Page 35 - Optical Communications Essentials

Optical Communication Systems Overview

Optical Communication Systems Overview 25

Figure 2.3. Illustration of a fiber-drawing process.

A problem with using single-mode fibers in the 1970s was that no light source
existed which could couple a sufficient amount of optical power into the tiny
fiber core (nominally 9µm in diameter). Therefore, multimode fibers with larger
core diameters ranging from 50 to 100 m were used first. The main light sources
were light-emitting diodes (LEDs) and laser diodes that emitted at 850nm. The
combination of early sources, multimode fibers, and operation at 850nm limited
the optical fiber links to rates of about 140Mbps over distances of 10km.
To overcome these limitations, around 1984 the next generation of optical
systems started employing single-mode fibers and operated at 1310nm where
both the fiber attenuation and signal distortion effects are lower than at 850nm.
Figure 2.4 shows this attenuation difference in decibels per kilometer as a func-
tion of wavelength. The figure also shows that early optical fibers had three
low-loss transmission windows defined by attenuation spikes due to absorption
from water molecules. The first window ranges from 800 to 900nm, the second
window is centered at 1310nm, and the third window ranges from 1480 to
1600nm. In 1310-nm systems the transmission distance is limited primarily by
fiber loss and not by other factors that might not allow a longer and faster
transmission link. Therefore the next evolutionary step was to deploy links at
1550nm where the attenuation was only one-half that at 1310nm. This move
started a flurry of activity in developing new fiber types, different light sources,
new types of photodetectors, and a long shopping list of specialized optical com-
ponents. This activity arose because transmitting in the 1550-nm region and
pushing the data rate to higher and higher speeds brought about a whole series

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