Highly efficient dual-grating 3-channel spectral beam combining of narrow-linewidth monolithic cw Yb-doped fiber amplifiers up to 5.5 kW

We present highly efficient three-channel dual-grating spectral beam combining with a combined signal output power of 5.5 kW at an excellent beam quality of M2 = 1.5. Three 2-kW all-fiber narrow-linewidth continuous-wave Ytterbium-doped fiber amplifiers at 1050 nm, 1070 nm and 1090 nm were combined using in-house fabricated polarizationindependent dielectric reflection gratings. The total combining efficiency was 94% at full power level, which is close to the expected value referred to the incorporated grating's efficiency in a dual-grating setup.