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Pulsed ytterbium-doped fiber amplifiers (YDFA) with ns-level signal width are important devices for obtaining high-power pulsed lasers. When some components in the amplifier are ineffective, e.g., the isolator or fiber is damaged, extra feedback light is generated and coupled into the gain fiber. The dynamic thermal distribution and waveform evolution of amplifiers with extra continuous-wave (CW) or pulse-wave (PW) feedback are theoretically analyzed in this work. The CW feedback can not only reduce the gain of the amplifier but can also change the thermal distribution of the gain fiber, while the PW feedback can reduce the leading or trailing edge of the output pulse by 3–4 ns, depending on the direction of the feedback light transmission. The theoretical analysis provides a reference for optimizing the thermal management and the fault diagnosis of a typical fiber amplifier with an output of several tens of watts. [ABSTRACT FROM AUTHOR]

The performance of doped fiber amplifiers can be enhanced significantly with the help of multi-stage pumping technique provided that various critical parameters of pumps including their optical power and wavelength are optimized. We report the performance enhancement of a ytterbium doped fiber amplifier (YDFA) for a 1.02–1.08 μ m spectral region with an optimized design based on a novel dual-stage in-band asymmetrical pumping scheme. By accurately adjusting the optical power and wavelength of pumps in both the stages, a record peak gain of around 62.5 dB and output power of 4.5 W are achieved for a signal wavelength of 1.0329 μ m at an optimized length of Ytterbium-doped silica fiber and optimized doping concentration of Yb 3 + . Moreover, a minimum noise figure (NF) of 4 dB is observed for a signal wavelength of 1.0329 μ m at the optimized parameters. Similarly, the effect of using high and low pump powers at the first and the second stage, respectively, on NF of the amplifier is also investigated at different values of signal powers. It is observed that the value of NF increases significantly by using high pump power at the first stage and low pump power at the second stage. [ABSTRACT FROM AUTHOR]

• Proposes the theoretical model of the ytterbium fiber amplifier based on the conservation laws. • Physical limits of this approach applicability are shown. • Presents a comparison of the proposed model and the models of a fiber amplifier. Based on conservation laws, kinetic equations for changes in ion populations in different energy states are written down, as well as equations how optical radiation intensity (and power) is changing in the optical fiber core is sequentially derived. Physical limitations of the given approach are also considered. A comparison of the results of a numerical analysis of the formation of luminescence in the ytterbium-doped optical fiber (with different representations of the probability of "survival" of spontaneous emission photons) with experimental data is presented. [ABSTRACT FROM AUTHOR]