Page 97 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
6

Light Sources and Transmitters

When you go to a large hardware store, you will see dozens of different light-
bulbs ranging in size and power from flashlight to floodlight applications. A
similar situation holds for light sources used in optical communications. In this
case the sources are much smaller, but they also range from simple, inexpensive
light-emitting diodes (LEDs) to costly, high-power laser diodes with complex
semiconductor structures.
Normally a user does not obtain an isolated light source, but purchases it as
part of an optical transmitter. However, it is important to know the character-
istics of the source in order to choose the transmitter properly. Thus this chap-
ter first presents the characteristics of LED and laser diode light sources.
Following this we will see how the sources are incorporated into transmitters.
The transmitter can range from a simple inexpensive, LED-based package that
fits on a circuit board for short-distance links, to expensive, laser diode-based
modules that also contain sophisticated electronics for controlling the tempera-
ture, wavelength stability, and optical output power level of the laser diode.

6.1. General Source Characteristics
To understand the differences between LEDs and laser diodes, let us first look
at some general characteristics. These include the operating wavelengths of
various device materials, the source output spectral width (the range of wave-
lengths that are contained in an optical output pulse), and the device modula-
tion capability (how fast a device can be turned on and off).

6.1.1. Materials
Semiconductor-based light sources are about the size of a grain of salt. This size
allows efficient coupling of their light output into the small diameters of fibers.
In addition, their semiconductor structure and low-power dissipation charac-
teristics make them compatible with integrated-circuit electronics. Let us first
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