## 5 Conclusions  
Specialty optical fibers for the Brillouin-scattering-based distributed sensor application were briefly reviewed. While most may not consider Corning's LEAF non-zero dispersion shifted fiber to be a \"specialty fiber\", its use in certain distributed sensor configurations lies well outside the reasons for its development. Therefore, it has been included in this discussion, but with the consideration that acoustic waveguide design could further be optimized and developed for distributed sensors. The  
Figure 8: Example acoustic and refractive index profiles for the equalization of Brillouin gain between the L\({}_{01}\) and higher order acoustic modes. In this configuration, the optical and acoustic boundaries are decoupled [138]. optical and acoustic waveguide properties of silicate glass optical fibers first were discussed. Next, material properties were described in the context of how they modify the relevant Brillouin scattering attributes of the fiber. Finally, the literature was surveyed for fiber configurations that give rise to novel methods and arrangements for this distributed sensing application. It has been suggested that these fibers enable systems capable of discriminative sensing (for example, distinguishing changes in \(\varepsilon\) and \(T\)), 3-D shape sensing, simple frequency-domain sensing, systems that are immune to changes in one or more environmental conditions (such as with athermal or atensic fibers), and fibers with enhanced thermo-mechanical response. Acoustic waveguide design, while leading to novel fiber configurations, such as a multi-peaked BGS with potential for discriminative sensing, has been limited to a few common dopants developed for conventional fabrication methods (such as modified chemical vapor deposition (MCVD)), namely GeO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and F. This has limited the range of fiber properties that are feasible or even possible. As such, the design of the BGS, the relative strength of \(g_{B}\), and thermal and strain coefficients are limited to a narrow range with the use of these dopants. The use of less conventional materials, even in the silicates, can enrichen the set of tools that can be used in the design of such optical fibers. For instance, the use of less conventional materials such as alkali metal oxide dopants can lead to fibers immune, or even several times more sensitive, to changes in temperature. Such materials also can find value in multi-core fibers and multi-material fibers [153], especially for high-sensitivity discriminative, multivariable sensing. However, due to technological limitations related to the material in an entrenched fabrication process, such new glass systems have not widely been studied utilizing the more conventional fabrication methods found in the commercial sector [154]. For instance, many of the novel silicate systems studied by these authors were fabricated using the molten core process (more details can be found in [155]) which differs significantly from the MCVD process in terms of both thermal and processing history, and ranges of compositions that can be produced. Although not currently commercially pervasive, it is the contention of these authors that the maturation of this and other fiber and glass fabrication methods [156; 157; 158] can give access to a wider range of the Periodic Table and fiber embodiments, and more widely-varied thermo-mechanical responses. This can lead to simpler and less expensive sensing systems (such as through beat signal measurements found in [44; 143], bringing the measurement frequency to a potentially more cost-effective region in the RF [159]). While fiber development cost may initially be high, the long-term payoff could be economies of scale. Conceptualization, P.D. and J.B.; Investigation, P.D. and J.B.; Writing-Original Draft Preparation, P.D. and J.B.; Writing-Review & Editing, P.D. and J.B.; Funding Acquisition, P.D. and J.B.