CO2 laser technology has revolutionized various industries, offering unprecedented precision and versatility. From medical procedures to industrial applications, CO2 lasers have become an invaluable tool. In this article, we will delve into the workings of CO2 lasers, explore their diverse applications, and highlight their advantages. So, let’s embark on a journey into the world of CO2 lasers.
CO2 laser (Carbon Dioxide Laser)
Active Medium
In a CO2 laser, the active medium is a mixture of CO2, N2 and He gases. In this CO2 laser, the transition between various vibration levels of the same electronic state of CO2 molecules leads to the production of the laser beam. This is a four-level molecular laser.
The Vibrational Modes of CO2 Molecule
A CO2 molecule consists of a central C-atom with two )-atoms attached on either side. A molecule of this type can vibrate with three different independent modes of vibration, as shown in the figure below. The three states of vibration of the molecule are represented by three quantum numbers (m, n, q).
There are three types of vibration modes in CO2 molecules.
| Sr. No | Vibration modes in CO2 molecules |
|---|---|
| 1. | Symmetric Stretching Mode (m) |
| 2. | Bending Mode (n) |
| 3. | Asymmetric Stretching Mode (q) |
1. Symmetric Stretching Mode
- In this mode, the oxygen atm oscillates along the axis of the molecule by simultaneously departing and approaching the carbon atom in between, which is stationary as shown in the figure above. The corresponding frequency is called symmetric stretching frequency and is denoted by m.
2. Bending Mode
- In this mode, all three atoms of the molecule undergo vibrational oscillations perpendicular to the axis of the molecule. But the movement of the oxygen atom and the carbon atom will be in opposite directions, as shown in the figure. The corresponding frequency is called bending frequency and is denoted by n.
3. Asymmetric Stretching Mode
- In this mode, the three atoms oscillate along the axis of the molecule with both the oxygen atoms moving in the same direction while the carbon atom moves in the opposite direction, as shown in the figure. The corresponding frequency is called asymmetric stretching frequency and is denoted by q.
Note: At any instant, the Co2 molecules can be vibrating in a linear combination of these three fundamental modes.
Also Read: Nd:YAG Laser
CO2 Laser Construction and Working
In this section, we will first learn the construction of a CO2 laser and then we will discuss the working of CO2 laser in detail.
Construction of CO2 laser
The discharge tube is 5.5 cm in diameter and 5 m in length. The discharge tube is filled with a mixture of carbon dioxide, nitrogen, and helium gases in 1: 2: 3 proportion, respectively. The active centers are CO2 molecules, DIshcarge is produced by D.C. excitation, as shown in the figure below.
Sodium chloride windows are placed at the end of the discharge tube inclined at the Brewster angle to the resonator axis. The windows are inclined at Brewster’s angle to obtain a plane-polarized laser beam. A pair of confocal mirrors acts as the optical resonator cavity.
Working of CO2 Laser
The CO2 laser is excited by a d.c. excitation. As the current passes through the mixture of gases, the N₂ molecules get excited to the metastable state.
N₂+ē→ N₂
The excited N₂ molecules interact with the ground state CO2, molecules through inelastic collisions and make them get excited to the level E, as shown in the energy-level diagram (Figure 5.8). The N₂ molecules return to the ground state after the inelastic collisions with the ground state CO2.
Level E4 is the upper laser level. E3, and E2, levels act as the lower laser levels. Due to continuous excitation, population inversion is achieved between E4, level, and the levels at E3, and E2.
Any of the spontaneously emitted photons will make the molecules undergo a transition between E4→E3, thereby generating a stimulated photon. These photons traveling along the resonator axis undergo multiple reflections at the optical resonator mirrors. As a result, the transition E4→E3 yields an intense and coherent laser output of wavelength order 10.6 µm. Similarly, the transitions between E4→E2 generate a laser beam of wavelength 9.6 µm. Both wavelengths fall in the far IR region.
The transitions between E3→E1 and E2→E1 occur due to the elastic collisions between the excited CO2 molecules with the unexcited CO2 molecules. Finally, by elastic collision with helium atoms, the CO2 molecule makes the transition E1→E0. i.e., helium helps in depopulating the lower levels. In addition to this, helium helps to conduct heat away from the walls of the discharge tube, which keeps CO2 cold, and this is achieved due to the high thermal conductivity of helium.
The role of N2 gas is to increase the population of the upper level of CO5.
The power output of this laser is 50-60 W/m when there is a longitudinal flow of gases.
Note: A set of three quantum numbers (010) means that the CO2 molecule in this energy state is in a pure bending mode of vibrations. Similar explanations can be given for other set of quantum numbers like (100) and (001).
But, if the flow is perpendicular to the discharge tube the power output can be raised to 10 kW/m, and this type of CO2 laser is known as a Transversely Excited Atmospheric Pressure laser or TEA laser.
Also Read: Q Switched Nd Yag Laser
Disadvantages of CO2 Laser
The Co2 molecules of the active medium dissociate into CO after certain hours of operation and the CO may contaminate the active medium. Thus, the CO has to be periodically removed away by the vacuum pumps.
Application of CO2 Laser
- It is widely used in material processing such as drilling, cutting, etching, welding, melting, and annealing.
- It is widely used in open-air communication.
- It is used in pollution monitoring, remote sensing, and in LIDAR (LIght Detection And Ranging).
- It is used in the medical field to perform microsurgery and bloodless surgery
FAQs
What does a CO2 laser do?
Carbon dioxide lasers can precisely remove thin layers of skin with minimal damage to the surrounding structures. These lasers treat sun damage, wrinkles, scars, warts, birthmarks, and other skin conditions.
Is CO2 laser a surgery?
A CO2 surgical laser is a type of laser that is used in dermatological and gynecological surgery.
Is CO2 laser painful?
The co2 does cause some discomfort.
co2 laser cost
Rs. 3000 to Rs. 7000
Also Read: Band Theory of Solids
