Which one of the following gases is the lasing or active medium in the laser tube?

Explanation

A gas laser is a laser in which an electric current is discharged through a gas inside the laser medium to produce laser light. In gas lasers, the laser medium is in a gaseous state. Gas lasers are used in applications that require laser light with very high beam quality and long coherence lengths. In gas lasers, the laser medium, or gain medium, is made up of a mixture of gases. This mixture is packed into a glass tube. The glass tube filled with a mixture of gases acts as an active medium or laser medium. The gas mixture is mostly helium, so helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to a state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states, responsible for non-laser lines.