Underwater laser transmission system usually uses seawater as the transmission medium,and laser is the carrier of information transmission.The underwater laser transmission system has broken the bottleneck of traditional underwater acoustic communication,and its performance is superior.However,this method is affected by the complex environment of seawater,and the optical transmission channel is more complex.Channel time dispersion will cause multipath effect during laser signal transmission,which will eventually increase the communication error rate and degrade the transmission performance of the system.Therefore,in order to achieve high-quality signal transmission,it is necessary to study the optical transmission channel in seawater.The main contents of this paper include:1.The two main factors affecting the transmission of laser signals in seawater,namely absorption and scattering,are introduced.The absorption and scattering effects of various substances in the underwater link on the transmitted laser beam are analyzed in detail,and the empirical formulas of absorption coefficient and scattering coefficient and the underwater radiative transfer equation of laser signal are given respectively.It lays a theoretical foundation for the subsequent research on underwater transmission of laser signals.2.The establishment of the single scattering geometric model and the time delay characteristics of laser transmission in underwater channels are studied.The single scattering geometric model of non-line-of-sight underwater particles was established in ellipsoidal coordinate,taking into account the effects of forward scattering and back scattering.Based on the Two Term Henyey-Greenstein scattering phase function,the impulse response expression of single scattering channel for laser underwater transmission in non-line-of sight mode is derived.The scattering process is regarded as the interaction process between molecules and particles in the underwater links and photons.Based on Monte Carlo method to simulate the photon transmission trajectory,the validity of the established channel theoretical model is verified.Finally,the time delay characteristics of the laser pulse at the transmitter after passing through the coastal and port water environment are compared and analyzed.3.The multiple scattering geometric model for laser transmission in underwater channel is established,and the pulse delay characteristics are studied.Three parameters,scattering distance,scattering zenith angle and scattering azimuth,are used to describe the position of photons in space,and the probability distribution functions of different parameters are given respectively.The multiple scattering geometric model of underwater particles is constructed.The impulse response expression of underwater laser multiple scattering channel based on probability is derived,and Monte Carlo method is used to verify the effectiveness of the channel theoretical model.Finally,the pulse broadening characteristics under different scattering orders are studied.The pulse delay characteristics under different water environment,laser pulse width,transmission distance and asymmetric factor are analyzed. |