laser systems

1.1.8 Chemical Laser
The chemical laser is an example of a laser where the pump energy comes from a chemical reaction between two atoms. The chemical laser is a member of the family of Gas Dynamic Lasers. Gas dynamic lasers are based on rapid expansion of hot, high pressure gas, through nozzles into a near vacuum.
Since the transfer of the molecules to the ground state takes more than the time of rapid expansion, we get at low temperature many molecules at excited levels. Thus, "population inversion". The gas usually flow through the nozzles in a transverse flow (perpendicular to the optical axis of the laser), so many nozzles can operate at the same time, yielding high power from the laser. The lasing action of the chemical laser is usually based on vibrational transitions of diatomic molecule.

The Material in a Chemical Laser
Most chemical lasers are based on Hydrogen halides.
HF
The most well known member of this family is Hydrogen Fluoride (HF).
The emitted radiation is in the Infra-Red (IR), in the spectrum range: 2.6 - 3.0 ?m.
DF
When Hydrogen is replaced by its heavier isotope - Deuterium, another member of the family: Deuterium Fluoride (DF) is created and emits in the spectrum range: 3.5 - 4.2 ?m. Other halides such as Hydrogen Chloride (HCl) and Hydrogen Bromide (HBr) have demonstrated lasing in the lab, but are not common. Because Fluorine and Hydrogen are very reactive gasses, Hydrocarbons are used as a Hydrogen source, and Fluorine compounds such as SF6 or NF3 are used as a source for Fluorine.
Fluorine extraction is done by electrical discharge which separates the SF6 molecule into Fluorine and Sulfur. In commercial chemical lasers, Oxygen is added to the reaction chamber, to react with the Sulfur to create SO2 molecules. Helium gas is added as a dilution gas and sometimes other gasses as well.
The Chemical Reaction
The reaction between Hydrogen and Fluorine can be ignited by an electric spark or by chemical means. In the reaction between Hydrogen molecules and Fluorine atoms, the highly active Fluorine reacts with the Hydrogen molecule (H2) to create free Hydrogen plus a molecule of HF*. Then the free Hydrogen reacts with the Fluorine molecule:
H2 + F ? HF* + H
H + F2 ? HF* + F
These reactions will continue as long as there are molecules of Fluorine and Hydrogen. Thus, gas flow into the laser cavity creates continuous laser emission. HF and DF molecules have a series of vibrational energy levels. The energy difference between successive energy levels, decreases at higher levels. This means that when the transition is between two high energy levels (such as E7-E6), the emitted photon will have lower energy than the photon emitted from the transition between lower energy levels (E2-E1). Since every vibrational level has a few rotational sub-levels, we have the explanation for the range of wavelengths emitted by these chemical lasers.
Chemical Laser Structure
The structure of a chemical laser is shown in figure 1.9.

Figure 1.9: The Basic Structure of the Chemical Laser
The gasses are injected into the laser through pipes with pinholes at their ends. The design of the pinholes is critical to avoid thermodynamic equilibrium of the gas. The gas flows rapidly out of the pinholes and creates a turbulent flow. This results in excited Hydrogen-halide molecule. The excited gas enters the laser optical cavity at right angle to the laser optical axis.
Advantages of Chemical Lasers:
• The source of energy is conveniently stored .
• Very high output power.
The atmosphere is more transparent to the emitted spectrum out of DF lasers than for HF lasers, so the DF laser is more developed, although its efficiency is lower, and the price of the Deuterium isotope is higher.
Disadvantages of Chemical Lasers:
• Fluorine is a very reactive gas.
• Hydrogen gas can explode easily.
Chemical Laser operation
In a commercial chemical laser, high voltage of about 8,000 Volts is applied to the electrodes of the laser tube. Some lasers use Ultra-Violet (UV) radiation before the electric discharge to pre-ionize the gas and increase the efficiency of the chemical reaction. The chemical reaction between free Fluorine and Hydrogen releases a large amount of heat while creating the molecule HF* which is in an excited state.
Chemical Laser Applications:
Most of the applications of chemical laser are military applications. It is designed to destroy enemy missiles in the air.