A low-power, low-cost optical transmission method could accelerate underwater communications.
Optical underwater communication systems are severely limited by the fact that water is essentially opaque to electromagnetic radiation, except in the visible band. Even then, light penetrates only a few hundred metres in the clearest waters and less in sediment-laden or highly populated waters.
Acoustic communications systems have been developed to overcome this issue and are now the predominant means of underwater communications between ships and smaller, autonomous and robotic vehicles.
However, such acoustic systems – while capable of long-range communication – transmit data at limited speeds owing to the relatively slow speed of sound in water. This makes the control of an undersea vehicle troublesome because of the delayed response of the vehicle to any control signal acoustically sent to it by a human operator.
Now, Woods Hole Oceanographic Institution (WHOI) scientists and engineers in Woods Hole, Massachusetts, led by senior engineer Norman Farr, have developed an optical communication method that complements existing acoustic systems to enable data to be transmitted at rates of up to 10-20Mbit/sec over a range of 200m using small, low-cost transmitters and receivers that consume relatively low battery power.
This isn’t the first time that such optical communication systems have been built for the task. The US Navy has, in the past, deployed systems using lasers to communicate under water. But Farr said that his team rejected the laser technology in favour of a lower-cost system that would not require the laser’s complicated aiming and tracking system, nor consume such a large amount of power.
Conceptually, the WHOI system comprises a pair of transmitters and receivers that can bi-directionally communicate with one another through the water. Modulated light from an array of from three to 20 high-power light-emitting diodes (LEDs) is spread in a hemispherical fashion by a diffuser and transmitted through the water to a photomultiplier tube at the receiver, where the light is amplified and the signals demodulated.
Each optical transmission system built around the new Woods Hole technology is custom built to meet the needs of a specific application that the Woods Hole engineering team might be called on to tailor it to.
The number of LEDs used, for example, is dependent on the demands of the undersea vehicle the system is designed for, which is contingent on the distance that the power needs to be transmitted. ’To transmit to a reasonable range, you need to put a fair amount of optical power in the water,’ said Farr. ’A system that can transmit power over 150m uses between 5W and 10W.’
The new system can also be configured to transmit the data from the LEDs using either time-division multiplexed (TDM) or wavelength-division multiplexing (WDM) signalling methods.
According to Farr, the TDM technique is more suited to applications where one set of control signals might be sent to an undersea vehicle while video data is simultaneously transmitted back from the seafloor. WDM, on the other hand, where multiple signals on the LED beam are combined at various wavelengths, is a technique more suited to transmitting high data rates bi-directionally.
Whatever choice of communication technique is required from the optical system, if both an optical system and a conventional acoustic system are deployed at a short range, the optical system can be used for high data rate transmission, while at longer ranges – when optical communication is impossible – the system can still rely on the acoustic transmission system.
This July, the WHOI engineers deployed one version of the system at the Juan de Fuca plate off the north-western coast of the US. The team employed a human-occupied vehicle (HOV) called Alvin to install a WDM optical system on a subsea data concentrator that collects and transmits geophysical data from wellheads situated at the undersea ridge.
The undersea wellheads are instrumented to detect seismic waves that propagate through the Earth as a result of earthquakes – each of them has a data-acquisition system on top of them that is acquiring and storing data. To date, however, the only way to retrieve that data has been to use a submersible such as Alvin to go down, plug into the system, offload the data and then return to the surface.
The new system will obviate the need for the Alvin submersible to perform that function and free up its time for more scientific missions. The optical transmitters deployed on the wellheads will transmit the data back to a surface ship through a standard University-National Oceanographic Laboratory System (UNOLS) optical communications unit lowered down to within a couple of hundred metres of the seafloor stations, where it acoustically wakes up the seafloor devices, after which they start to transmit data to it.
Aside from its use at the Juan de Fuca Ridge, Farr said that the broader benefit of the new optical transmission system will be to allow undersea vehicles to work in an untethered fashion, enabling them to be directly manoeuvred by an operator.
’The untethering of battery-operated vehicles is really our holy grail, and the new communications system makes that possible, allowing us to transmit high-bandwidth data optically,’ he said.
As it enables communications without the heavy tether-handling equipment required for a remotely operated vehicle (ROV), a combined optical/acoustic system could allow smaller, less expensive ships and fewer personnel to perform complex undersea missions.
Indeed, in one such an application, autonomous underwater vehicles (AUVs) could be used to collect sonar images and other data at hydrothermal vent sites and then transmit the data through an optical modem to receivers stationed on moorings in the ocean. The moorings could be connected to a cabled observatory and the data could then be sent back to scientists on shore. Scientists, in turn, could send new instructions to the AUVs via the optical modem.
Farr claims that this would enable scientists and researchers to more easily investigate numerous properties of the ocean, such as the acidity of water, species of marine life and erupting vents and seafloor slides.