Research On Optical Communication Technology Based On Quadrature Phase Shift Keying External Modulation

Wireless optical communication system transfer information through the atmospheric channel by using laser light beam as a data carrier. Compared with the traditional radio communication, the wireless optical communication has advantages such as non-electromagnetic interference, wide frequency band, high confidentiality, small size, easy to set-up and without laying communication link before use. It is suitable for high security institutions, satellite communication, and temporary terminal communication across obstacle etc. Therefore it is one of the most important topics of communication in China.Firstly, the overall structure of the wireless optical communication system was presented. By analyzing the principle of key components in this system, Bragg acousto-optic modulator was chosen as the external modulator, a semiconductor continuous laser which has a wavelength of 637 nm as the light source and a photomultiplier tube as photo detector. The combination of the semiconductor continuous laser and AOM can effectively reduce the chirp phenomenon in the modulation. Selecting Quadrature Phase shift keying and BCH code as signal processing method. The signal was processed by FPGA development board, which is reliable practical, and make the design effectively.Secondly, the signal processing by FPGA development board of the transmitter and receiver parts in the system were presented. According to the design process of FPGA, introducing the top module first, then introducing the design ideas and the results of the sub module in detail, verify whether the function of each module is right.Then, testing the performance of the selected AOM, laser and the signal processing modules of FPGA development board respectively, to verify the feasibility of each part of the system. Building a complete wireless optical communication system experiment platform. The original information was sent to FPGA to modulate and channel coding by the serial ports of a PC, and the baud rate is 115.2k Bd, which can be used to control the AOM to modulate the beam. Finally, a wireless optical communication using light intensity modulation/ direct detection method was set up for the indoor test in different light environments and analysis of the experimental results, the data rate is 230.4kbit/s, and the probability of error among received information is 10-5.In the end of this paper, a summary and analysis has been made, then discussed the disadvantages of this system and probable improvements in the future.