Fiber Optic Essentials

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

over 120



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

29