laser systems design

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.