Question

Explain construction and working of He:Ne laser with neat label diagram. 

Answer 1

A Helium–Neon laser or He-Ne laser, is a type of gas laser whose gain medium consists of a mixture of 85% helium and 15% neon inside of a small electrical discharge. The best-known and most widely used HeNe laser operates at a wavelength of 6328 A^o, in the red part of the visible spectrum. 

CONSTRUCTION OF He-Ne LASER :

1.The tube where the lasing action takes place consists of a glass envelop with a narrow capillary tube through the center.

2.The capillary tube is designed to direct the electrical discharge through its small bore to produce very high current densities in the gas.

3.The outer coupler and the HR(high reflective mirror) are located at the opposite ends of the plasma tube.

4.In order to make laser tubes more economical and durable manufacturers often attach the mirrors directly to the ends of the capillary tube.This is very common with small low power LASERS.

5.With high power tubes or when optically polarized output is desired,the capillary tubes ends are cut at an angle and sealed with glass planes called Brewster windows.

6.The plasma tube has a large cylindrical metallic cathode and a smaller metallic anode.The current is directed from cathode to anode.

7.In He-Ne LASER active medium is low pressure gas mixture of Helium and Neon which is contained in the plasma tube.

1.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.

2.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.

3.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.

4.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 E₆ and E₅, E₆ and E₃ and E₄ and E₃.

5.Consequently, three laser transitions take place.

`E_6  "to"  E_5   33900 A^°` (far IR region)

`E_6  "to"   E_3  6328 A^°` (visible)

`E_4  " to "  E_3  11500 A^°` (IR region)

6.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.

7.From E5 and E 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.

8.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.