Research On Optical Heterodyne Detection Technology For Acoustically Induced Water Surface Capillary Waves

The water surface capillary wave which is excited by underwater sound wave brings some imformation of submarine objects.So,detection of this acoustically induced water surface capillary wave provides a new idea to solve problems,such as submarine communication,underwater target detection,identification of characteristic parameters for fluid medium,and the like.The acoustically induced water surface capillary wave plays a role of brige that transmits under water imformation to the in-air equipment,so it holds very important research significance.To investigate the feasibility of detection method for water surface capillary wave based on optical heterodyne technology with theoretical and experiment analysis,this dissertation concentrates on the detection technique for acoustically induced water surface capillary wave.Main works and innovative researches in this dissertation are as the following:First of all,the mathematical model for acoustically induced water surface capillary wave was developed.The boundary condition for water surface capillary wave was analysised by considering the effect of surface tension,and the initial condition for the harmonic sound wave induced water surface capillary wave was also developed.The general solution for the acoustically induced water surface capillary wave governing equation was deduced,and taking the water surface capillary wave which was induced by the sound wave with a pressure of 1 Pa and the frequency of 100 Hz as an example,the numerical solution for the governing equation was also shown by using the finite difference method.The theoretical model of laser scattering characteristic for water surface was also studied,and a laser heterodyne detection method based on the high frequency carrier modulation upon the reference arm was proposed.The detection system was setup,in order to support the phase demodulation of heterodyne detection signal,the reference arm was phase-modulated by a high frequency carrier signal that was induced by a piezoelectric device,further more,a nother water surface that hosts similar roughness with detection surface was taken as the reference light reflecting surface.The experiments show that the heterodyne detection system performed a good capacity of resisting disturbance.Secondly,a signal demodulation based on the modulation depth analysis was proposed to recognize the characteristic parameters of medium-high frequency water capillary wave whose frequency is higher than 1 k Hz.In the frequency domain of detection signal,the frequency of water surface capillary wave can be estimated by the centre frequency of the effective spectrum components which were selected by a spectrum threshold in the certain frequency band.The phase-modulation depth caused by water surface capillary wave can be estimate from the spectrum attenuation degree between the low frequency components and the corresponding frequency-shift components,and the amplitude of water surface capillary wave can be calculated from the phase-modulation depth.Detection experiments for water surface capillary with frequency form 1 k Hz to 10 k Hz were conducted to demonstrate the efficiency of this method,and the results show that this method can determine the amplitude of water surface capillary wave with a high detection repeatability(2?)less than 2 nm.And this method performs good frequency detection repeatability(2?)with 2 Hz,and the lower limit of detectable frequency is 0.5 k Hz.The detectable amplitude range of this amplitude estimation method is from 0.5 nm to 121 nm.Thirdly,an improved phase generated carrier demodulation method was proposed to detect the medium-low frequency water capillary wave.This method obtains 4 orthogonal heterodyne signals by using 4 carrier signal s with different phase and carrier frequency to mix with the original detection signal,4 phase demodulation signals can be acquired by using differential cross multiplication algorithm upon 4 different orthogonal signal groups.The amplitude of water surface capillary wave can be estimate from the intensities of 4 phase demodulation signal s,the envelope value of the original detection signal and the modulation depth of carrier signal.A phase demodulation signal with a largest intensity was selected to do filtering and Fourier analysis,the frequency of water surface capillary wave can be determined by the maxium amplitude component.Detection experiments for water surface capillary with frequency form 0.1 k Hz to 2 k Hz were conducted to demonstrate the efficiency of this method,and the results show that this improved PGC demedulation method can determine the amplitude of water capillary wave with high detection repeatability(2?)of 4 nm.And this method provided a very high frequency detection repeatability(2?)which is better than 1 Hz,and the lower limit of detectable frequency is 50 HzFinally,experiments for the acoustically induced water surface capillary waves were conducted under laboratory conditions.Heterodyne detection experiments for water surface capillary waves induced by different intensities and different frequencies acoustic signals were conducted to investi gate the effectiveness of the modulation depth analysis method and the improved PGC method.Moreover,the detection experiments under condition of large scale disturbations were also conducted,the results show that even under such condition the improved PGC method can well determine the frequencies of water surface capillary.Theoretical simulation and experimental researches demonstrated the efficiency of optical heterodyne detection for acoustically induced water surface capillary wave to catch the characteristic parameters of underwater acoustic source,as well as the accuracy of demodulation methods to estimate the parameters of water surface capillary wave proposed in this dissertation.These works laid a technical foundation for underwater acoustic source water-surface opto-acousto sensing technology,and promoted the development of opto-acousto sensing technology.