Page 226 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

220 Advanced Topics

9.7 Summary
The optical fiber is a key element in the telecommunications revolu-
tion that has changed the lives of people around the world during
the last 15 years. The possibility of using optical fibers to transmit
signal over long distances had to wait until low-loss fibers were
demonstrated in the early 1970s. Realistic widespread deployment of
optical fibers had to wait until a compact optical source was avail-
able. Coincidentally, room-temperature continuous operation of
GaAs laser diodes was demonstrated at the same time. Shortly af-
terward, a whole new class of laser diode materials—GaInAsP—was
developed to permit transmission at the low-loss window for optical
fibers at 1550 nm. After this innovation, further development of
GaAs lasers for communications applications was largely abandoned.
By 1990, optical fiber telecommunications links were installed under
the ocean using GaInAsP lasers and electronic repeater amplifiers.
Signal modulation was accomplished by direct-current modulation of
the gain. The Er-doped optical amplifier was rediscovered. This am-
plifier is pumped by GaAs-based lasers, which reentered the optical
communications industry after a 10 year absence. The explosive
growth of the Internet put huge pressure on network operators to in-
crease capacity. This could be accomplished by raising the modula-
tion rate of the laser and by multiplexing many wavelengths into
one fiber. These two improvements were only possible because of the
Er-doped fiber amplifier. Increasing the modulation rate of lasers
has proved to be difficult because of chirp, which we discussed in
Chapter 8. The optical wavelength of the emission changes in time
when the laser is switched on. Since this feature is fundamental to
laser operation; modulation is now being achieved by an external
waveguide modulator that is independent from the laser—a kind of
very fast chopper. The emphasis in transmission laser development
is no longer modulation speed but on spectral purity and output
power.
This brief history illustrates that the technology in this field has
not developed in a straight line path over the years. Optical ampli-
fiers were developed before people had any ideas about optical fiber
communications. GaAs lasers have gone in and out of style. Electronic
signal regeneration has fallen out of favor because of optical amplifi-
cation. However, we should all keep our eyes on the silicon VLSI in-
dustry, which is starting to take optical fiber communications serious-
ly. The power of VLSI is legendary and it is not hard to imagine new
VLSI chips with some optical functionality hybridized with advanced
fast signal-processing circuits. Such developments may reintroduce
electronic signal processing as the technology of choice for optical

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