Analysis of Brillouin frequency shift and longitudinal acoustic wave in a silica optical fiber with a triple-layered structure

We report a thorough analysis on the Brillouin frequency shift as a function of geometrical parameters in a silica optical fiber consisting of triple-layered structure, GeO₂-doped core, P₂O₅, and F co-doped inner cladding, and pure silica outer cladding. General characteristic equations for the Brillouin frequency shift were analytically derived and analyzed for various fiber parameters. In experiments, three-layered optical fibers were fabricated and their Brillouin frequency shifts were measured in the wavelength region of 1.55 μm by a pump-probe technique. The longitudinal acoustic velocity in each layer was found significantly affected by the thermal stress as well as the dopant concentrations. We confirmed both in theory and experiment that the inner cladding of a three-layered optical fiber does provide a new degree of freedom in precise control of the Brillouin frequency shift.