The Characteristics Of Structured Beam And Its Applications In Optical Communications

The rapid development of 5th generation mobile networks and the arrival of the era of big data has caused sharp growth of Internet data demand,and thus brings unprecedented pressure and challenge to the current communication systems.How to further improve system capacity and spectral efficiency based on the existing communication technology has been a key problem for communications industry.Compared with traditional microwave communication,mobile communication and satellite communication,optical communication has aroused considerable interest because of its many advantages,such as high-bandwidth and dispense with frequency permission.Remarkably,the basic resources of light waves such as amplitude,phase,frequency/wavelength,time and polarization have been fully developed,which lead to a new capacity bottleneck for communication systems.To exploit the remaining space domain dimensional resources of light waves has become a research hotspot.Due to the properties of phase singularity,orbital angular momentum(OAM),specific polarization and approximation non-diffracting mechanism,structured beams are potentially applied in the field of optical communication.Based on this fact,the theory and experiment researches of optical field manipulation are conducted in this dissertation.Then,two novel diffraction gratings are designed to efficiently detect of the OAM modes and radial indices for structured beams,respectively.Subsequently,the influences of atmospheric turbulence and oceanic turbulence on the propagation properties of light beam are illustrated in detail with numerical simulations.Finally,the validity of encoding and decoding with structured beams is verified by establishing an optical wireless communication system.The main contents and results are as follows: 1.The high-quality structured beams are produced based on the computer-generated holograms(CGHs)which calculated by computer and then displayed on spatial light modulator(SLM).Both the intensity distribution curves of theoretical and experimental results are plotted.The experimental observations are in good agreement with the theoretical predictions,which indicate that high-quality beams are obtained experimentally.Additionally,a simple and practical improved experimental setup for detection of structured beams is built.By observing the patterns after the modulated beam interferes with a plane wave or a spherical wave with the aid of the improved detection experiment setup,it is demonstrated that the structured beams are generated.2.Actually,the process of structured beam coding/decoding communications is accomplished by using the light mode,for example,OAM modes.Therefore,to recover the coded information with different OAM states at the receiver side of an optical transmission system,an accurate measurement of the OAM mode associated with a received optical field is of great significance.To date,±120 OAM modes of the maximum detectable area is derived in the world.To further overcome the issue of limited detection range for OAM modes,we present a new scheme to determine the OAM modes of structured beams by using gradually changing-period spiral spoke grating(GCPSSG).The experimental results indicate that the detection of OAM states up to ±160 can be realized with this scheme.Meanwhile,it is found that this scheme is robust and effective because the GCPSSG shows good tolerance to environment vibration and beam misalignment for the OAM diagnostics.In addition,it is worth noting that the published literature focused on the diagnostics of the azimuthal index l of Laguerre-Gaussian(LG)beams,with the assumption that the radial index p(28)0 which are not applicable for priori unknown radial high-order LG beams.Therefore,how to detect the radial index associated with an incident vortex beam,apart from the detection of the azimuthal index becomes a task of great importance.Furthermore,transfer of information utilizing both radial and azimuthal indices of LG beams can provide more communication modes,and thus further increase the system capacity.We present a method to determine the mode indices of LG laser beams by using a spiral phase grating.Both simulation and experimental results demonstrate that the detection of the mode indices p and l can be achieved simultaneously.3.The phase screen of atmospheric turbulence and oceanic turbulence are derived by employing power spectrum inversion method based on fast Fourier transform.The multiple random turbulent phase screens method is adopted to simulate turbulent environment on the intensity and phase profiles of structured beams.The influences of link distance and turbulence parameters on the optical attenuation and phase distortion of light beam are discussed.Furthermore,the effect of turbulence on the intensity distribution is observed by using semi-experimental method,that is,uploading the turbulent phase screen to SLM.4.Here a transmission model of structured beams in turbulence is established.The mathematical expressions of average intensity,and OAM state detection probability,spiral spectrum,and channel capacity for structured beams are obtained.The influences of beam source parameters(wavelength,beam waist,source spatial coherence length,and mode number),turbulence strength,and propagation distance on the propagation properties of light beam are illustrated in detail with numerical simulations. 5.By combining theories and experiment,this dissertation studies the progress of structured beam encoding/decoding communications.The image is encoded with a single structured beam and composite structured beams by coaxial superposition of two LG beams at the transmitter,respectively.The original image information can be recovered at the receiver by conjugate phase hologram based on the coherent detection method for the single structured beam,or directly recognized by observation of spot patterns for composite structured beams.In the experiments,a 2-bit gray-scale image and an 8-bit gray-scale image of 64′64 pixels are transmitted by four different orbital angular modes and composite structured beams,respectively.6.In order to save the decoding time and improve encoding/decoding speed,a structured light beam array with special spatial position distribution is designed,i.e.,central rotational symmetry twisted light array.The light array is produced by CGHs.The information of the gray-scale image can be encoded as a one-to-one mapping from the gray scale to the light modes and space position distribution.At receiver,a cylindrical lens acts as the decoding optical element for signal decoding.The information of gray scale for transmitted image would be recovered with a single cylindrical lens by directly observing the far-field diffraction patterns.The feasibility of the scheme by exploiting a 256-ary coding/decoding free-space optical communication to transmit a 24-bit true color bitmap of 64′64 pixels is experimentally demonstrated.The results show that a zero bit error rate performance has been achieved,among the 98.304 kilobits data transmission in this experiment.