Carbon dioxide (CO2) gas Laser

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers invented by Kumar Patel of Bell  Labs in 1964, is one of the most useful. Carbon dioxide gas lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.6 and 10.6 micrometers. For the laser action, two points are important one is the population inversion between the two levels and second increase density of the incident radiation. The principle of a Carbon-di-oxide laser is the transition between Vibrational states of the same electronic state by achieving Population Inversion between these states. There are three normal modes of vibration in the CO2 molecule:

a.) The Asymmetric Stretch Mode

b.) The Bending Mode

c.) The Symmetric Stretch Mode

It consists of a discharge tube of size about 2.5 cm in diameter and 5.0 cm in length. Two optically plane and parallel mirrors. The discharge tube is filled with a mixture of carbon dioxide, nitrogen and helium gases in the ratio 15%: 15%: 70% respectively at a pressure of a few mm of mercury.

A high value of dc voltage is used for electric discharge in the tube due to which CO2 molecules break into CO and O. To maintain the equilibrium of CO2 molecules, a small amount of water is added to regenerates the CO2 molecules.

The vibrational and rotational modes of the CO2  cannot be excited themselves by photons.

When a voltage is placed across the gas, electrons collide with the N2 molecules and excite them to their lowest vibrational levels.

These vibrational levels happen to be at energy very close to the energy of the asymmetric vibrational states in the CO2    molecule. Now,   the exciting N2 molecules populate the asymmetric vibrational states in the  CO2 molecule through collisions.

The infrared output of the laser is the result of transitions between rotational states of the CO2  molecule of the first asymmetric vibrational mode (001) to rotational states of both the first symmetric stretch mode (100) and the second bending mode. (020).

Because of the high power CO2  lasers are frequently used in industrial applications for cutting and welding.

They are also very useful in surgical procedures because water absorbs this frequency of light very well.

Some examples of medical uses are laser surgery, skin resurfacing