Enhanced direct measurement of underwater sound velocity via optical-delay-induced envelope synthesis.

This Letter proposes an optical-delay-induced envelope synthesis method for enhanced direct underwater sound velocity measurement. By coherently synthesizing acousto-optic signal replicas with deterministic time delays, a differential envelope with a pronounced transition is generated that compresses the cross-correlation peak and improves time-of-flight (TOF) localizability. Experimental results show that the full width at half maximum of the cross-correlation peak is reduced by 32%, while the standard deviation of the TOF measurement decreases from 5.25 ns to 2.07 ns. Combined with an independently calibrated propagation distance, the method achieves an uncertainty of 0.0305 m/s for a single measurement. These results demonstrate that optical-domain envelope synthesis improves TOF localizability under narrowband excitation, providing a potential direct velocimetry approach for underwater acoustic positioning and communication.