Chemical lasers are lasers that emit radiation originating from a chemical reaction that has been used to excite a material into thermal excitation. The emitted radiation tends to be in the infrared part of the spectrum. This class of lasers was first hypothesized in the 1960s and pursued by U.S. military contractors for many decades due to their potential to produce power in the kilowatt and even megawatt range.

Despite the performance advantages of chemical lasers, the Department of Defense (DoD) stopped all development of chemical laser systems with the termination of the Airborne Laser Testbed in 2012. The DoD’s desire for a renewable power source, as well as not having to supply unusual chemicals like fluorine, deuterium, hydrogen peroxide or iodine, led to a push for electrically-pumped lasers such as diode-pumped alkali lasers (DPALS) instead.

In spite of their recent fall out of favor with the military, chemical lasers are interesting in their ability to produce large power outputs with unique lasing mechanisms. For instance, the chemical oxygen iodine laser (COIL) is an infrared chemical laser with an output at 1.315 micrometers and a power output up to hundreds of kilowatts. The COIL works by reacting hydrogen peroxide with chlorine and potassium hydroxide, producing potassium chloride, heat and oxygen in an excited state. The excited oxygen is unable to transition to its ground state and so it instead rapidly transfers its energy to injected iodine molecules since they are nearly resonant. The excited iodine then undergoes stimulated emission while the laser produces waste products of Image Credit: Wikipediapotassium salt, water and oxygen. Originally developed by the Air Force in the seventies, it was soon replaced by the all gas-phase iodine laser (AGIL).

AGIL uses chlorine atoms with gaseous hydrazoic acid resulting in excited molecules of chloronitrene (NCl) which pass their energy to the iodine atoms much like the oxygen in the COIL. The emission was the same, but the device was lighter and thus better suited for aircraft.

Another interesting chemical laser is the deuterium fluoride (DF) laser. The DF laser has a combustion chamber where ethylene is burned in nitrogen trifluoride, producing excited fluorine radicals. A mixture of helium and deuterium gas is injected into the exhaust stream of the combustion chamber and reacts with the fluorine radical, producing excited molecules of deuterium fluoride. The excited molecules then undergo stimulated emission and produce a coherent emission at around 3.8 micrometers with power output as high as those in the megawatt range. Originally conceived as a hydrogen fluoride (HF) laser, the resulting emission of 2.7 to 2.9 micrometers was impractical since it could be absorbed by the atmosphere. The switch to deuterium (heavy hydrogen) shifted the emission wavelength to a more usable area of the spectrum while maintaining the high power output.

The Mid-infrared Advanced Chemical Laser (MIRACL), the first megawatt class continuous wave laser, was a DF laser. It was first created in 1980 by TRW and proved very reliable with 70 seconds of maximum lasing duration. Originally designed to track and destroy cruise missiles, it later was studied as a possible option for an anti-satellite laser weapon. Although it has demonstrated effectiveness against certain targets at White Sands Missile Range, it was never configured for actual service as a field weapon.

With the DoD's move away from chemical lasers, the future of this industry is uncertain. There remains the potential for even larger continuous output powers with different reaction mechanisms, but without defense funding it is hard to see how these lasers will come to be.

In the past technologies have been discarded only to find renewed interest decades later and this may be the case for chemical lasers. Certainly the potential for megawatt continuous wave lasers is appealing with the promise of perhaps tens of megawatt lasers being possible with the right reaction mechanism (and laser size). Such power will always create a draw to the technology and we may find a renewed interest in these types of lasers in the future when computers are better at calculating and predicting effective reaction mechanisms. For now chemical lasers will have to progress through niche use in industry and hobbyists until the military once again comes calling.