LASERs have been around for almost 50 years now, and their usefulness as tools to probe the physical constantly increase. The History of Lasers is a long one starting with Einstien who first theorized about Stimulated Emmisions . This process is an important one for the Helium-Neon Laser, for without it, it would not work.

The Chamber of the He-Ne laser is filled with 90% Helium and 10% Neon, and these gases are held in a state of Population Inversion where the majority of Helium atoms are held into excited states as opposed to their ground state. Neon has an energy tranition that matches the Helium excited state, which because of quantem mechanical selection rules can not de-excite, so that when an excited Helium and Neon atom colide the Neon atom is put into a high energy meta stable state. The Neon atom is allowed to de-excite emmitting a photon and this photon is allowed to stimulate emmisions from other excited Neon atoms it interacts with. Inside the laser cavity there are two mirrors along the lengths, one with a 1% or lest trasmission rate. That way most of the light is reflected back to stiumlate more emissions of the same photons.

The cavity of He-Ne laser has resonance for different frequencies equal to n*c/(2*L), where n is an interger, c is the speed of light, and L is the length of the cavity. Even though all Neon atoms are emitting the same photon from their rest frame, they are all moving with respect to each other. This produces a doppler shift in their photons, cause the range of frequencies seen to broaden. The resonance and the doppler broadening are is the reasons why in actual lasers more than one wavelength is seen in their light. There reasons also contribute to the coherence length that is found with lasers. Some of these Helium Neon Lasers are also found with that the laser light has a poliarization associated with it. For more links with laser properties one can also go Here, or for a good over view of lasers you may also go Here .

Because of their properties, lasers have a wide range of applications. The can be used to produce Holograms, to catch Molecules, Machining, and applications in Biology.