| Free space optical communication is a new type of communication mode,which takes laser light wave as signal carrier and atmosphere as transmission medium.Combining the advantages of optical fiber communication and microwave communication,it has the advantages of large communication capacity,high transmission rate and security,good direction,no need to lay optical fibers,and is widely used in various fields.However,atmospheric turbulence has a serious impact on the performance of the communication system.The selected wavelength of free space optical communication system is mostly between 0.78 and 1.55 microns,i.e.near infrared band.According to the theory of laser propagation in the atmosphere and a large number of field experiments,the atmospheric effect of the mid-far infrared band laser with wavelength from 3 to 14 microns is smaller than that of the near infrared band.If the mid-far infrared band is used as the carrier of free space optical communication,it will help to reduce the atmospheric impact.In this paper,the mechanism of light intensity fluctuation caused by atmospheric turbulence is expounded,and the formula for calculating scintillation index under weak fluctuation is given.The revised scintillation index is given.The intensity distribution of laser in the atmosphere is simulated by Gamma-Gamma distribution model,and the distribution state of laser in different atmospheric conditions is obtained.When the transmission distance is constant,the propagation characteristics of laser are analyzed,that is,the Rytov variance decreases with the increase of wavelength.The trend diagram of the influence of mid-far infrared laser wavelength on Rytov variance is drawn.It can be seen that mid-far infrared laser has more advantages than short-wave infrared laser in atmospheric turbulence and has stronger resistance to disturbance.This paper introduces the phase screen generation algorithm,compares the advantages and disadvantages of the non-added harmonic and the added harmonic algorithms,and finally uses the added harmonic method to generate the phase screen.When the atmospheric coherence lengths are 50 cm,10 cm,5 cm and 0.1 cm respectively,512 x 512 phase screens are generated,On this basis,the propagation of laser in the atmosphere is simulated.The laser beam propagation characteristics are simulated when the propagation distance is 1 km,3 km and 5 km,and the atmospheric coherence length is 0.1 cm-80 cm.The correctness of the model is proved by the fitting degree of theoretical analytical solution and simulated numerical solution.The results show that with the increase of atmospheric coherence length,the spot breakage is serious.Then,the phase screen model is revised according to the wind speed.Based on this,the time-dependent waveform of light intensity and the probability distribution statistics are obtained.Gamma-Gamma model is used to fit the probability distribution of light intensity,but it is found that Gamma-Gamma model can not effectively describe the probability distribution of light intensity in weak turbulence.Therefore,the Gamma-Gamma model is modified by G-G model.The simulation results show that the probability density distribution curves of G-G model can well fit the theoretical curves under strong,weak and medium atmospheric turbulence.The equivalence of the atmospheric turbulence simulated by the hot air convection atmospheric turbulence simulator is analyzed,and relevant experiments are designed.The performance of the hot air atmospheric turbulence simulator is verified by using four bands of laser as light source.The experimental results show that the device can be used to simulate moderately weak atmospheric turbulence(coherence length is 5-20 cm,ranging from 1.81×10-1616 to 1.81×10-15)efficiently.The equivalent distance of intensity scintillation is 661.2m.Moreover,the spectral and probability distributions of the scintillation effect and the angle of arrival fluctuation effect of the simulated atmospheric turbulence are consistent with the actual atmospheric turbulence.It can simulate atmospheric turbulence very well.3.5?m laser light source was designed and manufactured.The atmospheric turbulence simulation device and related devices described in Chapter 5 were used to verify the atmospheric turbulence propagation characteristics of mid-far infrared beams indoors.The experimental results show that the fluctuation variance of 3.5?m is smaller than that of 1.55?m and 0.532?m in atmospheric conditions of,nC2?28?4.22?10-1515 m-2/3Cn2?28?1.02?10-1313 m-2/3and equivalent transmission distance of 5000m.That is to say,with the increase of wavelength,the scintillation factor decreases,which indicates that far-infrared laser is more suitable for atmospheric turbulence transmission than short-wave infrared laser;and with the increase of communication distance,mid-and far-infrared band slows down atmospheric turbulence.The effect is more obvious.The simulation results in Figure 3-4 in Chapter 3 are verified.It shows that the long wavelength light wave is more noise-resistant than the short wavelength light wave in the atmospheric channel.Secondly,experiments show that the G-G probability density function can better describe the intensity distribution of far-infrared light after passing through atmospheric turbulent channel.We use the G-G probability density function to fit,and the fitting efficiency is 90%.The correctness of G-G model is verified. |
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