Research On Random Phase Screen Model For Simulating Underwater Turbulence

Underwater wireless optical communication(UWOC)has attracted much attention with the growing demand for underwater information transmission.As a new type of optical multiplexing freedom,UWOC with Orbital angular momentum(UWOC-OAM)has,to tremendously increased the capacity.However,the underwater turbulence(UT)seriously affects the performance of a UWOC-OAM system.Therefore,it's necessary to establish an effective UT model.In this thesis,we establish an effective UT model with the power spectrum inversion method and the Zernike polynomial generation method.The main work are as follows:(1)The random phase screens model for simulating UT is established based on the power spectrum inversion method at first,and the low frequency part of the random phase screen has been compensated by the subharmonic compensation method.First,with the power spectrum inversion method,the random phase screen that meets the statistical characteristics of UT is produced by filtering a Gaussian white noise with the refractive index power spectrum,followed by Fourier transform.Then,the low frequency part of the random phase screen is improved by using the subharmonic compensation method.The results show that the improved random phase screen can meet the statistical characteristic of UC,with a higher compensation order and compensation level,the phase structure function of the random phase screen is closer to the theoretical one.Finally,the propagation of Laguerre-Gaussian(LG)vortex beam for the underwater environment is used to test the proposed phase screen.The results show that the performance evaluated by the improved random phase screen is closer to the theoretical one when the subharmonic order and sample level increase,(2)The random phase screens model for simulating UT is established based on Zernike polynomials,and the high frequency part of the random phase screen is improved by high frequency compensation.First,the random phase screen that meet the statistical characteristics of UT is generated by the superposition of different orders Zernike polynomials with their orthogonality.Then,the phase structure function of this random phase screen with different Zernike polynomial orders is subsequently analyzed.After that,the random phase screen is proposed to generate by linear superposition of Zernike polynomial method and the power spectrum inversion method with advantages of the both methods.The results show that the new phase screen has the best performance,and the phase structure function using the new phase screen is closest to the theoretical one.Simultaneously,compared with the phase screen generated by the power spectrum inversion method and the Zernike polynomial method,individually,the performance evaluated by the new phase screen is better.