Question

4. Explain the processes of lasing in the Co2 laser. What is the function of Nitrogen, and Helium in the gas mixture of the co2 laser?

Answer 1

Carbon dioxide lasers are gas lasers which emit infrared radiation. They are used for a variety of high power industrial applications. As discussed in the Lasers Selection Guide, all lasers consist of three components: an energy source (also known as a pump), a gain (or laser) medium, and an optical resonator. These components are labeled in the diagram below. The pump serves to provide energy which is amplified by the gain medium. This energy is eventually converted into light and is reflected through the optical resonator which then emits the final output beam.

• Electrical current — serving as the laser pump — which excites the gas medium.
• A mixture of gases — serving as the gain medium — comprised of carbon dioxide, nitrogen, hydrogen, and helium. The carbon dioxide, nitrogen, and helium make up the vast majority of the mixture, although the specific concentrations vary depending on the laser's intended use. Typical gas mixtures have an N₂:CO₂:He ratio of 1:1:8.
• A specialized optical resonator. Because CO₂ lasers operate solely within the infrared spectrum and can attain high power outputs, their optical components are typically made of specialized (and often expensive) materials such as germanium, zinc selenide, silver, gold, and diamond.
• When electrical current is introduced to the gain medium, the nitrogen molecules are excited to a vibrational state. Because these molecules are comprised solely of nitrogen, they will retain this vibrational energy for long periods. Next, the vibrating nitrogen molecules excite the carbon dioxide molecules, to the point that the gain medium becomes an effective amplifier for the pumped energy. As the nitrogen molecules come into contact with cold helium atoms, they gradually become less excited and transfer energy to the helium in the form of heat. The hot helium atoms must then be cooled to maintain a population inversion (a sufficient difference between excited and lower energy atoms to produce optical gain) with the excited carbon dioxide atoms
• the power panel, transformer, and rectifier provide electrical current as the pump, while the tank in the center feeds the gas mixture into the laser. The water pump system provides cooling for the sides of the laser, so that hot helium atoms become less excited when they collide with the water-cooled walls.

• CO₂ lasers feature many characteristics which make them ideally suited to industrial and materials processing applications. Some of the attributes include:

  • Low cost in relation to power capabilities (often less than $100 per watt)
  • High efficiency (output to pump ratio of up to 20%)
  • Wide possible variation in output power
  • Long lifetime
  • Many possible output waveforms
  • Minute alterations in gas concentration allows for the possibility of selecting from hundreds of discrete infrared wavelengths

  Carbon dioxide lasers are most frequently used in laser cutting and welding, or — for lower power devices — laser engraving and marking. Also, because water (which makes up the majority of biological tissues) absorbs infrared radiation well, CO₂ lasers are used in medical applications such as laser surgery, skin resurfacing, dermabrasion, and more recently to "weld" human tissue in place of sutures.

• It is relatively easy to excite a nitrogen (N2) molecule to its fi rst vibrational energy level via an electric fi eld or discharge. This fi rst energy level of nitrogen has almost the same energy content as the upper level of CO2. (see Fig. 2). Transfer of vibrational energy from N2 to CO2 is achieved through collision between the two molecules. Thus it is easier to excite the upper laser level of CO2 by introducing nitrogen. Adding nitrogen to CO2 results in higher laser power.

• The primary role of helium is to help CO2 to relax, i.e. to move from the lower to the lowest energy level and re-enter excitation process. Helium atoms collide with CO2 molecules and vibrational energy is transferred from CO2 molecules to the helium atoms. As a result, higher laser powers can be obtained. Helium also helps to conduct heat away from the electric fi eld or the electric discharge (it has the highest heat-conduction coeffi cient of all gases). This is essential for slow-fl ow CO2 lasers, where excess heat must be conducted to the walls of the discharge tube. For fast-fl ow CO2 lasers, helium gives a more stable electric discharge and supports the excitation effi ciency