Precise underwater distance measurement using laser frequency comb

Underwater distance measurement has been playing an important role in the fields of underwater navigation, search and rescue, and bathymetric survey. However, limited by the measurement sources (e.g. acoustic pulses or incoherent optical pulses), the precision can only achieve centimeter level at best, which greatly obstructs the advancement of underwater science and technology. Recent developments of optical frequency combs have given rise to revolutionary progress in metrology, spectroscopy, and optical distance measurement in air. However, frequency comb-based underwater distance measurement has rarely been reported. In this work, we describe a spectral interferometry-based method using a laser frequency comb at 518 nm (green light), which is capable of underwater distance measurement with high precision and accuracy. Due to the inherently dispersive characteristic of water, the measurement pulse will be chirped, and the spectrograms exhibit unstable oscillations, the spectral phase of which features a quadratic law. Distances up to 4.4 m and displacements down to 5 mu m underwater can be determined by virtue of the spectral phases. The experimental results show the differences within +/- 4 mu m at 4.4 m range, compared to the reference values. The Allan deviation is 1.596 mu m at 5 s averaging time, 481 nm at 100 s averaging time, and can achieve 261 nm with 540 s averaging time. Our work could provide a promising and powerful tool for underwater distance measurement with high precision at long range, and open the door to a host of underwater applications related to the measurement of distance, e.g. mapping and positioning.