Research On Scintillation Supression Techniques For Free-Space Optical Communications

Due to the fast development of internet and multimedia equipment, the communication network is becoming crowded. The existing wireless communication techniques based on microwave are running out of band resources to meet the requirement, which renews the concern of free space optical communication (FSO) systems. With a laser as a signal carrier, FSO can provide a long-distance communication link with wireless and broadband data transmission. With "wireless" as its one of the important advantages, FSO can be widely used in Local Area Network (LAN) and (Metropolitan Area Network) MAN. Another advantage is its potential for offering a high-speed communications between satellites and ground. Due to the bandwidth limitations in the microwave communication, FSO is considered to be one of the key technologies in the future ultra-high-speed (Gb/s) and high-capacity space communications.The biggest drawback of FSO is that the transmitting signal is susceptible to the atmospheric environment, wherein the turbulence-induced intensity fluctuation, known as optical scintillation, is considered to be the main factor in limiting system performance. To solve this problem, this dissertation studies the performances of partially coherent beam and space diversity techniques in scintillation reduction. The main innovations are as follows:1. The general scintillation index model for time-averaging reception of Gaussian-Schell model (GSM) beam is extended from the case of point receiver to the case of aperture receiver. The analytic expression is derived including the effects of the receiver aperture diameter, the correlation time τs of the source phase variation and the integration time τD of the detector. There exists a maximum value rmax of the relative detection speed (RDS) τs/τD. When the RDS is lower than τmax, the GSM beam can reduce the scintillation compared with the coherent counterpart. An approximate expression for τmax is derived, which is useful for making choice of the detector speed under different link conditions. It is also found that under the influence of finite RDS, increasing receiver aperture size can degrade the performance of the PCB in scintillation reduction, compared with the coherent beam in weak turbulence regime.2. The general model of beam wander for a coherent Gaussian beam propagating through atmospheric turbulence is extended to the case of a partially polarized electromagnetic Gaussian-Schell model (EGSM) beam. The expression of the beam wander is obtained by characterizing the EGSM beam with the effective beam parameters. The effects of initial spatial coherent lengths, degree of polarization and phase curvature are examined in detail. A condition is derived under which beams with different spatial coherence and degrees of polarization will generate the same beam wander. The model is potentially useful in beam parameter optimization.3. Based on the plane-wave model and the extended Rytov theory for ABCD ray-matrix formulation, expressions for the channel correlation are derived under general atmospheric conditions. The effects of antenna spacing, turbulence strength, receiver aperture size and wavelength are examined in detail and the corresponding physical mechanisms for the results are discussed. An approximation for estimating the channel correlation length under strong turbulence conditions is obtained. From the applications point of view, our formulation provides a theoretical way to evaluate the channel correlation under general link conditions, which is important for the prediction of system performance.