Laser power scaling and beam quality improvement with a tapered rod amplifier

In order to achieve an improved beam quality at high laser power in an optical amplification setup we investigated the effect of tapering a large mode area, Yb3+-doped all-solid double-clad fiber. The fiber with a 56μm core and a 460μm pump cladding was fabricated by the powder sintering technology (Repusil) to achieve a homogeneous step index profile. A 0.6m non-tapered fiber and a 0.45m tapered fiber were tested in a three-stage ns-pulsed fiber master oscillator power amplifier (MOPA) system. With the non-tapered fiber, we achieved amplified pulses with 540kW peak power and 1.4mJ pulse energy. The beam quality (M2) is about 10. In order to improve the beam quality, the fiber was tapered down to single-mode size and spliced to the MOPA system for seed coupling to maintain most seed power in fundamental mode without exciting much higher order modes. The results show that the beam quality (M2) was improved to 4. Both amplifier fibers showed relative low stimulated Raman scattering and amplified spontaneous emission. In summary, we demonstrated that local adiabatic tapers for seed coupling can improve the beam quality roughly by a factor of 2.5.