Explain He-Ne Laser under following heads: Construction – working with Diagram.
(i) Active medium:
It is a gas laser, which consists of a narrow quartz tube filled with a mixture of helium and neon gases in the ratio 10:1 respectively, at low pressure (~0.1 mm of Hg). Ne atoms act as active centers and responsible for the laser action, while He atoms are used to help in the excitation process. The length of the quartz tube is about 50 cm and the diameter is about 1 cm.
(ii) Optical resonator:
To construct the optical resonator cavity, two parallel mirrors are placed at the ends of the quartz tube one of them is partly transparent while the other is fully reflecting. The spacing between the mirrors is adjusted such that it should be equal to the integral multiple of half-wavelengths of the laser light.
(iii) Pumping system:
The pumping is done through electrical discharge by using electrodes that are connected to a high frequency alternating current source.
It is a four energy level laser system. The electrons produced from electric discharge collide with He and Ne atom and excite them to the higher energy levels He2 and Ne4 at 20.61 eV and 20.66 eV respectively. These two states are metastable so the atoms may stay there for a longer time. They are very close to each other, thus some of the He atoms at He2 state may transfer their energy to ground-state Ne atoms through collisions and excite them to the higher energy level Ne4. The kinetic energy of atoms provides the additional energy of 0.05 eV. Thus, He atoms help to achieve population inversion in Ne atoms.
Now some of the Ne atoms will decay spontaneously to the lower state Ne3 at 18.70 eV by emitting photons of wavelength 6328 Å. The photons that are moving parallel to the axis of the tube will reflect back and forth by the end mirrors of the rod and stimulate other excited Ne atoms to radiate another photon with the same phase. Thus, due to successive reflections of these photons at the ends of the tube, the number of photons multiplies. After a few microseconds, a monochromatic, intense, and collimated beam of red light of wavelength 6328 Å emerges through the partially silvered mirror.
The atoms at state Ne3 are unstable and decay spontaneously to a lower state Ne2 and finally, they come to the ground state by losing their remaining energy through collision with the tube walls. He-Ne laser operates continuously.