28 CHAPTER 2. OPTICAL FIBERS
Figure 2.4: Variation of intermodal dispersion∆T/Lwith the profile parameterαfor a graded-
index fiber. The scale on the right shows the corresponding bit rate–distance product.
of lightwave systems used graded-index fibers. Further improvement is possible only
by using single-mode fibers whose core radius is comparable to the light wavelength.
Geometrical optics cannot be used for such fibers.
Although graded-index fibers are rarely used for long-haul links, the use of graded-
indexplasticoptical fibers for data-link applications has attracted considerable atten-
tion during the 1990s [24]–[29]. Such fibers have a relatively large core, resulting in
a high numerical aperture and high coupling efficiency but they exhibit high losses
(typically exceeding 50 dB/km). TheBLproduct of plastic fibers, however, exceeds
2 (Gb/s)-km because of a graded-index profile [24]. As a result, they can be used to
transmit data at bit rates>1 Gb/s over short distances of 1 km or less. In a 1996
demonstration, a 10-Gb/s signal was transmitted over 0.5 km with a bit-error rate of
less than 10−^11 [26]. Graded-index plastic optical fibers provide an ideal solution for
transferring data among computers and are becoming increasingly important for Eth-
ernet applications requiring bit rates in excess of 1 Gb/s.
2.2 Wave Propagation
In this section we consider propagation of light in step-index fibers by using Maxwell’s
equations for electromagnetic waves. These equations are introduced in Section 2.2.1.
The concept of fiber modes is discussed in Section 2.2.2, where the fiber is shown to
support a finite number of guided modes. Section 2.2.3 focuses on how a step-index
fiber can be designed to support only a single mode and discusses the properties of
single-mode fibers.