Research On Phase Modulation Of Coherent And Partially Coherent Beams Applied In Optical Communications

The"14th Five-Year Plan for National Informatization"points out that informatization has entered a new stage of accelerating digital development and building a digital China in five years.In the past fifty years,it has greatly promoted social progress and economic development due to the vigorous development of information and communication technology.Optical communication is a rising communication technology for information transmission using laser carrying information.Compared with traditional microwave communication,optical communication has distinct advantages,such as low loss,large bandwidth and high fidelity.Driven by a series of technological updates and developments,optical communication has moved into a period of rapid development.Communication capacity and communication quality are the key to achieve the sustainable development of optical communication.Beams modulated by phase exhibit unique advantages in the field of optical communication.The orbital angular momentum modes carried by vortex beams have multi-stage and orthogonality,and they have proved to be a new degree of freedom for the beam.The technology of mode division multiplexing is able to effectively alleviate the shortage of spectrum resources and improve the spectrum efficiency of optical communication systems,which can be used for large-capacity optical communication.In addition,random transmission media is an inevitable problem in optical communication systems.The issues of orbital angular momentum crosstalk,scintillation and beam wander that cause signal interruption occur when signal beam modulated by phase propagates in random media,such as atmospheric turbulence,ocean turbulence.Low coherent beams（i.e.partially coherent beams）not only possess the characteristics of coherent beams,but also are insensitive to the disturbance of random media.It has an important advantage in suppressing the negative effects caused by random media.Therefore,the application of partially coherent beam modulated by phase to optical communication is one of the main research contents of this thesis.Based on the above background,this thesis mainly focuses on the application of coherent and partially coherent beams modulated by phase in optical communication.The main research contents of the thesis are as follows:Firstly,the mode purity of signal orbital angular momentum states and crosstalk orbital angular momentum states of Bessel Gaussian beams in underwater communication system and free space optical communication system was studied.In underwater optical communication system,a theoretical model for the receiving probability of signal orbital angular momentum states and crosstalk orbital angular momentum states due to perturbation of Bessel Gaussian beams was constructed.It is found that the mode purity of signal orbital angular momentum states can be improved by adjusting the size of the received aperture.In free optical communication system,an adaptive optical communication system based on Gerchberg-Saxton phase recovery algorithm was built.Meanwhile,the weight coefficients of the orbital angular momentum spectrum were measured simultaneously applying a vortex Darman axis cone grating and an adaptive compensation system when Bessel-Gaussian beams propagated in atmospheric turbulence.The results indicate that the wavefront distortion caused by turbulence of Bessel Gaussian beams in atmospheric turbulent channels can be compensated by an adaptive system.it provided theoretical and experimental support for improving the quality of optical communication system.Secondly,the application of traditional correlated partially coherent vortex beams and non-uniform correlated partially coherent vortex beams in free space optical communication was studied.The beam wander of Gaussian Schell-mode vortex beams in atmospheric turbulence simulated by a hot plate was experimentally measured.A free space optical communication system with multiplexed Gaussian Schell-mode vortex beams was constructed,and the bit error rate of single/multiplexed Gaussian Schell-mode vortex beams in this communication system was measured.The bit error rate curves of all channels were lower than the forward error correction threshold of 1×10^-3 in the communication system.The research results indicate that with the decrease of coherence,beam wander,power loss and bit error rate decrease.In other world,the lower the coherence,the stronger the turbulence resistance of the Gaussian Schell-mode vortex beams,and the more robust the free space optical communication system.Furthermore,the communication performance of non-uniform partially coherent vortex beams in atmospheric turbulent channels was studied.The results indicate that the detection probability of the signal orbital angular momentum state can be increased,the detection probability of the crosstalk orbital angular momentum state can be reduced,and the average bit error rate of communication system can be reduced by adjusting the ratio of the receiving aperture radius to the beam waist width.The above methods and systems can provide a way to significantly increase communication capacity while improving the reliability of optical communication systems.Finally,the scintillation index of Gaussian Schell-mode beams with cross phase in atmospheric turbulent was studied.Based on tensor optical methods,a theoretical model of the scintillation index of Gaussian Schell-mode beams with cross phase in atmospheric turbulence was constructed.The effects of turbulence intensity and beam parameters on scintillation index were analyzed.It is found that the scintillation index of Gaussian Schell-mode beams with cross phase can be reduced by phase modulation in atmospheric turbulence,thus improving the communication quality in atmospheric turbulence.Moreover,the scintillation index of Gaussian Schell-model beams with cross phase propagated in turbulent atmospheric was measured experimentally,and the trend of the measured scintillation index was consistent with the numerical results.