Underwater lidars

At present, acoustics is the most commonly used means of imaging the seabed at long range (a few hundred meters) (figure A), while the use of optics is restricted to short-range identification (a few meters) using conventional video cameras. This division of roles between acoustics and optics in underwater imaging is partly due to the different physical properties of acoustic and optical waves in the marine environment, but also to the different maturity of the associated technologies. As a result, the advent of coherent, amplitude- and frequency-modulated light sources (lasers), combined with the use of signal processing techniques inspired by radar techniques, means that the role of optics in underwater imaging can be extended, and new systems (figure B) can be developed that take advantage of the ability of light waves, unlike acoustic waves, to pass through the air/water interface with very high efficiency: $T=\frac{4n_1{n}_2}{{\left(n_1+n_2\right)}^2}\simeq \mathrm{0,98},$

at normal incidence for: $n_{\text{air }}\simeq 1$ , $n_{\text{eau }}\simeq \mathrm{1,333}$ .