Chemical laser

Chemical laser

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A chemical laser is a laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 that obtains its energy from a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

. Chemical lasers can achieve continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...

 output with power reaching to megawatt levels. They are used in industry for cutting and drilling.

Common examples of chemical lasers are the chemical oxygen iodine laser
Chemical oxygen iodine laser
Chemical oxygen iodine laser, or COIL, is an infrared chemical laser. As the beam is infrared, it cannot be seen with the naked eye. It is capable of output power scaling up to megawatts in continuous mode...

 (COIL), all gas-phase iodine laser
All gas-phase iodine laser
All gas-phase iodine laser is a chemical laser using gaseous iodine as a lasing medium. Like the chemical oxygen iodine laser , it operates at the 1.315 µm wavelength....

 (AGIL), and the hydrogen fluoride laser
Hydrogen fluoride laser
The hydrogen fluoride laser is an infrared chemical laser. It is capable of delivering continuous output power in the megawatt range.Hydrogen fluoride lasers operate at the wavelength of 2.7-2.9 µm. This wavelength is absorbed by the atmosphere, effectively attenuating the beam and reducing its...

 and deuterium fluoride laser, both operating in the mid-infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 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 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
George C. Pimentel
George Claude Pimentel was the inventor of the chemical laser. He also developed the modern technique of matrix isolation in low-temperature chemistry. In theoretical chemistry, he proposed the three-centre four-electron bond which is now accepted as the best simple model of hypervalent...

 jn 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
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 was first demonstrated in 1969, and subsequently patented, by researchers at The Aerospace Corporation in El Segundo
El Segundo, California
El Segundo is a city in Los Angeles County, California, United States. Located on the Santa Monica Bay, it was incorporated on January 18, 1917, and is one of the Beach Cities of Los Angeles County and part of the South Bay Cities Council of Governments...

, California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. . This device used the mixing of adjacent streams of H2 and F, within an optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...

, 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
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 mixing of adjacent supersonic streams, multiple non equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

 chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s and the interaction of the gain medium
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...

 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 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. A. 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.


The early analytical models coupled with chemical rate studies led to the design of efficient experimental CW HF laser devices at The Aerospace Corporation (D. J. Spencer, H. Mirels, D. A. Durran, "Performance of cw HF Chemical Laser with N2 or He Diluent", J. Appl. Phys., Vol. 43, No. 3, March 1972). Power levels up to 10 kW were achieved. DF lasing was obtained by the substitution of D2 for H2.

The TRW Systems Group in Redondo Beach, California
Redondo Beach, California
Redondo Beach is one of the three Beach Cities located in Los Angeles County, California, United States. The population was 66,748 at the 2010 census, up from 63,261 at the 2000 census. The city is located in the South Bay region of the greater Los Angeles area.Redondo Beach was originally part of...

, 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
MIRACL, or Mid-Infrared Advanced Chemical Laser, is a successful directed energy weapon developed by the US Navy. It is a deuterium fluoride laser, a type of chemical laser....

 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 focused beam. This type of COIL laser is used today in the ABL
Boeing YAL-1
The Boeing YAL-1 Airborne Laser Testbed, weapons system is a megawatt-class chemical oxygen iodine laser mounted inside a modified Boeing 747-400F. It is primarily designed as a missile defense system to destroy tactical ballistic missiles , while in boost phase. The aircraft was designated...

 (Airborne Laser, the laser itself being built by Northrop Grumman) and in the ATL
Advanced tactical laser
The Advanced Tactical Laser program is a US military program to mount a high energy laser weapon on an aircraft, initially the AC-130 gunship, for use against ground targets in urban or other areas where minimizing collateral damage is important. The laser will be a 100 kilowatt-class chemical...

 (Advanced Tactical Laser) produced by Boeing. Meanwhile, a lower power HF laser was used for the THEL
Thel is a commune in the Rhône department in eastern France....

(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.