Fundamentals of Optical Fiber Sensors

CHAPTER 2

FUNDAMENTALS OF OPTICAL FIBERS

Chapter focuses on the fundamentals of optical fibers, which is the basis of fiber sensors and fiber devices involved in this book. Section 2.1 gives a brief introduction to the fiber, including its structure and fabrication, basic characteristics, and classifications of the main fiber productions. Section 2.2 expounds the electromagnetic theo- ry of step-index fiber, which is the theoretical basis of the fiber wave- guide. Gradient-index fibers are analyzed in Section 2.3 by ray optics, which gives a clearer and more explicit view on the light propagation in fiber. Section 2.4 introduces briefly special optical fibers, including rare-earth-doped fibers and double-cladding fibers (DCFs), polarization-maintaining fibers (PMFs), and photonic crystal fibers (PCFs).

2.1 INTRODUCTION TO OPTICAL FIBERS

2.1.1 Basic Structure and Fabrication of Optical Fiber

Optical fiber is a thready material with a circular cross section that makes use of optical total internal reflection (TIR) to guide light waves. Figure 2.1(a) is a typical structure of a step-index fiber, which consists of a core with refractive index n1 and a cladding layer with index n2 slightly lower than n1. The light propagating inside the core will be totally reflected at the interface between the core and cladding layer when the incident angle is larger than the critical angle θc, and will be well confined in the core. According to Snell's law, the critical angle is determined by the ...