Research On Key Technologies In Free-Space Optical Communications Based On Orbital Angular Momentum Multiplexing

Optical vortex(OV),also known as vortex beam,is a singular beam whose wavefront has a helical phase.In such beam,each photon possesses a well-defined orbital angular momentum(OAM).In theory,it has an infinite number of eigenstates.It is similar to the other characteristics of the light beam,such as wavelength,time slot,polarization,amplitude and phase,OAM provides a brand new dimension of multiplexing for promoting the transmission rate and information capacity of optical communications.Therefore,the OAM multiplexing based on optical vortex beams has received great attention from the communication circles.In this paper,we focus on some key issues of orbital angular momentum in free-space optical communications.According to the characteristics of spatial light modulator(SLM),a new method to improve the conversion efficiency of vortex beam energy is proposed.And a novel communication link based on OAM spatial multiplexing is realized based on the theory of perfect vortex beam.At the same time,the performance of the communication link under the atmospheric turbulence disturbance is studied.The research contents and innovations of the thesis mainly include:A new method is proposed to improve the conversion efficiency of vortex beam energy.The quality and conversion energy efficiency of the OV are effectively improved by introducing the digital blazed grating into the computer-generated hologram(CGH).The relationship between SLM performance parameters,holograms and OV energy conversion efficiency is studied in detail.A mathematical model,based on the digital blazed grating theory,is established to analyze the diffraction angle and energy conversion efficiency of+1 order.Benefit from the full utilization of SLM performance,the quality of the vortex beam has been effectively improved.They are well suited to the requirements of free space communication.A novel free-space optical communication(FSO)system with perfect vortex beam(POV)multiplexing is realized.Based on OFDM-16QAM and POV,a dual-channel OAM multiplexed FSO link is set up.The link includes signal transmitter,OAM transfer and signal receiver.OFDM-16QAM optical signals sent form the signal transmitter are modulated into perfect vortex beam and propagate through a distance in the free space for spatial demodulation.After a series of offline processing on the Gaussian bright spot demodulated from perfect vortex beam,the constellations and mean bit error rates(BER)of subcarriers are achieved.The results show that,the using of POVs as carriers greatly reduces the systems' sensitivity to the change of OAM topological charge numbers,and the range of topological charge number multiplexing is expanded in real communication systems.The performances of free-space optical communication links based on POV multiplexing under the influence of atmospheric turbulence are seriously studied.And the performance comparison test was carried out with the Laguerre-Gaussian vortex beam(LGOV)multiplexed communication link under the same received power.The distortions of perfect vortex beams under different turbulence intensities arc observed through the spatial modulation and demodulation.The OAM spectra of the two multiplexed signals are obtained,and the crosstalk caused by different intensity turbulence to the OAM mode(topological charge number)is analyzed.OFDM-16QAM signals are used to measure the performance,which is also compared with the FSO link based on LGOV multiplexing in the same experimental environment.The results show that,the FSO link employing POVs multiplexing can bring a better performance to the system,which can always obtain a lower BER under the same received power.