He-Ne laser

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 of 10:1. Electrodes are provided to produce a discharge in the gas and they are connected to a high voltage power supply. The tube is sealed by inclined windows arranged at its end. On the axis of the tube, two reflectors are fixed which forms a resonator.

He Ne gas laser employs 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 the electric field, the electrons and ions will be accelerated towards anode and cathode. Since electrons 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 $F_2$ and $F_3$ which lie at 19 EV and 20 EV above the ground state. These are metastable states for helium.

Though the radiative transitions are forbidden, the excited He atom can return to the ground state by transferring its energy to Ne atoms through collision. Such an energy transfer can take place only when the two colliding atoms have identical energy states. $E_6$ and $E_4$ level of Ne atom nearly coincides with $F_3$ and $F_2$ of Helium.

Ne atoms acquire energy and go to an excited state and helium atoms return to the ground state by transferring their energy to Ne atoms. This is the 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. $E_6$ and $E_4$ states are metastable states as collision goes on neon atoms accumulate in these states whereas $E_5$ and $E_3$ level of neon are sparsely populated.

Therefore, a state of population inversion is achieved between $E_6$and $E_5$, $E_6$ and $E_3$ and $E_4$ and $E_3$. 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 the 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 $E_5$ and $E_3$ level neon atom can make downward transition to $E_2$ 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. $E_2$ is again a metastable state.

Therefore, Ne atoms tend to accumulate at this level again. However, they are made to collide with the walls of the discharge tube and they give up their energy and return to the ground state.

Role of Helium atoms:

Being a good conductor of heat, He acts as a coolant, and no separate cooling system is required.

The 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.

Merits:

• Continuous output laser source
• Highly stable
• No separate cooling is required

Demerits :

• Low efficiency and low power output
• Gases are a novel medium for laser as gases are found in the purest form so their optical properties are well defined.