Chapter 2 Transmitters
In this chapter we will discuss optical transmitters for fiber optic systems.
This includes basic fiber optic communications light sources, such as
LEDs, different types and wavelengths of lasers, and optical modulators.
Our focus in this chapter is primarily on communication and sensor
systems. Certain types of optical transmitters have become optimized
for fiber optic communications, and other sources used for illumination
and imaging have very different specifications and operating principles.
While we discussed the many types of optical fiber in a single chapter,
because a number of different applications use the same type of fiber, we
cannot do the same with light sources; a fiber optic link is fundamentally
different from a physician’s fiberscope. This chapter describes the type
of transmitters used in fiber optic links; sources for medical, industrial
and lighting applications have different characteristics and will be treated
in a separate chapter.
Following a review of basic terminology, we will discuss LEDs, lasers,
and optical modulators. LEDs are frequently used for multimode fiber
optic links. The two types of LEDs, surface emitting LEDs and edge
emitting LEDs (ELEDs), have high divergence of the output light (mean-
ing that the light beams emerging from these devices can spread out
or more); slow rise times (>1 ns) that limit communication to
speeds of less than a few hundred MHz; and output powers on the order
of 0.1–3 mW. However, LEDs are less temperature sensitive than lasers
and are very reliable.
For higher data rates, laser sources at short wavelengths (780–850 nm)
are typically used with multimode fiber, and longer wavelengths