Free-Space Optical Communication Link Across 16 km to a Modulated Retro-reflector Array

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Free-space optical (FSO) links provide secure, high bandwidth data communications. Links to retro-reflectors have the advantage of a simple retroreflector terminal design. However, the simplicity of the retro-reflector terminal comes at the cost of the complexity of the interrogator terminal. This paper describes a 16 km free-space laser communications link between a high power interrogator terminal and a modulated retro-reflector array at 2 Mbps. This link was the longest high data rate (>1 Mbps) modulated retro-reflector link ever reported at the time this paper was written.

The 16 km free-space optical link to a modulated-retro-reflector was a tremendous technical challenge. The link range was substantially longer than previous similar FSO links. The extreme link range resulted in much higher link losses than incurred on shorter links. The higher link losses were mitigated through interrogator strategies that included a high power Erbium doped fiber amplifier (EDFA) transmitter, an extremely small transmitter divergence, and sophisticated pointing and tracking. To maximize the optical power projected across the link, the transmitter divergence was only 100 µrad. The small transmitter divergence required stringent optical alignment, and high speed pointing and tracking. The high speed pointing and tracking system was a custom design employing two fast steering mirrors and a tone tracking quadcell detector. The link losses were also mitigated by the use of an array of 3 modulated retro-reflectors. The array both increased the optical link margin as well as reduced the atmospheric channel fading. The interrogator terminal was developed by the author at the Laboratory for the Physical Sciences. The author was also responsible for the link calculations and logistics. The modulated retro-reflector array was developed by researchers at the Naval Research Laboratory under the supervision of William Rabinovich.

This paper describes the 16 km FSO link scenario, the interrogator system, the modulated retro-reflector array, as well as link performance results. The link performance results presented include pointing and tracking performance, channel fading due to atmospheric scintillation, and the communication link performance using various data protocols.