HELIUM–NEON   LASERS

       The first gas laser, the helium–neon (HeNe) laser, is still an important source of coherent red light with uses ranging from bar-code scanning to alignment. At one time the most popular laser, the dominance of this laser in the market is decaying rapidly, due to inexpensive, reliable, and small semiconductor lasers operating in the same range of wavelengths and power levels as the HeNe. Although the semiconductor cousin of the HeNe lacks the beam quality of the HeNe, this is not important for many applications.

       HeNe lasers normally operate with the familiar red (632.8 nm) beam, but multiple transitions are possible, allowing the laser to operate (with suitable optics) at wavelengths in the infrared, orange, yellow, and green. Power output for commercially available HeNe tubes ranges from under 1 mW for a small HeNe tube to just over 100 mW for a large behemoth unit. These lasers typically feature excellent spectral and coherence characteristics, so are a popular choice for uses in holography, where they are still the laser of choice, preferred over semiconductor types. They are also the de facto standard laboratory laser, used for a myriad of odd jobs around the lab, including alignment and testing of optical components.