Carbon-di-Oxide [CO2] Laser

CARBON‒DI‒OXIDE [CO₂] LASER

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: Metallic mirror of gold (or) silicon mirrors coated with aluminium

Power output: 10 KW

Nature of output: Continuous (or) pulsed

Wavelength of the output: 9.6 μm & 10.6 μm (96000Å & 106000Å)

Introduction

An Indian Engineer C.K.N.Patel designed the CO₂ laser. We know in the case of atoms, electrons can be excited to higher energy levels. The distribution of electrons in the shells and sub‒shells define the electronic state of the molecule (eg. He‒Ne laser).

Besides these electronic energy levels, the molecule can have other energy levels also due to rotation and vibration of the molecule (CO₂) they give rise to various rotational and vibrational energy levels, as shown in Fig.10.12.

E₁E₂ ‒ Electronic energy levels

v',v"‒ Vibrational energy levels

j, j' ‒ Rotational energy levels.

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 to CO₂ 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.

The molecular gas laser can have two types of transitions

(i) Transition between vibrational states of the same electronic state (Fig.10.13).

(ii) Transition between vibration levels of different electronic state (Fig.10.14).

NOTE: No need to write the introduction and to draw these figures, if CO₂ LASER is asked for 8 marks.

CO₂ laser of satisfies the first condition (i.e.) here the laser transition occurs between vibrational energy levels of the same electronic state.

Fundamental modes of vibration of the CO₂ molecule

There are three fundamental modes of vibration.

1. Symmetric stretching mode (10°0)

2. Bending mode (01°0, 02°0)

3. Asymmetric stretching mode (00°1, 00°2)

i) Symmetric stretching mode (10°0)

Here the carbon atom is stationary and the oxygen atoms oscillate (or) vibrate along the axis of the molecule as shown in Fig.10.15 (simultaneously it approaches (or) departs with respect to the carbon atom). The state of vibration is given by 3 integers (mn'q) here (10°0), which corresponds to the degree of excitation.

ii) Bending mode (01°0, 02°0)

Here the atoms will not be linear, rather the atoms will vibrate perpendicular to the molecular axis as shown in Fig 10.16. This gives rise to two quanta of frequency represented by (01°0, 02°0).

iii) Asymmetric stretching mode (00°1, 00°2)

Here all the three atoms will vibrate. Here the oxygen atoms vibrate in the opposite direction to the vibration direction of carbon atom as shown in Fig.10.17, which gives the quanta of frequency (00°1 and 00°2)

NOTE: In general the Quanta of frequency is represented as mn'q

m → Quanta of frequency, when CO₂ is in symmetric stretching mode.

n → Quanta of frequency, when CO₂ is in bending mode.

q → Quanta of frequency, when CO₂ is in Asymmetric stretching mode.

l → Angular momentum about the axis of the molecule.

Since rotational energy is not considered I = 0.

Construction

It consists of a discharge tube in which CO₂ is taken along with nitrogen and helium gases with their pressure level of 0.33:1.2:7 mm of Hg for CO₂, nitrogen and the He respectively. Nitrogen helps to increase the population of atoms in the upper level of CO₂, while helium helps to depopulate the atoms in the lower level of CO₂ and also to cool the discharge tube.

The discharge is produced by D.C. excitation. At the ends of the tube Sodium chloride/Brewster windows are placed as shown in Fig. 10.18. Confocal silicon mirrors coated with Aluminium (or) metallic mirror of gold is employed for proper reflection, which form the resonant cavity. The output power can be increased by increasing the diameter of the tube.

Working

1. The discharge is passed through the tube first, the Nitrogen atoms are raised to excited state

e^‒ + N₂ → N₂^*

2) The excited N₂ atoms undergo resonant energy transfer with CO₂ atom and raises CO₂(00°1) to excited state due to closer energy level of CO₂ (00°1) and Nitrogen.

 N₂*+CO₂ → CO₂*+N₂

3) When transition takes place between 00°1 to 10°0, laser of wavelength 10.6 μm is emitted as shown in Fig.10.19.

4) Similarly, when transition takes place between 00°1 and 02°0 laser beam of wavelength 9.6 μm is emitted as shown in Fig.10.19

5) Since 00°1 → 10°0 has a higher gain than 00°1 → 02°0 transition, usually the laser beam of wavelength 10.6 μm is produced more.

6) When the gas flow is longitudinal power output is 50 to 60 watts but if the gas flow is perpendicular to the discharge tube the output power may be raised to 10 kilowatt/m.

7) This type of CO₂ laser is known as TEA laser (Transversely Excited Atmospheric Pressure Laser).

8) 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 bas discharge tube.

NOTE: The upper energy level of Helium cannot be seen in the energy level diagram, because it occurs very far from the ground state energy level.

Applications of CO₂ Laser

1. This laser has applications in medical field such as neurosurgery, microsurgery, treatment of liver, lungs and also in bloodless operations.

2. It is widely used in open air communication.

3. This laser also have wide applications over military field.