Turbid-harbor demonstration of transceiver technologies for wide dynamic range undersea laser communications

Undersea laser communications represent a promising area of research with a large set of applications. Wide dynamic range receivers are necessary to operate through a range of possible water qualities and link distances. In the signal-starved regime, photon-counting photomultiplier tubes (PMTs) are a key technology for high-sensitivity communications. When more signal is available, linear avalanche photodiodes (APDs) provide an opportunity for higher-rate communication. We have designed a receiver terminal employing both kinds of detectors to show robust operation over nearly two orders of magnitude in power and data rate. An optical link including this receiver terminal was submerged in Narragansett Bay, RI to demonstrate underwater optical communication over several days. The PMT receiver demonstrated robust, error-free performance over channel rates from 1.302 Mbaud to 10.416 Mbaud for received optical power levels ranging from -84.1 dBm to -75.3 dBm. The PMT link demonstrated an error-free user rate of 8.68 Mb/s. This corresponded to nearly-ideal detector efficiency on the order of one detected photon per bit. The PMT receiver was contained entirely within the submerged enclosure and demonstrated full real-time decoding, including strong forward error correction. A low-power transmitter was used to demonstrate a link with loss equivalent to 18 extinction lengths. With moderately-powered transmitters, this distance could be extended to 22.4 extinction lengths. The PMT receiver was capable of operating at near-theoretical limits during the day and night. Its multi-rate operation demonstrated the capability of trading sensitivity for data rate efficiently. With the same low-power transmitter, the APD receiver achieved a bit error rate less than 1×10-9 at 125 Mbaud. Furthermore, it achieved an error rate correctable by forward error correction for a link with loss equivalent to 9 extinction lengths.