The most common and inexpensive gas laser, the helium-neon laser is usually constructed to operate in the red at 632.8 nm. It can also be constructed to produce laser action in the green at 543.5 nm and in the infrared at 1523 nm.
The collimation of the beam is accomplished by mirrors on each end of the evacuated glass tube which contains about 85% helium and 15% neon gas at 1/300 atmospheres pressure (Metrologic). These mirrors could be both flat, but this requires great precision in alignment, so the common laboratory He-Ne lasers are manufactured with the semiconfocal mirror arrangement shown.
The helium gas in the laser tube provides the pumping medium to attain the necessary population inversion for laser action.