14.2 TECHNICAL CHALLENGES IN HIGH-SPEED,
LONG-DISTANCE OPTICAL COMMUNICATIONS
14.2.1 An Overview of Transmission Impairments
in WDM Systems
This section briefly explains the problems which have to be solved when performing
high-speed, long-distance optical transmission. One essential limitation which has
been encountered since the early days of optical communications is optical fiber loss.
Thanks to the progress of fiber manufacturing, the attenuation of fibers has dropped
to slightly below 0.2 dB/km, in the wavelength range where the attenuation is at its
minimum, i.e., at 1.55 mm. Even low as it is, loss must be compensated for by
optical amplifiers, such that an optical signal traveling into the fiber can be recovered
after the typical long distan ...