The acoustic interaction of optical pulses in optical fibers is investigated directly by time-resolved pump-probe measurements of the transmission of a fiber Sagnac-loop interferometer. Resonant enhancement of the refractive-index change deltan(ac) induced by the acoustic waves is observed when the repetition frequency of the pulse train is close to a vibrational eigenfrequency of the fiber. For standard fiber deltan(ac) is enhanced by a factor of ~3 at the 465-MHz eigenmode frequency, and this factor increases to ~10 when the polymer jacket is removed from the fiber. The guided acoustic wave Brillouin scattering spectrum that arises from spontaneous thermal excitation of the acoustic eigenmodes of the fibers is also measured. The results suggest that the resonant enhancement of deltan(ac) is limited by dampling that is due to the polymer jacket and by inhomogeneous broadening that is due to fiber diameter variations.
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