http://www.strategypage.com/htmw/htsub/ ... 90910.aspx
Underwater Lasers Change Everything
September 10, 2009: Finally, there are lasers that can be used communicating underwater. This is done by using a laser pulse tuned to ionize water, and generate an acoustic pulse. Thus surface ships or aircraft could communicate with suitably equipped subs. This stuff is still in the lab, but given the need for underwater communications, there's lots of incentive to get it into service. If this survives development and testing, it will revolutionize submarine operations.
For years, researchers have been trying to find ways to use lasers to detect submarines, or to enable underwater communications. So far, it's been found that blue-green lasers can reach some ten meters beneath the surface, and be used for detection and communication. Not terribly useful for subs (which typically stay farther down than ten meters), although work continues on using this capability to search for bottom mines in shallow waters.
Two years ago, the U.S. Navy completed development of this system, which enabled nuclear subs to communicate with the rest of the world that, normally, could not be done until the boat came close to the surface and poked a radio antenna above the surface. The Deep Siren, or "tactical paging system", provided a practical solution to the problem of communicating with a submerged sub. The system consists of a disposable buoy, that is dropped in the water, by an aircraft or over the side of a ship, in the general area (within about 90 kilometers) where the sub is believed to be. The buoy sends out an acoustic signal that U.S. subs are equipped to automatically pick up. This coded message either orders the sub to get a radio antenna above water and call home, or simply delivers a brief message. The buoy also has a satellite telephone capability, so that additional messages can be sent from anywhere, to the sub. The sub cannot send messages to the buoy (because powerful sensors are required to pick up the signals). In the past, the only way to "page" submerged subs was via a large, shore based, low frequency, transmission system. This system was less reliable than the new one, although it had a much longer range.
The navy recently successfully tested the other end of the system. To do this, the sub releases a similar buoy through its garbage chute. The buoy hovers for a while (so the sub can move away), then rises to the surface and sends its messages. Thus the buoy signal will not give away the exact location of the boat. The buoy then receives messages (short ones) and uses a sonar type device to send the data acoustically, for up to 90 kilometers, to the sub. Outgoing messages, which are sent via satellite, can be longer, and even include outgoing email from the crew to family. But the acoustically transmitted messages are much shorter, and include orders from the surface ships, or anyone in the chain of command, to the sub commander.
Deep Siren can also be useful for American carrier task forces, which are usually accompanied by at least one SSN (nuclear attack sub.) Because thermal layers make underwater transmissions vary a great deal in range, the buoy sends the command messages several times to insure at least one gets through. The buoy from the sub can stay active for several days, if the sub is remaining in the area. But eventually, the buoy sinks itself.
The U.S. Navy has spent about $10 million on Deep Siren so far, mainly to install it in some subs and test it. These tests continue, to see how reliable it would be under realistic conditions. Raytheon apparently believes the Deep Siren isn't ready for prime time yet, but for security reasons, isn't discussing what the problems are.