Chemical Lasers

In these lasers the energy of excitation comes from a chemical reaction that takes place in the medium itself. In this sense, then, chemical lasers are self-pumped. Even more interesting is the operation of one of the most common of these lasers, the hydrogen-fluoride laser that operates in the IR at 4.6 microns. The added intrigue is due to the chain reaction that takes place to excite the laser molecule, HF . A mixture of hydrogen (molecular) gas, H2, and fluorine gas, F2, is subjected to an electric discharge to start the chain reaction, which results in the production of a hydrogen-fluoride molecule in an excited vibrational level (excited state is denoted by a starred superscript: HF∗) and the dissociative production of H or F for the next reaction:

etc.

So, the medium in these lasers is used up as a fuel to generate their laser light. Therefore, practical limitations aside, the power of these lasers depends on the amount of the chemical (gas volume) that is used in the laser. Because of their conceivably limitless power output, these lasers have been studied mostly for their military applications. The most famous of these is the US Army's Mid-Infrared Advanced Chemical Laser, MIRACL, located at White Sands Missile Range, New Mexico. This is a formidable chemical laser that has been developed as part of the Department of Defense's efforts for its Strategic Defense Initiative (SDI).

MIRACL is the US's most powerful laser. It operates in a band from 3.6 to 4.2 microns producing megawatts of CW output for as long as 70 seconds. For its fuel it burns ethylene, C2H4

, with nitrogen trifluoride, NF3. The resulting free fluorine atoms combine with deuterium gas that is injected to this burning fuel to form deuterium fluoride molecules (DF), which ultimately provide the laser light. In this laser the beam within the optical cavity is about 21 cm high and 3 cm wide, resulting in a 14 cm2 output beam. For testing purposes this laser is used with an aiming and focusing telescope system called the SEALITE Beam Director (SLBD) which was first developed by the Navy. The moving part of SLBD that is capable of fast rotations and high accelerations weighs about 18,000 pounds. Its telescope can focus the beam to any target that is located within a range of a minimum of 400 meters up to infinity.