Chemical Laser - Origin of The CW Chemical HF/DF Laser

Origin of The CW Chemical HF/DF Laser

The possibility of the creation of infrared lasers based on the vibrationally excited products of a chemical reaction was first proposed by J. C. Polanyi in 1961. A pulsed chemical laser was demonstrated by Jerome V. V. Kasper and George C. Pimentel in 1965. First, hydrogen chloride was pumped optically so vigorously that the molecule disassociated and then re-combined, leaving it in an excited state suitable for a laser. Then hydrogen fluoride and deuterium fluoride were demonstrated. Pimentel went on to explore a DF - CO₂ transfer laser. Although this work did not produce a purely chemical continuous wave laser, it paved the way by showing the viability of the chemical reaction as a pumping mechanism for a chemical laser. Pimentel was awarded a patent for a scalable overtone HF laser (United States Patent 4,760,582) in 1971.

The continuous wave (CW) chemical HF laser was first demonstrated in 1969, and subsequently patented, by researchers at The Aerospace Corporation in El Segundo, California. . This device used the mixing of adjacent streams of H₂ and F, within an optical cavity, to create vibrationally excited HF which lased. The atomic fluorine was provided by dissociation of SF₆ gas using a DC electrical discharge. Later work at US Army, US Air Force, and US Navy contractor organizations (e.g. TRW) used a chemical reaction to provide the atomic fluorine, a concept included in the patent disclosure of Ref. 3. The latter configuration obviated the need for electrical power and led to the development of high power lasers for military applications.

The analysis of the HF laser performance is complicated due to the need to simultaneously consider the fluid dynamic mixing of adjacent supersonic streams, multiple non equilibrium chemical reactions and the interaction of the gain medium with the optical cavity. The researchers at The Aerospace Corporation developed the first exact analytic (flame sheet) solution, the first numerical computer code solution and the first simplified model describing CW HF chemical laser performance.

Chemical lasers stimulated the use of wave-optics calculations for resonator analysis. This work was pioneered by E. A. Sziklas (Pratt & Whitney Aircraft) and A. E. Siegman (Stanford University.) An example of an early paper on this subject is E. A. Sziklas and A. E. Siegman, "Mode calculations in unstable resonator with flowing saturable gain. II. Fast Fourier transform method," Appl. Opt., vol. 14, pp. 1873–1889, August 1975. Part I of this was a companion paper that dealt with Hermite-Gaussian Expansion and has received little use compared with the Fourier Transform method which has now become a standard tool at United Technologies Corporation (SOQ), Lockheed Martin (LMWOC), SAIC (ACS), Boeing (OSSIM), tOSC, MZA (Wave Train), and OPCI. Most of these companies competed for contracts to build HF and DF lasers for DARPA, the U.S. Air Force, the U.S. Army, or the U.S. Navy throughout the 1970s and 1980s. General Electric and Pratt & Whitney dropped out of the competition in the early 1980s leaving the field to Rocketdyne (now ironically part of Pratt & Whitney - although the laser organization remains today with Boeing) and TRW (now part of Northrop Grumman.)

Comprehensive chemical laser models were developed at SAIC by R. C. Wade, at TRW, by D. Bullock, and at Rocketdyne by D. A. Holmes,. Of these, perhaps the most sophisticated was the CROQ code at TRW, outpacing the early work at Aerospace Corporation.