Page 23 - Fiber Bragg Gratings

4 Chapter 1 Introduction

central role in more than one way. The field remains an active area for
research.

1.2 Materials for glass fibers

Optical fiber for communications has evolved from early predictions of
lowest loss in the region of a few decibels per kilometer to a final achieved
1
value of only 0.2 dB km" . The reason for the low optical loss is several
fortuitous material properties. The bandgap of fused silica lies at around
9 eV [22], while the infrared vibrational resonances produce an edge at
a wavelength of around 2 microns. Rayleigh scatter is the dominant loss
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mechanism with its characteristic A~ dependence in glass fibers indicat-
ing a near perfect homogeneity of the material [23]. The refractive index
profile of an optical fiber is shown in Fig. 1.1. The core region has a higher
refractive index than the surrounding cladding material, which is usually
made of silica. Light is therefore trapped in the core by total internal
reflection at the core-cladding boundaries and is able to travel tens of
kilometers with little attenuation in the 1550-nm wavelength region. One

Figure 1.1: Cross-section of an optical fiber with the corresponding refractive
index profile. Typically, the core-to-cladding refractive index difference for single-
3
mode telecommunications fiber at a wavelength of 1.5 /am is —4.5 X 10~ with a
core radius of 4 /nm.