A highly nonlinear optical fiber having an improved stimulated Brillouin scattering threshold is provided. The fiber includes a central core region made substantially from silica doped with aluminum, a trench region surrounding the central core region, and a silica cladding surrounding the trench region. The refractive index profile of the fiber is optimized. A refractive index difference of the central core region relative to the cladding (n+) is less than 26×103, and more preferably at or near 21×103. A refractive index difference of the trench region relative to the cladding (n) is less than 5×103. The trench region is preferably doped with fluorine. The aluminum doping level of the central core region is preferably less than 14 wt % Al. A fiber doped with aluminum having this refractive index profile exhibits a significantly higher figure of merit (PthLeff) than conventional germanium-doped fibers.
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