CO2 Molecular gas laser: Principle, Construction, Working, Characteristics, Advantages, Disadvantages and Applications

Molecular Gas laser

In a molecular gas laser, laser action is achieved by transitions between vibrational and rotational levels of molecules. Its construction is simple and the output of this laser is continuous.

In CO2 molecular gas laser, transition takes place between the vibrational states of Carbon dioxide molecules.

CO2 Molecular gas laser

It was the first molecular gas laser developed by Indian born American scientist Prof.C.K.N.Pillai.

It is a four level laser and it operates at 10.6 μm in the far IR region. It is a very efficient laser.

Energy states of CO2 molecules.

A carbon dioxide molecule has a carbon atom at the center with two oxygen atoms attached, one at both sides. Such a molecule exhibits three independent modes of vibrations. They are

a)     Symmetric stretching mode.

b)    Bending mode

c)     Asymmetric stretching mode.

a. Symmetric stretching mode

In this mode of vibration, carbon atoms are at rest and both oxygen atoms vibrate simultaneously along the axis of the molecule departing or approaching the fixed carbon atoms.

b. Bending mode:

In this mode of vibration, oxygen atoms and carbon atoms vibrate perpendicular to molecular axis.

c. Asymmetric stretching mode:

In this mode of vibration, oxygen atoms and carbon atoms vibrate asymmetrically, i.e., oxygen atoms move in one direction while carbon atoms in the other direction.

Principle:

The active medium is a gas mixture of CO2, N2 and He. The laser transition takes place between the vibrational states of CO2molecules.

Construction:

It consists of a quartz tube 5 m long and 2.5 cm in the diameter. This discharge tube is filled with gaseous mixture of CO2(active medium), helium and nitrogen with suitable partial pressures.

The terminals of the discharge tubes are connected to a D.C power supply. The ends of the discharge tube are fitted with NaCl Brewster windows so that the laser light generated will be polarized.

Two concave mirrors one fully reflecting and the other partially form an optical resonator.

Working:

Figure shows energy levels of nitrogen and carbon dioxide molecules.

When an electric discharge occurs in the gas, the electrons collide with nitrogen molecules and they are raised to excited states. This process is represented by the equation

N2 + e* = N2* + e

N2 = Nitrogen molecule in ground state e* = electron with kinetic energy

N2* = nitrogen molecule in excited state e= same electron with lesser energy

Now N2 molecules in the excited state collide with CO2 atoms in ground state and excite to higher electronic, vibrational and rotational levels.

This process is represented by the equation N2* + CO2 = CO2* + N2

N2* = Nitrogen molecule in excited state. CO2 = Carbon dioxide atoms in ground state CO2* = Carbon dioxide atoms in excited state N2 = Nitrogen molecule in ground state.

Since the excited level of nitrogen is very close to the E5 level of CO2 atom, population in E5 level increases.

As soon as population inversion is reached, any of the spontaneously emitted photon will trigger laser action in the tube. There are two types of laser transition possible.

1.Transition E5  to E4 :

This will produce a laser beam of wavelength 10.6μm

2.Transition E5  to E3

This transition will produce a laser beam of wavelength 9.6μm. Normally 10.6μm transition is more intense than 9.6μm transition. The power output from this laser is 10kW.

Characteristics:

1.     Type: It is a molecular gas laser.

2.     Active medium: A mixture of CO2 , N2 and helium or water vapour is used as active medium

3.     Pumping method: Electrical discharge method is used for Pumping action

4.     Optical resonator: Two concave mirrors form a resonant cavity

5.     Power output: The power output from this laser is about 10kW.

6.     Nature of output: The nature of output may be continuous wave or pulsed wave.

7.     Wavelength of output: The wavelength of output is 0.6μm and 10.6μm.

Advantages:

1.     The construction of CO2 laser is simple

2.     The output of this laser is continuous.

3.     It has high efficiency

4.     It has very high output power.

5.     The output power can be increased by extending the length of the gas tube.

Disadvantages:

1.     The contamination of oxygen by carbon monoxide will have some effect on laser action

2.     The operating temperature plays an important role in determining the output power of laser.

3.     The corrosion may occur at the reflecting plates.

4.     Accidental exposure may damage our eyes, since it is invisible (infra red region) to our eyes.

Applications:

1.     High power CO2 laser finds applications in material processing, welding, drilling, cutting soldering etc.

2.     The low atmospheric attenuation (10.6μm makes CO2 laser suitable for open air communication.

3.     It is used for remote sensing

4.     It is used for treatment of liver and lung diseases.

5.     It is mostly used in neuro surgery and general surgery.

6.     It is used to perform microsurgery and bloodless operations.