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 the Co2 molecular gas laser, the transition takes place between the vibrational states of Carbon dioxide molecules.
Carbon Dioxide Laser: Definition, Principle
In carbon dioxide laser, Co2 gas molecules are used as the active medium and population inversion is achieved by the electrical pumping method.
The active medium is a gas mixture of CO2, N2 and He. The laser transition takes place between the vibrational states of CO2 molecules.
Energy states of CO2 molecules
Carbon dioxide (Co2) is a symmetric molecule (O=C=O) and it has three modes of vibration:
- Symmetric stretching.
- Antisymmetric stretching is shown in the figure.
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.
In this mode of vibration, oxygen atoms and carbon atoms vibrate perpendicular to molecular axis.
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.
Components of Co2 Laser
Active medium ( or Carbon dioxide laser gain medium )
The main component of a Carbon dioxide laser is a medium in the form of CO2 gas molecules called an active medium. The main characteristics of the active medium are as follows:
- It must have a pair of energy levels separated by a certain amount of energy. The energy level having energy is known as an upper energy level or higher excited energy level and the energy level having low energy is known as low energy or ground state.
- It must allow a population inversion between two energy levels.
It is an external source of energy that provides the necessary energy to the active medium to produce a state of population inversion essential for lasing action.
A resonant cavity
Population inversion is achieved to amplify the signal by ( or Photon ) stimulated emission. However, in practice, most of the atoms in the excited state emit spontaneously and do not contribute to the overall output. Only a few atoms in the excited state emit via stimulated emission and the hand’s overall gain of the output is small.
Therefore, we require a positive feedback mechanism to make most of the atom in the excited state to emit via stimulated emission for contributing to the current output. A device used to you have a positive feedback mechanism for Maximum coherent output is known as resonator or resonant cavity.
Thus, a resonant cavity or resonator is a feedback device that makes the photon to move back and forth through the active medium. In this process, the number of photons emitted due to stimulated emission are multiplied.
A resonant cavity consists of a pair of plane or spherical mirrors placed parallel to each other at the end of the active medium. One of the mirrors is a fully reflecting mirror and the other is a partially transmitting mirror. The laser output is taken out through the partially transmitting mirror which is also called the output coupler mirror.
Population inversion in Carbon Dioxide laser
Population inversion in Carbon dioxide (CO2) laser is achieved by electric discharge pumping. In this case, voltage is applied across the electrodes of the gas discharge tube which is filled with a low-pressure gas mixture known as the gain medium.
The applied voltage produces an electric field within the tube. This electric field accelerates electrons within the gas. These electron collides with the gas atom or gain medium and excite their atom to higher energy levels or excited energy levels.
If the atom in the lower-lying energy level makes the transition to the excited state faster than the atom in the higher-lying energy level makes the transition to the lower-lying energy levels, then the population of atom in the higher energy level is more than the population of atom in lower energy levels. Hence, population inversion in gases is achieved.
Carbon Dioxide Laser Construction and Working
Construction of Carbon Dioxide Laser
it consists of a tube 5m long and 2 cm in diameter. The discharge is produced by d.c excitation. The resonant cavity is formed of confocal silicon mirrors coated with aluminum. Pressured He is about 7 Torr, P (N2)~ 1.2 Torr, and P (CO2)~0.33 torr. The gain of E(0,0,1) – E (1,0,0) transition is higher, so laser oscillator at 10.6 µm.
To obtain an oscillation of 9.0 µm, some selective device such as a diffraction grating is placed in the cavity. The maximum power of this laser is about 50 – 60 w/m. CO2 lasers are widely used in the industries.
Working of Carbon Dioxide Laser
To have a CO2 laser, a mixture of Co2 and N2 in the ratio of about 0.8:1 is filled in a gas discharge tube. Also, helium is part of the mixture. Helium is more than N2 in the mixture. Co2 molecule act as an active or laser medium and N2 molecule help in achieving the population inversion in the same way as helium is used in He-Ne laser.
When an electric discharge is passed through the tube, the number of electrons is emitted, which pumps nitrogen molecules to V = 1 state.
In CO2 laser, the energy difference between the vibrational energy levels of Nitrogen and carbon dioxide is very small ( i.e. about 0.3 eV ) and hence there are a large number of electrons in the gas discharge tube having the energy of more than 0.3 eV. The energy levels of CO2 molecules are shown in the figure.
The vibrational state for N2 is a metastable state. Therefore, it provides a longer time for the collision between the excited N2 molecules and atom molecule of Co2 in the ground state.
As a result of this, the Co2 molecules are excited to the energy state E (0,0,1). Hence, population inversion is achieved as the number of CO2 molecules in the energy state E (0,0,1) are much more than the Co2 molecules in other vibrational states like E (1,0,0), E (0,0,2), and E (0,1,0).
Transition through stimulated emission process from energy level E (0,0,1) to the energy level E (1,0,0)and E (0,2,0) give rise to infrared Photons of wavelength10 µm and 9.6 µm respectively.
The function of adding Helium in the mixture of Co2 and N2 is many folds.
- It influences the lifetime of the energy label and does not spoil the population inversion.
- Helium has high thermal conductivity and so helps to conduct heat away to the walls of the tube keeping CO2 cold. In other words, helium helps to depopulate the lower level.
Characteristics of Carbon Dioxide Laser
|Type||It is a molecular gas laser.|
|Active Medium||A mixture of CO2 , N2, and helium or water vapor is used as an active medium.|
|Optical resonator||Two concave mirrors form a resonant cavity.|
|Pumping Method||The electrical discharge method is used for Pumping action.|
|Power output||The power output from this laser is about 10kW.|
|Nature of output||The nature of output may be continuous wave or pulsed wave.|
|Wavelength of output||The wavelength of output is 0.6μm and 10.6μm.|
Advantages of Carbon Dioxide Laser
- The construction of CO2 laser is simple.
- The output of this laser is continuous.
- It has high efficiency.
- It has very high output power.
- The output power can be increased by extending the length of the gas tube.
Disadvantages of Carbon Dioxide Laser
- The contamination of oxygen by carbon monoxide will have some effect on laser action.
- The operating temperature plays an important role in determining the output power of laser.
- The corrosion may occur at the reflecting plates.
- Accidental exposure may damage our eyes since it is invisible (infrared region) to our eyes.
Application of Co2 laser
- Due to the high power of CO2 laser, it has frequently used in industrial areas such as for cutting and welding.
- it is used for soft tissue surgery.
- it is used in fabricating.
- used in skincare problems to treat different non-cancerous (benign) and cancerous (malignant).
- It is used to perform microsurgery and bloodless operations.