Fiber lasers form the basis for fiber amplifiers and correspond in structure to an optical fiber. They consist of a core glass only a few micrometers thick, which is concentrically surrounded by one or more cladding glasses. The core glass has a higher refractive index than the cladding glasses.
This structure is identical for fiber lasers and optical fib ers. The differences between the core glass of an optical fiber and that of a fiber laser lie in the doping. The core glass of fiber lasers is doped with rare earth ions such as erbium (Er), ytterbium( Yb), neodymium( Nd), and thulium (Tm), and the cladding glass usually consists of multiple layers. Each doping emits a different wavelength. Examples of corresponding fiber lasers include the erbium-doped EDFA amplifier, which emits infrared light with a wavelength of 1.5 µm, the ytterbium-doped YDFA amplifier with an infrared emission of 1.1 µm, and the thulium-doped TDFA amplifier with a light emission between 1,460 µm and 1,530 µm.
By doping the fiber core with ions, it can amplify light and thus perform the functions of a fiber amplifier. The light emission is activated by powerful pump lasers whose pump beam is coupled into the cladding glass. The pump beam is coupled over the entire fiber length from the pump core into the core glass, where it activates the laser process. Before activation with the pump beam, the electrons are in the valence band, from where they exceed the energy potential of the band gap due to the pump beam energy and are transported into the conduction band. From there they fall back into the valence band, they recombine with the holes and emit photons. Since the pump beam is active over the entire fiber length, an interaction is achieved over the entire fiber length.
Since fiber lasers consist of several concentrically arranged cladding glasses - double clad fiber ( DCF) - the light remains in the inner cladding glass because it is repeatedly reflected by the outer cladding glass. It is reflected from the interface between the inner and outer cladding glass and passes through the core glass, where it triggers the light amplification.