Measurement of the refractive-index modulation generated by electrostriction-induced acoustic waves in optical fibers

The refractive-index modulation generated in optical fibers by electrostriction-induced acoustic waves is investigated directly by a pump-probe measurement technique in a 1-km-long fiber Sagnac-loop interferometer. Pump pulses propagating unidirectionally around the loop generate transverse acoustic waves that produce a time-dependent relative phase shift for the probe pulses. The consequent interferometer transmission changes are used as a measure of the acoustic interaction. The acoustic impulse response function of the fiber is measured by use of short optical pulses with a 1-MHz repetition rate. The response consists of a series of peaks separated by ∼21 ns with a maximum value of refractive-index modulation of -1.1 × 10^-11 generated by the 230-pJ pump pulses. At a higher pulse repetition rate of 100 MHz additional temporal structure is generated in the 0-5-ns period after the pump pulse, leading to a response similar to that observed in long-range soliton interaction.