Research On Laser Induced Acoustic Underwater Detection Technology

As a new ocean detection method,laser acoustic underwater detection technology has the advantages of cross-media detection,high detection mobility and high positioning accuracy.In this paper,the mechanism of laser acoustic and the propagation characteristics of laser and acoustic waves are simulated respectively,Then,a laser acoustic underwater detection experimental device is built,and the characteristics of laser acoustic source,laser acoustic transmission,target echo and target detection method are studied experimentally and the test results are analyzed,Finally,sea test is carried out on the laser acoustic underwater detection system.The main work and innovation points of the paper are as follows:（1）The mechanism of laser acoustic is analyzed theoretically,and the propagation characteristics of laser and acoustic wave are modeled and analyzed.Based on the acoustic mechanism of thermal expansion,vaporization and optical breakdown,the simulation analysis of photoacoustic conversion is carried out.The simulation results show that the excitation acoustic characteristics of laser are related to laser energy,laser time domain waveform,physical properties of water medium and heating area.The photoacoustic conversion efficiency of different mechanisms ranges from 0.01%to30%,and the optical breakdown mechanism has the highest conversion efficiency and the best effect.The propagation characteristics of laser in atmosphere and sea water and the propagation characteristics of acoustic wave in sea water are simulated and analyzed.The simulation results show that at a distance of 700m from the water surface,the sea water can still be vaporized by using a pulse laser with a peak power of4×10¹⁰W.In order to improve the efficiency of underwater detection,the wide spectrum characteristics of laser acoustic signal,the influence of Marine background noise,the propagation attenuation of acoustic wave in seawater and the multipath effect should be considered comprehensively.（2）An experimental device for laser acoustic underwater detection is designed.The acoustic source characteristics of the laser under different excitation conditions are studied experimentally.When the energy of 1064nm laser is 300m J,the pulse width is8ns,and the underwater focusing depth is 7cm,the sound pressure level of the laser acoustic signal can reach 190.5d B,the pulse width is about 0.05ms,and the acoustic spectrum range is 0-600k Hz.The intensity of acoustic pulse is proportional to the laser energy,and the intensity of acoustic signal excited by different focusing depth is different.The transmission characteristics of underwater laser sound signal are experimentally studied.When the laser sound propagates from 1m to 10m,the signal sound pressure level decreases from 190.7d B to 173.3d B,the pulse width decreases from 0.05ms to 0.013ms,and the main spectrum distribution range decreases from 0-600k Hz to 0-467k Hz.The echo characteristics of laser acoustic signal on different targets are explored and a target recognition method based on energy feature extraction in frequency domain is proposed.After analyzing the target positioning at 5m underwater distance,it is concluded that the multipath effect caused by surface reflection and systematic random error are the main reasons for the positioning error.（3）The experiment of laser acoustic underwater detection was tested at sea.An underwater detection experiment was conducted in the sea area near Tianhengdao Wharf in Qingdao.The experimental results show that the source signal excited by the laser with a wavelength of 1064nm and an energy of 227m J in seawater can reach a sound pressure level of 184.0d B and a pulse width of about 0.03ms,the spectrum distribution ranges from 375-500k Hz.And the sound propagation distance can reach100 meters.The time-frequency ridge feature extraction method can still detect objects with spacing less than 7.5cm.The sea depth was measured to an error of less than 3%.