A novel technique to suppress the stimulated Raman scattering (SRS) effect in high-power ytterbium-doped fiber amplifier was 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 depletes 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 boosts the gain of the long-wavelength signal laser, enabling it to undergo rapid amplification towards the end of the amplifier. Compared to 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 shortening the effective length and alleviating the intensity of the interaction between the signal laser and Stokes wave for higher SRS threshold.
Through numerical simulation, the SRS threshold of a 20/400 μm fiber amplifier is investigated under different wavelength and power ratio of the auxiliary laser with signal laser. The results indicate that incorporating an auxiliary laser with an appropriate wavelength and power level can significantly decrease the interaction intensity between the signal and Stokes wave, thereby enhancing the SRS threshold of the amplifier efficiently. Specifically, in a 1080 nm fiber amplifier utilizing a 20/400 μm 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, a substantial increase in the SRS threshold from 3.14 kW (singular signal laser) to 8.42 kW can be anticipated. Moreover, based on the auxiliary laser amplification technique to suppress 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 designed to mitigate the SRS effect, which is promising to promote further power scaling of all-fiber amplifier.