Optical propagation through the ocean encounters significant absorption and scattering; the impact is exponential signal attenuation and temporal broadening, limiting the maximum link range and the achievable data rate, respectively. MIT Lincoln Laboratory is developing narrow-beam lasercom for the undersea environment, where a collimated transmit beam is precisely pointed to the receive terminal. This approach directly contrasts with the more commonly demonstrated approach, where the transmit light is sent over a wide angle, avoiding precise pointing requirements but reducing the achievable range and data rate. Two advantages of narrow-beam lasercom are the maximization of light collected at the receiver and the ability to mitigate the impact of background light by spatial filtering. Precision pointing will be accomplished by bi-directional transmission and tracking loops on each terminal, a methodology used to great effect in atmospheric and space lasercom systems. By solving the pointing and tracking problem, we can extend the link range and increase the data throughput.
Scott A. Hamilton, Cathy E. DeVoe, Andrew S. Fletcher, Igor D. Gaschits, Farhad Hakimi, Nicholas D. Hardy, Thomas Howe, Nathan Mittleman, Hemonth G. Rao, Marvin S. Scheinbart, and Timothy M. Yarnall, "Undersea narrow-beam optical communications field demonstration," Proc. SPIE 10186, Ocean Sensing and Monitoring IX, 1018606 (Presented at SPIE Defense + Security: April 11, 2017; Published: 22 May 2017); https://doi.org/10.1117/12.2264733.
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