Study On Propagation Characteristics Of Blue-green Laser In Seawater With Different Attenuation Coefficients

Underwater detection technology plays an important role for human beings to develop and explore the ocean,and sonar is a common equipment for underwater detection using sound waves.However,acoustic waves have the disadvantages of the low bandwidth utilization rate,slow propagation speed and large time delay.In order to make up for the shortcomings of sonar detection,underwater laser detection technology employs blue and green lasers to detect the seawater with strong penetration ability instead of sound wave,which plays a crucial role in the field of underwater detection.Underwater laser detection technology has the advantages of fast propagation rate,strong directivity and high imaging resolution.However,the attenuation effect of seawater on laser energy and the reflection performance of different types of targets are important factors affecting the underwater laser detection system.The heterogeneity and anisotropy of seawater’s optical property are caused by the presence of yellow dissolved substances,non-pigment suspended particles,and the attenuation of light energy in the process of laser propagation in water is more serious than that in the atmosphere.The size and complex refractive index of suspended particles in seawater result in different attenuation coefficients of seawater.The attenuation of seawater with different attenuation coefficients on laser propagation process greatly affects the operating distance and detection performance of underwater laser detection system.In addition,the reflection characteristic of the target is an important factor affecting the underwater laser detection system.Therefore,the study of the propagation characteristics of lasers in seawater is beneficial to the design of the underwater laser detection system.In this paper,the optical model of underwater blue-green laser propagation is established.Monte Carlo algorithm was used to simulate the walking process of underwater photon.The scattering and absorption of blue-green laser by seawater medium with different attenuation coefficients are studied,and the scattering and polarization characteristics of suspended particles in seawater are analyzed.The fluctuation of light intensity under strong and weak ocean turbulence is derived,and the propagation model of lasers under ocean turbulence with different attenuation coefficients is established.The underwater lidar equation is given,and the expression of underwater laser reflection intensity of specific targets are derived and its reflection characteristics are given.The main research contents and achievements of this paper include:1.The underwater laser propagation model is established by photonic process of Monte Carlo simulation.Using the blue-green laser with wavelength 532 nm,the impulse response has been discussed both in theorem and simulation in typical seawater.Besides,the channel parameters such as the water type,distance,system parameters of the transceiver have been taken into account by computer simulations.Results shows that in clear water,when the propagation distance is less than 40 meters,the system parameters of the receiver have little influence on the received light power.However,in harbor water,the spread of time delay increases with the increase of the FOV and the laser divergence angle,which reduces the propagation efficiency of light path.Researchshows that when the attenuation length is less than or equal to the propagation length,the received light power is approximately equal to or less than results of Beer law,and the received light intensity decreases with the increase of the received aperture.However,when the attenuation length is greater than the propagation length,the received light power is greater than results of Beer law,and the received light intensity increases first and then remains constant with the increase of the received angle.2.A method based on a combination of Mie scattering theory and Monte Carlo numerical simulation is used to establish the transmission model of underwater photons.The effects of oceanic suspended particles on underwater laser propagation are given in the round.The characteristics of two typical underwater suspended particles and the relationship between incident wavelength and optical coefficient are analyzed.The effects of particle size and complex refractive index on the normalized energy,the intensity of the received light,the length of the optical path and the delay of the optical channel are given.Results show that the optical coefficient of particles increases with the increase of particle size,the received normalized energy of the same propagation length decreases,the scattering number increases,the received light intensity decreases,and the channel delay increases.It is verified that the smaller the imaginary part of the complex refractive inde,the stronger the normalized received energy,and the larger the peak of the received light intensity.Particles of the same size have the same imaginary part and different real parts of the complex refractive index,and the received light intensity depends on the albedo.3.Based on the light scattering characteristics of oceanic suspended particles,the laser depolarization is generated under the action of oceanic suspended particles.Monte Carlo method is proposed to establish the polarization characteristics model of underwater photon transmission.The Stokes vector and polarization degree of four different types of incident polarized laser are calculated.Theoretical analysis and simulation results indicate that a depolarization phenomenon with respect to the laser will be generated with theparticle size increases,and it has a greater effect on linearly polarized light than circularly polarized light.Therefore,circularly polarized light maintains good polarization characteristics in the underwater laser transmission process.4.After analyzing the attenuation effect of calm seawater on lasers,the influence of ocean turbulence on laser propagation characteristics is studied.The oceanic turbulence refraction power spectrum model with a function of the eddy diffusivity ratio is adopted.By using the Rytov method,the scintillation index(SI)is derived using the aperture averaging technique for Gaussian beam under weak to strong oceanic turbulence.Variations of the SI in weak to strong oceanic turbulence are evaluated versus optical length and the oceanic turbulence parameters such as the ratio of temperature to salinity contributions to the refractive index spectrum,the rate dissipation of the mean squared temperature,and the rate of dissipation of kinetic energy per unit mass of fluid.Considering the eddy diffusivity ratio of salinity to temperature,the effects of the ratio of temperature to salinity contributions to the refractive index spectrum on the performance of underwater wireless optical communication systems in weak to strong turbulence are analyzed.The study of the Gaussian beam scintillation index theory under the strong oceanic turbulence is a further improvement of the light intensity fluctuation theory under the strong ocean turbulence.Results show that the effects of some oceanic turbulence parameters on the SI of Gaussian beam under strong and weak turbulence are opposite.5.Based on Monte Carlo numerical simulation method,the laser beam is equivalent to a large number of photons according to the particle characteristics of the optical beam.Oceanic turbulence is considered as a number of different size but isotropic turbulent vortex cells.A channel model for Gaussian beam propagation under ocean turbulence with different attenuation is established and the influence of oceanic turbulence on the propagation characteristics of Gaussian beam is analyzed.It provides a new method for studying the turbulence effect of laser in seawater more comprehensively.Results show that under the ocean turbulence with absorption and scattering,the probability density function of the received light intensity in both clear and harbor waters conforms to lognormal distribution.And it also proves that oceanic turbulence can also weaken the energy of light.6.Based on the lidar equation of atmosphere,the laser echo power equation of underwater target is derived.The effects of water optical attenuation coefficient and incident angle on target echo power are investigated.The single-station model is used to study the reflection characteristics of the target.The influence of the correlation coefficient of specular and diffuse components on the reflected light intensity of three special axisymmetric shapes of target are given.Results show that when the target has a special axisymmetric shape,the reflected light power in the specular component will increase with the increase of the surface slope.The reflected light power in the diffuse reflection component decreases with the increase of the diffuse reflection coefficient.The research work in this paper provides an important theoretical basis for the design of underwater laser detection system,and provides reference value for the selection of equipment,the detection of underwater suspended particles,and the promotion of the theory of light propagation under oceanic turbulence.