Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 451

15.1 History

Fiber optics is the branch of optical technology con-
cerned with the transmission of light through fibers
made of transparent materials, such as glass, fused sil-
ica, or plastic, to carry information.
Fiber optics has been used by the telephone industry
for over thirty years, and has proved itself as being the
transmission medium for communications. Past history
shows audio follows the telephone industry, therefore
fiber optics will soon be a force in audio.
The founder of fiber optics was probably the British
physicist, John Tyndall. In 1870 Tyndall performed an
experiment before the Royal Society that showed light
could be bent around a corner as it traveled in a rush of
pouring water. Tyndall aimed a beam of light through
the spout along with the water and his audience saw the
light follow a zigzag path inside the curved path of the
water. His experiment utilized the principle of total
internal reflection, which is also applied in today’s
optical fibers.
About ten years later, William Wheeler, an engineer
from Concord, Massachusetts invented a scheme for
piping light through buildings. He used a set of pipes
with a reflective lining and diffusing optics to transmit
light (bright electric arc) through a building, then
diffuse it into other rooms. Although Wheeler’s light
pipes probably didn’t reflect enough light to illuminate
the rooms, his idea kept coming up again and again until
it finally coalesced into the optical fiber.
At about the same time Alexander Graham Bell
invented the photophone, Fig.15-1. Bell demonstrated
that a light ray could carry speech through the air. This
was accomplished by a series of mirrors and lenses
directing light onto a flat mirror attached to a mouth-
piece. Speech vibrating the mirror caused the light to
modulate. The receiver included a selenium diode
detector whose resistance varied with the intensity of
light striking it. Thus the modulated light (sunlight, etc.)
striking the selenium detector varied the amount of
current through the receiver and reproduced speech that
could be transmitted over distances of approximately
200 meters.
In 1934, an American, Norman R. French, while
working with AT&T, received a patent for his optical
telephone system. French’s patent described how
speech signals could be transmitted via an optical cable
network. Cables were to be made out of solid glass rods
or a similar material with a low attenuation coefficient
at the operating wavelength.

Interest in glass waveguides increased in the 1950s,
when research turned to glass rods for unmodulated

transmission of images. One result was the invention of

the fiber scope, widely used in the medical field for

viewing the internal parts of the body. In 1956 Brian

O’Brien, Sr., in the United States, and Harry Hopkins

and Narinder Kapany, in England, found the way to

guide light. The key concept was making a two-layer

fiber. One layer was called the core the other layer was

called the cladding (see section on light). Kapany then

coined the term fiber optics.

An efficient light source was needed but it wasn’t

until 1960 when the first laser light was invented that it

became available. A Nobel Prize was awarded to Arthur

Schawlow and Charles H. Townes of Bell Laboratories

for developing the laser, which was first successfully

operated by Theodor H. Maiman of Hughes Research

Laboratory. The manufacturing process of lasers from

semiconductor material was recognized in 1962. At the

same time semiconductor photodiodes were developed

for receiver elements. Now the only thing left was to

find a suitable transmission medium.

Then in 1966 Charles H. Kao and George A.

Hockham, of Standard Telecommunication Labs,

England, published a paper proposing that optical fibers

could be used as a transmission medium if their losses

could be reduced to 20 dB/km. They knew that high

losses of over 1000 dB/km were the result of impurities

in the glass, not of the glass itself. By reducing these

impurities a low-loss fiber could be produced for tele-

communications.

Finally in 1970, Robert Maurer and associates at

Corning Glass Works, New York, developed the first

fiber with losses under 20 dB/km, and by 1972 lab

samples were revealed as low as 4 dB/km. Since then

the Corning Glass Works and Bell Telephone Labs of

the United States; Nippon Sheet Glass Company and

Nippon Electric Company of Japan; and at AEG-Tele-

funken, Siemens and Halske in Germany, have devel-

oped glass fibers with losses at about 0.2 dB/km. There

is also some plastic materials as well as glass being used

for shorter distances.

Figure 15-1. Alexander Graham Bell’s Photophone.

Mouthpiece

Transmitter

Diaphragm

modulated

beam of

light

Sunlight

Receiver

Selenium cell Earpiece

200 m