The Nobel Prize in Physics 2009

M.V.N. Murthy, The Institute of Mathematical Sciences, Chennai

There was a time, not too long ago, that the announcement of Nobel Prize would be know only through the newspapers the next day or even a few days later. Limited amount of signals were transmitted through ordinary copper/aluminium cables. But in recent times, the announcement reaches a large part of the world almost instantly thanks to the work of Charles Kao on optical fibres more than 40 years ago.
Kao was born in Shanghai in November 1933, and is widely regarded as the father of modern communication. He was educated in London and continues to live and teach in Hong Kong.
"Let there be Light"-- Optical Fibres
Guiding of light through a medium is based on the simple concept of total internal reflection. This occurs when light gets reflected back into a medium of higher density at its edge with a medium of lower density.
In the 19th century the concept was used to guide light through a water jet following a suggestion by Michael Faraday. This is shown as an illustration (by Daniel Colladon, 1884) in the figure. Here water comes out of a short spout on a water tank and then falls through open air, as in a fountain. A device on the illustration's left hand side produces light and directs a beam of light into the water tank. The light appears to flow out with the water, getting internally reflected all the way. This demonstration of this "light fountain" needs to be done in a darkened room to see the effect.
Later the same effect was demonstrated in bending glass and through small glass rods.
All this changed with the invention of lasers which is a stable, intense and focussed beam of light. Information can be coded into extremely fast pulses forming the basis for digital communication. One problem that remained was transmitting these signals over very long distances without loss. This is where optical fibres play a central role, in communication.
In 1952, Indian physicist Narinder Singh Kapany, widely considered the father of fibre optics, conducted experiments that led to the invention of the optical fibre. However the loss of signal was a major problem.
Reducing this loss became a challenge for Charles Kao. Initially he set a modest aim of transmitting at least one percent of light over a thousand metre distance. Kao and his colleague, George A. Hockham, studied the properties of glass fibres and concluded that the main problem was the impurity of glass and not the loss due to scattering. To provide good transparency so that the loss of light can be minimised the glass has to be ultra pure. Yet another problem is the fragility of normal glass- it breaks easily. However, it becomes strong, light and flexible when it is drawn out in a long thread.
Today's optical fibres are typically a few hundred micron thick. Unlike copper cables, glass is made out of quartz, the most abundant mineral on Earth and also very cheap. It is also not sensitive to lightening or bad weather unlike normal cables. Attenuation in modern optical cables are far less than those in copper cables.
The first optical fibre, 6000km long, was the trans-atlantic cable laid in 1988. The total length of optical fibre today can be wrapped around the Earth more than 25,000 times (one billion km long), and the length is increasing every hour at an amazing speed. Compared to one percent survival rate set by Kao initially, today more than 95 percent of light remains after a full kilometre.
Infrared laser light of wavelength 1.55 micrometers is used for all long distance communication since the losses are the lowest here. Each fibre can carry many independent channels, each using different wavelength of light. Thus transferring thousands of gigabits of information per second is no longer a "pipe dream".
The digital Eye
Almost every one has seen the astonishing images taken by space telescopes of planets and even far away stars. At the heart of these pictures lies the digital eye invented by Willard Boyle and George Smith. Boyle was born in Amherst, Nova Scotia in Canada in 1924 and Smith was born in 1930 in White Plains, New York in USA. Both of them worked for Bell Laboratories, Murray Hill, New Jersey and are now retired.
While the fibre optic cables make up the back bone of modern communications, the bulk of the traffic or content is made up of digital images. The digital eye that captures these images is based on the invention of a digital sensor, called charge-coupled device or CCD, by Boyle and Smith in the year 1969. When Boyle and Smith started out on their invention their goal was actually to create a better electronic memory device. But this is now long forgotten and the invention has found unintended use as an image sensor.
To understand how this is done, let us look at an ordinary photograph. If you magnify the picture many times over, all you will see is an array of dots of differing colours and intensities. Each dot contains some information projected on the photographic plate by a lens of an object in its sight. Processing of this information by chemical means produces the desired image or the photograph.
Digital imaging works on the same principle but each dot is now replaced by a cell (small capacitor). When light falls on the cell, electrons are emitted through the photo-electric effect (first discussed by Albert Einstein in his famous paper of 1904). The cell collects the electrons emitted. The number of electrons collected in each cell is proportional to the intensity of light. By applying voltage to the cell, the electron content of each cell or what is usually called a pixel can be read out. A pixel is an image point much like the dots in a photographic plate. More electrons are interpreted as regions in the picture which had more light while dark areas yield fewer electrons.
A CCD chip contains an array of such pixels and its capacity is measured by the number of pixels it holds. For example an array of 2048 pixels by 1024 pixels has the capacity of 2.1 million (mega) pixels (their product). Such a CCD is shown in the figure. Each pixel information through electric signals is subsequently translated into digital ones (1s) and zeros (0s) which can be read by any computer to reconstruct the image. Such an image is usually in greyscale (black and white). Filters are used in order to obtain the information about the colour of light falling on each cell.
The first invention of the photographic film was presented by Louis Daguerre in 1839 to the French Academy of Science. We have now entered a new era with digital cameras equipped with image sensors instead of film. A few years ago the 100 million pixel barrier was breached with new technologies.
CCD has opened the eyes of science to the previously unseen- it is now indispensable to the field of astronomy. The array of 30 CCDs used on the Sloan Digital Sky Survey telescope imaging camera is shown in the figure.
The amazing pictures sent by the Hubble space telescope were made possible by the revolution in digital imaging technology. A CCD colour image of the Pelican Nebula is shown in the figure. Now CCD technology is routinely used in a host of medical applications, for example, imaging the inside of the human body, both for diagnostics and for surgical operations. It can reveal fine details in very distant and in extremely small objects.

BOX: Total Internal Reflection
It is well known that a ray of light falling on the surface of water or glass bends towards the vertical due to refraction. This can be inverted so that a ray of light travelling from glass to air bends away from the vertical. By changing the angle suitably it is possible to bend the ray of light such that it does not leave the glass medium at all. This is known as the total internal reflection in a medium whose refractive index is more than that of air.
The angle at which total internal reflection begins is called the critical angle. This is the principle that forms the basis for optical waveguide technology where light is captured inside a glass fibre continuously bounces of its walls and moves forward.
The gastroscope, used to look inside the stomach, in medicine, and the flexible periscope used in defence were early applications of this idea.
One of the biggest problem was the loss of light in transmission. Only a small percentage of the original intensity could be recovered. In the initial days there was not much use for this method for transmitting information. Early telephones used ordinary cables and developed further with wireless communication technologies using radio waves.

End of Box

Box on How an optical fibre transmits light

Different types of optical fibres in use today are shown in the figure. Usually they consist of a core coated with some material of lower refractive index called the cladding. The input pulse is sent in from the left and goes through the fibre. The shaded bands at the outer edge of the fibre have lower density than the central core and hence total internal reflection takes place. The shape of the output pulse that comes out of the fibre is shown on the right.
The figure at the top illustrates the principle of propagation using total internal reflection. There is a single core surrounded by a less-dense outer cover. The output signal is highly distorted.
In the middle figure the core consists of continuously varying refractive index while the cladding is a single medium. The output pulse shape is more like the input pulse shape but not exactly so.
The bottom figure illustrates a single mode fibre where there is a complex composition of core and cladding so that the signal remains undistorted.
Also see the answer to "How can optical fibres be used to transmit light" in the "Do You Know" section of the Mar-Apr 2009 issue of JM.

End of box