Master oscillator fiber amplifier

The gain can be as high as tens of dB. And many bulk amplifiers, especially with high average output power, have very low gain.

Figure 1: MOPA schematic of a single-stage pump core. For higher power, a second double-clad fiber amplifier needs to be added. Seed laser diodes can work in the pulsed domain

However, the use of optical fibers also has some disadvantages:
Due to the existence of various fiber nonlinear effects, it is difficult to obtain high peak power and pulse energy in pulsed systems. For example, for fiber optic devices, energies of a few millijoules are already high in nanosecond pulsed systems, and bulk lasers can deliver even higher energies. In single-frequency systems, stimulated Brillouin scattering can greatly limit the output power.
Due to their high gain, fiber amplifiers are particularly sensitive to back reflections. When the power is very high, it is difficult to solve this problem with Faraday isolators.
The polarization state is usually unstable unless polarization-maintaining fibers are used.
It is advantageous to employ gain-switched laser diodes as seed lasers in fiber MOPAs. This device can be compared to Q-switched lasers, for example, in applications in the laser market. Part of this advantage lies in the flexibility of the output form: not only the pulse repetition rate but also the pulse length and shape, and of course the pulse energy, can be adjusted.
An issue to be considered in MOFAs is the saturation power, which is low even in a large mode area double-clad fiber relative to the usual output power. Therefore, power extraction can be as efficient as fiber lasers, even at relatively low seed powers.