The effects of Surface Solitary Waves (SSWs), which are created by Internal Solitary Waves (ISWs), on optical imaging were investigated in this paper. We explored the connection between SSWs and optical images using simulations and experiments with calm water surface conditions. An experimental platform was built in a controlled laboratory environment where optical images and in‐situ wave data could be gathered simultaneously. SSWs created by different ISWs were successfully observed. The optical images revealed inverse bright and dark patterns for different ISW types (depression or elevation type). Based on theories and experimental results, simulation models of the ISWs and SSWs were built in Unigraphics NX. Using the illumination simulation function of Light Tools, the reason underlying the reversed patterns was explained by the irradiance distribution. Both the experiment and simulation indicated that the SSWs created by the ISWs affected the characteristics of optical images. To further explore how SSWs affect imaging, 16 experiments were designed to investigate the effects of ISW type, amplitude, and water density ratio. It is found that the relationship among the SSW amplitude, ISW amplitude, and density ratio was consistent with earlier studies. The relationships between imaging characteristics (grayscale differences and pattern widths) and the two types of SSWs were consistent.

Optical remote sensing has been an important method to investigate the internal solitary wave (ISW). However, acquiring the ISW amplitude from optical remote sensing images has been the most difficult problem. In this letter, a method is proposed for inverting the amplitude of the ISW. A simulation platform of optical remote sensing is established to detect ISWs in the laboratory, which offers corresponding ISWs and remote sensing images. The experimental results show that the characteristic parameters of optical remote sensing images are changed with ISW amplitude. The relationship between them is nonlinear and related to the hierarchical structure. Hence, based on the support vector machine (SVM), random forest (RF), convolutional neural network (CNN), and multilayer perceptron (MLP), four inversion models of ISW amplitude are built and tested by the in situ data of the Wenchang area. The inversion results indicate that the average relative error of the SVM model is the smallest at 12.4%. It can be applied to the area with a ratio of stratification limited to 0.1-0.6.

As an important means of research, the study of internal solitary wave (ISW) in the laboratory has attracted extensive attention in recent years. However, the observation of ISW in the laboratory is usually carried out by high-speed image recording system, which is confined to the laboratory research and difficult to truly applied to the in-situ observation. Therefore, we propose a new method for experimentally measuring ISWs amplitude and waveform based on the micro-nano fiber Mach-Zehnder interferometer (MZI), the experiments of rising-type and descending-type ISWs were carried out in the laboratory with water tank. Comparing the measurement results of MZI with the observation results of Charge-coupled devices (CCD), it was demonstrated that they are time-matched and the mean square error (MSE) of the rising-type ISWs and descending-type ISWs amplitude are 0.195 and 0.129 respectively, the relative error is less than 20%. The measurement result conforms to the sech2-type curve by comparing the ISWs waveform retrieved by MZI with the analytical solution of the KDV equation, and the waveform measured by MZI has a high coincidence degree with that measured by CCD, which indicated that the MZI can measure the complete ISWs shape with high accuracy. As a new type of sensor, MZI is not only suitable for ISW observation in laboratory, but also is expected to be applied to Marine field research in the future to realize in-situ measurement of ISW.

The optical imaging mechanism of convex mode-2 Internal Solitary Waves (ISWs) in calm water without wind is investigated based on experimental and simulation calculations. By constructing an optical imaging system in the laboratory, the optical images of ISWs, the matching in-situ data, and the free surface displacement caused by ISWs are acquired simultaneously. The experimental outcomes show that the convex mode-2 ISWs exhibit bright-dark stripes in the optical image and produce a depressed free surface displacement on the water surface. The physical model of free surface displacement is constructed in Unigraphics NX (UG) and imported into Light Tools (LT) for ray tracing. The consistency between simulation calculations and experimental results shows that the free surface displacement generated by the mode-2 ISWs will change the reflection of light, leading to bright-dark stripes in the optical image. A series of experiments were designed to explore the influence of amplitude on the optical imaging of mode-2 ISWs when the thickness of the intermediate layer is different or the same. The results show that the amplitude of the wave will affect the imaging effect of ISWs in optical image, and the large amplitude will make the stripes clearer in the image.