Since the central part of laser beam cannot be transmitted through the traditional Cassegrain antenna, thus a large energy loss is caused. To improve the transmission efficiency of traditional Cassegrain antenna, a pair of lenses is designed and set between the laser source and traditional Cassegrain antenna in this paper. The parameters of the pair of lenses and Cassegrain antenna system is determined and the energy distribution after the laser beam passing through the pair of lenses is discussed in detail. Based on three dimensional vector refraction and reflection theory, the ray tracing of the entire communication system and the chart of the transmission efficiency are simulated with MATLAB. From the chart, it can be concluded that proper laser beam shaping can improve the transmission efficiency of the entire communication system up to 100% at the wavelength of 1550 nm under ideal condition. After taking several practical factors such as the reflectivity of the mirror composing the Cassegrain antenna, the transmissivity and chamfering of the pair of lenses and the dispersion of the material into account, the transmission efficiency of the entire system can still be increased to 95.54% at the wavelength of 1550 nm. Compared with designing complex antenna systems, the method presented in this paper is more practical and convenient for optical communication.
We propose a hollow-core photonic bandgap fiber (HC-PBGF) with background composed of two materials to support orbital angular momentum (OAM) modes. Numerical models are set up to figure out the effective indexes and confinement losses over 1.3-2.0 μm. Simulation results show that this fiber can support more than 48 OAM modes, of which the effective indexes satisfy the condition for effective index separation (<10-4) and the confinement loss keeps under 10-7 dB/m over 1.3-2.0 μm. According to the comparison between fibers with same structure but comprising one or two background materials, adopting two materials to compose background is an effective method to significantly improve the performance of OAM-supporting HC-PBGF. The HC-PBGF proposed here is competitive in dealing for OAM multiplexing for optical communication systems.