10 Gb/s Ethernet laser communications using optical space-time division multiple access

The use of free-space laser communications for military applications will continue to increase because of its advantages with respect to bandwidth, low probability of detection, interception and jamming, and absence of spectrum restrictions. Multiple access, the ability of a terminal to simultaneously provide network access for multiple users, will become increasingly important as the number of users increases. Decreasing cost, size, weight, and power per user is essential to increasing the number and types of lasercom users. We have developed an approach to multiple-access lasercom that accomplishes this using the commercial paradigm of sharing the most expensive terminal resources among all the users. Space-time division multiple-access (STDMA), analogous to an optical spacetime switch, hops the transmit beam and receive direction among multiple users and exchanges data while the beam dwells on a user. A key enabler of STDMA is electronic beam steering, which provides fast, precise, agile beam re-pointing. We present the results of laboratory tests of STDMA for two users. The dead-time in re-pointing the beam between users has been measured under different conditions and found to be as low as 10 ms. This is low enough to enable efficient bandwidth utilization. 10 Gb/s Ethernet traffic has been transmitted to the terminals to directly measure bandwidth efficiency, which varied from ~85% to 98% depending on the transmission time per remote terminal.