Engineering Physics

10.6 Helium–Neon gas [He–Ne] laser

Helium–Neon gas laser is a continuous four level gas laser. It consists of a long, narrow cylindrical tube made up of fused quartz. The diameter of the tube will vary from 2 to 8 mm and length will vary from 10 to 100 cm. Flat quartz plates are sealed at the ends of the tube, the plates are sealed at Brewster angle with the axis of the tube to obtain polarized laser light as shown in Fig. 10.4. So, the plates are called Brewster windows. The tube is filled with helium and neon gases in the ratio of 10:1. The partial pressure of helium gas is 1 mm of Hg and neon gas is 0.1 mm of Hg, so that the pressure of the mixture of gases inside the tube is nearly 1 mm of Hg.

Laser action is due to the neon atoms. Helium ...