By Geoff Fein
While the Navy is developing communication buoys to enable submarines to pass data while at speed and depth, a Defense Advanced Research Projects Agency (DARPA) effort is demonstrating the feasibility to use lasers to achieve the same objective.
The goal of the Tactical Relay InformaTIOn Network (TRITON) program is to develop and demonstrate the feasibility of two-way laser communications between an aircraft and submarine while operating at speed and depth, Larry Stotts, DARPA program manager, told Defense Daily recently.
“The reason the Navy is looking at TRITON is its potential to facilitate duplex, submerged submarine communications over a broader range of speeds and depths than current towed buoy and/or cable systems do today,” he said. “Most of the Navy requirements involve two-way communications and TRITON has the potential to facilitate duplex communications over a much larger portion of the Navy’s submarines’ communications speed-depth envelope than either towed arrays, towed buoys or periscope-mounted antenna systems.”
TRITON will have its transmitter and receiver integrated into the submarine, he added.
Lockheed Martin [LMT] is developing a system of buoys to enable two-way communications between submarines and ships, aircraft and ashore assets, under Increment 1 of the Communications at Speed and Depth (CSD) program (Defense Daily, July 13).
While the focus of the CSD effort is to develop lines of data communication between tethered and untethered buoys, the Navy does have the ultimate goal of moving forward and away from buoys toward a laser technology, if it can be demonstrated to be effective, according to the Space and Naval Warfare Command.
But using lasers for communications is challenging, Stotts noted.
“To overcome the major channel losses created by particulate absorption in the ocean and particulate scatting in clouds, we need to maximize the amount of power at depth and minimize system noise created by daytime sunlight,” he said.
Laser communications works very well at night where the moonlight is effectively a million times weaker than the sun, Stotts said.
To overcome the major channel losses, TRITON proposes to use lasers that operate in the optimum portion of the blue-green regime, which is the optical window in open ocean water (minimum loss) and to use the resulting scattered laser light from the particulate scattering of clouds and the ocean water to close the optical link, he said.
“The impact of the scattered radiation use is that the data rate of the link is dictated by the time spreading of the cloud-scattered laser pulse; pulse broadening by the water column is small compared to that created by cloud scattering,” Stotts said.
To address the latter challenge, DARPA uses solid state lasers (that are more electrically efficient) and matched narrow spectral/Wide-field-of-view optical receivers, he added.
“The TRITON system is being proposed as modular, so we will not have catastrophic failure modes for either the transmitter or receiver, [for example] graceful degradation,” Stotts said.
DARPA and the Office of Naval Research (ONR), as well as industry, have programs to develop blue-green solid-state lasers for submarine laser communications (SLC), Stotts said.
“These lasers have been shown in previous Navy-DARPA SLC to potentially have the highest wall-plug efficiency–electrical-to-optical power conversion and Size, Weight and Power (SWaP), compared to other laser options,” he explained. “TRITON has chosen one design for its experiments. Industry is pursuing several other candidates and the final selection will be made if TRITON goes on to future development with the Navy after its final field trials.”
The Navy and DARPA have performed many experiments and sea trials over the past 30 years, Stotts added. The last major one was the Tactical Airborne Laser Communications (TALC) program in the early 1990s that demonstrated two-way SLC in a field environment off the coast of California.
“This program had such firsts as the first duplex communications between an aircraft and submarine, the first medium-power, high-efficiency, diode-pumped solid-state laser and the first duplex communications at depth out to a broad slant range from the aircraft,” he added.
Based on the results of the TRITON program, the Navy will hopefully proceed on taking TRITON technologies to the next stage, Engineering and Manufacturing Development, Stotts said.