With neat energy level diagram describe the construction and working of a He-Ne Laser. What are its merits and demerits? What is the role of helium atoms?
It consists of a long discharge tube of length 50 cm and diameter 1 cm. The tube is filled with a mixture of He and Ne in the ratio 10:1. Electrodes are provided to produce a discharge in the gas and they are connected to high voltage power supply. The tube is sealed by inclined windows arranged at its end. On the axis of tube two reflectors are fixed which forms resonator.
He Ne gas laser employ four level pumping schemes. When the power is switched on the electric field ionizes some of the atoms in the mixture of He and Ne gases. Due to electric field, the electrons and ions will be accelerated towards anode and cathode. Since electron have smaller mass they acquire higher velocity and He atoms are lighter in weight and therefore readily excitable.
The energetic electrons excite He atoms to excited states F2 and F3 which lies at 19 ev and 20 ev above the ground state. These are metastable states for helium.
Though the radiative transitions is forbidden, the excited He atom can return to the ground state by transferring their energy to Ne atoms through collision. Such an energy transfer can take place only when the two colliding atoms have identical energy states. E6 and E4 level of Ne atom nearly coincides with F3 and F2 of Helium.
Ne atoms acquires energy and goes to excited state and helium atoms return to ground state by transferring their energy to Ne atoms. This is main pumping mechanism. Ne atoms are active centers and Helium plays the role of pumping agent.
The probability of energy transfer from Ne to He atom is less as there are 10 Helium atoms to 1 Neon atom. E6 and E4 states are metastable states as collision goes on neon atoms accumulate in these states whereas E5 and E3 level of neon are sparsely populated.
Therefore, a state of population inversion is achieved between E6and E5, E6 and E3 and E4 and E3. Consequently, three laser transitions take place.
E6—–E5 33900 A° (far IR region) E6——-E3 6328 A° (visible) E4—-E3 11500 A° (IR region)
As the terminal levels of lasing transitions are sparsely populated the fraction of Ne atom that must be excited to upper level can be much less. As such the power required for pumping is low. Random photons emitted spontaneously sets stimulated emission and coherent radiation is produced.
From E5 and E3 level neon atom can make downward transition to E2 level. Incoherent light is emitted due to spontaneous transition. As lower levels depopulate faster than upper levels it is easier to maintain population inversion throughout laser operation. E2 is again a metastable state.
Therefore, Ne atoms tends to accumulate at this level again. However, they are made to collide with the walls of discharge tube and they give up their energy and returns to ground state.
Merits:
- Continuous output laser source
- Highly stable
- No separate cooling is required
Demerits :
- Low efficiency and low power output
- Gases are novel medium for laser as gases are found in the purest form so their optical properties are well defined.
Role of Helium atoms:
- Being a good conductor of heat, He acts as a coolant and no separate cooling system is required.
- He atoms being lighter than Ne atoms absorbs the energy from the high energy electrons easily and very fast.
- The ratio He : Ne=10:1 makes the probability of energy transfer for He atoms to Ne atoms much higher than that of the reverse.