## 1 Introduction  
A Fiber Bragg Grating (FBG) is a periodic modulation of the refractive index within the core of an optical fiber [1], which results in a wavelength-selective resonant behavior that resonates at the so-called Bragg wavelength [2]. The spectrum of an FBG results in a narrow bandwidth of reflected waves, while the remainder of the spectrum is transmitted through the grating. The FBG, as well described in [1, 2], implements the Bragg resonant condition within an optical fiber, and results in a compact device that finds broad applications in telecommunications and sensing. In fiber optics, FBGs are extremely important as they behave as narrow-band notch filters, or as passband filters when preceded by a circulator or fiber coupler [3]. An FBG substantially implements a similar function to microwave or electronic resonators, but at a much narrower resonance filters and at infrared wavelengths. Thus,FBGs find many applications in telecommunications [1], in signal equalization [3], and as reflectors for laser cavity in fiber lasers [4]. FBGs are also popular for sensing, as the Bragg wavelength shifts when temperature or strain variations are applied to the grating [5], making it a compact sensing device that finds numerous fields of application. The first generation of FBGs have been fabricated using photosensitive fibers [1], having a significant Ge-doping that facilitates the process of FBG inscription, and on standard single-mode fibers (SMFs) by means of H\({}_{2}\) loading [6]. More recently, the possibility to use a femtosecond (fs) laser focused on the fiber core has opened the possibility of inscribing an FBG on several types of fibers [7; 8]. SMF fibers have low attenuation and adhere to telecommunication standards (such as ITU-T G.657.A1 and G.652.D), and the possibility of inscribing FBGs into such fibers gives rise to wavelength filters and sensors at a relatively low cost, and easy to be interconnected. With the emergence of research on specialty fibers, engineered in order to achieve specific functions that SMFs cannot achieve, new sensing functions can be enabled. In this sense, the possibility of inscribing FBGs in specialty fibers has provided the backbone for advanced applications in sensing and fiber lasers, particularly thanks to direct inscription. Among others, [PERSON] et al. [9] reported FBGs in microstructured fibers which add refractive index sensitivity to the inherent temperature/strain sensitivity; [PERSON] et al. [10] reported FBGs directly inscribed with a point-by-point technique in the inner core of a dual-core fiber, which represents the end reflectors of a fiber laser cavity; [PERSON] et al. [11] reported FBGs inscribed in a polymer fiber in the infrared, which achieves a much larger sensitivity to temperature effects, and reports also a humidity sensitivity; [PERSON] et al. [12] reported FBGs inscribed in a bioresorbable fiber, which has the property of being potentially absorbed by the human body after use. Among the specialty fibers used in sensing applications, several efforts have been recently devoted to altering the Rayleigh scattering properties of fibers. In this scenario, three main approaches have given similar results. [PERSON] et al. [13] reported a method based on rapid pulses with a fs laser (300 nJ at 250 kHz repetition rate), increasing the scattering level of a SMF by up to 45 dB. [PERSON] et al. [14] have obtained similar results, with a scattering increment of 37 dB, by means of exposure to intense ultraviolet light; this method has also been used to generate random gratings [15]. More recently, [PERSON] et al. [16] have obtained a 36.5 dB scattering increment by using a MgO-nanoparticle-doped (MgO-NP) fiber as sensing medium. This setup has also been used to design a scattering-level multiplexing [16; 17], a new domain of multiplexing where the diversity is given by the scattering level at each sensing point.