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       Carbon-di-oxide laser
       CO2 laser was one of the earliest gas laser to be developed in BELL Labs in 1964 by Indian born CKN Patel.
       It is a molecular gas laser
       In molecular gas lasers laser oscillations are achieved by the transition between the vibrational and rotational levels of the molecule
       o/p of these lasers are more powerful
       In CO2 laser the transition occurring between different vibrational states are responsible for laser effect
       CO2 molecule has a central carbon and two Oxygen one at either ends
       Such a molecule can vibrate in three different modes of vibration, and in each mode of vibration the centre of gravity remains fixed
                    •       Sym. Stretching mode
                    •       Bending mode
                    •       Assym. Stretching mode.

 Symmetric stretching mode
       Carbon atom fixed in its position and each oxygen atom can vibrate along the axis of the molecule simultaneously departing or approaching the fixed carbon atom.
       The frequency corresponding to this mode of vibration is called symmetric stretching frequency
  Bending mode
       Oxygen atom and carbon atom may vibrate at right angles to the line passing through the centre of gravity.
       The frequency corresponding to this mode of vibration is called as bending frequency
Asymmetric stretching mode
       The two oxygen atom and the carbon atom at the centre vibrates asymmetrically or they vibrate in an opposite directions with respect to its mean position.
       The corresponding frequency is called asymmetric stretching frequency

       The experimental setup is as shown
       Consist of discharge tube made of fused Quartz of 2.5 cm in diameter and 5 m long.
       Special feature of the CO2 laser is the o/p is dependent on the diameter of the discharge tube.
       The discharge produced DC excitation
       Active medium consists of He, N2 and CO2, Brewster's windows at the ends,
       Near confocal silicon mirror coated with aluminium formed at the resonant cavity

       In CO2 lasers N2 plays the similar role of He in He-Ne laser
       N2 goes to excited state by collision with electrons
                                                N2 + e1 → N2* + e2
       The excited N2 transfers energy to CO2 and CO2 gets excited
                                                N2 + CO2 → CO2* + N2
       The lowest vibrational level of N2 has nearly as much energy as the symmetric stretching mode of CO2 molecule and so the excited N2 readily transfer energy to CO2 in resonant collisions.
       Operating temperature plays an important role in determining the o/p power of the laser.

Merits and demerits
            •       Has very high o/p power can be increased by increasing the length of the gas         tube
            •       o/p power of the laser depends upon the operating temperature.
       CO2  molecule’s dissociation into CO may contaminate the active medium. 

       Widely used in material processing such as drilling, cutting, welding, melting and annealing.
       It is widely used in open air communication.
       It is used in pollution monitoring and remote sensing.
       It is used in medical field to perform microsurgery and blood less surgery.         

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