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【趣闻】Sand from centuries past;Send future voices fast.
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Sand from centuries past;Send future voices fast. A Nobel Lecture organized by the Royal Swedish Academy of Sciences and The Prize Committee in Physics delivered by Mrs Gwen MW Kao on behalf of Prof Charles K Kao Nobel Laureate in Physics 2009 8 December 2009 Aula Magna Stockholm University 1. Introduction It is sad that my husband, Professor Charles Kao, is unable to give this lecture to you himself. As the person closest to him, I stand before you to honour him and to speak for him. He is very very proud of his achievements for which the Nobel Foundation honours him. As are we all! In the 43 years since his seminal paper of 1966 that gave birth to the ubiquitous glass fiber cables of today, the world of telephony has changed vastly. It is due to Professor Kao’s persistence in the face of skepticism that this revolution has occurred. In the 1970s the pre-production stage moved to ITT Corp Roanoke VA, USA. Whilst Charles worked there, he received two letters. One contained a threatening message accusing him of releasing an evil genie from its bottle; the other, from a farmer in China, asked for a means to allow him to pass a message to his distant wife to bring his lunch. Both letter writers saw a future that has since become past history. In the 1960s, our children were small. Charles often came home later than normal – dinner was waiting as were the children. I got very annoyed when this happened day after day. His words,maybe not exactly remembered, were –‘Please don’t be so mad. It is very exciting what we are doing; it will shake the world one day!’ I was sarcastic, ‘Really, so you will get the Nobel Prize, won’t you!’ He was right – it has revolutionized telecommunications. 2. The early days In 1960, Charles joined Standard Telecommunications Laboratories Ltd. (STL), a subsidiary of ITT Corp in the UK, after having worked as a graduate engineer at Standard Telephones and Cables in Woolwich for some time. Much of the work at STL was devoted to improving the capabilities of the existing communication infrastructure with a focus on the use of millimeter wave transmission systems. Millimeter waves at 35 to 70 GHz could have a much higher transmission capacity. But the waters were uncharted and the challenges enormous, since radio waves at such frequencies could not be beamed over long distances due to beam divergence and atmospheric absorption. The waves had to be guided by a waveguide. And in the 1950’s, R&D work on low loss circular waveguides –HE-11 mode – was started. A trial system was deployed in the 1960s. Huge sums were invested, and more were planned, to move this system into the pre-production stage. Public expectation for new telecommunication services such as the video phone had heightened. Charles joined the long-haul waveguide group led by Dr Karbowiak at STL. He was excited to see an actual circular waveguide. He was assigned to look for new transmission methods for microwave and optical transmission. He used both ray optics and wave theory to gain a better understanding of waveguide problems – then a novel idea. Later, his boss encouraged him to pursue a doctorate while working at STL. So Charles registered at University College London and completed the dissertation ‘Quasi-Optical Waveguides’ in two years. The invention of the laser in 1959 gave the telecom community a great dose of optimism that optical communication could be just around the corner. The coherent light was to be the new information carrier with capacity a hundred thousand times higher than point-to-point microwaves –based on the simple comparison of frequencies: 300 terahertz for light versus 3 gigahertz for microwaves. The race between circular microwave waveguides and optical communication was on, with the odds heavily in favour of the former. In 1960, optical lasers were in their infancy, demonstrated at only a few research laboratories, and performing much below the needed specs. Optical systems seemed a non-starter. But Charles still thought the laser had potential. He said to himself: ‘How can we dismiss the laser so readily? Optical communication is too good to be left on the theoretical shelf.’ He asked himself the obvious questions: 1. Is the ruby laser a suitable source for optical communication? 2. What material has sufficiently high transparency at such wavelengths? At that time only two groups in the world were starting to look at the transmission aspect of optical communication, while several other groups were working on solid state and semiconductor lasers. Lasers emit coherent radiation at optical frequencies, but using such radiation for communication appeared to be very difficult, if not impossible. For optical communication to fulfill its promises, many serious problems remained to be solved. 3. The key discovery In 1963 Charles was already involved in free space propagation experiments: the rapid progress of semiconductor and laser technology had opened up a broader scope to explore optical communication realistically. With a helium-neon laser beam directed to a spot some distance away, the STL team quickly discovered that distant laser light flickered. The beam danced around several beam diameters because of atmospheric fluctuations. The team also tried to repeat experiments done by other research laboratories around the world. For example, they set up con-focal lens experiments similar to those at Bell Labs: a series of convex lenses were lined up at intervals equal to the focal length. But even at the dead of night when the air was still and even with refocusing every 100 meters, the beam refused to stay within the lens aperture. Bell Labs experiments using gas lenses were abandoned due to the difficulty of providing satisfactory insulation while maintaining the profiles of the gas lenses. These experiments were struggles in desperation, to control light travelling over long distances. At STL the thinking shifted towards dielectric waveguides. Dielectric means a non-conductor of electricity; a dielectric waveguide is a waveguide consisting of a dielectric cylinder surrounded by air. Dr Karbowiak suggested Charles and three