetre range.

Even higher optical frequencies seemed a logical next step in 1958 to Alec Reeves who invented digital pulse-code modulation before World War II. Other people climbed on the optical 

communications bandwagon when the laser was invented in 1960. 

In 1961, Elias Snitzer, working with Hicks, demonstrated the similarity by drawing fibres with cores so small they carried light in only one wave-guide mode. However, virtually everyone 

considered fibres too lossy for communications; attenuation of a decibel per meter was fine for looking inside the body, but communications operated over much longer distances, and required loss no more than 10 to 20 decibels per
kilometre.

In 1966, Kao and Hockham’s forecast that fibre loss could be reduced below 20 decibels per kilometre attracted the interest of the British Post Office, which then operated the British telephone
network. FF Roberts, an engineering manager at the Post Office Research Laboratory, saw the possibilities, and persuaded others at the Post Office. His boss, Jack Tillman, tapped a new
research fund of 12 million pounds to study ways to decrease fibre loss.

With Kao almost evangelically promoting the prospects of fibre communications, and the Post
Office interested in applications, laboratories around the world began trying to reduce fibre loss.

It took four years to reach Kao’s goal, and the route to success proved different than many had expected. Most groups tried to purify the compound glasses used for standard optics, which are easy to melt and draw into fibres. At the Corning Glass Works (now Corning Inc.), Robert Maurer, Donald Keck, and Peter Schultz started with fused silica, a
material that can be made extremely pure, but has a high melting point and a low refractive index. They made cylindrical performs by depositing purified materials from the vapor phase, adding carefully controlled levels of dopants to make the refractive index of the core slightly higher than that of the cladding, without raising attenuation dramatically. In September 1970, they announced they had made single-mode fibres with
attenuation at the 633-nanometre helium-neon line below 20 decibels per
kilometre. The fibres were fragile, but tests at the new British Post OfficeResearch Laboratories facility in Martlesham Heath confirmed the low loss.

In 1970, Bell Labs and a team at the Ioffe Physical Institute in Leningrad made the first semiconductor diode
lasers able to emit continuous wave at room temperature. Over the next several years, fibre losses dropped dramatically, aided both by improved fabrication
methods and by the shift to longer wavelengths where fibres have inherently lower attenuation.

Not satisfied with the low bandwidth of step-index multimode
fibre, the researchers concentrated on multi-mode fibres with a refractive-index gradient between core and cladding, and core diameters of 50 or
62.5 micrometres. The first generation of telephone field trials in 1977 used such fibres to transmit light at 850 nanometres from
gallium-aluminum-arsenide laser diodes.

Those first-generation systems could transmit light several kilometres without repeaters, but were limited by loss of about 2 decibels per kilometre in the
fibre. A second generation soon appeared, using new InGaAsP lasers which emitted at 1.3 micrometer, where fibre attenuation was as low as 0.5 decibels per
kilometre, and pulse dispersion was somewhat lower than at 850 nanometers. Development of hardware for the first transatlantic fibre cable showed that single-mode systems were feasible, so when deregulation opened the long-distance phone market in the early 1980s, the carriers built national backbone
systems of single-mode fibre with 1300-nanometers sources. That technology has spread into other telecom
applications, and remains the standard for most fibre systems.

However, a new generation of single-mode systems is now beginning to find applications in submarine cables and systems serving large
numbers of subscribers. They operate at 1.55 micrometres, where fibre loss is 0.2 to 0.3 decibels per
kilometre, allowing even longer repeater spacings. More important, erbium-doped optical fibres can serve as optical amplifiers at that wavelength, avoiding the need for electro-optic regenerators. Submarine cables with optical amplifiers can operate at speeds of 5 gigabits per second, and can be upgraded from lower speeds simply to changing terminal electronics. 

The biggest challenge remaining for fibre optics is economical. Today
telephone and cable television companies can cost-justify installing fibre links to remote sites serving tens to a few 

hundreds of customers. However, terminal equipment remains too expensive to justify installing fibres all the way to homes, at least for present services. 

Instead, cable and phone companies
run twisted wire pairs or coaxial cables from optical network units to individual homes. Time will only tell how long
that lasts.