Experimental demonstration of influence of underwater turbulence on ghost imaging

It is difficult to obtain a clear image in underwater turbulence environment with classical imaging methods due to the absorption, scattering, and underwater turbulence on the propagation beam. However, ghost imaging (GI), a non-locally imaging technique, has shown the turbulence-free ability in atmospheric turbulence by exploiting the second-order correlation between the signal beam and the reference beam. In this paper, we experimentally investigate the imaging quality of GI affected by the underwater environment, where the underwater environment is simulated by a 1 m×0.4 m×0.4 m tank with distilled water. The water temperature is controlled by a heater inside the tank, and a temperature gradient is obtained by putting the heater at different positions of the tank. The water vibration is produced by a heavy force, and the turbid medium is obtained by dissolving very small specks of CaCO₃ in the water. A set of Hadamard speckle pattern pairs are generated and modulated on the incident beam, and then the beam illuminates on an unknown object after passing through the simulated underwater environment. With the second-order correlations, the image is reconstructed under different temperature gradients, water vibration, and turbid medium ratios. The results show that GI has the turbulence-free ability under lower temperature gradient, water vibration, and turbid media. The structural similarity image measurement (SSIM) values of the reconstructed images only start to decrease when the temperature gradient is greater than 4.0℃. The same temperature gradient produced at the different positions has a little effect on the quality of the underwater GI.