Navy adapts laser cannons to chat with stealth subs [SciencePunk]

Talking to submarines is a very a tricky business - most communication systems are based on radio or acoustic signals, but neither travel very far in water. This means that to pick up radio signals, submarines must surface or raise communication buoys very close to the surface, neither of which are appropriate for nuclear-powered stealth submarines that remain deep underwater for months at a time. Radio operators can overcome the problem of poor signal penetration by using giant arrays broadcasting in extremely-low frequency ranges, but these are expensive, difficult to build, and massively energy-intensive. The Russian ZEVS system operates at 82Hz, corresponding to a wavelength of 3658.5km - more than half the radius of Earth! Normally signals are transmitted using an aerial corresponding to half the wavelength of the signal, but in the case of ELF systems, such an aerial would stretch from New York to Omaha. Instead, a site with extremely low ground conductivity is found and two electrodes sunk into the ground, some 60km apart, in effect using Earth as part of the aerial. Then a dedicated power station must be built to supply the array. The result - signal that can broadcast just a few characters per minute at several watts, heard the world over. The system is so expensive and impractical that only two countries have ever built ELF arrays - the Soviet Union and the United States of America. However, physicists at the US Naval Research Laboratory in Washington are proposing a relatively cheap and portable alternative: laser cannons. A team led by Dr Ted Jones used flashes of intense laser light to superheat small packets of water. The resulting steam explosion generated a 210 decibel acoustic pulse - louder than a jet taking off at 25ft. The researchers hope to develop the laser cannon to encode information in the acoustic signal. Because the laser can travel hundreds of metres through clear air, the technology could be mounted on aircraft to talk to submarines below the surface. By studying the reflected sounds of the steam explosion, the technology could also be used by ships and submarines to gather information on underwater environments, from detecting mines to mapping the seabed. Jones will present the research later this month at the 157th meeting of the Acoustical Society of America in Portland, Oregon. All of which leaves only one problem with both the ELF and laser communication systems: although the Navy can chat to their submarines, the submarines can't talk back. Any bright ideas, SciencePunk readers? Read the comments on this post...