Helium-Neon Laser

Unfortunately, this is a low-resolution picture of a helium-neon gas laser. The red laser beam can be seen on the left emanating from the partial mirror at the end of the resonating cavity. Early He-Ne lasers had external mirrors and the discharge tube used ‘Brewster- angled’ windows at the end of the cavity. This prevents reflections from the gas tube ends instead of the mirrors. Later, the mirrors were glued to the cavity tube ends and sealing the tube. Although this allowed the mirror alignment to be factory set, there was an issue with helium escaping through the glue seals. This resulted in a relatively short life of the laser tube. Interestingly, a tube suffering from this would light as normal but not lase. This tube was of a later design where the seals were hardened and useful life extended considerably. Producing a red laser with a He-Ne discharge is now confined to history due to its low efficiency and the advent of semiconductor lasers.