A 100-W 1178-nm continuous-wave single-frequency linearly polarized Raman fiber amplifier

We report a 100-W continuous-wave (CW) 1178-nm narrowband polarization-maintaining (PM) Raman fiber amplifier (RFA) based on ESO’s patented RFA technology. A linearly-polarized 15-mW narrow-linewidth 1178-nm seed laser was amplified in a two-stage PM RFA counter-pumped by a PM 1120-nm fiber laser. Efficient suppression of stimulated Brillouin scattering (SBS) pushed the SBS threshold above 100 W. The Raman fiber types and lengths were chosen to maximize the Raman conversion efficiency in order to mitigate the thermal loads on critical optical components in the RFA. At 100 W of RFA output, which to the best of our knowledge is a record for such an RFA, the required 1120-nm pump laser power is only 180 W. Measurements of the linewidth of the amplified 1178-nm output at different power levels confirmed that the narrow linewidth of the 1178-nm seed laser is preserved even at 100 W of RFA output. The measured polarization extinction ratio (PER) at the 100-W level is > 24 dB, ensuring efficient second harmonic generation. Assuming a conservative conversion efficiency of ~ 80% by a resonant frequency doubler, narrow-linewidth CW guide star lasers with powers > 80 W at 589 nm can confidently be expected. The output spectrum measured at 100 W reveals that there is no risk of power migration from 1178 nm to the next Stokes order, which indicates that the RFA is still SBS-limited. With further optimization of the SBS suppression, it is expected that the RFA output power can be scaled beyond the current 100-W level.