In a two-node bi-directional Free Space Optical (FSO) communication system, the two FSO nodes exchange data encoded on optical carrier beams sent across an unobstructed line of sight (LOS) between the two nodes. As shown in FIG. 1, a conventional two-node bi-directional system is illustrated. As shown, a first node 2 and a second node 3 communication by transmitting and receiving a signal 6, 7 sent between the nodes. The data can be encoded on the signals in any matter; a binary, on-off, exemplary signal is illustrated for simplicity. Each node has an optical output 4 for transmitting the desired signal 6, 7, and also an optical input 5 for receiving the transmitted signal. Once received, the internal electronics of the node can decode the signal and obtain the transmitted data.
A Pointing, Acquisition, and Tracking (PAT) control system running on each FSO node maintains the optical alignment of the two beams in the FSO link using local beam steering elements. Each PAT system must keep its transmit (Tx) beam adequately pointed into the other node's receive (Rx) aperture while simultaneously keeping its Rx beam (arriving from the far node) adequately aligned with the local Rx detector. The FSO system is said to be “Tracking” when this two-way alignment is within tolerance for the exchange of data between the two nodes. FSO links require a non-trivial light propagation time proportional to the link distance, which must be considered in the design of the PAT control system. What is needed is a PAT control system that provides high-bandwidth performance for any link distance.
The traditional state-of-the-art PAT approach is the power-peaking approach. This approach requires handshaking of the two communication systems. For example, the transmitter needs to scan its laser beam while the receiver measures the received power. Let D(t) be the direction of the beam as a function of time. After the transmitter finishes the scanning, the receiver analyzes the received signals and determines the time, tmax, when the signal power was at its maximum. The receiver then sends the transmitter the peak-power time tmax. The transmitter then steers its beam to D(tmax). This process introduces time delay, which, for long distance links, degrades alignment performance, which, in turn, degrades communication performance. Also, this process requires the overhead of the handshaking layer itself.