Why helium is doped in carbon dioxide laser:
The CO2 molecules in the states E4 and E3 deexcite to state E2 through inelastic collision with unexcited CO2 molecules. This process is very fast so there will be accumulation of CO2 in this level and they can break the population inversion in upper levels because there is probability of excitation of molecules from E2 to E3 and/ or E4.
To stop the accumulation of CO2 molecules in E2 special additives like He and water vapors are added into the gas mixture. CO2 molecules return to the ground state E1 through collisions with the He to which it transfers the excitation energy.
Other function of He is to conduct the heat away to the walls keeping CO2 cold, this is because helium has high thermal conductivity.
Wavelength and output of carbon dioxide laser:
E5 to E4 with laser wavelength of 10.6 μm.
E5 to E3 with laser wavelength of 9.6 μm.
Thus, these transitions produce lasers of wavelength 10.6 μm and 9.6 μm which lie in the far infra-red region.
Output of 10KW is achieved with transition of 10.6 μm and it is in continuous wave mode. Efficiency of CO2 laser is approximately 30%. The CO2 laser is more efficient than other gas lasers because in CO2 laser, the levels taking part in laser transitions are the vibrational rotational levels of the lowest electronic level and as these levels are very close to the ground level, thus a large part of the input energy is converted into output laser energy. Thus CO2 laser is more efficient than other gas laser.
Applications of CO2 laser: CO2 laser have wide applications in industry for welding, cutting and for hole drilling. High energy CO2 lasers are used to destroy cancer tissues.
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Reference: This article is referred from my authored book “optics and lasers” having ISBN 978-81-272-2948-2