Research On Propagation Properties Of Partially Coherent Beams In Wireless Optical Communications

With the flourishing development of the communications market,the internet and multimedia services for individual users and enterprise clusters are ever-increasing.As a result,the network traffic exponentially grows and far exceeds the prediction of Moore's law.At a rough estimate,the data transmission rate rises tenfold every five years,whereas the overall network traffic at a factor of hundred within the next decade.Unfortunately,the current radio frequency(RF)communications is difficult to afford the explosive growth of data traffic due to the limitation of bandwidth.Consequently,wireless optical communications(WOC)using light waves as the carriers to convey information in free space(here “free space”means the air,water,outer space,vacuum or other similar media),has attracted considerable attention and becomes a hot topic in the field of communications.Compared with RF communication systems,WOC systems have many advantages,such as higher data rates,security,portability and scalability.Owing to these excellent characteristics,WOC has a great potential in different aspects from resolving the existing “last mile” access network issues to realizing the high-speed long-distance data communications with a narrow beam,such as the large-scale data exchange centers,campus and enterprise networks,space-to-ground communications,submarine-to-ship communications,etc.What is more,WOC systems are license-free and immune to electromagnetic(EM)interference.Hence,WOC can be used as an alternative or complementary technology to RF and optical fiber communications.However,the performance of outdoor WOC links is susceptible to the transmission media,such as atmosphere,ocean,and other fluids.For instance,the atmospheric channel is time-varying,unpredictable,and sensitive to the weather conditions.The fog,haze,rain,snow,sleet and other harsh weather conditions will significantly reduce the received optical power,deteriorate the link performance,and shorten the maximum link length.As a result,outdoor WOC systems are hard to reach the anticipant link availability of 99.999%(viz.,five-nine benchmark in the telecommunications business)under the low-visibility weather conditions.In this dissertation,the atmosphere and ocean are mainly considered as the transmission media.Light waves propagating in the atmosphere or ocean are mainly affected by three phenomena,i.e.,absorption,scattering,and refractive-index fluctuations(i.e.,optical turbulence).Absorption and scattering mainly caused by the gas molecules and particulates in the atmosphere(aquatic molecules and suspended particles in the ocean)are strongly wavelength dependent.Correspondingly,absorption and scattering can be minimized by selecting the appropriate optical wavelength.Optical turbulence generated by the random refractive-index fluctuations,will result in the beam spreading,beam wander,angle-of-arrival(AOA)fluctuations,and irradiance scintillation.The performance of outdoor WOC links are deteriorated to a great extent by the turbulence-induced fading,even under clear air conditions.Therefore,it is essential to analyze the influence of optical turbulence on the beam propagation in turbulent atmosphere/ocean,so as to find appropriate means to mitigate the turbulence-induced degradation.Besides the diversity,aperture averaging,adaptive optics,forward error correction(FEC),the partially coherent beam(PCB)is also an effective approach to mitigate the turbulence-induced fading.Due to this promising property,the PCB has a tremendous application potential in WOC communications.However,the reduction of turbulence-induced fading by using the PCB is at the expense of reducing the received optical power.For this reason,it is important to seek out a proper balance between the scintillation reduction and power loss in specific WOC systems.In recent years,laser beams carrying phase singularities(i.e.,optical vortex beams)have also attracted much attention in WOC communications as a result of their potential for being employed as information carriers.The data rate can be significantly increased by multiplexing optical vortex beams.Since Gori et al.established a sufficient condition for devising genuine spatial correlation functions,many different kinds of the non-conventional PCBs(i.e.,their spectral degrees of coherence are not Gaussian profiles)are introduced,such as the multi-Gaussian Schell-model(MGSM)beam,Bessel-Gaussian Schell-model(BGSM)beam,and cosine-Gaussian Schell-model(CGSM)beam,in which some can be used in the future partially coherent WOC systems.As a natural extension,the non-conventional PCB vortex beams can be introduced,e.g.,the MGSM vortex(MGSMV)beam.The aim of this dissertation is to investigate the statistical properties of non-conventional partially coherent beam(PCBs)or their corresponding vortex beams propagating through the turbulent atmosphere/ocean,to understand the influence of the source and turbulence parameters on beam quality in the receiver plane.In addition,the channel correlation of two PCBs in a transmitdiversity-based WOC system under the condition of weak turbulence is explored as well.In conclusions,some theoretical results that can be useful for the future partially coherent WOC systems are demonstrated.To summarize,the major research work of this dissertation are outlined in the following:Based on the extended Huygens-Fresnel principle,second-order moments of the Wigner distribution function(WDF),and generalized spatial power spectrum model valid in non-Kolmogorov turbulence,the closed-form analytical expressions for the cross-spectral density,average intensity,spectral degree of coherence,effective radius of curvature,and propagation factor of the BGSM beam in atmospheric turbulence,have been derived.The evolution properties of the BGSM beam propagating through non-Kolmogorov turbulence are investigated by a set of typical numerical examples.The influence of the generalized refractive-index structure constant,outer and inner scales,and index of spatial power spectrum of non-Kolmogorov turbulence on the evolution properties of the BGSM beam,are discussed in detail.Numerical results illustrate the turbulence-induced degradation can be remarkably reduced by using the BGSM beam with proper source parameters.Based on the extended Huygens-Fresnel principle,paraxial approximation and theory of coherence,the closed-form analytical expressions for the cross-spectral density,average intensity,normalized rms beam width,and angular spread of the MGSMV beam with topological charge l = ±1 on propagation in turbulent media,have been derived.Furthermore,the closed-form analytical expression for the third moment about zero of spatial power spectrum of oceanic turbulence,has been deduced.With the help of spatial power spectrum models valid in non-Kolmogorov turbulence and oceanic turbulence,and by using a set of typical numerical examples,this dissertation has quantitatively explored the evolution behaviors of the average intensity,normalized rms beam width,coherent vortices of the MGSMV beam with topological charge l = ±1 in turbulent atmosphere and in turbulent ocean,respectively.The dependence of the MGSMV beam propagation on parameters of the source and atmospheric/oceanic turbulence is discussed in detail.The impact of the beam index and fluctuations of atmospheric and/or oceanic turbulence on the conservation distance of the topological charge is stressed.Based on the Rytov approximation,approach of “effective beam”,and von K(?)rm(?)n spectrum model valid in atmospheric turbulence,and by use of a set of typical numerical examples,this dissertation has investigated the channel correlation characteristics of a transmitdiversity-based WOC system with two partially coherent-collimated Gaussian beams on propagation through an ABCD optical system in weak turbulence regime.The impact of the source coherence radius,beam separation distance,size of the receiver aperture,and optical wavelength on the channel correlation are analysed at great length.It is found that the PCB has an advantage over the fully coherent beam in mitigating the adverse effect of the channel correlation.However,the decrease of the channel correlation through reducing the spatial coherence properties of the transmit beams is finite.