THE US navy approved designs last week for a shipboard laser that it hopes will focus a 100-kilowatt beam through the ocean mist.

The system will be based on the free-electron laser, invented in the 1970s by John Madey, which generates light from a powerful beam of electrons.

A big attraction of the free-electron laser is the ability to adjust its output wavelength to improve transmission through the thick, moist air at sea, says Mike Rinn of Boeing, which produced the initial designs. Other laser weapons emit at fixed wavelengths. Also, the laser is electrically powered, so it can …