Stimulated Brillouin scattering (SBS) in optical-fiber gain media continues to be one of the major obstacles to laser-power scaling in fiber-based, narrow-linewidth optical amplifiers. But by physically straining the optical-fiber gain medium itself, researchers at the Chinese Academy of Sciences (Shanghai, China) have demonstrated a 170 W linearly polarized singlemode amplifier (10 μm fiber core) with a 7X increase in the SBS threshold—the highest output from a single-frequency, true-singlemode, polarization-maintaining, all-fiber amplifier, according to the researchers. The true singlemode output has high pointing stability, which is crucial for applications such as gravitational-wave detection and coherent beam combination.
A 50 mW, 1064 nm seed laser is first amplified to 3 W using conventional ytterbium (Yb) fiber amplifiers; next, ten 25 W laser diodes pump another length of fiber (210 W pump power total after losses). For the last stage of the amplifier, 3.9 m of PLMA-YDF-10/125 gain fiber from Nufern (East Granby, CT) is wound onto spools with variable tension to induce 20 different strain values or steps along the fiber length that have been mathematically determined to optimize SBS suppression for this fiber configuration and length. Amplifier output power for an unstrained fiber reaches just 24.5 W as the backward power (Rayleigh scattering + SBS) increases dramatically to 38 mW. With the strain applied to the fiber, however, power reaches 170 W as backward power is efficiently suppressed. Contact Yan Feng at [email protected].
