We deployed a narrow-beam optical measurement and communication experiment over several days in the shallow, turbid water of Narragansett Bay, Rhode Island (USA). The experiment consisted primarily of a transmitter module and a receiver module mounted on a metal framework that could be lengthened or shortened. The communication wavelength was 515 nm. The experiment characterized light propagation characteristics, including images of the received beam over time. The experiment included manual beam steering. Images obtained during the steering process provided insight into future development of an automated steering procedure. Water transmissivity was also measured. Over time and tides, the optical extinction length varied between 0.66 m and 1.07 m. The transmitter’s optical power was kept low at 0.25 mW. The receiver included a high-sensitivity photon-counting photomultiplier tube (PMT) and a high-speed linear avalanche photodiode (APD). Both links processed data continuously in real time. The PMT supported multiple channel rates, from 1.302 Mbaud to 10.416 Mbaud. It also included strong forward error correction (FEC) capable of operating at multiple code rates. The PMT link demonstrated near-theoretical channel performance at all data rates, error-free output after FEC, and robust operation during day and night. This link efficiently traded data rate for link loss. It demonstrated error-free performance for input powers as low as -84.1 dBm, or 18 extinction lengths. The APD receiver demonstrated a channel error rate of 1e-9 at 125 Mbaud. Furthermore, it demonstrated a channel error rate correctable by FEC at a link loss equivalent to 9 extinction lengths.
Hemonth G. Rao, Catherine E. DeVoe, Andrew S. Fletcher, Igor D. Gaschits, Farhad Hakimi, Scott A. Hamilton, Nicholas D Hardy, John G. Ingwersen, Richard D. Kaminsky, Marvin S. Scheinbart, and Timothy M. Yarnall, "Field demonstration of multi-rate narrow-beam undersea optical communication with photon-counting and linear receivers in a turbid harbor (Conference Presentation)," Proc. SPIE 10096, Free-Space Laser Communication and Atmospheric Propagation XXIX, 1009617 (Presented at SPIE LASE: February 02, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2256597.5389862371001.
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