Chemical laser

A chemical laser is a laser that obtains its energy from a chemical reaction. Chemical lasers can achieve continuous wave output with power reaching to megawatt levels. They are used in industry for cutting and drilling, and in military as directed-energy weapons.

Common examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the hydrogen fluoride laser and deuterium fluoride laser, both operating in the mid-infrared region. There is also a DF-CO2 laser (deuterium fluoride-carbon dioxide), which, like COIL, is a "transfer laser." The hydrogen fluoride (HF) and deuterium fluoride (DF) lasers are unusual in that there are several molecular energy transitions with sufficient energy to be above the threshold required for lasing. Since the molecules do not collide frequently enough to re-distribute the energy, several of these laser modes will operate either simultaneously, or in extremely rapid succession so that an HF or DF laser appears to be operating simultaneously on several wavelengths unless a wavelength selection device is incorporated into the resonator.

Origin of the CW chemical HF/DF laser
The motivation for a chemical laser was born out of the carbon dioxide laser program in the late 1960s and early 1970s. DF had been used as a chemical reaction to excite the carbon dioxide molecule through a near resonant match between one of the DF levels and one of the CO2 levels. This scheme was used by Navy researchers and their contractors, such as Pratt & Whitney Aircraft (a division of United Technologies Corporation), General Electric, Rocketdyne (now part of Pratt & Whitney), and TRW (now part of Northrop Grumman). This type of laser was called a "transfer laser" by the Navy. Eventually carbon dioxide was eliminated as an intermediary and DF was tried as a stand alone lasing medium. Very quickly, deuterium was dropped in favor of hydrogen, since it is far less costly and more readily available. However, later it was realized that HF produces infrared radiation in the 2.6 to 3.1 μm waveband, a region of the spectrum absorbed by water vapor in the atmosphere. Interest was renewed in DF, which produces radiation in the 3.7 to 4.2 μm band, which passes easily through the atmosphere.

As with many successful inventions, there are many fathers: Don Spencer, Jack Hinchen, George Pimentel, and Bob Freiberg, for example have been credited as being the inventor of the chemical laser, though these researchers worked independently. Clearly it depends on how you define chemical laser, and there were many steps along the way to a purely chemically pumped chemical laser.

A pulsed chemical laser was demonstrated by Jerome V. V. Kasper and George C. Pimentel by 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, and subsequently patented, by researchers at The Aerospace Corporation in El Segundo, California. This work was done in parallel with similar work at United Aircraft Research Laboratories (now United Technologies Research Center) by J.J. Hinchen. Similar work was started up very quickly by James A. Harrington at the University of Alabama in Huntsville. These devices used the mixing of adjacent streams of H2 and F2, within an optical cavity, to create vibrationally excited HF which lased. However, since the fluorine was provided by dissociation of SF6 gas with a DC electrical discharge, this also fell short of being a purely chemical laser. Later work at the US Army, US Air Force, and US Navy used only chemical reactions to drive a true chemical laser, meaning that only chemical energy from the exothermic reaction was used in producing the laser beam. This is very important for scaling up to high energy lasers for military or industrial use.

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 (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 1980's 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.)

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

Performance
Studies led to the design of efficient high-power experimental CW HF laser devices. Power levels up to 10 kW were achieved by The Aerospace Corporation researchers. DF lasing was obtained by the substitution of D2 for H2.

The TRW Systems Group in Redondo Beach, California, subsequently received US Air Force contracts to build higher power CW HF/DF lasers. Using a scaled-up version of an Aerospace Corporation design, TRW achieved 100 kW power levels. General Electric, Pratt & Whitney, & Rocketdyne built various chemical lasers on company funds in anticipation of receiving DoD contracts to build even larger lasers. Only Rocketdyne received contracts of sufficient dollar amounts to continue competing with TRW. TRW produced the MIRACL device for the U.S. Navy that achieved megawatt power levels. The latter is believed to be the highest power continuous laser, of any type, developed to date (2007).

TRW also produced a cylindrical chemical laser (the Alpha laser) for DARPA, which had the advantage, at least on paper, of being scalable to even larger powers. However, by 1990, the interest in chemical lasers had shifted toward shorter wavelengths, and the chemical oxygen-iodine laser (COIL) gained the most interest, producing radiation at 1.315 μm. There is a further advantage that the COIL laser generally produces single wavelength radiation, which is very helpful for forming a very well focussed beam. This type of COIL laser is used today in the ABL (Airborne Laser, the laser itself being built by Northrop Grumman) and in the ATL (Advanced Tactical Laser) produced by Boeing. Meanwhile, a lower power HF laser was used for the THEL (Tactical High Energy Laser) built in the late 1990s for the Israeli Ministry of Defense in cooperation with the U.S. Army SMDC. It holds the distinction of being the only fielded high energy laser to demonstrate effectiveness in fairly realistic tests against rockets and artillery. The MIRACL laser has demonstrated effectiveness against certain targets flown in front of it at White Sands Missile Range, but it is not configured for actual service as a fielded weapon. This may soon change with ABL and ATL, if current plans and funding hold out.