Simply put, a laser beam is one form of directed energy — similar in concept to sunlight focused through a magnifying glass, a directed burst of microwave energy, or a bullet fired from a gun.
The word “LASER” is an acronym for Light Amplification by Stimulated Emission of Radiation — scientific words used to describe the process that produces the unique form of energy.
The foundation for the process is the production of flashes of light, called photons, that are emitted when an excited atom releases the excess energy it has acquired and returns to its preferred ground-state energy level. A laser “beam” results when photons are produced nearly simultaneously by a large collection of the same type of atoms.
In a beam, the photons move in step (in phase) with each other and travel in the same direction (coherence). To concentrate the beam, these emitted photons are collected, optically manipulated and amplified. The beam also propagates at a specific wavelength, which depends on the type of atom used.
In the case of Airborne Laser (ABL), its four lasers were purposely chosen to best match the job each would be required to do. On the ABL, only the High Energy Laser (HEL) produces sufficient power to destroy a target.
Active Ranger System (ARS) — On a mission, it is the first to fire. A carbon dioxide (CO2) continuous laser that propagates at a wavelength of 11.15 microns, it sends a beam to the proposed target using the returned signal to produce continuous, high-resolution tracking data. Associated software prioritizes the sequence of targets to be destroyed.
Track Illuminator Laser (TILL)– A solid state laser called a Yb:YAG because its lasing medium is a ytterbium: yttrium aluminum garnet. It produces a pulsed laser beam that propagates at a wavelength of 1.03 microns. The TILL’s job is to locate and stabilize the nose in the field of view of highly specialized cameras that help track the missile.
Beacon Illuminator Laser (BILL) — A pulsed Nd:YAG laser that uses a neodym-ium:yttrium aluminum garnet as its lasing medium. The beam from the BILL bounces off the target and returns to the aircraft, where it is processed by sophisticated optical/software equipment to measure and compensate for the amount of distortion in the atmosphere between the aircraft and the target.
Both the BILL and the TILL are kilowatt-class lasers, producing energy on the order of 1,000 watts. In comparison, a common laser pointer produces energy of .001-.005 watts.
Chemical Oxygen Iodine Laser (COIL) — ABL’s real power producer is the megawatt (1,000,000 watt)-class COIL laser section composed of six identical modules. Each module weighs roughly 4,500 pounds and is the size of an SUV turned on its end. It is a continuous wave laser that propagates at a wavelength of 1.315 microns, a wavelength chosen specifically because its beam travels well through the atmosphere. That is, little of the beam’s energy is absorbed enroute to the target. Iodine is its lasing medium
The COIL works this way: Chlorine gas is injected into a extremely fine spray of Mixed Base Hydrogen Peroxide (MHP), a mixture of Basic Hydrogen Peroxide (BHP) and three salts — sodium, potassium and lithium hydroxides. When the chlorine and MHP interact excited oxygen atoms are produced. The excited oxygen atoms are then mixed with iodine atoms. The extra energy is transferred from the oxygen to the iodine atoms. When the excited iodine atoms return to their ground state, photons are given off which produce the laser beam.
One module (LM-1) of the six that will be installed in the ABL has been tested, producing 118 per cent of anticipated power. After it was tested, LM-1 was shipped to Edwards AFB, Calif., and five clones were constructed. After the six-module laser section has been tested it will be integrated into the ABL aircraft.
Airborne Laser System Program Office, Office of Public Affairs,
3300 Target Road, Building 760, Kirtland AFB, NM