What Are Chemical Lasers?

Chemical lasers are laser systems in which the population inversion required for stimulated emission is produced by an exothermic chemical reaction rather than by an external electrical discharge or optical pump. Because the energy stored in chemical bonds is released in the reaction, chemical lasers can generate very high continuous-wave output power from relatively compact systems, making them attractive for high-energy directed-energy applications where electrical power infrastructure would be prohibitive. Their development accelerated from the 1960s onward, driven largely by military interest in high-power coherent optical sources for long-range targeting and missile defense.

The operating principle distinguishes chemical lasers from gas lasers more broadly. Both classes use a gaseous gain medium, but conventional gas lasers, such as the CO2 and He-Ne systems, rely on electrical excitation to create population inversions. Chemical lasers instead ignite or sustain a chemical reaction within the resonant cavity, producing reaction products in electronically or vibrationally excited states. The excited-state population then undergoes stimulated emission, amplifying the optical field circulating between the cavity mirrors.

Lasing Mechanisms and Gain Media

The gain medium in a chemical laser is defined by the exothermic reaction that populates the upper laser level. In hydrogen fluoride (HF) and deuterium fluoride (DF) lasers, fluorine atoms react with hydrogen or deuterium molecules to produce HF or DF molecules in vibrationally excited states. The HF laser produces output across a comb of vibrational-rotational transitions in the 2.6 to 3.0 micrometer band, while the DF laser emits in the 3.6 to 4.2 micrometer band with better atmospheric transmission. The chemical oxygen iodine laser (COIL) uses a different mechanism: singlet-delta oxygen, produced by the reaction of chlorine with basic hydrogen peroxide, transfers its excitation energy to molecular iodine, populating the excited state that lases at 1.315 micrometers. This near-infrared wavelength propagates through the atmosphere with low loss and is well focused by optical systems, making COIL particularly suitable for long-range applications. Detailed treatment of these mechanisms appears in research archived through SPIE Digital Library.

Power Scaling and Beam Quality

Chemical lasers are among the highest-power continuous-wave lasers demonstrated. The Airborne Laser (ABL) program, built around a megawatt-class COIL system mounted in a modified Boeing 747, demonstrated that chemical laser power could be scaled to levels capable of engaging boost-phase ballistic missiles at ranges of hundreds of kilometers. Power scaling is achieved by increasing the flow rate of reactive chemicals through the gain region, which simultaneously maintains the chemical population inversion and removes waste heat by convective flow. Beam quality, quantified by the M² parameter, determines how tightly the output can be focused at long range; chemical flow lasers achieve near-diffraction-limited beams when the gas flow is uniform and the cavity alignment is maintained precisely. The US Air Force Research Laboratory led development of the COIL technology from its invention in 1977 through the ABL program and subsequent directed-energy research.

Applications

Chemical lasers have applications in a range of specialized fields, including:

• Missile defense and directed-energy weapons programs requiring megawatt-class continuous-wave power
• Long-range precision engagement and counter-unmanned aerial system applications
• Materials processing, including laser cutting and drilling of metals using COIL beams transmitted via optical fiber
• Atmospheric propagation research, where high-power beams probe turbulence and absorption effects
• Scientific studies of reaction dynamics and vibrationally excited molecular states relevant to gas-phase kinetics