Popping steam bubbles used for underwater communications
A new method of underwater communication relies on using lasers to make a series of tiny popping noises.
The Naval Research Laboratory says the technique can be used to expand and improve both naval and commercial underwater acoustic applications, including undersea communications, navigation, and acoustic imaging.
The laser ionizes a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound.
The optical properties of water can be manipulated with very intense laser light to act like a lens, allowing the laser beams actually to focus themselves. And because the slightly different colors of the laser travel at different speeds in water, they can be arranged so that the pulse also compresses in time as it travels through water, concentrating the light even more.
Because these two effects are much stronger in water than air, a properly tailored laser can travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water, allowing aircraft to communicate with submarines.
The technique could also be used for underwater acoustic imaging, by using a moveable mirror to create an array of pops whose echoes would give a detailed picture of underwater terrain.