Characteristics and Principle of (CO2) Laser

CO₂laser

Carbon dioxide laser is the first molecular gas laser and it was developed by C.K.N. Patel in the year 1963. It is a four level molecular gas laser. In molecular gas lasers, the laser oscillations are achieved by the transition between the vibrational and rotational levels of molecules. Output of these lasers is continuous.

Characteristics of CO₂ laser

Type	–	Molecular gas laser
Active medium	–	Mixture of CO₂ , N₂ and Helium (or) water vapour.
Active centre	–	CO₂
Pumping method	–	Electric discharge method.
Optical resonator	–	Silicon mirrors coated with aluminium.
Power output	–	10kW.
Nature of the output	–	Continuous wave and can be pulsed.
Wavelength of the output	–	9.6μm and 10.6 μm

Principle

The transition between these vibrational and rotational energy levels leads to the construction of molecular gas laser. Here the nitrogen atoms are initially raised to excited state. The Nitrogen atoms delivers the energy CO₂ to atoms which has closest energy level to it. Then, transition takes place between the vibrational energy levels of the CO₂ atoms and hence laser beam is emitted.

Fundamental modes of vibration of the CO₂ molecules

In order to understand the working of this laser one has to recall the rotational and vibrational spectrum of CO₂ molecules. The three atoms can be considered as a ring over a straight line, the end atoms being oxygen with a carbon atom C at the centre. There are three modes of vibration.

(i) Symmetric stretching mode

In this mode of vibration the carbon atom is fixed in its position and each oxygen atom can vibrate in opposite direction symmetrical to the carbon atom with each other along a straight line and is known as symmetric mode of vibration.

(ii) Bending mode

In this mode of vibration the oxygen atom and the carbon atom may vibrate at right angles to the line passing through the centre of gravity. This is known as bending mode.

(iii) Asymmetric stretching mode

In this mode of vibration asymmetric mode of operation, the two oxygen atoms may vibrate about the central atom asymmetrically, and at the same time, the carbon atom also vibrate from its mean position.

Asymmetric mode Fig. 1.1 CO₂ molecules-different modes of vibrations

In addition to these three vibration modes, the molecule can also rotate and therefore, quantized rotational energy levels are also possible. A series of rotational levels are associated with each vibration level.

Construction

It consists of a quartz discharge tube 1m long and 2.5cm in diameter and discharge is produced by d.c. excitation. The tube is filled with a mixture of CO₂, N₂ and He gases in 1:2:3 proportions respectively.

Along with CO₂ there are Nitrogen and Helium gases in the apparatus. The partial pressures of CO₂, N₂ and Helium around 0.33 torr, 1.2 torr, 7 torr respectively.

Sodium chloride Brewster windows are used at the ends. Two concave silicon mirrors coated with aluminium are provided of which one is completely reflecting and the other is partially reflecting. They form the optical resonator. The output power can be increased by increasing the diameter of the tube.

CO2 Laser

Fig. 1.2 CO₂ laser

Working

The CO₂ laser generally uses two additional gases N₂ and He. Nitrogen helps to increase the population of atoms in the upper level of CO₂ . Helium helps to cool down the discharge tube and also to depopulate the atoms in lower level of CO₂ .
When electrical discharge is passed through the gas, the electrons collide with Nitrogen molecules and are raised to their excited state. (N₂+ e^*→ N₂* + e )

Energy Level Diagram of CO2

Fig. 1.3 Energy level diagram of laser

The excited N₂ atom undergoes a collision of the second type and makes the CO₂ molecules e excited. N₂*+CO₂→ CO₂* +N₂
The frequency of the laser can be pictured in an energy level diagram as shown in (Fig.1.3).
Since the E₅ energy level of CO₂ is very close to the excited level of N₂ atom, the population of E₅ level of CO₂ increases rapidly.
When transition takes place between E₅ to E₄ laser of wavelength is emitted.
Similarly, when transition takes place between E₅ to E₃ laser beam of wavelength 9.6 μm is emitted.
The operating temperature plays an important role in determining the output power of laser.
The contamination of carbon monoxide and oxygen will also have some effect on the laser action. To avoid this the unused gases can be pumped out and fresh CO₂ must be pumped inside the discharge tube.
The power output coming from this laser is 10kW. CO₂ Laser is also called as Transversely Excited Atmospheric pressure laser (TEA LASER).

Advantages

The construction of CO₂ laser is simple.
The output of this laser is continuous
It has extremely high efficiency and very high output power.
The output power can be increased by extending the length of gas tube.

Disadvantages

Corrosion may occur in the reflecting plates.
The contamination of oxygen by carbon monoxide will have some effect on laser action.

Applications

Because of its high power output, it is used in material processing such as welding, drilling, cutting etc.,
It is also widely used in open air communication system.
It is used in laser remote sensing.
It is used in the treatment of liver, and also in neuro surgery.

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