Research On Phase Vortex And Polarization Vortex Optical Communications

The continuous growth of data traffic has led to the development and utilization of physical dimensions such as wavelength/frequency,time,amplitude/phase and polarization.To address the emerging capacity crunch,exploiting the spatial structure to improve the transmission capacity and spectral efficiency has attracted great attention.Vortex beam,which is regarded as the representative beam possessing spatial structure,is commonly known as phase vortex(orbital angular momentum,OAM)and polarization vortex(vector beam)characterized by phase singularity and polarization singularity,respectively.Due to the unique unbounded orthogonal states of vortex beams in principle,vortex beams carrying different information can be multiplexed together in optical communications.By employing vortex beam multiplexing together with the other multiplexing technologies and advanced modulation formats,one can realize sustainable expansion of transmission capacity,which provides a new approach to break the capacity limits.Therefore,in this dissertation,researches on applications in optical multiplexing,encoding/decoding and signal processing employing phase vortex or polarization vortex in free-space,fiber and integrated chips are studied both theoretically and experimentally.The major contents of this dissertation are summarized as follows.1)Researches on basic theories and key technologies of vortex beam.The basic theory of phase vortex and polarization vortex,the relationship between different vortics in fiber and key technologies of vortex beam generation/detection(multiplexing/de-multiplexing)are introduced.We propose a simple configuration incorporating a single polarization-sensitive phase-only liquid crystal spatial light modulator(SLM)to facilitate polarization-insensitive modulation.Furthermore,we experimentally demonstrate several polarization-insensitive optical communication subsystems employing phase vortices,e.g.single phase vortex transmission,phase vortices multicasting and multiplexing.2)Researches on optical communication systems based on integrated optical vortex emitters employing phase vortices and polarization vortices.(1)We experimentally demonstrate a few-mode fiber(FMF)link employing phase vortex based on a highly efficient silicon integrated optical vortex beam emitter.Using this device,seven vortex modes,each modulated by a 20-Gbit/s quadrature phase-shift keying(QPSK)signal,have been successfully transmitted through 2 km fiber and 3.6 km fiber,respectively.(2)We propose a novel and simple method to map amplitude modulation to spatial modes modulation.We experimentally demonstrate a 15-Gbit/s phase vortices encoding/decoding by employing an integrated phase vortices multiplexer.(3)We experimentally demonstrate a fiber link employing polarization vortices based on integrated vortex beam emitter.First,we evaluate the performance of single polarization vortex transmission in the chip-fiber link.Furthermore,we demonstrate the multiplexing and kilometer-scale data transmission of two polarization vortices generated via integrated optical vortex emitters.Two polarization vortices,each carrying a 10-Gbit/s or a 20-Gbit/s 16-ary quadrature amplitude modulation(16-QAM)signal are multiplexed for transmission though a 2-km fiber link.(4)We experimentally demonstrate and evaluate the performance of an analog signal transmission system with integrated optical vortex emitter and 3.6-km FMF link using phase vortices with topological charge l(28)-2,2.The spurious free dynamic range(SFDR)of the second-order harmonic distortion(SHD)is measured to estimate the analog link performance.Moreover,we study the impact of resonance wavelength shift of the optical vortex emitter induced by increasing the input optical power on the analog link performance.3)N-dimensional phase vortices multiplexing and fractional phase vortices multiplexing.(1)By employing N-dimensional multiplexing and modulation link,i.e.5.8-Gbaud Nyquist 32-QAM signal over pol-muxed 52 phase vortices(104 channels in total),we demonstrate a free-space data link with a total transmission capacity of 8.16-Tbit/s and an aggregate ultrahigh spectral efficiency of 435-bit/s/Hz.(2)To get more channels for phase vortex communication,inspired by dense wavelength division multiplexing(DWDM),we present free-space optical communications exploiting fractional phase vortices multiplexing.Using QPSK/16 QAM signals over fractional phase vortices,we experimentally demonstrate,for the first time to our best knowledge,fractional phase vortices multiplexing communication,achieving phase vortex channel spacing as small as 0.6.4)Relay amplification and signal processing of vortex optical communication.(1)We propose and demonstrate a phase vortex fiber amplifier based on a ring-core erbium-doped fiber(RC-EDF).The measured small signal gain is above 14 d B from 1530 to 1565 nm.We estimate the RC-EDF assisted phase vortex fiber amplifier performance by measuring the bit-error rate(BER)of the two phase vortices(l(28)-1,1)and four wavelength multiplexing system modulated with 10-Gbaud QPSK or 16-QAM signal.(2)We propose and demonstrate flexible all-fiber pre-and post-data exchange functions in a fiber-based phase vortex space-division multiplexing(SDM)network.Phase vortices l(28)-1,1 carrying 5-Gbit/s four-level pulse amplitude modulation(PAM-4)signals are multiplexed to transmit through a 1.1 km fiber in the system.Two simple and controllable data exchange control units are placed at the front and back ends of the fiber to demonstrate pre-and post-data exchange functions.(3)We propose and demonstrate space-selective switch employing phase vortices in the space domain for switching network.One is the phase vortex switching among different phase vortices possessing different spatial phase structures.The other is the space switching among different space locations.By using a single SLM,we experimentally demonstrate reconfigurable 4×4 phase vortex mode switching,space switching,and joint phase vortex mode and space switching.