Laser peak power scaling and beam quality improvement with Ytterbium rod-type fiber amplifiers made by powder sinter technology
Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. An approach of local adiabatic taper is proposed to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. The powder-sintering technology was employed to achieve rod-type fibers with excellent refractive index homogeneity. First experiments were performed with 56m core diameter rod fibers. While non-tapered fiber amplifier achieved a peak power of 544kW, tapered amplifier reached 230kW. For comparable average powers of 10W, the taper improves the beam quality from M2 values of about 10 to 3.5, while the monolithic seed coupling significantly improves the beam stability. It was observed that the dopants diffuse during the tapering process because of high temperature, possibly providing further sources for coupling to higher order modes. Second experiments with improved rod-type fiber amplifiers (reduce the Al3+-content of fiber core and use suitable material of outer clad to mitigate the diffusion problem) delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). An endcap was adopted for the tapered fiber amplifier and the peak power is scaled up to 375kW in the nearly diffraction limited region. For future work, the confined doped fibers and a picosecond seed laser source are envisioned.