A laser resonator that generates a laser beam by the existence of an Erbium doped fiber with light
This resonator type follows the basic principle of how to create a laser beam where a medium is "pumped" with energy to then emit photons when the medium returns to its natural state.
In a Fiber laser, it is usually an Erbium doped fiber that is the medium itself. The fiber itself consists of three different layers, the inner Erbium-doped core , a transparent intermediate layer  and the outer mantle .
The pumping is done by relatively inexpensive diode lasers which inject laser light into the fiber . The light is guided and reflected on the outer mantle and passes through the fiber core repeatedly, raising the electrons of the medium to a higher energy level (exites) and then being stimulated to return to its basic state (emitting), releasing the bundles of light (photons).
In this way, the laser beam is generated along the direction of the fiber and is then emitted  and passed on via fiber up to the application.
Several different types of fiber generators are available. For example, in some cases the beam is bounced between mirrors at the ends or the pumping is not done from the end but via one or more inlets along the length of the fiber.
An advantage of this laser source compared to CO2 and Nd: YAG lasers is that it is virtually maintenance-free.