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Free space optical (FSO) sensor networks using direct line of light (LOS) laser links can provide spatially efficient and
physically secure connectivity. The data rates can range from bits/s to hundreds of Mb/s with the complete optical
transceiver system consuming power in the tens of mW. These features are advantageous for low-power communication
networks over short distances in environments where LOS is available, and where radio frequency connectivity must be
avoided because of interference or security problems. The range of links in FSO networks is limited by power
requirements and angular coverage. In order for FSO directional networks to provide viable short-range connectivity, the
networks must provide signal coverage over a wide field of view and operate with efficient media access protocols to
minimize random access times for the independent transmitting nodes within the network. In this paper, the system
design of a FSO sensor network is presented. The system includes a network of small, low power (mW), integrated
systems, or "motes," that transmit data optically to a central "cluster head," which controls the network traffic of all the
motes and can relay data to another cluster head in a series of multi-hops to achieve data communication over longer
distances. To provide wide field of view signal coverage, each cluster head is equipped with multiple vertical cavity
surface emitting lasers oriented in different directions and controlled to diverge at 10°. To implement the proper media
access controls, a properly designed master-slave network connecting multiple motes to a cluster head was developed
and implemented. The network can handle multiple access from all motes within each cluster head's field of view, as
well as set up a directional network backbone between multiple cluster heads, so that signals collected from a mote can
be relayed through other cluster heads, until the signal is delivered to its destination. This paper presents the network
architecture and optical communication system hardware of our FSO sensor network, and some experimental
performance results of our multiple access protocol attempting to resolve channel contention between 10 motes and a
cluster head.
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