Manipulation Of Photonic Orbital Angular Momentum

An optical vortex is a beam of light whose phase varies in a corkscrew-like manner along the beam’s direction of propagation and carries both spin angular momentum and orbital angular momentum(OAM). This dissertation focuses on manipulating photonic OAM in experiment, including creating optical vortices in femtosecond pulses, OAM conversation and encoding for optical communication. The main content contains:1. A new method is proposed to generate extraordinary vortex beam with spiral arm diffraction pattern. Spatial light modulator(SLM) is widely used to create ordinary vortem beam with doughtnut intensity profile. We use a phase aperture with circlular(or polygonal) configuration to convert Gaussian beam to extraordinary vortex beam with spiral arms diffraction pattern.Ordinary and extraordinary vortex beam are studied based on diffraction model. Thoretical studys demonstrate that the extraordinary vortex beam is a combination of the diffraction of a Laguerre-Gaussian beam through a numerical aperture and the diffraction of a Gaussian beam through a counterpart apertrue. Simulation results are in good agreement with the experimental results, i.e. both the theoretical and experimental intensity pattern for the extraordinary vortex has l spiral arms, which is similar to that of sprial shape in its phase distribution.An optical experimental system consisting of phase-only SLM is built up. A 630 nm He-Ne laser is used to study vortex beam.The new method provides a convenient way to visualize a vortex’s topological charges.2. Rules in OAM exchanging are analyzed, including combining vortex beams with different topological charges and reservation of OAM during down-conversion in nonlinear crystal. Specifically, a femtosecond laser and a I-type BBO(Beta Barium Borate) crystal are used to conduct experiment on reservation of OAM during second harmonic generation.3. Application of OAM basis and angular basis in Quantum Key Distribution are studied. We also compare the ordinary and extraordinary vortex beam in the aspect of 16-digital Quadrature Amplitude Modulation. Differences between continue pulse laser and ultrashort pulse laser are compared. The results show that OAM encoding along with mode-division multiplexing is promising in increasing data capacity and security.The helically phased beam with l spiral arms in intensity distribution provides unique properties for application, such as mechanical torque action with applications for trapping and moving objects ranging in size from tens of nanometers to tens of micrometers, potential high-density information coding for optical communication systems and quantum bits encoded in orbital angular momentum at the single-photon level for quantum information processing.