JEE Main & Advanced Physics Communication System / संचार तंत्र Helium-Neon Laser

This laser contains a mixture of helium \[(\approx 90%)\] and neon \[(\approx 10%)\] at low pressure in a cylindrical tube with mirrors at each end. The energy level diagram in figure shows the important energy levels for the helium and neon atoms. A large electric field is established in the tube by electrodes connected to a high-voltage power supply, Electrons from ionized atoms are accelerated by the field and collide with atoms.

Because of the energy-level structure for helium, collisions often excite helium atoms to the level labeled \[{{E}_{3}}\] in the figure. In a process called collision transfer, energy is transferred from excited helium atoms to neon atoms during collisions, thus producing a population of neon atoms in the \[{{E}_{2}}\] level. The transition from level \[{{E}_{2}}\] to \[{{E}_{1}}\] in neon is forbidden, but the transition out of the \[{{E}_{1}}\] level is allowed. This means that the population of atoms in the \[{{E}_{2}}\] level builds up, and that of the \[{{E}_{1}}\] level is rapidly depleted.

Stimulated emission from \[{{E}_{2}}\] to \[{{E}_{1}}\] predominates and laser light is generated.

The mirrors at each end of the tube encourage emissions along the tube axis by reflecting the light back and forth inside the tube. One of the mirrors is slightly leaky, transmitting about 1 percent of the incident light. This transmitted light forms the laser beam which we find so useful.