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01 Feb 2006
2006 marks the 40th anniversary of Kao and Hockham's breakthough paper on fiber optic communication. Optics.org talks with Philip Hargrave of Nortel about how fiber won over the skeptics.
Forty years ago, the concept of transmitting data on light signals through glass was considered madness by many, including the media. However, given the limited capacity of ordinary copper to carry phone calls, engineers at Standard Telecommunication Laboratories (STL) in Harlow, UK, were committed to come up with an alternative technology.
"The first idea was to have co-focal lenses that refocused every 100 m or so, and to use servo loops to adjust the focus," Philip Hargrave, Chief Scientist, Nortel, told Optics.org "Then researchers thought about putting light down steel pipes one inch across lined with silver, but found that the technique was incredibly temperature sensitive."
Out of all the options, Charles Kao and George Hockham working at STL were the first to realize that the idea of sending light down thin bits of glass a few microns across was the most attractive idea. The duo's landmark paper, "Dielectric-fiber surface waveguides for optical frequencies", was published by the Institution of Electrical Engineers in 1966 (see below) and gave the green light to the concept of optical fiber communication.
"They essentially showed that it would be viable if the [transmission] loss could be reduced to less than 20 dB/km," explained Hargrave. "They proved that there was no fundamental mechanism that would stop this loss being achieved and that was really the crux."
At that time, the attenuation of light in glass stood at more than 1000 dB/km, which meant that light would travel just 3 m or so before halving in intensity. However, Kao, Hockham and their colleagues remained confident that if the magic figure of 20 dB/km could be met then fiber optic communications would take-off commercially.
Motivated by the news, Corning found a way of making ultrapure glass within just a few years of the announcement. "By 1973, the team at Harlow built a rig over 2 km with a loss of 25 dB/km - not quite down to that magic figure, but not far off," said Hargrave. "The key date was 1977 when the loss was down to 5 dB/km and British Telecom decided to install a field trial between the UK towns of Hitching and Stevenage, only a matter of 20 or 30 miles away from the Harlow site."
The system had gallium arsenide narrow stripe lasers installed at one end and silicon avalanche photodiodes at the other end, with roadside mounted regenerators placed every few kilometres. Critically, engineers were able to put real phone calls through the system and arguably that was when fiber optic communication really took off.
By 1980 the loss came down even further and firms were experimenting with putting cables under the sea. The first one to Belgium was installed in the early 1980s. "You enter a period that goes on until the year 2000 where everyone was increasing the capacity of fiber," remembered Hargrave. "In fact, there was a time where the capacity of the fiber doubled every nine months with no increase in cost."
Cost remains a pertinent topic today, with fiber-to-the-home looming large on the horizon. In this case, the barrier is proving to be not the price of optical components, but the expense of ducting, installation and civil engineering. "The challenge is getting that infrastructure investment and only accepting a return over a very long timescale for the civil engineering part, while trying to get a reasonable return over more realistic technology timescales for what goes down the pipe," concluded Hargrave.
Key events:
• 1966 STL researchers publish the first paper considering the possibility of using glass fibers to carry information.
• 1967 Video transmission is achieved over 20 metres of optical fiber at Harlow Laboratories in the UK.
• 1971 Digital colour TV signals are sent along a single fiber at the Harlow site in Essex, UK.
• 1977 Stevenage Field Experiment: first field demonstration of live telephone traffic carried on fiber in the UK.
• 1980 First undersea fiber cable systems trial by Nortel Networks.
• 1986 UK-Belgium undersea system installed.
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