2. Key Laboratory of Electromagnetic Space Information, Chinese Academy of Sciences, Hefei, Anhui 230027, China
Overview: Orbital angular momentum (OAM) of light as a degree of freedom provides an infinite number of orthogonality states with different mode topological charges. The application of OAM mode multiplexing technology in free space and fiber optical communication has become one of the most active research areas of communication. A huge challenge for OAM beam free space optical communication system is the disturbance of atmospheric turbulence. The effects include beam point jitter, intensity and phase fluctuation, damage beam pattern and crosstalk between OAM modes. Mode purity of OAM beam is very important for free space OAM mode multiplexing optical communication system. Therefore, it is significant to carry out experimental research on the actual free space transmission of OAM beams.
The short-distance free-space transmission characteristics of the OAM beam were experimentally studied. The transmission distance is 0~50 m indoors. The atmospheric environment is weak and stable. A digital micromirror device (DMD) was used to generate the OAM beam in the experimental setup. The DMD model is DLP4500 with micromirror numbers of 912×1140. At the receiver, a spatial beam analyzer was used to measure the intensity pattern of the OAM beam. The beam analyzer uses a 12-bit 1.4 megapixel CCD as the detector. The detection aperture size is 9.5 mm and the pixel size is 6.45 μm×6.45 μm. The beam broadening effect of the OAM beam at different transmission distances was studied. The measured data was compared with the theoretical results of the OAM beam propagation broadening calculated by the Fresnel diffraction model, which verifies they are consistent in trend. The phase pattern of the OAM beam was studied by the interferometric method. At the receiver, a single path Sagnac interferometer (SPSI) was used to separate and detect the intensity of modes of the OAM beam. The SPSI has higher stability than the Mach-Zehnder interferometer. The effects of energy coupling between the sending mode and the sideband modes of the OAM beam were studied. An electric heater was used to generate the strong turbulence to simulate the influence to the OAM beam mode transmission characteristics. The average pointing deviation was characterized turbulence strength approximately.
The experimental results show that the longer transmission distance and stronger turbulence cause larger energy coupling of the OAM beam from the sending mode to the sideband modes, and the OAM beam with larger mode topological charge has more deterioration of the mode purity after transmission in the strong turbulence. The results are helpful for analyzing the bit error rate (BER) characteristics of the OAM mode multiplexed optical communication system.