Research On Partially Coherent Beam And Its Propagation Characteristics And Application In Atmosphere

With the increasing demand for data transmission,optical wireless communication has gradually become a hot research topic of the next generation mobile network communication technology because of its high data rate and large communication capacity.Near-ground optical wireless communication uses laser as the information carrier and atmosphere as the transmission medium,which has the advantages of anti-interference,anti-eavesdropping and high bandwidth.However,the atmosphere is a complex medium for light transmission,and the atmospheric turbulence existing along the propagation path will lead up to beam’s wavefront distortion,and cause beam drift,beam dispersion and light intensity fluctuation,which has a bad impact on stable transmission of optical communication and limits the transmission rate.This paper studies partially coherent beam and its propagation characteristics and applications in atmosphere,and provides an effective solution for the stable data transmission of broadband optical wireless communication.Partially coherent beam is significantly less affected by turbulence in optical intensity stability and beam dispersion than fully coherent beam,showing its property of resistance to turbulence.Besides,the special form of coherence distribution brings special propagation properties.Therefore,partially coherent beams have become a hot spot in the research of beam regulation.However,due to the influence of light source fluctuation,the application of partially coherent beam in optical wireless communication is limited,and wireless communication schemes based on partially coherent beam are rarely reported.This paper studies Bessel-Gaussian Schell-model beam which has small intensity fluctuation,and then proposes a space division multiplexing scheme based on this beam for the first time to achieve high-speed and stable communication in turbulent environment.In order to solve the problem of channel crosstalk caused by beam dispersion of partially coherent beam on the space division multiplexing scheme,the circular airy function is introduced into the amplitude profile of partially coherent beams and the influence of this kind of amplitude profile on the transmission characteristics is studied.The main contents of this paper include:(1)Considering the importance of the beam’s continuously changing coherence property in the simulation accuracy,based on the multistep propagation theory,the change of beam coherence is added to the simulation sampling process as a control factor to realize the adaptive control of beam sampling and increase the simulation accuracy.The improved simulation method can meet the simulation requirements when the beam radius and coherent length change unevenly in the propagation.(2)Based on the extended Huygens Fresnel principle,the theoretical results of the facula evolution,the field coherence and the scintillation index of Bessel-Gaussian Schell-model beam in turbulent environment are given.The beam is generated in the experiment,and the propagation characteristics of the beam in the turbulent environment are studied indoors through the turbulence simulator.The results show that Bessel-Gaussian Schell-model beam has a circular far-field facula,and the radius of the ring is strictly controlled by the transmission beam parameters.The light intensity distribution at the point on the ring is more stable than that of Gaussian Schell-model beam,showing better ability to resist the turbulence.In addition,the beam propagation has somehow self-healing characteristics,and it maintains the reasonable circular facula profile when the scale of the unobstructed area is much larger than the coherence length.(3)In this paper,a space division multiplexing scheme based on Bessel Gaussian Schell mode beam is proposed.The channel capacity is analyzed by simulation method,and the feasibility of the scheme is verified by experiments.By controlling the beam parameters of the signal light at the transmitter,the scheme can realize the self separation of each channel at the receiving end and multiply the total communication capacity.Due to the anti-turbulence ability of Bessel Gaussian Schell mode beam,the channel capacity still has a 6 d B gain compared with the traditional Gaussian optical communication scheme even in strong turbulent environment.(4)To deal with the problems in the partially coherent optical space division multiplexing scheme,such as the beam dispersion which increases the crosstalk,and reduces the spatial multiplexing degree and the total channel capacity,this paper studies in detail the influence of the annular airy amplitude profile which introduces self-focusing effect on the propagation characteristics of uniform partially coherent beams.The annular airy amplitude can effectively suppress the beam divergence,forming local energy convergence,and the focusing distance can be adjusted within kilometers.And even in strong turbulent environment,it can still show good focusing effect.In addition,the introduction of annular airy amplitude can further alleviate the light intensity disturbance at the focusing position.These characteristics provide a feasible scheme for further optimization of wireless optical communication performance.The space division multiplexing scheme based on the Bessel-Gaussian Schell-model beam proposed in this paper can provide a new direction for the future development of wireless optical communication.The relevant research on the beam propagation characteristics can also be used as a supplement to the turbulent optical communication theory and optical field regulation technology,and provide reference for the development of the next generation of wireless optical communication technology.