High-energy and high-peak-power nanosecond pulse generation with beam quality control in 200-μm core highly multimode Yb-doped fiber amplifiers

We explored high-energy and high-peak-power pulse generation in large-core multimode fiber amplifiers, achieving what is to our knowledge the highest reported energies, up to 82 mJ for 500-ns pulses, 27 mJ for 50-ns pulses, and 2.4-MW peak power for 4-ns pulses at 1064 nm, using 200-mum-diameter and 0.062-N.A. core Yb-doped double-clad fiber amplifiers. The highly multimode nature of the fiber core was mitigated by use of a coiling-induced mode-filtering effect to yield a significant improvement in output-beam quality from M-2 = 25 from an uncoiled fiber to M-2 = 6.5 from a properly coiled fiber, with the corresponding reduction in number of propagating transverse modes from greater than or equal to200 to less than or equal to20. (C) 2005 Optical Society of America.