Fundamentals of Optical Fiber Sensors

CHAPTER 3

FIBER SENSITIVITIES AND FIBER DEVICES

Characteristics of light propagation in the fiber will be affected by physical conditions that are encountered by the fiber. On the other hand, the fiber's sensitivity to physical conditions is the basis of fiber sensors and most intrinsic to fiber devices. In this chapter, the sensitivities and their physical mechanisms are expounded in Section 3.1. Characteristics and basic theory of fiber couplers are stated in Section 3.2; and devices incorporated with couplers are introduced in Section 3.3. Polarization characteristics are discussed in Section 3.4; and polarization-related fiber devices are presented in the last section.

3.1 FIBER SENSITIVITIES TO PHYSICAL CONDITIONS

It is interesting to investigate the fiber sensitivities to various physical conditions and to understand their mechanisms. Such sensitivities are the basis fiber devices and fiber sensors. In this section, sensitivities of single-mode fibers (SMF) to strain, stress, and temperature are analyzed. Sensitivity of germanium-doped silica fiber to ultraviolet laser irradiation, which is the basis of fiber gratings, will be discussed in Chapter 4. Nonlinear optical (NLO) effects, which are used in distributed sensors, will be analyzed in Chapter 5.

3.1.1 Sensitivity to Axial Strain

When a force is applied to a section of fiber in its axial direction, it will be stretched or compressed, that is, an axial strain occurs. According to the theory of elasticity, both strain ...