Dependence of Frequency Shift of Depolarized Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers

We study through experiment, calculation and simulation the frequency dependence of depolarized GAWBS observed in four different photonic crystal fibers and extend the results to others through simulation. In two of the fibers studied, high frequency modes (>; 1 GHz) are observed in addition to the usual lower frequency torsional radial (TR) modes. The acoustic modes responsible for GAWBS are characterized through a combination of simulations and experiments. The calculations are done using the silica rod approximation, which is valid for PCFs with a low to intermediate air-fill fraction or with a very high air-filling fraction. Further simulations extend the range of photonic crystal fibers in which acoustic modes can be characterized. These are used to study the dependence of the acoustic mode frequency on key fiber parameters such as the ratio of air to silica across the whole cross section (κ) and the lattice pitch (Λ) .