Study On 2 ?m Band Spatial Beam Shaping And Its Performance Based On Optical Antenna

Free Space Optics(FSO)is a method of communication that uses laser as a carrier of data signals to transmit data in free space.As an important field of optics,free space optical has a broad research prospect and market value in recent years due to its advantages of low energy consumption,high confidentiality,high integration and wide availability.At the same time,free space optical has excellent application value in aerospace,military detection and sensing.For free space optics,2?m band serves as its atmospheric window,the atmospheric transmittance is over 80%,and the absorption depth in the middle atmosphere is about 10 times that of the 1.55?m band.Therefore,the temporal resolution of atmospheric sounding is relatively high,which is conducive to the study of stratospheric turbulence measurement and has high spatial transmission and utilization value.Gaussian beam,as the carrier of transmitting signals in free space optics,has a large divergence angle,after it transmits over a long distance,the aperture of the light spot at the receiving end expands seriously,which is not good for relay and utilization.In this thesis,two kinds of shaping schemes are designed to compress the divergence angle and reshape the optical field distribution respectively,considering that the 2?m band beam transmitting in free space will diverge very wide and high loss at the recieving end.The main contents of this thesis are as follows:(1)In order to solve the problem that the beam parameter product of 2?m band is too large and the collimation accuracy of a single element is low,a two-level collimation structure is designed.The semi-divergence angle of the laser through aspheric lens shaping is compressed to 0.261 mrad.The semi-divergence angle of the laser recompressed by the cassegrain optical antenna was 32.5?rad.At the same time,I analyse the relationship between the minimization of the light spot at the receiving end and the amplification factor at 1km.When the amplification factor of the optical antenna is from7 to 23,the radius error of the light spot at the receiving end is within 8mm.(2)According to the needs of the receiving end,the Galileo optical antenna is designed to compress the spot radius to 4.941 mm,so as to build an integrated transceiver antenna system.The effects of atmospheric turbulence,receiver radius,receiver sensitivity,transmitting power and beam propagation loss on the system performance are analyzed.The errors of defocusing,tilt and radius of curvature in launcher are analyzed.(3)In order to solve the problem of central energy loss in cassegrain optical antenna,based on the theory of diffraction optics,the diffraction efficiency is taken as the algorithm cycle condition,and the GS algorithm is improved,so as to design the binary optical element that produces ring flat-top beam.Theoretically,it can achieve 100% energy utilization,with a diffraction efficiency of 99.2% and more than 800 cycles,which can overcome the problem of local optimal solution of ordinary GS algorithm.