Underwater ghost imaging with detection distance up to 9.3 attenuation lengths.

Underwater ghost imaging LiDAR is an effective method of underwater detection. In this research, theoretical and experimental investigations were conducted on underwater ghost imaging, combining the underwater optical field transmission model with the inherent optical parameters of a water body. In addition, the Wells model and the approximate Sahu-Shanmugam scattering phase function were used to create a model for underwater optical transmission. The second-order Glauber function of the optical field was then employed to analyze the scattering field degradation during the transmission process. The simulation and experimental results verified that the proposed underwater model could better reveal the degrading effect of a water body on ghost imaging. A further series of experiments comparing underwater ghost imaging at different detection distances was also conducted. In the experimental system, gated photomultiplier tube (PMT) was used to filter out the peak of backscattering, allowing a larger gain to be set for longer-range detection of the target. The laser with a central wavelength of 532 nm was operated at a frequency of 2 KHz, with a single pulse energy of 2 mJ, a pulse width of 10 ns. High-reflective targets were imaged up to 65.2 m (9.3 attenuation lengths (ALs), attenuation coefficient c = 0.1426 m, and scattering coefficient b = 0.052 m) and diffuse-reflection targets up to 41.2 m (6.4 ALs, c = 0.1569 m, and b = 0.081 m). For the Jerlov-I (c = 0.048 m and b = 0.002 m) water body, the experimentally obtained maximum detection distance of 9.3 ALs can be equivalent to 193.7 m under the same optical system conditions.