Sensitivity of Distributed Acoustic Sensing (DAS) to underwater sound

Application of DAS to telecommunication cables on the seafloor provides a new, powerful modality of active and passive acoustic sensing of the ocean. The measurand in DAS is a time-resolved variation of phase of the Rayleigh-scattered coherent light propagating in an optical fiber. The phase is coupled to ocean processes through the strains and stresses in the fiber. Despite the exponential proliferation of DAS applications, physics-based understanding of the transfer function between the acoustic field and the DAS measurand is lacking. We seek to partially fill this gap by considering diffraction of acoustic waves on unclad and clad fiber suspended in water. The theory is simplified by the cable radius being small compared to acoustic wavelength. DAS proves sensitive to acoustic pressure in the incident wave rather than radial or axial particle displacement. DAS sensitivity differs drastically from the one previously predicted for fiber-optic hydrophones B. Budiansky et al., Appl. Opt., 18, 4085–4088 (1979) because of the cable extent. Angular dependence of the DAS transfer function is strongly affected by resonance scattering of sound by the cable. Cladding can shift the resonance scattering from propagating to evanescent acoustic waves, which has important implications for acoustic remote sensing.