In this paper, we investigated the SBS suppression potential in a fiber amplifier with both cylindrical vector beam output and nonlinear spectral compression method applied. By applying different kinds of modes output and different spectral management schemes, the SBS threshold of a same fiber amplifier was measured to be 67 W, 95 W and 104 W. These schemes correspond to LP01 mode output with spectrum maintained, TM01 mode output with spectrum maintained and TM01 mode output with spectrum narrowed, respectively. Experimental results indicated that the SBS threshold of our fiber amplifier with cylindrical vector beam output and spectrum compressed can be enhanced by 1.56 times, comparing to the traditional Gaussian beam one. The combination of the two techniques can pave the way for mitigating the SBS effect in high-power narrow-linewidth fiber amplifiers.
For powers exceeding a threshold the spectral broadening in fiber amplifiers becomes a significant challenge for the development of high power narrow bandwidth fiber lasers. In this letter, we show that the spectral broadening can be partly caused by four-wave mixing(FWM) process in which the power of the central wavelength would transfer to the side ones. A practical FWM induced spectral broadening theory has been derived from the early works. A numerical model of fiber amplifier has been established and FWM process has been added to the model. During the simulation process, we find that when a 10 GHz, several watts narrow bandwidth laser is seeded into a few modes fiber laser amplifier, the FWM induced spectral broadening effect might continually increase the FWHM of the spectra of the continuum laser to 100 GHz within the amplification process to several hundred watts which has been convinced by our experiments. Some other results have also been analyzed in this paper to complete the four-wave mixing induced spectral broadening theory in fiber amplifiers.
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