Experimental Zernike-mode analysis of wavefront distortions in underwater Rayleigh–Bénard turbulence

In this study, wavefront distortions induced by underwater Rayleigh–Bénard convective turbulence are experimentally investigated using an optical modal-based approach. For the first time, to our knowledge, a two-channel moiré wavefront sensor is employed to directly reconstruct the wavefront of a laser beam propagating through an underwater convective medium. The reconstructed wavefront is expanded in terms of the first 28 Zernike polynomial modes. The variance and cumulative variance of the Zernike coefficients are calculated and analyzed for different temperature differences and various beam propagation paths. The results indicate that increasing the temperature difference enhances the overall turbulence strength and increases the magnitude of phase distortions, while the relative distribution of energy among different spatial scales undergoes only minor changes. Low-order Zernike modes, corresponding to large-scale convective structures, dominate wavefront distortions in regions close to the hot and cold plates. In contrast, in the central region of the tank, the contributions of low- and high-order modes approach comparable values, resulting in a more balanced energy distribution and reduced spatial inhomogeneity. This behavior reflects the absence of a dominant spatial scale in the core of the convective medium. The main novelty of this work, to our knowledge, lies in providing a quantitative and modal-based experimental description of underwater Rayleigh–Bénard turbulence using real reconstructed wavefront data and in experimentally separating the contributions of large- and small-scale turbulence. These results offer physical insight into the multiscale structure and anisotropy of underwater optical turbulence and can be utilized in developing more accurate light propagation models and improving the performance of underwater optical imaging and communication systems.