City of Light: The Story of Fiber Optics

‘‘THE ONLY THING LEFT IS OPTICAL FIBERS’’ 115
of the prestigious Bell Labs. Responsible journalists knew they should trust
such authoritative sources. When the Kao and Hockham paper finally appeared,
it also made no discernible mark even in the technical press.
No Silver Medals for Invention
The French followed the same trail after Spitz talked the French Ministry of
Defense out of a small grant, but they lagged behind STL. Werts modeled light
transmission along a fiber ‘‘surface’’ with 1.4-millimeter microwaves, the
shortest wavelength available. Then he calculated how light should travel
through a clad single-mode optical fiber. Only after Werts started did Spitz
discover Snitzer’s careful analysis of fiber modes. Werts had to make his own
laser for fiber-transmission experiments, buying mirrors from America and
filling a glass tube with helium and neon.
The Decibel Score
Decibels are very handy units for engineers evaluating signal strength but can
be quite confusing to other people. If you want to avoid complexity, you can
think of decibels as a way of keeping score. In fiber optics, they usually measure
how much of the signal is lost when being transmitted. Lower loss is better, so
lower decibel numbers are better. Engineers usually measure transmission loss
per kilometer of fiber, so the standard units are decibels per kilometer. Metric
units are standard for the research and development community, but you needn’t
worry too much about them. The relative scores for fibers are the same whether
the loss is measured per kilometer, per foot, per mile, or per light year. (The
numbers, however, are quite different. Kilometers are used here because they’re
the most common scale.)
Strictly speaking decibels measure the ratio of output power to input power
on a logarithmic scale. The formula used for fiber optics is
� �
power out
decibels � 10 log .
power in
For optical fibers, the number is negative because power output is less than
input, but the sign is usually ignored. (The only way to have higher output is in
an optical amplifier.)
Decibels greatly simplify engineering calculations because you can find total
loss by multiplying fiber loss in decibels per kilometer by the length of a fiber.
For example, 50 kilometers of 0.5 decibel/kilometer fiber has a 25-decibel
loss. However, because the scale is logarithmic, it’s easy to underestimate the
impact of loss measured in decibels. Loss of 10 decibels means one-tenth of the
signal remains. Loss of 20 decibels means only one percent remains, while loss
of 100 decibels means only 10 �10 (0.000,000,000,1) remains. The table
below gives some examples, with physical analogies that may be helpful.