Free-Space Optical Communication Link Across 16 km to a Modulated Retro-reflector Array
Free-Space Optical Communication Link Across 16 km to a Modulated Retro-reflector Array
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Date
2007-04-25
Authors
Plett, Mark Lewis
Advisor
Goldhar, Julius
Citation
DRUM DOI
Abstract
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.