**1. Introduction**

Fiber optic Bragg gratings (FBGs) were first produced in 1978 by Ken Hill et al., during an experiment where an optical fiber was exposed to intense radiation from an argon laser. The interference process started between the incident wave and the 4% light reflected at the opposite end of the fiber [1]. It was not until a decade later, in 1989, that G. Meltz and his team proposed Bragg gratings production process by exposing the fiber transversely to two coherent beams of ultraviolet light, forming an interference pattern in the fiber core [2, 3].

Over the last decades, Bragg gratings have been used as sensors [4, 5] or as optical filters in wavelength division multiplexing (WDM) communication systems [6]. They have found application in devices with routing capacity in the optical domain, as is the case of optical add drop multiplexers (OADM) and optical cross connect (OXC) nodes. They are also used in frequency stabilizers of semiconductor lasers [7], on tunable fiber lasers [8], and in the gain equalization of optical amplifiers [9]. Fiber dispersion compensation [10] is another realization with tunable fiber Bragg gratings.

The evolution of transmission systems from point-to-point connections to dynamically configurable networks brought the need for tunable optical filters. Furthermore, the increasing transfer of information over fiber optic networks causes an increase in the number of optical channels in Dense (DWDM) or even in Coarse WDM (CWDM). The expansion in the bandwidth necessity range can cover the S, C, L, and U bands, corresponding approximately to 215 nm.

During the last 3 decades, but also very recently, the use of spread spectrum techniques in the optical domain, such as optical code division multiplexing (OCDMA), has stirred the interest of the scientific community, as an option in reconfigurable optical networks [11]. This process allows the various channels to share the same spectral band; each one being identified by a specific code. Other features of this technique should be highlighted: Greater safety in the data transmission, more flexible use of available bandwidth, reducing crosstalk between adjacent channels, and enabling asynchronous communication [12]. The recent use of spectral amplitude coding OCDMA (SAC-OCDMA) in optical coding and decoding systems has brought the ability to eliminate multiple access interference (MAI) [13]. For this technique to be attractive from the point of view of implementation in an optical network, the definition of the codes will have to be flexible [14], so that tunable FBGs are a key element of the process. One of the features that make FBGs attractive is that the reflection spectrum can be tuned from a few nanometers, heating the Bragg grating or applying mechanical tension to the ends (compression or extension).

The objective of this chapter is to describe the implementation of two tuning methods for fiber Bragg gratings, as a function of variations in temperature or in the length of the FBG. Among the methods of mechanical deformation, the deformation of the grating by stretching or compressing the fiber and the deformation by curvature of a flexible blade connected to the Bragg grating are experimentally implemented and discussed.

The rest of the chapter is structured as follows: Section 2 presents the operating principle of FBG tuning techniques using mechanical and temperature techniques. Section 3 describes the experimental measurement setup for FBG tuning characterization. Section 4 presents the main experimental results and discusses the potential of each tuning technique. Finally, Section 5 gives concluding remarks.
