Fundamentals of Optical Fiber Sensors

CHAPTER 5

DISTRIBUTED OPTICAL FIBER SENSORS

It is a unique feature that fiber itself can play the role of sensing. Moreover, it provides the possibility of constructing a distributed sensor system. This chapter is devoted to distributed fiber sensors. The basic physical principles of optical scatterings, including elastic and inelastic scatterings, are introduced in Section 5.1. Section 5.2 describes the optical fiber time domain reflectometer (OTDR) and related distributed fiber sensors, which are based on the Rayleigh scattering effect. Section 5.3 introduces the distributed Raman temperature sensor system. Section 5.4 is engaged in distributed Brillouin sensors for both temperature and strain. The last section introduces interferometric distributed sensors.

5.1 OPTICAL SCATTERING IN FIBER

Early in the 1970s, soon after the breakdown of low-loss silica optical fiber, backscattering effect was investigated in detail [1–4]. The effect was used to characterize loss and imperfections of fiber [5]; and the special technology of OTDR was invented [6,7], which has been used widely in fiber communication technology. The fiber in OTDR is not only the medium for transporting optical probes and signals, but also the sensing element giving information about the status of the fiber. Shortly later, nonlinear optical scattering effects, including Raman scattering and Brillouin scattering, were also utilized to develop various distributed sensors. In this section, the basic physical mechanisms ...