Numerical Simulation Study On The Effect Of Imaginary Part Fluctuation Of Equivalent Refractive Index On Optical Transmission

Turbulence is a century-old problem.The study of transmission of light in turbulence is a problem that people are concerned about.When light waves propagate in turbulent atmosphere,they will be affected by atmospheric turbulence,resulting in atmospheric turbulence effects such as scintillation,fluctuation of the angle of arrival,beam jitter,and beam expansion.Analysis of these light signals can yield the state of atmospheric turbulence.For the problem of light transmission in the turbulence,the main research methods are field experiments and numerical simulation.In the field of field experiments,the research tools include instruments such as large aperture scintillators.In terms of numerical simulation,researchers have developed a variety of numerical simulation schemes.Theory and experiments show that temperature fluctuation is an important factor causing changes in the amplitude and phase of light waves.However,the research and application of light transmission in the atmosphere are mostly carried out in the atmospheric boundary layer,especially in the urban boundary layer.Aerosol particles are almost always present in the urban boundary layer.However,for light transmission,there are still few studies on the fluctuation of the extinction effect caused by aerosol particles and other substances.Extending the refractive index to a complex number can comprehensively represent the refraction and extinction of light in the atmosphere,which is the equivalent refractive index.The fluctuation of the real part of the equivalent refractive index corresponds to the fluctuation of the refraction caused by the temperature fluctuation,while the fluctuation of the imaginary part of the equivalent refractive index corresponds to the fluctuation of the extinction effect.In this paper,an existing numerical simulation model of optical transmission is developed,which takes into account the effect of fluctuations in the imaginary part of the equivalent refractive index on light transmission.The work carried out and the results obtained are as follows:1.The imaginary part of the equivalent refractive index is introduced in the numerical simulation experiment of plane wave transmission.The simulation results show that the imaginary part of the equivalent refractive index contributes to the fluctuation of the light intensity.The one-dimensional power spectrum of the logarithmic light intensity is in good agreement with the theoretical spectrum.This illustrates the rationality of the plane wave propagation simulation scheme.The contribution of the fluctuation of the imaginary part of the equivalent refractive index to the scintillation variance is quantitatively analyzed based on the numerical simulation of typical atmospheric parameters.The results show that the effect is greater than 8%at 1000m.At the same time,the comparison with the field experiment results is carried out,and the consistency is good.The effect of the fluctuation of the imaginary part of the equivalent refractive index on the optical transmission is separately analyzed by numerical simulation.In the usual measurement and simulation range(kilometer scale)and the common range of the structure constant of the imaginary part of the equivalent refractive index,the fluctuation of the imaginary part of the equivalent refractive index will not cause scintillation saturation.2.The numerical simulation experiment of spherical wave transmission is carried out in the divergent coordinate system,and the situation of point-emitting point receiving and point-emitting surface receiving is considered.The resulting logarithmic intensity one-dimensional power spectrum is similar to the theoretical spectral shape.The contribution of the fluctuation of the imaginary part of the equivalent refractive index to the scintillation variance under different turbulence parameters and measurement conditions common in scintillator experiments is theoretically calculated.The results show that the contribution of the fluctuation of the imaginary part of the equivalent refractive index to the scintillation variance is often greater than 10%at the transmission distance of the kilometer level.