Chemical oxygen iodine laser

Overview
Chemical oxygen iodine laser, or COIL, is an infrared chemical laser. As the beam is infrared, it cannot be seen with the naked eye. It is capable of output power scaling up to megawatts in continuous mode. Its output wavelength is 1.315 µm, the wavelength of transition of atomic iodine.

The laser is fed with gaseous chlorine, molecular iodine, and an aqueous mixture of hydrogen peroxide and potassium hydroxide. The aqueous peroxide solution undergoes chemical reaction with chlorine, producing heat, potassium chloride, and oxygen in excited state, with spontaneous lifetime of about 45 minutes. This singlet delta oxygen transfers its energy to the iodine molecules injected to the gas stream; they are nearly resonant with the singlet oxygen, so the energy transfer during the collision of the particles is rapid. The excited iodine then undergoes stimulated emission and lases at 1.315 µm in the optical resonator region of the laser.

The laser operates at relatively low gas pressures, but the gas flow has to be nearing the speed of sound at the reaction time; even supersonic flow designs are described. The low pressure and fast flow make removal of heat from the lasing medium easy, in comparison with high-power solid-state lasers. The reaction products are potassium salt, water, and oxygen. Traces of chlorine and iodine are removed from the exhaust gases by a halogen scrubber.

COIL laser was developed by the US Air Force in 1977, for military purposes. However, its properties make it useful for industrial processing as well; the beam is focusable and can be transferred by an optical fiber, as its wavelength is not absorbed much by fused silica but is very well absorbed by metals, making it suitable for laser cutting and drilling. Rapid cutting of stainless steel and hastelloy with a fiber-coupled COIL was demonstrated. In 1996, TRW Incorporated managed to get a continuous beam of hundreds of kilowatts of power that lasted for several seconds.

RADICL, Research Assessment, Device Improvement Chemical Laser, is a 20 kW COIL laser tested by Air Force in around 1998.

COIL is the main weapon laser for the military airborne laser and advanced tactical laser programs.

All gas-phase iodine laser (AGIL) is a similar construction using all-gas reagents, more suitable for aerospace applications.