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In this paper, two monolithic 1018 nm Yb-doped fiber laser systems-one as an oscillator (OSC) and another as a specially-designed amplifier (SDA)-are investigated using 20/400 μm Yb-doped fibers (YDF). 394 W of output power and a slope efficiency of 39.4% was obtained from the OSC setup. The SDA setup resulted in 407 W of output power with a slope efficiency of 39.9%, which is the highest reported output power via the 20/400 μm YDF. The beam quality factor M 2 is also measured to be ~1.5 for both setups. Considerations regarding the gain fiber length were taken into account so as to suppress any detrimental effects such as ASE and self-pulsing. The output power behavior of both setups indicate that further power scaling would be possible.

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The behavior of the high-order harmonics and output attosecond pulses from hydrogen molecule ions with various internuclear distances that are exposed to high intensity incoming pulses are investigated. The incoming pulses that are spectrally wide yield from a superposition of monochromatic beams with a constant frequency distance. Our simulations show that the most intense and shortest attosecond pulses can result from hydrogen molecular ions with large internuclear distances which are exposed to irradiation of intense pulses with a frequency width greater than 0.03 a.u.