A narrow-beam undersea laser communications field demonstration

We report a demonstration of narrow-beam laser communication through the waters of Narragansett Bay in Rhode Island, USA. The transmitter and receiver were mounted on an aluminum truss and placed in the water alongside a pier operated by the Naval Undersea Warfare Center. The transmitter consisted of a real-time modulator and encoder, a 515 nm wavelength commercial laser, collimating optics, and a steering mirror. The receiver included a steering mirror, a focal plane camera, a linear-mode avalanche photo-diode (APD), a photo-multiplier tube (PMT) single photon detector, a large area imaging camera, an iris to vary the field of view, optics to split the beam between the various detectors, and field-programmable gate array (FPGA) electronics for real-time demodulation and decoding. The PMT and APD detectors were used for communications demonstrations; the imaging and focal plane cameras were used for channel characterization measurements and system alignment. Communications and characterization data were collected through a variety of conditions over the five day field experiment, including day and night, calm and high winds, and flood and ebb tide. In the experiment, the transmit power, receiver field of view, and link distance were varied. The water transmissivity and volume scattering function were measured throughout the experiment to calibrate the results. Real-time communications demonstrations with the PMT were carried out between 1 megabit-per-second (Mbps) and 8.7 Mbps at 7.8 meters, which represented between 8 and 12 beam extinction lengths. With the APD, 125 Mbps were demonstrated at 4.8 meters, representing approximately 5 extinction lengths.