1. Introduction

Non-invasive sensors for temperature and strain measurements in explosive enviroments and with immunity to electromagnetic interference are constantly required. In this context, optical fibers have become a key piece to develop temperature sensors in such hazardous applications. As an example of these applications, we can mention petroleum pipeline monitoring and hydraulic fracturing evaluations [1, 2]. Consequently, many efforts have been made to improve the performance of fiber optic temperature sensors based on different principles and designs.

The development of temperature and strain fiber sensors for environmental measurements where immunity to electromagnetic interference and high personal safety are required has been studied with great interest until today. This fact has allowed the development of a large number of fiber optic temperature sensors based in different principles and desgins; among them, we can distinguish two principal groups which are based in doped [3–10] and un-doped fibers [11–17]. In the first group, we have sensors that use the temperature dependence of the fluorescence lifetime and involve techniques as the fluorescence intensity ratio. In these sensors, the key parameter is the temperature dependence of the absorption and emission cross-sections of the pump and signal in the doped fiber amplifier [18–22]. On the other hand, the second group of temperature sensors are based on un-doped fibers, and these use principally interference techniques. As examples, we can mention sensors based on Fabry-Perot interferometers, fiber Bragg gratings, long-period gratings, optical fiber couplers, and tapered fibers [11–17]. The key parameter in these sensors is the temperature dependence of the dielectric material that modifies the modal behavior of the signal that propagates in the un-doped fiber. It consequently changes the interferometer condition and generates a power variation at the end of the fiber device. Currently, several works have been performed to improve the sensitivity of these temperature sensors employing a combination of these sensing techniques in doped and un-doped fibers, respectively. Additionally, these combinations have allowed discerning between simultaneous strain and temperature measurements [3–6]. In this context, the gradual progress to develop improved temperature fiber sensors requires the necessity to explore continuously novel configurations based on the sensing techniques described above. At this point, one interesting proposal is to consider the use of a tapered fiber, amply used as a temperature fiber sensor [11–17], inscribed simultaneously in a doped fiber amplifier, which poses an additional temperature response caused by its cross-sections [3–10, 18–21]. At this respect, only the efficiency of pump absorption in tapered doped fiber lasers has been studied [23, 24], but a detailed analysis of its temperature response has not be performed. Therefore, in this chapter, we present an analysis of the temperature sensing characteristics of a tapered Yb- and Tm-doped fiber amplifier, and we explore its feasibility to be used as sensing element in temperature fiber sensors. In the present analysis, we study the thermal response of the tapered doped fiber with different tapered fiber structures and pump schemes to find an optimized design for temperature sensing.
