Charles K. Kao - Work and Related History - Fiber Optics and Communications

Fiber Optics and Communications

In 1960s at STL, Kao and his co-workers did their pioneering work in the realisation of fiber optics as a telecommunications medium, by demonstrating that the high-loss of existing fibre optics arose from impurities in the glass, rather than from an underlying problem with the technology itself.

Initially Kao worked in the team of Antoni E. Karbowiak (aka Toni Karbowiak), who was working under Alec Reeves to study optical waveguides for communications. Kao's task was to investigate fiber attenuation, for which he collected samples from different fiber manufacturers and also investigated the properties of bulk glasses carefully. Kao's study primarily convinced himself that the impurities in material caused the high light losses of those fibers.

In 1963, Kao was appointed head of the electro-optics research group at STL. He took over the optical communication program of STL in December 1964, because his supervisor, Karbowiak, left to take the Chair in Communications in the School of Electrical Engineering at the University of New South Wales (UNSW), Sydney, Australia. Karbowiak now is regarded as one of the pioneers of photonics technology of Australia.

Although Kao succeeded Karbowiak as manager of optical communications research, he immediately decided to abandon Karbowiak's plan (thin-film waveguide) and overall change research direction with his colleague George Hockham. They not only considered optical physics but also the material properties. The results were first presented by Kao to the IEE in January 1966 in London, and further published in July with George Hockham (1964–1965 worked with Kao)a. This study first theorized and proposed to use glass fibers to implement optical communication, the ideas (especially structural features and materials) described are largely the basis of today's optical fiber communications.

In 1965b, Kao with Hockham concluded that the fundamental limitation for glass light attenuation is below 20 dB/km (decibels per kilometer, is a measure of the attenuation of a signal over a distance), which is a key threshold value for optical communications. However, at the time of this determination, optical fibers commonly exhibited light loss as high as 1,000 dB/km and even more. This conclusion opened the intense race to find low-loss materials and suitable fibers for reaching such criteria.

Kao, together with his new team (members including T.W. Davies, M.W. Jones, and C.R. Wright), pursued this goal by testing various materials. They precisely measured the attenuation of light with different wavelengths in glasses and other materials. During this period, Kao pointed out that the high purity of fused silica (SiO₂) made it an ideal candidate for optical communication. Kao also stated that the impurity of glass material is the main cause for the dramatic decay of light transmission inside glass fiber, rather than fundamental physical effects such as scattering as many physicists thought at that time, and such impurity could be removed. This led to a worldwide study and production of high-purity glass fibers. When Kao first proposed that such glass fiber could be used for long-distance information transfer and could replace copper wires which were used for telecommunication during that era, his ideas were widely disbelieved; later people realized that Kao's ideas revolutionized the whole communication technology and industry.

Kao played a leading role in the early stage of engineering and commercial realization of optical communication. In spring 1966, Kao traveled to the U.S. but failed to interest Bell Labs, which was a competitor of STL in communication technology at that time. He subsequently traveled to Japan and gained support. Kao visited many glass and polymer factories, discussed with various people including engineers, scientists, businessmen about the techniques and improvement of glass fiber manufacture.

In 1969, Kao with M.W. Jones measured the intrinsic loss of bulk-fused silica at 4 dB/km, which is the first evidence of ultra-transparent glass. Bell Labs started considering fiber optics seriously.

Kao developed important techniques and configurations for glass fiber waveguides, and contributed to the development of different fiber types and system devices which met both civil and militaryc application requirements, and peripheral supporting systems for optical fiber communication. In mid-1970s, he did seminal work on glass fiber fatigue strength. When named the first ITT Executive Scientist, Kao launched the "Terabit Technology" program in addressing the high frequency limits of signal processing, so Kao is also known as the "Father of the Terabit Technology Concept". Kao has published more than 100 papers and was granted over 30 patents, including the water-resistant high-strength fibers (with M.S. Maklad).

At an early stage of developing optic fibers, Kao already strongly preferred single mode for long-distance optical communication, instead of using multi-mode systems. His vision later was followed and now is applied almost exclusively.

Kao is also a visionary of modern submarine communications cables and largely promoted this idea. He predicted in 1983 that world's seas would be littered with fiber optics, five years ahead of the time that such a trans-oceanic fiber-optic cable first became serviceable.

Ali Javan’s introduction of a steady helium-neon laser and Kao’s discovery of fiber light-loss properties now are recognized as the two essential milestones for the development of fiber-optic communications.