Advancing Undersea Optical Communication

A remotely operated vehicle and undersea terminal emits a coarse acquisition stabilized beam after locking onto another lasercom terminal. (Source: N. Fandel)

Nearly five years ago, NASA and Lincoln Labora­tory made history when the Lunar Laser Communi­cation Demon­stration LLCD used a pulsed laser beam to transmit data from a satellite orbiting the moon to Earth at a record-breaking download speed of 622 megabits per second. Now, researchers at Lincoln Labora­tory are aiming to once again break new ground by applying the laser beam tech­nology used in LLCD to underwater communi­cations.

“Both our undersea effort and LLCD take advantage of very narrow laser beams to deliver the necessary energy to the partner terminal for high-rate communi­cation,” says Stephen Conrad, a staff member in the Control and Auto­nomous Systems Engineering Group, who developed the pointing, acqui­sition, and tracking (PAT) algorithm for LLCD. “In regard to using narrow-beam tech­nology, there is a great deal of similarity between the undersea effort and LLCD.”

However, undersea laser communi­cation presents its own set of challenges. In the ocean, laser beams are hampered by signi­ficant absorption and scattering, which restrict both the distance the beam can travel and the data signaling rate. To address these problems, the labora­tory is developing narrow-beam optical communi­cations that use a beam from one underwater vehicle pointed precisely at the receive terminal of a second underwater vehicle.

This technique contrasts with the more common undersea communi­cation approach that sends the transmit beam over a wide angle but reduces the achievable range and data rate. “By demon­strating that we can success­fully acquire and track narrow optical beams between two mobile vehicles, we have taken an important step toward proving the feasi­bility of the laboratory’s approach to achieving undersea communi­cation that is 10,000 times more efficient than other modern approaches,” says Scott Hamilton, leader of the Optical Communi­cations Tech­nology Group.

Most above-ground auto­nomous systems rely on the use of GPS for positioning and timing data; however, because GPS signals do not pene­trate the surface of water, submerged vehicles must find other ways to obtain these important data. “Underwater vehicles rely on large, costly inertial navi­gation systems, which combine accelero­meter, gyroscope, and compass data, as well as other data streams when available, to calculate position,” says Thomas Howe of the research team. “The position calcu­lation is noise sensitive and can quickly accu­mulate errors of hundreds of meters when a vehicle is submerged for significant periods of time.”

This posi­tional uncer­tainty can make it difficult for an undersea terminal to locate and establish a link with incoming narrow optical beams. For this reason, “We implemented an acqui­sition scanning function that is used to quickly translate the beam over the uncertain region so that the companion terminal is able to detect the beam and actively lock on to keep it centered on the lasercom terminal’s acqui­sition and communi­cations detector,” researcher Nicolas Hardy explains. Using this metho­dology, two vehicles can locate, track, and effec­tively establish a link, despite the inde­pendent movement of each vehicle underwater.

Once the two lasercom terminals have locked onto each other and are communicating, the relative position between the two vehicles can be determined very precisely by using wide bandwidth signaling features in the communi­cations waveform. With this method, the relative bearing and range between vehicles can be known precisely, to within a few centi­meters, explains Howe, who worked on the undersea vehicles’ controls.

To test their underwater optical communi­cations capa­bility, six members of the team recently completed a demon­stration of precision beam pointing and fast acquisition between two moving vehicles in the Boston Sports Club pool in Lexington, Massa­chusetts. Their tests proved that two under­water vehicles could search for and locate each other in the pool within one second. Once linked, the vehicles could poten­tially use their established link to transmit hundreds of giga­bytes of data in one session.

This summer, the team is traveling to regional field sites to demonstrate this new optical communi­cations capability to U.S. Navy stake­holders. One demonstration will involve underwater communications between two vehicles in an ocean environment. The team is planning a second exercise to demon­strate communi­cations from above the surface of the water to an underwater vehicle. The undersea communi­cation effort could tap into inno­vative work conducted by other groups at the labora­tory. For example, inte­grated blue-green opto­electronic tech­nologies, including gallium nitride laser arrays and silicon Geiger-mode avalanche photo­diode array tech­nologies, could lead to lower size, weight, and power terminal imple­mentation and enhanced communi­cation func­tionality.

In addition, the ability to move data at megabit-to gigabit-per-second transfer rates over distances that vary from tens of meters in turbid waters to hundreds of meters in clear ocean waters will enable undersea system appli­cations that the labora­tory is exploring. Howe, who has done a signi­ficant amount of work with under­water vehicles, both before and after coming to the labora­tory, says the team’s work could transform undersea communi­cations and operations. “High-rate, reliable communi­cations could completely change under­water vehicle operations and take a lot of the uncer­tainty and stress out of the current operation methods.” (Source: MIT)

Link: Optical Communications Technology (S. Hamilton), Lincoln Laboratory, Massachusetts Institute of Technology MIT, Cambridge, USA

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