Theoretical And Experimental Study On The Optical Frequency Domain Detection For Ship Wake

For more than a decade, optical detection for ship wake bubbles has become a hot research topic. The basic principle is based on the different propagation characteristics of the laser inside and outside the wake. By emitting laser into the water, detecting changes in the characteristics of the laser parameters can indirectly determine the presence of the wake and the characteristics of wake. Due to the light wave length high speed, good direction, optical detection for wake has the advantages over the acoustic detection of high sensitivity, anti-jamming, detection distance. And it avoids the disadvantage of a large propagation loss in the water. Optical detection for ship wake also can make conventional acoustic interference and confrontation means useless. In addition, studies of the optical detection for ship wake can also provide a reference for the optical detection such as Hemingway, aerosols and other marine phenomena as well as other gas-liquid two-phase process. This dissertation is mainly concerned with frequency domain detection of ship wakes. The author’s major contributions are outlined as follows:1. The relationship between Wake bubble’s features and backscatter laser Doppler shift signal characteristics is theoretically researched. A simulation of the Doppler shift spectrum caused by the moving bubbles in wake is performed with Monte Carlo method. The influences of the bubble population and the velocity distribution of bubbles on the laser Doppler shift spectrum are analyzed. Theoretical simulations show that the amplitude and the cutoff frequency of the shift spectra will increase with the increasing bubble population and range of velocity distribution.2. The relationship between imitated wake and backscatter laser Doppler shift signal characteristics is experimentally studied. On the basis of theoretical analysis, using the differential Doppler method, the differential optical system for measuring the backscattered light signal Doppler shift of spectral is set up. The influences of imitated wakes with different bubble population and different velocity distribution on the Doppler shift spectrum are studied experimentally in the laboratory sink and anechoic pool. Experimental results agree with the theoretical analysis.3. Statistical properties of the experimental data are analyzed. The collected experimental data is statistically processed. The change characteristics of spectral width, spectral peak and spectral area of the scattered laser Doppler shift signal spectrum are quantitatively analyzed. This provides a quantitative reference standard for the optical detection method for wake, which is based on the laser Doppler frequency shift changes.4. A new method for ship wake detection is presented. The existing four kinds of optical detection of wake are concluded, including the continuous laser scattering intensity detection, the imaging detection, the pulse return detection and the polarization detection. On the basis of theoretical and experimental studies, a novel optical detection method for ship wake is presented which is based on the variation of the Doppler shift parameters characteristics of the laser backscatter signal, such as the spectral width, spectral peak and spectral area.