Research On Free Space Optical Communication System And Its Major Components

Atmospheric free space optics (FSO) is a communication technology that transmits data though atmosphere, and one of the new techniques in short-haul broadband applications, especially in the "last mile" communication. FSO provides high bandwidth with low cost, high secrecy, convenient deployment and flexibility. But because FSO is a technology of line-of-sight and atmosphere is an open medium, FSO is subject to outside disturbance, such as atmospheric effects and building base-motion. Atmosphere conditions cause signal attenuation and random fluctuation, shorten link range, and make it difficult to achieve all-weather communication. Building base-motion poses great challenge for transceiver alignment. The purpose of the whole project is to look for a low-cost feasible way to settle these problems. In this dissertation the main assignment is to design a FSO system with basic and key components, and establish the foundation of further studies. An FSO communication system is presented in this paper firstly, including optical antenna, transceiver module, and pointing, acquisition and tracking (PAT) subsystem. Some new components are adopted such as vertical cavity surface emitting laser (VCSEL), two dimension position sensitive detector (PSD), and fast steering mirror (FSM) drive by piezoelectric ceramic micro-positioner. Optical antenna is one of the important parts of FSO. A bi-static dioptric optical antenna is designed and evaluated with optical design software ZEMAX. This design eliminates backscatter from transmitter to receiver and simplifies the beam pointing and tracking subsystem. The transmitting antenna is mainly composed of a VCSEL collimator and a beam expander; ZEMAX simulation results show that after collimating with this antenna, a divergence angle (full angle) of 1mrad is obtained. In receiving antenna, when a cone-channel condenser is located at the exit pupil of a Keplerian telescope, proper field of view is obtained without tracking system at the receive end, because the receiving optical power of APD will keep stable to a great extent when the incident beam deflects. In transceiver module, VCSEL is easy to be drive and modulated with low power consumption because of its low threshold current. As to the VCSEL driver and APD amplifier, specific integrated circuits are adopted to simplify the design and increase stability. Meanwhile, based on the analysis of the relation between signal-to-noise ratio (SNR) and optimal multiplication gain of APD, and the relation between ambient temperature and multiplication gain, temperature sensing circuit and adjustable APD bias circuit are designed. A PAT subsystem is designed in order to mitigate the effect of building base-motion. The flow process chart is presented with a detail description of pre-pointing, coarse adjusting, precise adjusting and tracking. The position error signal is obtained by PSD, and the executive components are pan-tilt head and FSM. Processing circuit of PSD and the inner structure of FSM are also described. In addition, some metrics to indicate FSO performance are introduced, including link power budget and two curves: link margin versus range and attenuation versus range. With parameters of the FSO system designed in this dissertation, power budget and both of the two curves are calculated and analyzed.