Transmitting light. Filled with light

Transmitting light

The invention of the laser at the beginning of the 1960s was a decisive step forward for fiber optics. The laser was a stable source of light that emitted an intensive and highly focused beam of light, and could be pumped into a thin optical fiber. The first lasers emitted infrared light and required cooling. Around 1970 more practical lasers were developed which could work continuously at room temperature. This was a technological breakthrough that facilitated optical communication.

All information could now be coded into an extremely fast flashing light, representing digital ones and zeros. However, how such signals could be transmitted over longer distances was still not known — after just 20 meters, only 1 percent of the light that had entered the glass fiber remained.

Reducing this loss of light became a challenge for a visionary like Charles Kuen Kao. Born in 1933 in Shanghai, he had moved to Hong Kong together with his family in 1948. Educated as an electronics engineer, he defended his Ph. D. -thesis in 1965 in London. By that time he was already employed at the Standard Telecommunication Laboratories, where he meticulously studied glass fibers together with his young colleague George A. Hockham. Their goal was that at least 1 per­cent of the light that entered a glass fiber would remain after it had travelled 1 kilometer.

In January 1966, Kao presented his conclusions. It was not imperfections in the fiber thread that was the main problem, instead it was the glass that had to be purified. He admitted that this would be feasible but very difficult. The goal was to manufacture glass of a transparency that had never been attained before. Kao’s enthusiasm inspired other researchers to share his vision of the future potential of fiber optics.

 Glass is manufactured from quartz, the most abundant mineral on Earth. During its produc­tion, different additives such as soda and lime are used in order to simplify the process. How­ever, in order to produce the purest glass in the world, Kao pointed out that fused quartz, fused silica, could be used. It melts at almost 2 000 °C, a heat difficult to control but from which one would draw out ultra-thin threads of fiber. After four years, 1971, scientists at the Corning Glass Works in the USA, a glass manufacturer with over 100 years experience, produced a 1 kilometer long optical fiber using chemical processes.

Filled with light

Ultra-thin fibers made out of glass may seem very fragile. However, when glass is cor­rectly drawn out in a long thread, its properties change. It becomes strong, light and flex­ible, which is a prerequisite if the fiber is to be buried, drawn under water or bent around corners. Unlike copper cables, glass fiber is not sensitive to lightning, and unlike radio communication, it is not affected by bad weather.

It took a fair share of time to coil the Earth in fiber. In 1988, the first optical cable was laid out along the bottom of the Atlantic Ocean between the United States and Europe. It was 6 000 km long. Today, telephone and data communication flows in a network of opti­cal glass fiber, the length of which totals over 1 billion km. If that amount of optical fiber was wrapped around the globe it would span the world more than 25 000 times — and the amount of fiber is increasing every hour.

Even in a high purity glass fiber, the signal is slightly reduced along the way and has to be reinforced when it is transmitted over longer distances. This task, which previously required electronics, is today performed by optical amplifiers. This has brought an end to unnecessary losses that occur when light is transformed to and from electronic signals.

Today 95 percent of the light remains after having been transmitted a full kilometer, a number that should be compared to Kao’s ambition of having 1 percent left after that same distance. Furthermore, it is not possible to speak of only one single kind of fiber. Choosing which fiber to use is subject to many different technical considerations, com­munication needs and costs.

The fibers consist of a sophisticated interplay between size, material properties, and wave­lengths of light. Semiconductor lasers and light diodes the size of a grain of sand fill networks of optical fibers with light which carries almost all of the telephone and data communication around the world. Infrared light with a wavelength of 1. 55 micrometers, is nowadays used for all long distance communication where the losses are lowest.

The capacity of optical cable networks is still growing at an amazing speed – transfer­ring thousands of gigabits per second is no longer a dream. Technological development is heading in the direction of more and more interactive communication, where optical fiber cables are designed to reach all the way into the homes of each and every one of us. The technology already exists. What we do with it is an altogether different question.