8.1   INTRODUCTION

As we discussed in Chapter 3, in optical fiber communication systems, the information
is coded in the form of optical pulses (a 1 or a 0) which propagate along optical
fiber links. In a point-to-point long-distance optical fiber communication link, these
optical pulses would have to propagate over very long distances, such as hundreds to
thousands of kilometers. We know that as optical pulses carrying information propagate
through an optical fiber, they get attenuated and lose power and get broadened
in time due to dispersion. For retrieving information, the optical pulses need to be
detected and converted to electrical signals for further processing. Optical detectors
need to receive a minimum optical power to be able to decipher the sent bit, whether
it is a 1 or a 0. Also, if the pulse dispersion is large, the adjacent pulses may start to
overlap, resulting in nonresolvable pulses and leading to errors in detection.

To propagate over long distances, an actual system uses regenerators, which are
placed periodically along the link and compensate for the accumulated loss and
dispersion (Fig. 8.1). In the case of electronic regenerators, the incoming optical
pulses are first converted into electrical pulses, which are then processed in the
electronic domain to retime and reshape the pulses. This process also removes any
noise that may have accumulated in the pulses. The resulting electrical pulses are
then amplified and used to drive a laser diode, resulting in a fresh optical pulse
stream. In this way the pulse stream leaving the regenerator is almost as good as it
was when it started from the transmission end. When loss and dispersion accumulate
due to further propagation, another regenerator compensates, and in this way the
information-carrying pulse stream is able to propagate over very long distances
without much accumulation of errors.

If an optical communication system employs wavelength-division multiplexing
with multiple signal wavelengths carrying information, at each regenerator site we
would first need to demultiplex (separate) the various channels, use as many regenerators
as the number of wavelengths, and after regenerating the channels would
need to be multiplexed (combined) into a single output for further transmission. Such
electronic regenerators would be very expensive solutions for WDM systems.