Non-mechanical Compact Optical Transceiver for Optical Wireless Communications

Abstract

The advantages of optical communications as compared to radiowave (RF) communications include broader bandwidth, larger capacity, lower power consumption, more compact equipment, higher security against eavesdropping, better protection against interference, and the absence of regulatory restrictions (Hyde & Edelson, 1997). Moreover, the demand for high-data-rate transmission from spaceborne observation platforms is steadily increasing (Toyoshima, 2005a). Free-space optical (FSO) communications systems are expected to play an important role in providing such high-data-rate communications, and optical technologies for satellite networks are expected to revolutionize space system architecture (Chan, 2003). For terrestrial optical wireless communications, a transmitter with a 3 × 3 square array of vertical-cavity surface-emitting lasers (VCSELs) was evaluated in Parand et. al. (2003). Multiple-input multiple-output (MIMO) systems were presented based on optical code-division multiple-access (OCDMA), imaging diversity receivers, and white LEDs (Hamzeh et. al., 2004; Djahani et. al., 1999; Zeng et. al., 2009; Minh et. al., 2009). For longrange free-space laser communications, however, maintaining a line of sight between transceivers is particularly difficult because of the small divergence angle of the laser beams. Minimizing the requirements for the tracking system and ensuring the steady operation of the onboard optical terminal are therefore important for realization of commercial applications. FSO links are also well known for their susceptibility to adverse weather conditions such as cloud and fog, which pose great challenges to link availability. For terrestrial FSO links, RF links can be backed up to ensure continuous availability (Wu et. al., 2007). Optical terminals for long-distance communications tend to have large mass because optical tracking systems require mechanically movable parts for coarse laser pointing and tracking. Reductions in mass, power, and volume can decrease interference with other missions on satellites. Non-mechanical movable architecture is extremely attractive for robust and lifelong operation of an optical terminal in orbit. The small satellite community still uses 9.6kbps communication links by employing ham radio communications due to resource constraints in nano-class satellites (Nakaya et al., 2003; Miyashita et al., 2006). Compact terminals can be used in nano-class satellites that have a mass of the order of a few tens of kilograms. There is also a significant advantage concerning frequency-licensing problems faced by satellites, and in this regard optical frequency carriers will be of great use to the small satellite community. Research and development at National Institute of Information