Pump noise transfer in a highly efficient, in-band pumped thulium-doped fiber amplifier

A wide range of applications in material’s processing, remote sensing and frequency conversion processes for mid-IR or EUV generation benefit from high-power laser sources in the 2 µm wavelength region [1-3]. Thulium (Tm-) doped fiber lasers have shown already a tremendous scaling potential in terms of average power and high-energy extraction [4,5]. Usually, high-power, Tm-doped fiber amplifiers are pumped at 793 nm. Due to the large quantum defect of ~ 60 % such systems face considerable thermal challenges [5]. This large quantum defect can be overcome by applying in-band pump sources in the wavelength range between 1550 nm and 1900 nm [6-8]. A promising, power scalable concept is to use Raman fiber lasers as pump sources emitting at 1692 nm [9]. Due to their nonlinearity-based design, these pump sources contribute a certain amount of amplitude fluctuations. If the noise of the pump source is transferred to the signal noise within the amplification process, this could be detrimental and a disadvantage. Therefore, in this work we present our results in terms of efficiency and relative amplitude noise transfer to the amplified signal, while in-band pumping in the context of Thulium-doped fiber amplifier compared to the state-of the art pumping scheme at 793 nm.