free electron laser

Illustration of a free electron laser. A beam of electrons is sent through an undulator -- an array of magnets with alternating north and south poles. The magnetic field in the undulator forces each bunch of electrons to oscillate back and forth, causing them to emit a laserlike beam of light.

Image courtesy Flavio Robles/Creative Services Office, Lawrence Berkeley National Lab

Military Lasers

There are many different types of lasers:

  • Solid state lasers have a lasing medium that is solid crystal, like the ruby laser or the neodinium YAG laser, which emits 1.06 micrometer wavelength.
  • Gas lasers have a lasing medium that is a gas or combination of gases, such as helium-neon laser or carbon dioxide laser, which emits 10.6 micrometer wavelengths (infrared).
  • Excimer lasers have a lasing medium that is a combination of reactive gases, like chlorine or fluorine, and inert gases, like argon or krypton. The argon fluoride laser emits ultraviolet light of 193 nanometer wavelengths.
  • Dye lasers have a lasing medium that is a fluorescent dye, such as rhodamine. They can be tuned to a variety of wavelengths within a certain range. The rhodamine 6G dye laser can be tuned from 570- to 650-nanometer wavelengths.
  • Carbon dioxide lasers are being explored by the military because they're powerful infrared lasers that can be used for cutting metal.

There are several lasers currently being used for military purposes. One that's being researched and developed is the free electron laser (FEL). In the 1970s, Stanford physicist John Madey invented and patented the FEL, which consists of an electron injector, a particle accelerator and a magnetic undulator or wiggler. It works like this:

  1. The electron injector injects a pulse of free electrons into the particle accelerator.
  2. The particle accelerator accelerates the electrons to near the speed of light (300,000 km/s)
  3. The electrons move through the undulator or wiggler, which is a series of magnets with alternating north-south directions.
  4. Inside the wiggler, the electrons oscillate back and forth. With each bend, they emit light of a specific wavelength.
  5. The spacing of the magnets within the wiggler controls the wavelength of emitted light. So, the FEL laser can be tuned by changing the magnet spacing.
  6. In theory, the FEL can be tuned from the infrared region to the X-ray region of the electromagnetic spectrum.

FELs have been used to produce high-energy infrared light and synchrotron X-rays for research purposes. The FEL was also a laser of interest for the Defense Department's Strategic Defense Initiative (President Reagan's "Star Wars" program). Recently, the U.S. Naval Postgraduate School acquired Madey's original FEL developed at Stanford University, to use for military research.

In 1977, the U.S. Air Force developed a chemical oxygen-iodine laser (COIL). The energy source for the COIL is a chemical reaction, and the lasing medium is molecular iodine. Here's how it works: atoms, heat and byproducts, including water vapor and potassium chloride.

  1. A chemical reaction occurs between chlorine gas and liquid mixture of hydrogen peroxide and potassium hydroxide.
  2. The chemical reaction produces single oxygen
  3. Molecular iodine gets injected into the laser. The singlet oxygen provides the energy to get the iodine atoms to lase and emit infrared light at a wavelength of 1.3 micrometers.
  4. The laser can emit light continuously or the light can be pulsed, which increases the efficiency of the laser.

The COIL laser is used aboard the Air Force's Airborne Laser, which we'll talk about next.