What Are Chemical Oxygen Iodine Lasers?

Chemical oxygen iodine lasers (COIL) are high-power, continuous-wave chemical lasers that emit at a wavelength of 1.315 micrometers by exploiting energy transfer from electronically excited oxygen to atomic iodine. First demonstrated in 1977 at the Air Force Weapons Laboratory, COIL was the first chemical laser to achieve power levels in the kilowatt and ultimately megawatt range while operating at a wavelength short enough to be focused to small spot sizes over long atmospheric paths. The combination of scalable power, good beam quality, and near-infrared wavelength made COIL the leading candidate for airborne directed-energy weapon systems throughout the 1990s and 2000s.

The designation distinguishes COIL from other chemical laser families such as hydrogen fluoride and deuterium fluoride systems, which emit in the mid-infrared band where atmospheric absorption is substantially higher. COIL's 1.315 micrometer output falls in a window of low atmospheric absorption and can be transmitted through fused-silica optical fiber, properties that expand both its range and its industrial utility relative to longer-wavelength chemical lasers.

Operating Principle and Chemical Reactions

COIL operates through a two-stage chemical energy conversion chain. In the first stage, an aqueous mixture of hydrogen peroxide and potassium hydroxide reacts with chlorine gas to produce singlet-delta oxygen, written O2(1Δ), along with potassium chloride and water vapor. Singlet-delta oxygen is a metastable excited state of molecular oxygen that carries approximately 0.98 eV of excitation energy and has a radiative lifetime long enough to survive transport into the laser gain region. In the second stage, gaseous molecular iodine is injected into the flow of singlet-delta oxygen; rapid dissociation produces atomic iodine, and energy transfer from the singlet oxygen populates the upper laser level of atomic iodine. Stimulated emission at 1.315 micrometers then occurs as the iodine atoms return to their ground state. The reaction chemistry is described in detail in the SPIE conference proceedings on COIL technology and development.

Power Scaling and Beam Quality

COIL power scales with the volumetric flow rate of singlet-delta oxygen through the gain region, and megawatt-class devices achieve this by operating the singlet oxygen generator at very high chemical throughput and flowing the gain medium at supersonic velocities to suppress optical absorption losses. Near-diffraction-limited beam quality is maintained by careful management of flow uniformity and thermal gradients in the gain region. The 1.315 micrometer wavelength is efficiently transmitted by fused-silica optical fiber, enabling beam delivery through flexible optical paths that are impractical for mid-infrared chemical lasers. This property also makes COIL suitable for industrial laser cutting and drilling of metals, since the wavelength is strongly absorbed by common engineering alloys. Information on beam propagation and atmospheric transmission relevant to COIL is available through publications of the AFRL Directed Energy Directorate.

The Airborne Laser Program

The most ambitious deployment of COIL technology was the Airborne Laser Testbed (ALTB), also designated YAL-1, a megawatt-class COIL system installed in the nose of a modified Boeing 747-400F. Developed under a joint effort by Boeing, Northrop Grumman, and Lockheed Martin, the ALTB was intended to engage tactical ballistic missiles during their boost phase at ranges of several hundred kilometers. Ground testing demonstrated successful ignition of the laser at full power in 2004, and the system destroyed a liquid-fueled target missile in a flight test in 2010. The program was subsequently canceled by the Department of Defense in 2011 due to cost and operational constraints, but the ALTB remains the most powerful aircraft-mounted directed-energy system ever flight-tested.

Applications

Chemical oxygen iodine lasers have applications in a range of demanding fields, including:

• Missile defense and theater directed-energy weapons requiring megawatt continuous-wave power
• Industrial laser processing, including metal cutting and precision drilling via fiber delivery
• Atmospheric propagation studies and adaptive optics research for long-range beam delivery
• Scientific research on chemical kinetics and electronically excited molecular oxygen states