Characteristics of Q-switched cladding-pumped ytterbium-doped fiberlasers with different high-energy fiber designs

摘要

We theoretically and experimentally analyze Q-switched claddingnpumped ytterbium-doped fiber lasers designed for high pulse energies. Wencompare the extractable energy from two high-energy fiber designs: (1)nsingle- or few-moded low-NA large mode area (LMA) fibers and (2)nlarge-core multimode fibers, which may incorporate a fiber taper fornbrightness enhancement. Our results show that the pulse energy isnproportional to the effective core area and, therefore, LMA fibers andnmultimode fibers of comparable core size give comparable results.nHowever, the energy storage in multimode fibers is mostly limited bynstrong losses due to amplified spontaneous emission (ASE) or evennspurious lasing between pulses. The ASE power increases with the numbernof modes in a fiber. Furthermore, spurious feedback is more difficult tonsuppress with a higher NA, and Rayleigh back-scattering increases withnhigher NA, too. These effects are smaller in low-NA LMA fibers, allowingnfor somewhat higher energy storage. For the LMA fibers, we found thatnfacet damage was a more severe restriction than ASE losses or spuriousnlasing. With a modified laser cavity, we could avoid facet damage in thenLMA fiber, and reached output pulse energies as high as 2.3 mJ, limitednby ASE. Theoretical estimates suggest that output pulse energies aroundn10 mJ are feasible with a larger core fiber, while maintaining a goodnbeam quality