| Free space optical communication technology is a research hotspot in optical communication industry,which has many advantages such as large communication capacity,strong anti-interference ability and high confidentiality.When the signal light is transmitted in random medium,it is easily affected by the weather environment,leading to random fluctuations of the amplitude and phase of the light wave and causing distortion of the beam wavefront.At present,the Adaptive Optics is used to correct the distortion phase caused by atmospheric turbulence,but there is still a technical bottleneck of low coupling efficiency.Planar optical waveguide,as the medium of optical signal transmission in integrated optical electronic devices,provides a solution for spatial optical coupling.The grating coupler is etched on the optical waveguide,which has the advantages of large effective receiving area,large alignment tolerance and possible on-chip test.This paper will make a detailed theoretical analysis of the coupling of spatial light to grating structure in atmospheric turbulence.The main work is as follows:(1)Based on the atmospheric turbulence model and the refractive index fluctuation power spectrum model,the average optical field distribution of the transmitting and receiving surfaces of Gaussian beams transmitted through turbulence is solved from the Helmholtz equation in free space and the paraxial approximation of the optical wave transmission.The free-space light-lensgrating coupling model in atmospheric turbulence is established,and the coupling efficiency of space light and grating waveguide coupler is calculated,and the expression of coupling efficiency is given.(2)Under the modified Von Karman turbulence spectral model proposes a grating structure for the coupling between spatial light and waveguide under atmospheric turbulent channels.Based on the average optical field model of Gaussian beam transmitted through atmospheric turbulence,the coupler model of waveguide grating is established.The transmission mode of space optical signal in optical waveguide and the corresponding propagation constants of each mode are analyzed based on Maxwell equations.The finite difference time domain numerical method is used to calculate the electromagnetic field distribution of grating at space points.Analyzing the influence of the period length,etching depth and duty cycle of the traditional grating structure on the coupling efficiency,and the grating coupler was designed and optimized.When the transmission distance is 8000 m,the turbulent outer scale is 1 m,and the atmospheric turbulence coefficient Cn2=10-15 m-2/3,the coupling efficiency of the optimized device is 50.5%.Analysis shows that the coupling efficiency is not ideal because most of the light transmitted to the substrate,resulting in energy loss.(3)A method of adding distributed Bragg mirrors to the cladding under the grating structure is proposed.The light diffracting to the substrate is reflected,so that as much spatial light as possible is coupled into the gate optical waveguide to further improve the coupling efficiency.Considering the uniformity of the material,three sets of Si/SiO2 mirror structures were selected.The simulation results show that when the grating period is 660 nm,the etching depth is 100 nm,and the bottom cladding thickness is 1.45 μm,the coupling efficiency of the grating coupler is 74%at the wavelength of 1550 nm.The problems of low efficiency of spatial optical coupling and difficult alignment in the atmospheric turbulence channel can be effectively solved.Finally,the fault tolerance analysis of the main parameters affecting the grating coupling performance is carried out.When the etching width is between 330 nm～390 nm,and the light source coupling angle is between 15°～11.5°,the coupling efficiency of the designed coupler is maintained above 70%.With good process error tolerance,it has important theoretical significance to improve the free space optical coupling performance in random media. |
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