• thermal annealing
• high temperature
• harsh environments
• Fiber Bragg Grating Sensors
• FBG strain sensor
• FBG
• thermal regeneration

In the last few years an increased demand in the sensor market has been occurred and nowa- days the interest in harsh environment sensing application represents the driving force for the development of novel sensing elements based on fiber optic. Indeed, there are several industrial sectors (Aerospace, Automotive and Oil&Gas are few examples) that can benefit from the adoption of optical fiber based sensing systems to measure physical parameters such as temperature, deformation, pressure and humidity. Moreover, there are several requirements for the sensing element such as size, weight, electromagnetic immunity and resistance to very high temperature environments, making the most common electronic sensors on the market useless.For these main reasons the market focused on alternative solutions based on different physical principles where the fiber is used either as transmission medium and sensing element. Fiber Bragg Grating Sensors (FBGS) are among the most widespread and promising fiber optic sen- sors and, in this work, limits and the improvements about FBGS subjected to high temperatures have studied in detail. The Fiber Bragg Grating (FBG), in its simplest form, is a permanent periodic refractive index modulation inscribed in the optical fiber core through writing techniques that exploit the photosensitivity of the optical fiber. This grating structure acts as a band-rejection optical filter passing all wavelengths of light that are not in resonance with it and reflecting wavelengths that satisfy the resonance Bragg condition at the so-called Bragg wavelength λB. FBG based sensors exploit the λB dependency on temperature and deformation of the grating and these quantities can be monitored through interrogation techniques. It has been noted that one of the major shortcomings in the market is a stable dynamic strain gauge based on FBGs sensor in high temperature environments. In fact, the major man- ufacturers of FBG strain sensor gauge guarantee a maximum operating temperature of about + 80◦C and only a few manufacturers guarantee a maximum temperature not exceeding 150◦C-200◦C. This lack stimulated this thesis which is focused on the following main tasks:
Analyze the behavior of commercial and low-cost FBGs beyond their operating temperature limits and the physical principles describing their behavior. Hence, exploit high temperature processes to define a method that makes FBG stable at very high temperatures. Analyze and experimentally demonstrate the feasibility of FBG based optical fiber sensor for dynamic strain measurements withstanding high temperatures.

The thesis is structured as follows: in the Chapter 1 (Introduction) a brief preface to the problem and the motivations of the work are given; the basis and the tools to understand how an optical fiber is made, how it works and what are the physical mechanisms involved are provided in the Chapter 2. In the Chapter 3 an useful overview of FBG technology is given to understand the physics of these photonic devices, how they are fabricated, the classification and the principles of operation as sensors of temperature and deformation.
Then in the Chapter 4, trying to address the first task, experimental tests have been carried out to observe the behavior of standard FBG at temperatures much higher than their operating limits (tests up to 600◦C - 1000◦C). This allowed to observe and characterize mainly two thermal-optical processes for FBGs and fibers: the thermal annealing and the thermal regeneration. The main goal was to exploit these processes to bring FBGs naturally designed for low temperatures beyond their operational limits.
In the second part of this work, a possible simple strain sensor design for high temperatures is studied and tested. In Chapter 5 all the materials used, the assembly procedures and the construction of the high temperature sensor deformation apparatus is reported. In fact, it was necessary to design a specific set-up for sensing element characterization within the laboratory furnace and according with the interrogation system. The two main objectives are to observe what kind of problems can occur in sensors of this type at high temperatures (over 200◦C) and try to use FBGs written in highly birefringent optical fibers to realize high temperature dynamic strain sensors. FBGs written in this type of fibers seem to be promising in the world of optical fiber sensors. In the Chapter 6 the main results and analysis of the tests carried out on the sensor have been reported. In the Conclusions, the main results and difficulties of this work have been summarized. Furthermore, a brief outlook for future research and possible improvements is also reported.