others to work on his idea of a thin film waveguide. But thin film waveguides failed: the confinement was not strong enough and light would escape as it negotiates a bend. When Dr Karbowiak decided to emigrate to Australia, Charles took over as the project leader and he then recommended that the team should investigate the loss mechanism of dielectric materials for optical fibers. A small group worked on methods for measuring material loss of low-loss transparent materials. George Hockham joined him to work on the characteristics of dielectric waveguides. With his interest in waveguide theory, he focused on the tolerance requirements for an optical fiber waveguide; in particular, the dimensional tolerance and joint losses. They proceeded to systematically study the physical and waveguide requirements on glass fibers. In addition, Charles was also pushing his colleagues in the laser group to work towards a semiconductor laser in the near infrared, with emission characteristics matching the diameter of a single-mode fiber. Single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier. The laser had to be made durable, and to work at room temperatures without liquid nitrogen cooling. So there were many obstacles. But in the early 1960s, esoteric research was tolerated so long as it was not too costly. Over the next two years, the team worked towards the goals. They were all novices in the physics and chemistry of materials and in tackling new electromagnetic wave problems. But they made very credible progress in considered steps. They searched the literature, talked to experts, and collected material samples from various glass and polymer companies. They also worked on the theories, and developed measurement techniques to carry out a host of experiments. They developed an instrument to measure the spectral loss of very low-loss material, as well as one for scaled simulation experiments to measure fiber loss due to mechanical imperfections. Charles zeroed in on glass as a possible transparent material. Glass is made from silica –sand from centuries past that is plentiful and cheap. The optical loss of transparent material is due to three mechanisms: (a) intrinsic absorption, (b)extrinsic absorption, and (c) Rayleigh scattering. The intrinsic loss is caused by the infrared absorption of the material structure itself, which determines the wavelength of the transparency regions. The extrinsic loss is due to impurity ions left in the material and the Rayleigh loss is due to the scattering of photons by the structural non-uniformity of the material. For most practical applications such as windows, the transparency of glass was entirely adequate, and no one had studied absorption down to such levels. After talking with many people, Charles eventually formed the following conclusions. 1. Impurities, particularly transition elements such as iron, copper, and manganese, have to be reduced to parts per million or even parts per billion. However, can impurity concentrations be reduced to such low levels? 2. High temperature glasses are frozen rapidly and therefore are more homogeneous, leading to a lower scattering loss. The ongoing microwave simulation experiments were also completed. The characteristics of the dielectric waveguide were fully defined in terms of its modes, its dimensional tolerance both for end-to-end mismatch and for its diameter fluctuation along the fiber lengths. Both the theory and the simulated experiments supported the approach. They wrote the paper entitled, ‘Dielectric-Fibre Surface Waveguides for Optical Frequencies’ and submitted it to the Proceedings of Institute of Electrical Engineers. After the usual review and revision, it appeared in July 1966 – the date now regarded as the birthday of optical fiber communication. 4. The paper The paper started with a brief discussion of the mode properties in a fiber of circular cross section. The paper then quickly zeroed in on the material aspects, which were recognized to be the major stumbling block. At the time, the most transparent glass had a loss of 200 dB/km, which would limit transmission to about a few meters – this is very obvious to anyone who has ever peered through a thick piece of glass. Nothing can be seen. But the paper pointed out that the intrinsic loss due to scattering could be as low as 1 dB/km,which would have allowed propagation over practical distances. The culprit is the impurities: mainly ferrous and ferric ions at these wavelengths. Quoting from the paper: ‘It is foreseeable that glasses with a bulk loss of about 20 dB/km at around 0.6 micron will be obtained, as the iron-impurity concentration may be reduced to 1 part per million’. In layman terms, if one has a sufficiently ‘clean’ type of glass, one should be able to see through a slab as thick as several hundred meters. That key insight opened up the field of optical communications. The paper considered many other issues: ? The loss can be reduced if the mode is chosen so that most of the energy is actually outside the fiber. ? The fiber should be surrounded by a cladding of lower index (which became the standard technology). ? The loss of energy due to bends in the fiber is negligible for bends larger than 1 mm. ? The losses due to non-uniform cross sections were estimated. ? The properties of a single-mode fiber (now a key technology especially for long distance and high data rate transmission) were analyzed. It was explained how dispersion limits bandwidth; an example was worked out for a 10 km route – a very bold scenario in 1966. It may be appropriate to quote from the Conclusion of this paper: The realization of a successful fiber waveguide depends, at present, on the availability of suitable low-loss dielectric material. The crucial material problem appears to be one which is difficult but not impossible to solve. Certainly, the required loss figure of around 20 dB/km is much higher than the lower limit of loss figure imposed by fundamental mechanisms. Basically all of the predictions pointed accurately to the paths of developments, and we now have 1/100 of the loss and 10,000 times the bandwidth then forecast – the evolutionary proposal in the 1966 paper was in hindsight too conservative. |
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