Longitudinal acoustic modes and Brillouin-gain spectra for GeO_2-doped-core single-mode fibers

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Abstract
Guided longitudinal-acoustic modes, which give rise to Brillouin gain, are theoretically clarified for a single-mode fiber with a GeO2-doped core and pure-silica cladding. Longitudinal-acoustic L0m modes are found from the theoretical analysis to interact with the electromagnetic field of the HE11 mode. Brillouin-gain spectra are measured for clarifying the theory. A few gain peaks in the Brillouin-gain spectra are successfully explained by taking account of the phase-velocity characteristics of the L01, L02 and L03 modes guided in the GeO2-doped core region. Furthermore, Brillouin frequency shifts per unit dopant concentration for GeO2 and F are experimentally obtained to confirm the dispersion characteristics of the guided acoustic modes. The evaluated frequency shifts per unit dopant concentration are 107 and 356 MHz/mol% for GeO2 and F, respectively, at a wavelength of 1550 nm.