Abstract
In this work, mode evolution in the presence of stimulated Raman scattering (SRS) effect has been studied in a 2 kW polarization-maintained (PM) fiber amplifier. It proved that the SRS-induced mode distortion can be observed at 40 dB of signal to Raman noise ratio in a PM fiber amplifier, which is significantly lower than that in a non-PM system. By using mode decomposition (MD) method, the SRS-induced mode distortion has been analyzed quantitatively by decomposing the signal and the Raman light individually, and the signal mode evolution shows a dissimilar phenomenon compared with the SRS-induced transverse mode instability. The SRS effect in large mode area fiber lasers result in continuous excitation of signal high order modes (HOMs), which leads to the signal beam quality deterioration and the output power rollover. The power fraction of HOMs increases by 30% with the Raman power ratio only being 1.67% at the maximal output power, and the Raman Stokes light mainly exists in fundamental mode distribution, which can be attributed to the beam cleanup effect.
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