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Fibre optics and communications

In the 1960s at Standard Telecommunication Laboratories (STL) based in Harlow, Essex, England, Kao and his coworkers did their pioneering work in creating fibre 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.

In 1963, when Kao first joined the optical communications research team he made notes summarising the background situation and available technology at the time, and identifying the key individuals involved. Initially Kao worked in the team of Antoni E. Karbowiak (Toni Karbowiak), who was working under Alec Reeves to study optical waveguides for communications. Kao's task was to investigate fibre attenuation, for which he collected samples from different fibre manufacturers and also investigated the properties of bulk glasses carefully. Kao's study primarily convinced him that the impurities in material caused the high light losses of those fibres. Later that year, 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.

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). This study proposed the use of glass fibres for optical communication. The concepts described, especially the electromagnetic theory and performance parameters, are the basis of today's optical fibre communications.

"What Kao did in Harlow transformed the world and provided a backbone for the internet. He was the father of fiber optics."

—Harlow Museum's David Devine on Kao's pioneering work in fiber optics at STC 's Standard Telecommunication Laboratories in Harlow

In 1965, 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 fibres 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 fibres 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 (SiO) 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 fibre, 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 fibres. When Kao first proposed that such glass fibre 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.

He also 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 fibre 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 fibre optics seriously. As of 2017, fibre optic losses (from both bulk and intrinsic sources) are as low as 0.1419 dB/km at the 1.56 μm wavelength.

Kao developed important techniques and configurations for glass fibre waveguides, and contributed to the development of different fibre types and system devices which met both civil and military application requirements, and peripheral supporting systems for optical fibre communication. In mid-1970s, he did seminal work on glass fibre 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 fibres (with M. S. Maklad).

At an early stage of developing optic fibres, 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 was 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 fibre optics, five years ahead of the time that such a trans-oceanic fibre-optic cable first became serviceable.

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