1. Introduction

The discovery of lasers in the 1960s and development of low loss silica optical fiber opened a new era of fiber optic sensors. Intrinsic insensitivity to electromagnetic interference (EMI), remote detection, operational ability in hazardous environment, and potential for distributed sensing make them especially useful for monitoring large nuclear infrastructures such as coolant monitoring, reactor containment buildings, nuclear waste storage sites etc. [1–5]. Radiation tolerant fibers can be used in various configurations for distributed sensing of temperature, strain, and several other parameters avoiding the requirement of positioning many discreet sensors [6–11]. Further, the radiation sensitive fibers can be used for radiation dose monitoring for local dose deposition measurements, hot spot dose monitoring in waste storage facilities, surveillance at airports and ports of entry, etc. With the

availability of lasers, fibers, and low noise detectors in the mid-IR region, it has become possible to design novel distributed sensor devices for sensing hazardous volatile compounds for homeland security especially at airports, underground metro stations, and big event areas.

A wide range of techniques such as intensity modulation, wavelength encoding, and polarization provide powerful sensing capabilities. Further, several detection techniques have been investigated for development of optical fiber-based distributed sensors. Radiation-induced absorption, scintillation, fluorescence, optically stimulated luminescence, and induced refractive index changes have been used for real-time dose measurements. Optical fiber grating [12–15]-based specialty sensors have been used for distributed strain measurements in very low temperature, very high temperature, or high radiation environment. Raman and Brillouin scatteringbased techniques are used for distributed temperature measurements for fire and hot spot detection. Mid-IR and near-IR absorption measurements coupled with hollow core fibers are used for leak detection of hazardous gases. This chapter will describe the basic principles, main components, various sensing systems for advanced applications, and future potential of distributed fiber sensors.
