Kilowatt-level narrow-linewidth cascaded Yb-Raman fiber amplifier at 1178 nm

A kilowatt-level narrow-linewidth all-fiber amplifier at 1178 nm based on cascaded Yb-Raman hybrid gains is demonstrated for the first time, to the best of our knowledge. Three phase-modulated single-frequency fiber sources operating at 1071 nm, 1120 nm, and 1178 nm are applied in this amplifier. They can extract Yb ion gain and function as a temporally stable 1st-order Raman-pumped laser, a 2nd-order Raman-pumped laser, and a Raman signal laser, respectively. This configuration enables simultaneous cascaded Raman conversion and narrow-linewidth operation. By theoretically selecting the suitable length of the large-mode-area Yb-doped fiber in the main amplifier, efficient power conversion to the laser at 1178 nm is realized. A 1.14-kW narrow-linewidth cascaded Yb-Raman fiber amplifier at 1178 nm is obtained with a slope efficiency of 77%. The 3-dB linewidth of the amplifier at 1178 nm broadens little during amplification. At the maximum output power, the higher-order Raman is not observed in the output spectrum, and the beam quality factors of the amplifier are M_x² = 1.29 and M_y² = 1.31. Our study proves that using cascaded Yb-Raman hybrid gains in cooperation with temporally stable fiber sources is promising for the power scaling of narrow-linewidth fiber amplifiers at the long-wavelength extreme of the Yb gain spectrum.