Institute of Physics                                    Division of High Power Systems

COIL - Laser for future advanced technologies

Division of High Power Systems of the Institute of Physics AS CR in Prague is focusing on research related to powerful iodine lasers - a pulsed photodissociation laser system PALS (Prague Asterix Laser System), and a continuous-wave chemical laser system COIL (Chemical Oxygen-Iodine Laser). Both these lasers radiate on a laser transition of iodine atom in the near infrared region at a wavelength of 1.315 mm, however the pumping mechanism of the upper laser level is quite different. While the PALS is pumped by means of UV light of flash lamps, the COIL is pumped by energy of excited molecular oxygen originating primary from a chemical reaction.

Chemical lasers

Chemical lasers are a special category of gas lasers, in which a primary chemical energy is transferred very efficiently into energy of coherent laser radiation. For example, energy released in a chain spontaneous or initiation chemical reaction can initiate at suitable conditions the inversion population in atoms or molecules on vibrational or vibration-rotational transitions accompanied by laser radiation. This principle is typical for instance for halogen HF/DF lasers with a wavelength in the range of 2.7 - 3.4 mm. Chemical lasers with advantageous shorter wavelengths operate on energetically more rich electronic transition between electronic and ground states of atom or molecule. This excited state is usually metastable with a long radiative lifetime, and due to this fact, a stimulated emission in this species may run only very slowly. A great energy reservoir of such species can be however utilized for energy pumping of other suitable molecule or atom in a collisional transferring process, which is typical for energy transfer electronic lasers like a COIL.

Principle and mechanism of COIL operation

Chemical Oxygen-Iodine Laser (abbreviated as a COIL) operates on the principle of electronic energy transition from molecular metastable oxygen in electronic excited singlet delta state, O₂(¹D_g), to iodine atom by a fast near resonant process. Singlet oxygen as a primary energy source for laser pumping is generated by chemical way. Mechanism of COIL operation can be simply described by the following three processes: a) process of singlet oxygen generation by chemical reaction of gaseous chlorine with liquid basic hydrogen peroxide

Cl₂ + H₂O₂ + 2KOH ® O₂(¹D_g) + 2H₂O + 2KCl

b) process of laser pumping of atomic iodine

O₂(¹D_g) + I(²P_3/2) Û I(²P_1/2) + O₂(³D_g)

c) process of stimulated emission of atomic iodine accompanied by laser radiation

I(²P_1/2) Þ I(²P_3/2) + hn (1.315 mm).

Singlet oxygen has exception properties, and one of the most important for a COIL is its extremely long radiative (collisionless) lifetime (~70 min). It can be therefore generated in a separate device, so called a singlet oxygen generator. The above chemical reaction can only provide O₂(¹D_g) in such high concentrations and at enough high partial pressure to meet requirements for high-power laser operation. In a conventional COIL, a part of the O₂(¹D_g) energy is needed also for dissociation of molecular iodine serving as a source of iodine atoms for laser action. A supersonic expansion of flowing laser medium in the resonator region is beneficial for several reasons. Let's name e.g. an effect on the equilibrium constant (temperature dependent) of pumping reaction, which results in much efficient utilization of primary energy of singlet oxygen for laser gain. Supersonic laser devices can be more compact at the same power than former subsonic systems. Supersonic expansion of gas medium provides also more homogeneous laser beam. Main components of supersonic COIL are schematically shown in the given presentation.

Merits of COIL for applications

Some characteristics of COIL are beneficial for special advanced applications of this laser system. Let's mention e.g. its high efficiency (up to 40%) and attainable high powers (up to a megawatt level). In comparison with technological lasers CO₂ (l = 10.6 mm) and CO (l = 4-8 mm), a much shorter wavelength of COIL allows utilizing an optical fibre technique for a laser beam transition to the end-user, and is advantageous also for a more efficient absorption of laser beam by metal materials. The Nd:YAG system with a similar wavelength (l = 1.06 mm) can be also equipped with the fiber technique but this laser cannot provide as high powers as a COIL.

Potential COIL application projects

Multi-kilowatt high-power COIL facilities are developed currently in the USA, Germany, Japan, Russia, Korea, China, and India. Other small COIL devices up to 2 kW exist in many laboratories and universities all over the world (including the Prague COIL Laboratory), used for basic research of particular problems in the COIL operation. Several application projects have been elaborated for application of COIL as a technological laser in the form of mobile unit, e.g. for dismantling and decommissioning of obsolete nuclear reactors or plants, and some hazardous parts of chemical plants, further in a shipbuilding industry for cutting of thick metal plates, and in a petrol industry for extraction from the sea bottom. The COIL is developed mainly for military aims as a part of the airborne ballistic defence system, Airborne laser.

COIL research in the Institute of Physics AS, Prague

The Department of Chemical Lasers contributes to the COIL development through a basic research of experimental and theoretical problems related to a COIL operation. In the past, our activities concerned mainly physical and chemical processes involved in the singlet oxygen generation by different types of generators, and we succeeded for example in determining of the Einstein coefficient for O₂(¹D_g) emission used for singlet oxygen optical detection. Further, we performed research on a pulsed regime of subsonic COIL by gain modulation at magnetic field application, and studied fluid dynamic processes in the supersonic laser device. A current scientific program of the department is aimed at two main problems: 1/ generation of atomic iodine for lasing by two methods excluding energy of singlet oxygen for this process - by a chemical method from gaseous reactants directly in the laser, and by an electric discharge in iodine donors, and 2/ development of the original singlet oxygen generator with parameters satisfying the Airborne laser conditions. Results of performed theoretical estimations based on computational modelling of both problems show that successful solving of these problems will contribute to an increase of the laser efficiency (and also power), and to a simplification of laser operation technology. Currently, the department has at its disposal a developed supersonic COIL device of the output power up to 1 kW, and several pilot small-scale devices including appropriate diagnostic techniques.

Partnership of co-operation

US Air Force Research Laboratory/DED at Kirtland AF Base, NM, USA
Institute of Thermomechanics AS CR
Institute of Inorganic Chemistry AV CR
Lebedev Physical Institute RAS, Samara Branch, Russia

Current research financing

Grants of US Air Force Research Laboratory/DED via USAF EOARD (European Office of Aerospace Research and Development), 2001 - 2007
Grant of the Grant Agency CR, 2005 - 2007
Grant of Ministry of Education, Youth and Sports CR, 2006 - 2008

Last changes: Miroslav Censky 26.10.2006