Backscatter Signal in Underwater Lidars with a Complexly Modulated Probe Beam

The results of statistical modeling of the backscatter signal in lidars when probing the water column with pulses with internal modulation by complex frequency modulated signals and their matched processing in the receiving path of the lidar are presented. The simulation results are compared with analytical calculations in the small-angle approximation. It is shown that the photons spread along their paths, associated with multiple scattering in the medium, does not prevent effective compression of the complex signal, and the small-angle approximation well describes the energy-carrying part of the signal backscattered by the water column. A comparison was made of the levels of backscatter signals when probing water with a short pulse and a complexly modulated pulse. It is shown that the use of complexly modulated illumination pulses makes it possible to reduce the power emitted by the source while maintaining the level of the backscattering signal in the lidar and its range resolution. Calculations of the levels of the signal backscattered by a localized diffusely reflecting object are performed and it is shown that at a delay value corresponding  to the arrival time of ballistic photons, the compressed pulse is not distorted. At long delay times, a pulse tail is formed due to the spread of photons along their paths. An example of calculating the backscattering pulse in the presence of a non-reflecting object in the water is given.

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