汽化激光致声跨界通信的三维数值特性

Three-dimensional numerical characteristics of optical-to-acoustic transboundary communication in vaporization mechanism

激光致声技术可以将空气中的激光信号转化为水下的水声信号,将两种最佳信道高效地结合起来,从而解决空中-水下跨界通信问题。为了提高激光致声跨界通信的保密性和光声转化效率,采用了数值计算方法对不同条件下,激光在海面三维散射特性和透射特性进行了研究。首先,为了提高仿真计算的准确性,建立了三维激光海面散射场的数学模型。其次,采用数值计算方法求解三维激光海面散射的表面积分方程,并根据入射光波的特点,对入射界面进行强、弱区划分,以提高计算效率。最后,通过室内模拟试验对仿真结果进行了验证。研究结果表明,入射角度对于汽化激光致声通信的保密性和效率有着重要的影响,为后续的系统设计和相关应用研究提供了有意义的参考。

Objective Due to the strong attenuation of electromagnetic waves by seawater and the complex and variable characteristics of ocean acoustic channels,a single communication method can not meet the long-distance communication needs of aerial platforms for underwater targets.Optical-to-acoustic technology can convert laser signals in the atmosphere into underwater acoustic signals,combining two optimal channels to overcome the bottleneck problems,that is,large attenuation of single electromagnetic waves in water and strong divergence in the atmosphere.And this technology does not require sound waves to be transmitted remotely through underwater channels,resulting in minimal interference to the signal and extremely high transmission efficiency.However,different incident angles play an important role in the photoacoustic conversion efficiency and confidentiality of air-underwater transboundary communication.And vaporization mechanism is more feasible than other mechanisms.Therefore,it is necessary to use numerical calculation methods to study the three-dimensional scattering characteristics of laser on rough sea surfaces at different sea conditions and incidence angles,in order to improve the confidentiality of optical-to-acoustic transboundary communication in vaporization mechanism and accurately grasp the variation characteristics of laser transmission energy intensity with incidence angle.Methods To improve the accuracy of simulation results,mathematical model of three-dimensional laser scattering field on sea surface was established(Fig.1).To enhance the calculation efficiency,three-dimensional surface integral equation of laser on sea surface was solved with preconditioned conjugate gradient method.And the interface was divided into strong zone and weak zone according to the characteristics of incident light wave to enhance the computing efficiency(Fig.2).And different rough surface in different sea state are simulated(Fig.3).In order to obtain the three-dimensional energy distribution characteristics of the laser scattering field on a fluctuating water surface,we used the Monte Carlo method to statistically analyze the effects of different incident angles and sea conditions on the energy distribution of the scattering field.The number of Monte Carlo simulations was 5000 times.Finally,the simulation experimental system was established,and the simulation results were validated by indoor simulation experiment(Fig.8).Results and Discussions The effect of different incident angles and different sea conditions on the energy distribution characteristics of scattering field and transmitting field are statistically analyzed(Fig.4-7).The simulation results show that the incident angle have important effect on the conversion efficiency and confidentiality of air-underwater transboundary communication.A simulation experimental system is built in the laboratory to test the conversion efficiency of optical-to-acoustic in vaporization mechanism at different incident angles and the experiment results are shown(Tab.1).The experiment results show that the optimal range of incidence angle isθi∈[0°,60°].The feasibility of transboundary communication with optical-to-acoustic in vaporization mechanism has been proven by the laser induced acoustic characteristic parameters at vertical incidence after convergence(Tab.2).Conclusions It is found that as the incident angle increases,the water surface spot area and transmittance will change so that spot power density and convergence efficiency decrease continuously through the analysis of threedimensional numerical calculation results and indoor simulation test results.When the incident angle is greater than 60°,the sound source level and photoacoustic conversion efficiency measured in the experiment begin to decrease significantly.The spectrum analysis results of the acoustic pulse signal show that the energy peaks of the acoustic pulse appeared at 126 kHz,36.8 kHz,and 4.647 kHz,with most of the energy concentrated around200 kHz.The experiment results show that the optimal range of incidence angle isθi∈[0°,60°].When the power density of the light spot decreases,the vaporization phenomenon on the water surface also weakens.Which means the power density of the water surface spot area cannot reach the vaporization threshold so as to make the vaporization phenomenon and the thermal expansion phenomenon happen together.The experimental result shows that the photoacoustic conversion efficiency is almost 10~(-3)in the above case.