Acoustic coefficients of P 2 O 5 -doped silica fiber: acoustic velocity, acoustic attenuation, and thermo-acoustic coefficient

We present measurements and modeling of the effect of P2O5 doping on the acoustic damping and temperature sensitivity coefficients of silica fibers. In particular, the Brillouin gain spectrum of a highly P-doped fiber is measured and investigated at different temperatures. It is found that the acoustic damping coefficient (proportional to the Brillouin spectral width) of phosphorus pentoxide (1.41 x 10(5) m(-1) for bulk P2O5 at 11 GHz) is similar to, but larger than, that of germanium dioxide. Additionally, the acoustic velocity (and thereby the Stokes. shift) is found to be much less dependent on temperature in P2O5 (+ 0.12 m/s/degrees C) than in SiO2 (+ 0.56 m/s/degrees C). Using these coefficients (the thermo-acoustic coefficients), the modeled and unique slopes of the Stokes.-shift-versus-temperature curves for the four observed acoustic modes each lie within 3% of the measured values. Finally, utilizing both the thermo-optic and thermo-acoustic coefficients, a design example is presented where a composition is determined for which the dependence of the Brillouin frequency shift on temperature is minimized. In this example, the calculated temperature sensitivity is less than 5 kHz/degrees C over the temperature range -100 degrees C < T < 100 degrees C for the molar composition 0.54P(2)O(5):0.46SiO(2). (C) 2011 Optical Society of America