From the article Fiber Lasers: Not Just for Materials Processing Anymore

In its simplest form, a fiber laser can be viewed as a brightness enhancer for diode lasers. Although diode lasers have electrical efficiencies of 70 percent or higher, they generate low-brightness, divergent beams. Even so, diodes serve as efficient pumps for fiber lasers, which effectively convert their output into high-brightness, narrow-output beams.

When doped with various active elements, the fiber component can produce a variety of wavelengths in the infrared. Erbium, ytterbium and thulium, for example, generate outputs at 1, 1.5, and 2 μms, respectively.

The flexibility of fiber lasers is particularly exemplified when configured in a master oscillator power amplifier (MOPA) architecture. Such configurations begin with a seed source such as a simple, narrow-linewidth semiconductor laser that can modulate its repetition rate and pulse duration electronically. Injecting the seed pulses from this source into a well-designed MOPA amplifies them to higher power levels without compromising any of the base qualities of those pulses.

This is not as trivial a design task as it sounds. MOPAs can generate sufficient power to self-lase, to distort the pulse and to degrade spectrally. So care must be taken to avoid such issues as well as the spontaneous emission in the amplifier chains. In a well-designed system, however, the pulse duration, peak power, spectral content, polarization and beam quality are all at your fingertips