Brillouin scattering spectrum in photonic crystal fiber with a partially germanium-doped core

The Brillouin scattering spectrum in a photonic crystal fiber (PCF) with a partially Ge-doped core is measured with a pump–probe technique at a wavelength of 1320 nm. One main peak and four subpeaks are observed. The main peak has a Lorentzian shape with the bandwidth $\mathrm{\Delta }{\nu }_B=66 \mathrm{MHz}$. Its intensity is six times higher than that from a standard single-mode fiber measured under the same conditions, which is consistent with the ratio of $\left(1/A_{\mathrm{eff}}^{\mathrm{PCF}}\right)/\left(1/A_{\mathrm{eff}}^{\mathrm{SMF}}\right)$, where $A_{\mathrm{eff}}$ is the effective area of the fibers. The temperature coefficient for the main peak is 0.96 MHz/°C. We believe that the subpeaks are caused by an interaction between light-wave and guided modes of longitudinal acoustic waves in the graded-Ge-doped region, the silica region, and the microstructured cladding. An analysis of the guiding and antiguiding properties of the PCF for acoustic waves is presented.