Using a fiber as a laser medium gives a long interaction length, which works well for diode-pumping, and also can used in . This geometry results in high photon conversion efficiency, as well as a rugged and compact design. When fiber components are spliced together, there are no discrete optics to adjust or to get out of alignment.

The fiber-based laser design is highly adaptable. It can be adapted to do anything from welding heavy sheets of metal to producing femtosecond pulses. Many variations exist on the fiber-laser theme, as well as some configurations that are not, strictly speaking, fiber lasers. Fiber amplifiers provide single-pass amplification; they're used in telecommunications because they can amplify many wavelengths simultaneously. Fiber amplification is also used in the master-oscillator power-amplifier (MOPA) configuration, where the intent is to generate a higher output from a fiber laser. In some circumstances, an amplifier is used even with a continuous-wave (CW) laser.

Another example is fiber-amplified spontaneous-emission sources, in which the stimulated emission is suppressed. Yet another example is the Raman fiber laser, which relies on Raman gain that essentially Raman-shifts the wavelength. This is an application that's not in wide use, although it's certainly being used in research. In fact, some new research is going on into using fluoride glass fibers for Raman lasing and amplification rather than the standard silica fibers.

However, the fiber host is usually silica glass with a rare earth dopant in the core. The primary dopants are ytterbium and erbium. Ytterbium has center wavelengths ranging from about 1030 to 1080 nm and can emit in a broader range of wavelengths if pushed. Using pump diodes emitting in the 940 nm range can make the photon deficit very small. Ytterbium has none of the self-quenching effects that occur in neodymium at high densities, which is why neodymium is used in bulk lasers and ytterbium is used in fiber lasers (they both provide roughly the same wavelength).

Erbium fiber lasers emit at 1530 to 1620 nm, which is an eye-safe wavelength range. This can be frequency-doubled to generate light at 780 nm—a wavelength that's not available from fiber lasers in other ways. And finally, ytterbium can be added to erbium so that the ytterbium absorbs pump light and transfers that energy to erbium. Thulium is another dopant that emits even deeper into the near-infrared (NIR; 1750 to 2100 nm), and is thus another eye-safe material.