Special Light Beam Generation And Its Applications In Optical Communications

The basic physical dimensions of lightwaves contain amplitude,phase,polarization,time,wavelength/frequency and transverse space.However,by using the polarization,time,amplitude/phase,wavelength/frequency of lightwaves for optical communications,the transmission capacity is rapidly approaching the fundamental limit.In addition to these traditional dimensions,lightwaves also have transverse spatial dimension.By using the space domain,one can increase the number of parallel communication channels and greatly improve the communication capacity of the system,which could break the capacity limits of existing technology and realize sustainable expansion of the communication capacity.A light beam with a special transverse spatial distribution is known as special light beam.Due to its special physical properties,it has attracted much attention in many fields of optics.Nowadays,several kinds of special light beams have been reported in free space and optical fiber.In optical communications,the special light beams not only can be used to improve the communication capacity,but also can make the communication system much more flexible.In this thesis,we mainly research on the generation of special light beams and its applications in optical communication.The major contents of this thesis are summarized as follows:?1?Free-space special light beam generation and its applications in optical communications.?1?By designing simple configurations with phase-only spatial light modulators?SLMs?,arbitrary manipulation of the spatial full field information?i.e.amplitude and phase?of a light beam is achieved.Using this approach to facilitating arbitrary and independent control of spatial amplitude and phase,one can flexibly generate different special kinds of light beams for different specific applications.Multiple collinear orbital angular momentum?OAM?beams,Laguerre-Gaussian?LG?beams,Bessel beams and arbitrary beams with odd-shaped intensity,having both spatial amplitude and phase distributions,are successfully generated in the experiments.?2?Based on the conventional iterative algorithm,a pattern search assisted iterative?PSI?algorithm to simultaneously generate multiple OAM modes using a single phase-only element is proposed.The PSI algorithm shows favorable operation performance for generating 100 randomly spaced OAM modes and 50 evenly spaced OAM modes with high diffraction efficiency?>93%?,low relative root-mean-square error?R-RMSE?and low standard deviation.Moreover,one can also manipulate the relative power distribution of the generated OAM modes.?3?By designing and optimizing complex phase pattern combining with axicon phase distribution using PSI algorithm,data multicasting from a single Gaussian mode to multiple Bessel modes is demonstrated in the experiment.Under the obstructed path conditions,obstruction-free data-carrying Bessel mode multicasting is also demonstrated.All the multicasted Bessel modes show relatively low crosstalk from their neighboring modes and achieve bit-error rate?BER?less than 1e-3.?4?By exploiting curved light beams for line-of-sight light communications,free-space data-carrying bendable light communications along arbitrary trajectories is proposed and demonstrated in the experiment.Moreover,multiple functionalities of free-space bendable light communications,including bypass obstructions transmission,self-healing transmission,self-broken trajectory transmission,and movable multi-receiver transmission are evaluated.The observed results indicate that bendable light beams can make free-space optical communications more flexible,more robust and more multifunctional.?2?Special light beam communications in traditional few-mode fiber?FMF?and multi-mode fiber?MMF?.?1?Spatial mode encoding/decoding in km-scale FMF is proposed.Three grayscale images are successfully transmitted through a 1.1-km FMF by employing either 4 modes,i.e.three linearly polarized?LP?modes of LP₀₁,LP_11a,LP_11b and one OAM mode of OAM_-1,or 2 modes(OAM₊₁1 and OAM_-1).The bit-error rate is evaluated and zero error among all received data is achieved,showing favorable fiber link communication performance using the spatial modes of fiber for encoding/decoding.?2?A novel and simple method to convert conventional amplitude modulation to spatial mode modulation is realized for high-speed mode encoding/decoding transmission.By using two traditional phase-only SLMs,10 Gbit/s data transmission over a 2.6-km MMF link based on amplitude-to-mode(OAM₀ and OAM_-1)modulation mapping is demonstrated in the experiments.?3?By analyzing the mode properties in traditional MMF,data transmission using four OAM modes(OAM_0,1,OAM_-1,1/OAM_+1,1,OAM_+2,1,and OAM_+3,1)in 2.6 km OM3 MMF is experimentally demonstrated.Moreover,two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF is also achieved with optimized inter-and intra-mode-group crosstalk.?4?By combining FMFs and MMF,OAM based mode-division multiplexing in heterogeneous fiber-optic network is proposed and demonstrated.40 Gbit/s data transmission employing two OAM beams multiplexing is achieved.?3?Special light beam communications in elliptical-core few-mode fiber?EC-FMF?and ring-core fiber.?1?By analyzing the mode properties in a two mode EC-FMF,20 Gbit/s data transmission over a 1.1-km EC-FMF link based on amplitude-to-mode(LP₀₁ and LP_11a)modulation mapping is demonstrated with low crosstalk in the experiment.?2?By exploiting a graded-index ring-core fiber with large separation of high-order OAM mode groups,8.4-Tbit/s data transmission using two OAM modes in an 18-km fiber is successfully demonstrated with low crosstalk.Multiple-input multiple-output digital signal processing?MIMO DSP?is not required in the experiment because of the large high-order mode group separation of the OAM fiber.?3?An OAM distributed Raman amplifier?DRA?over 18-km graded-index ring-core fiber is proposed and demonstrated.Within the band from 1530 nm to 1565 nm,the maximum on-off gains are 3.5 dB for both OAM modes(OAM₊₄ and OAM₊₅),and the differential modal gain is less than 0.3 dB.The measured optical signal-to-noise ratio?OSNR?with DRA is 1.2 dB higher than the one without DRA.The obtain results show favorable performance for long distance OAM based optical fiber communications.