Fiber lasers usually refer to lasers that use optical fibers as the gain medium, although some lasers that use semiconductor gain media (semiconductor optical amplifiers) and fiber resonant cavities can also be called fiber lasers (or semiconductor optical lasers). In addition, some other types of lasers (e.g., fiber-coupled semiconductor diodes) and fiber amplifiers are also called fiber lasers (or fiber laser systems).

The gain medium in most cases is a rare-earth ion-doped fiber, such as erbium (Er3+), ytterbium (Yb3+), thorium (Tm3+), or praseodymium (Pr3+), and needs to be pumped by one or more fiber-coupled laser diodes. Although the gain medium of fiber lasers is similar to that of solid-state bulk lasers, waveguide effects and small effective mode areas result in lasers with different properties. For example, they typically have high laser gain and resonant cavity losses. See the terms fiber laser and body laser.

1. In a laboratory setup, a common dichroic reflector can be used at the vertically cut fiber port, as shown in Figure 1. However, this solution cannot be used for mass production and is not durable.

2. The Fresnel reflection at the end face of the bare fiber is sufficient to act as an output coupler for the fiber laser. An example is given in Fig. 2.

3. It is also possible to deposit a dielectric coating directly on the fiber port, usually by evaporation. Such coatings give a large reflectivity over a wide range.

4. For commercial products, fiber Bragg gratings are usually used, which can be prepared directly from doped fibers or by fusing undoped fibers to active fibers. Figure 3 shows a Distributed Bragg Reflection laser (DBR laser) that contains two fiber gratings, and a distributed feedback laser exists where there is a grating in the doped fiber with a phase shift in between.

5. If the light exiting the fiber is collimated using a lens and reflected back through a dichroic reflector, better power handling can be obtained (e.g., Figure 4). The light obtained by the reflector will have a greatly reduced intensity due to having a larger beam area. However, a slight misalignment can cause significant reflection losses and additional Fresnel reflections at the fiber end face can create a filtering effect. The latter can be suppressed by using tilt-cut fiber ports, but this introduces wavelength-dependent losses.

6. An optical loop reflector can also be formed (Figure 5), utilizing a fiber coupler and passive fiber.

Most optical lasers are pumped by one or more fiber-coupled semiconductor lasers. The pump light is coupled directly into the core, or at high power into the pump cladding (see Dual Cladding Fibers), as discussed in more detail below.