Research On Optical Transmission Characteristics In Ocean-Atmosphere Mixed Channel

Whether in the ocean or the atmosphere,under the action of velocity injection or mechanical mixing of water,the refractive-index field with the fluctuation of time and space appears,which is called optical turbulence.Its existence makes the refractive index not change or change slowly in the adjacent time or space,but it has obvious cumulative effect of refractive index change in a certain long time or long distance of beam propagation path.It makes optical systems have to consider the general propagation effects related to the medium when light propagates through,and the effects related to the wave itself.For laser detection and measurement systems,the cumulative effect of long exposure time or long distance transmission cannot be avoided,so a better understanding with the statistical characteristics of the turbulent field is needed.This article focuses on the characteristic statistic of the refractive index fluctuation in ocean-atmosphere turbulence channel.Based on the wide-range Prandtl/Schmidt number ocean refractive index power spectrum of optical turbulence,the outer scale model is introduced.And the Pierson-Moscowitz sea-spectrum model is used to discuss the laser transmission problem caused by the rough sea surface originated from the wind.Finally,taking the Gaussian-Shell mode beam into the laser uplink transmission channel.The second-order statistical of transmission characteristics represented by the spectral density and coherence are discussed,and the conclusion are drawn.In the ocean,whether the refractive index power spectrum is dominated by single temperature,salinity,temperature-salinity coupling term,or that is considered by the whole three,the value of coherence radius in the turbulence described by the outer-scaled wide-range number refractive index power spectrum is smaller than that without outer scale.The increase in the mean square temperature,the decrease in transmission distance,or the increase in wavelength of the incident beam will gradually increase the value of the coherence radius.As the temperature-salinity ratio increases,the beam coherence radius does not follow the linear trend,because it is modulated by the non-linear parameter the eddy diffusivity ratio.For the Gaussian mode beam,the increase of the coherence radius will further make the spectral density broadening after it is transmitted through the turbulent field relatively small,and the decrease of the coherence degree is also relatively small,and the signal beam will maintain a relatively good beam quality.With the increase of sea surface wind speed,the energy of the phase fluctuation brought to the interface also increases,which will cause serious attenuation and decoherence of the signal beam.As the wind speed increase,the cross spectral density function will be different,and the larger the energy injection will make the cross spectral density correlation relatively large at the same correlation distance.In the near-low range of atmospheric turbulence,the beam is less disturbed by turbulence than ocean,and the value of coherent radius is relatively large,so the signal beam can maintain relatively stable spectral width and coherence in the near-low range.