Suppression effect of auxiliary laser on stimulated Raman scattering effect of high-power Yb-doped fiber laser amplifier

A novel technique to suppress the stimulated Raman scattering (SRS) effect in high-power ytterbium- doped fiber amplifier is proposed and theoretically investigated by introducing an auxiliary laser to manipulate the gain distribution in the amplifier. By injecting an auxiliary laser with shorter wavelength than the signal into the amplifier, the auxiliary laser, owing to its larger stimulated emission cross-section, initially extracts a significant portion of the laser gain. At this point, the gain of the longer-wavelength signal laser is suppressed to a certain extent. As the pump power is depleted in the rear segment of the gain fiber, the amplified auxiliary laser, which has larger absorption cross-section than the signal, is gradually absorbed by the active fiber and transfers its power to the signal laser. This process enhances the gain of the long-wavelength signal laser, enabling it to be rapidly amplified at the end of the amplifier. Compared with the amplification of the singular signal laser, the introduction of an extra auxiliary laser shifts the high-gain region of the signal laser to the rear portion of the amplifier, thereby reducing the effective length and alleviating the interaction strength between the signal laser and Stokes wave, in order to obtain a higher SRS threshold. The SRS threshold of a 20 mu m/400 mu m fiber amplifier is investigated by using numerical simulation under different wavelengths of the auxiliary laser and different power ratios of the signal laser to auxiliary laser. The results indicate that incorporating an auxiliary laser with an appropriate wavelength and power level can significantly reduce the interaction strength between the signal and Stokes wave, thereby enhancing the SRS threshold of the amplifier efficiently. Specifically, in a 1080 nm fiber amplifier utilizing a 20 mu m/400 mu m ytterbium-doped large mode area fiber, if the total power of the 1080 nm signal and 1040 nm auxiliary laser is set to 200 W, while with a power ratio of 1:25, the SRS threshold increasing from 3.14 kW (singular signal laser) to 8.42 kW can be anticipated. Moreover, based on the auxiliary laser amplification technique that suppresses the SRS effect, the output power enhancement of fiber lasers with the structure of master oscillator power amplifier (MOPA) is also analyzed. This technical solution is relatively straightforward to implement and can be seamlessly integrated with other techniques aimed at reducing the SRS effect, which is promising to promote further power scaling of all-fiber amplifier.