A New Method for Experimentally Measuring the Internal Solitary Wave Based on Optical Fiber Sensing

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 sech(2)-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.