How the use of fiber optics in
electronic communications developed?
In 1880, only four years after his
invention of the telephone, Alexander Graham Bell used light for the
transmission of speech. He called his device a Photophone.
It was a tube with a flexible mirror at
its end. He spoke down the tube and the sound vibrated the mirror.
The modulated light was detected by a photocell placed at a distance
of two hundred meters or so. The result was certainly not hi-fi but
the speech could at least be understood.
Following the invention of the ruby
laser in 1960, the direct use of light for communication was re
investigated. However the data links still suffered from the need for
an unobstructed path between the sender and the receiver.
Nevertheless, it was an interesting idea and in 1983 it was used to
send a message, by Morse code, over a distance of 240 km (150 miles)
between two mountain tops.
Enormous resources were poured into the
search for a material with sufficient clarity to allow the
development of an optic fiber to carry the light over long distances.
The early results were disappointing.
The losses were such that the light power was halved every three
meters along the route. This would reduce the power by a factor of a
million over only 60 meters (200 feet).
Obviously this would rule out long
distance communications even when using a powerful laser. Within
ten years however, we were using a
silica glass with losses comparable with the best copper cables.
The glass used for optic fiber is
unbelievably clear. We are used to normal ‘window’ glass looking
clear but it is not even on the same planet when compared with the
new silica glass. We could construct a pane of glass several
kilometers thick and still match the clarity of a normal window.
If water were this clear we would be
able to see the bottom of the deepest parts of the ocean. We
occasionally use plastic for optic fiber but its losses are still
impossibly high for long distance communications but for short links
of a few tens of meters it is satisfactory and simple to use.
It is finding increasing applications
in hi-fi systems, and in automobile control circuitry. On the other
hand, a fiber optic system using a glass fiber is certainly capable
of carrying light over long distances.
By converting an input signal into
short flashes of light, the optic fiber is able to carry complex
information over distances of more than a hundred kilometers without
additional amplification. This is at least five times better than the
distances attainable using the best copper coaxial cables.
The system is basically very simple: a
signal is used to vary, or modulate, the light output of a suitable
source — usually a laser or an LED (light emitting diode). The
flashes of light travel along the fiber and, at the far end, are
converted to an electrical signal by means of a photo-electric cell.
Thus the original input signal is recovered.
When telephones were first invented, it
took 75 years before we reached a global figure of 50 million
subscribers. Television took only 13 years to achieve the same
penetration and the Internet passed both in only four years. As all
three of these use fiber optics it is therefore not surprising that
cables are being laid as fast as possible across all continents and
oceans.
Optic fibers carry most of the half
million international telephone calls leaving the US everyday and in
the UK over 95% of all telephone traffic is carried by fiber.
Worldwide, fiber carries 85% of all communications.
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