On Divergence-Angle Efficiency of a Laser Beam in Free-Space Optical Communications for High-Speed Trains

We compare narrow and wide laser beams in a free-space optical (FSO) communications system for high-speed train communications and analyze the tradeoffs among the received power, coverage area, and the complexity of the acquisition-tracking-pointing (ATP) mechanism. We propose to employ a divergence angle of a wide beam in the range [0.07°, 2.002°] to relax the steering speed of the fast steering mirror, which is a major component of the ATP mechanism in an FSO transceiver. In addition, we overcome the negative effects of train vibrations by using a proposed divergence-angle range for a wide beam. The proposed range of divergence angles provides a large link range and effective coverage length, and contact time as compared to a narrow beam.