Fig. 1. Experimental setup of the coherent-detection-based distributed acoustic impedance sensing system enabled by a grating array. SOA, semiconductor optical amplifier; EOM, electro-optic modulator; EDFA, erbium-doped optical fiber amplifier; FC, fiber coupler; CFBGs, chirped fiber Bragg gratings; FUT, fiber under test; ISO, isolator; AOM, acousto-optic modulator; PC, polarization controller; PD, photodetector; DAQ, data acquisition card.

Fig. 2. Partial temporal waveform of the excitation and probe pulses, presenting 10-ns guard interval.

Fig. 3. (a) Beat signal segment and (b) normalized spectrum corresponding to a sensing fiber section between two CFBGs, when the excitation pulse is turned off.

Fig. 4. (a) Beat signal segment and (b) normalized spectrum corresponding to a sensing fiber section between two CFBGs, when the excitation pulse is turned on.

Fig. 5. (a) Beat signal of Rayleigh scattering as the excitation pulse is turned on and (b) frequency-domain diagram. (c) Part of the beat signal and (d) its corresponding frequency-domain diagram.

Fig. 6. Beat pulses generated by the entire sensing fiber.

Fig. 7. Spectra of beat pulses from the entire sensing fiber.

Fig. 8. Evolution of amplitudes of frequency components at pΩF−ωLO(p=0,1,2) along the fiber.

Fig. 9. (a) Averaged additional phase factor ξ(zr) and (b) difference of additional phase factor Δξ(zr) along the fiber.

Fig. 10. Distributed FSBS spectra when the stripped fiber section is placed in (a) air, (b) anhydrous ethanol, and (c) water.

Fig. 11. FSBS spectra of the stripped fiber section when it is placed in (a) air, (b) anhydrous ethanol, and (c) water.

Table 1. Beat Frequencies Corresponding to p=0,±1,2 for R_0,9 to R_0,13 Modes with f_LO = 200 MHz^a