One of the most common types of
Fibre lasers is the Erbium-Doped Fibre Laser. We use an otherwise-normal optical fibre, made out of silica glass. We add to it very small amounts of the rare-earth element Erbium. The small particles of Erbium are mixed in to the core of the fibre when it is made. This process of introducing small amounts of another element is called doping.
The reason we use Erbium is because the Erbium atoms have very useful energy levels. There is an energy level that can absorb photons with a wavelength of 980nm, and this then decays to a meta-stable state equivalent to 1550nm.
This means that we can use a cheap diode laser 'pump source' at 980nm and we get a very high quality, and potentially very high power beam out at 1550nm.
