Study Of The Characteristics Of Blue-green Laser Propagating Through Oceanic Turbulence And Downlink Channel

Since the discovery of blue-green laser underwater transmission window,airborne/satellite laser communication become one of the most promising technologies,which have received widely attention.The main factor limiting the development of laser communication is the complex characteristics of its transmission channel,including the atmospheric channel,the air-sea interface and the seawater channel.Compared with the single channel mentioned above,the study on the transmission characteristics of laser in the whole channel with foam covered rough sea surface is relatively less.At the same time,the influences of the oceanic turbulence caused by the random fluctuations of refractive index result from the temperature fluctuation and the salinity fluctuation on the transmission laser have become the focus of research in recent years.Compared with single beam coherent beam,the study of the propagation characteristics of the partially coherent beam and its array with anti-turbulence effects through the oceanic turbulence is of great significance.In this paper,the influences of oceanic turbulence on the propagation characteristics of the partially coherent array beams are investigated in detail.At the same time,the energy attenuation characteristics of laser through downlink channel are analyzed quantitatively under different sea conditions.Firstly,this paper introduces the oceanic turbulence model,the influences of turbulence parameters(the rate dissipation of kinetic energy?,the rate of loss of temperature variance?_T,the radio of temperature gradient to salinity gradient?)on turbulent flow is systematically analyzed,the results show that the influence of salinity on turbulence is several times higher than that of temperature,which is different from the characteristics of atmospheric turbulence only considering the effects of temperature.The influences of turbulence on the laser propagation is further verified through establishing the experiment that laser transiting through liquid turbulent,which will be of great significance for further theoretical verification.Based on the extended Huygens-Fresnel principle,considering the turbulence parameters on the averaged intensity and the characteristics of spatial coherence are investigated for both coherent and incoherent superposition GSM array beams,the results show that under different turbulence intensity conditions,the influence distance is from tens of meters to hundreds of meters.By calculating the average intensity and beam broadening of Gaussian beam,GSM beam,Gaussian array beams as well as GSM array beams,the superiority of partially coherent beam propagation in oceanic turbulence is further proved.At the same time,the characteristics of the spatial coherence and the polarization characteristic of EGSM array beams are studied,the results show that with a short transmission distance,the spatial coherent and polarization properties of the beam mainly decide by the initial state of the beam.With the distance become larger,the spatial coherent and polarization properties of the beam will be decided by the initial state,the turbulence intensity and the cumulative effect of the turbulence along the path.Secondly,based on the Mie scattering theory and the rough surface scattering theory,The energy attenuation of laser beam through sea fog,the rough sea surface with foams as well as the seawater considering the vertical non-uniform distribution of chlorophyll is studied,the results show that the effect of chlorophyll on the energy attenuation of laser beam is the most significant,at the receiving plane of 100m in the sea water,the energy attenuations caused by different chlorophyll concentration distributions range from-59dB to-110dB,which is much greater than that of the oceanic turbulence.Under the condition of thick sea fog(V=100m),the energy attenuation of laser beam is relatively large,at the receiving plane of 100m in the sea water,the energy attenuation extreme of laser beam will be10.7dB less than that of mist(V=1000m).With the visibility of sea fog reaching a certain range(V=500m),the variations of the roughness of sea surface as well as the covered bubbles have significant influences on the laser transmission angle domain,at the same time,exert a certain influences on transmission extreme of laser power.