Construction

• A quartz glass tube containing a mixture of helium and neon gases in the ratio 10 : 1 at a low pressure of about 0.6 mm Hg.
• At the two ends of the tube are two parallel mirrors perpendicular to the axis: one has a reflection coefficient about 99.5 %, the other is semitransparent (reflection about 98 %), from which the laser beam leaves.
• A source of high-frequency electric field outside the tube, or a high DC voltage between two electrodes inside, produces an electrical discharge in the gas.

Why helium and neon?

• The laser light comes from neon, the active atoms.
• Helium is added because one of its metastable excited levels has almost exactly the same energy as one of the metastable levels of neon.
• Helium atoms are excited easily by the discharge, and since they are ten times more numerous they transfer energy to neon atoms efficiently by collisions (resonant energy transfer).
• Thus helium acts as the energy "pump" for neon, and is the reason the population inversion of neon can be achieved.