Page 211 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 205

was 1550 nm, at which losses are at a minimum. The maximum mod-
ulation rate of these lasers was initially 565 megabits/sec. In long-
haul networks, signals could be carried for 70 to 100 km before they
needed to be regenerated. This means cutting the fiber and coupling it
to a photodiode followed by an amplifier and a lot of electronics to re-
condition the signal, and using it to drive another laser that launches
the signal back into the fiber. This kind of repeater was the exact ana-
log of electrical repeaters that the telecommunications companies
used in the days of transmission by copper cables.
In only a few years after the first installations, progress in laser de-
velopment led to the direct modulation laser at 2.5 Gbit/sec, more
than four times faster than 565 megabits. That means that four times
as much information could be carried over the same fiber. However,
the change meant replacing all the repeater amplifiers. This could be
done in principle for a long-haul terrestrial link, but is totally imprac-
tical for submarine optical cable. Soon thereafter, fiber optical engi-
neers began to propose transmission systems that could carry several
wavelengths of light simultaneously. This was a way to boost the ca-
pacity of the optical fiber, but it meant redesigning the repeater so
that there is a complete detection and using reconditioning electronics
for each wavelength of light. This wavelength-division multiplexing
sounded like a great idea, but no practical solutions were in sight. In
1987, laser light amplifiers were rediscovered. Using this technique,
the light wave signals could be amplified optically without having to
use detectors or electronic amplifiers. Just like optical fibers, this am-
plification is completely independent of the modulation frequency. It
can also be used over a significant range of wavelengths. This “just-in-
time” solution meant that the operator could install such an amplifi-
er, even under the ocean, and it would continue to perform in just the
same way, even if more wavelengths were added or if the bit rate were
upgraded. A short history of this discovery has been written by Jeff
Hecht (see Bibliography).
This development occurred during the same time as the birth of the
internet. Telecommunications network companies asked for more and
more capacity to meet the demand. There are basically two ways to
increase capacity:

1. Increase the modulation rate of the channel
2. Increase the number of channels

Raising the modulation rate means creating optical pulses that are
shorter, so that more of them can be sent per second. However, it was
immediately discovered that short pulses launched into an optical
fiber do not stay short. They spread out in time. This is called disper-

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