laser systems design

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.
1.1.9 Far Infra-Red (FIR) Lasers
Far Infra-Red (FIR) lasers emit radiation in the Far-Infra-Red spectrum (wavelength range 12-1000 ?m. The wavelength range greater than 100 ?m is sometimes called sub-millimeter wave.
Far Infra-Red (FIR) lasers are gas lasers, and their lasing action occur between rotational levels of the gas molecules of the active medium. Usually these transitions are within the same vibrational level. The active medium in FIR lasers is usually a gas of simple organic molecule such as: C2H4, CF4, NH3,
Because of the very narrow width of each energy level of these materials, it is inefficient to optically pump them with ordinary light sources. The best way to achieve population inversion in these lasers is to pump them with another laser at shorter wavelength. Usually CO2 laser is used for pumping.
Properties of FIR Lasers
A schematic drawing of FIR laser is shown in figure 1.10.



Figure 1.10: schematic drawing of FIR laser
The main research use of FIR lasers is for spectroscopic measurements. It is possible to use the same FIR laser system for different laser gasses, and each gas has usually some lasing lines.
Structure of Far Infra-Red (FIR) lasers
The lasing gas is confined within a tube (similar to CO2 or He-Ne lasers).The gas can either flow through the tube, or the tube can be sealed off. The gas pressure within the tube is 30-300 torr.
Optical pumping is usually done along the optical axis of the laser. The mirror through which the pumping is done is coated so that the pumping wavelength passes through, and the laser wavelength is blocked. Thus the laser radiation is trapped inside the tube, passing many times through the active medium, and being amplified.
Since the optical pumping is done by a laser, the pumping wavelength is determined precisely, so specific energy levels can be excited. The main problem in using FIR lasers is to find optical components which are transparent at these long wavelengths, since most optical materials are not transparent at wavelength more than 40 ?m.
1.2 Solid State Lasers
The atoms in a solid are close to each other, and the interaction between neighbors is strong. Thus, the absorption and emission spectrum ranges in solids are much wider than those of gasses. Wide absorption spectrum allows pumping of the active medium with a "conventional" light source, which has a wide emission spectrum.
In Optical Pumping the active medium is excited by illuminating it with external electromagnetic source. The photons from the external source are absorbed by the material of the active medium, thus transferring energy to its molecules. Two types of electromagnetic sources are used in optical pumping:
• Source of wide band electromagnetic spectrum- such as Flash lamps, incandescent lamps, arc lamps, etc.
• Source of narrow band electromagnetic spectrum - another laser.
Structure of the active medium in Solid State Laser

The active medium in solid state lasers is a medium of one solid material, in which impurity ions of another material are embedded. These impurity ions are replacing atoms of the solid background, and the energy levels which participate in the lasing process are those of the ions of impurity.
The solid background influence on the energy level structure is minor. Thus, the same impurity ion embedded in different host material will emit at very close wavelengths. The optical properties of the laser are dictated mostly by the impurity ion. On the other end, the physical properties of the active medium such as thermal conductivity, thermal expansion, are determined by the solid host. Thus, the solid host determines the maximum power levels which can be emitted from the laser.

Optically Pumped Solid State Lasers
The active medium in these lasers is a crystal or a glass. The shape of the active medium is usually a rod with circular or square cross section. The pumping beam usually enters the active medium via its surface area along the rod, while the laser radiation is emitted through the ends of the rod. The ends of the rod are usually at right angles to the rod axis, and are optically polished.
Solid state lasers emit radiation in either pulsed mode or in continuous mode. The pump lamps for pulsed lasers are usually Xenon (or Krypton) flash lamps, in which a low pressure gas is contained within quartz tube. The pump lamps for continuous lasers are usually Halogen lamps, or high pressure Mercury discharge lamps.
Arrangement of Pump and Laser Rod
There are many ways to transfer as much pump light as possible from the lamp to the active medium. The most common method is to use an elliptic optical cavity (A cavity created by an ellipsoid of revolution).The lamp is at one focus of the ellipsoid, and the rod of the active medium at another, as described in Figure 1.11.


Figure 1.11: Methods of Optical Pumping of Solid State Lasers.
The inner surface of the cavity are coated with a reflective coating (usually Gold), such that all the radiation emitted from the lamps ended at the active medium.
Diode Pumped Solid State Lasers (DPSSL).

During the last few years, with the new developments of diode lasers at high powers, a new pumping method is being developed for solid state lasers. Instead of broad spectrum pumping source, Diode Lasers are used as pumping sources.
The wavelength in these diode lasers can be adjusted to fit the absorption spectrum of the active medium. These diode lasers are very efficient sources, and almost all their light is absorbed by the active medium.
Thus, very little energy is lost (converted into unwanted heat).These solid state lasers which are pumped by diode lasers are called: Diode Pumped Solid State Lasers (DPSSL).