Control of underwater noise radiated by naval vessels relies on two factors. First, ship design for quiet operation, and second, provision of additional control features to monitor and manage the acoustic signature according to operational requirements. While the first is the most crucial factor, the second gains in significance when quiet to ultra-quiet operation is required. Self-monitoring of farfield acoustic signature is inherently problematic because of the difficulty in providing real-time information on radiated sound at large distances from the vessel. A variety of less direct, but currently more effective methods may be employed. However, the one proposed here is based on direct optical sensing of underwater sound through laser Doppler velocimetry in the acoustic nearfield of a vessel. The merits and shortcomings of the technique are discussed, including some of the problems contributing to the presently poor signal-to-noise ratio. While it appears that the technique may be feasible at low frequencies of 10 to less than 1000 Hz, some serious noise limitation such as those due to Brownian diffusive motion need to be overcome first.
"Direct optical sensing of underwater sound fields: naval applications", Proc. SPIE 3707, Laser Radar Technology and Applications IV, (28 May 1999); doi: 10.1117/12.351350; https://doi.org/10.1117/12.351350