Abstract

To achieve a high spectral purity Raman laser in a wide range of pump power, we propose the adjustable Raman threshold method and experimentally demonstrate a 6.85 kW ytterbium-Raman (Yb-Raman) fiber amplifier in this paper. The Raman threshold is adjusted to coordinate with pump power level via manipulating the Raman proportion from the master oscillator. The spectral purity of Raman laser at 1120 nm reaches 86.0 $\%$ at the highest output power and keeps over 65 $\%$ throughout. And the 3 dB bandwidth is maintained within 2.5 nm. The system shows potential in further power scaling-up because the four-wave-mixing-induced high-order Raman noise is circumvented by designing wavelengths of the seeders. This work not only reports the highest output power of the Yb-Raman fiber lasers (to the best of our knowledge), but also provides an approach to achieve an agile lasing threshold in the Yb-Raman fiber amplifiers without any structural adjustments. The Yb-Raman fiber amplifiers with high spectral purity over a broad range of power will have promising applications in communications, industry and scientific researches.

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