High-power ytterbium-doped fibre master-oscillator power-amplifier at 1018 nm

A high-power master-oscillator power-amplifier (MOPA) at 1018 nm employing ytterbium (Yb)-doped fibres as a gain medium is reported. Utilizing a diffraction grating as a reflector, we could successfully suppress the influence of the broadband amplified spontaneous emission on the master-oscillator or the power-amplifier, resulting in stable amplification of the laser signal at 1018 nm. Based on a simple theoretical simulation on gain spectra and experimental investigation on parasitic lasing thresholds, the Yb fibre MOPA constructed in-house yielded 220 W of output at 1018 nm with a beam propagation factor (M2) of 1.1. The prospects for further power scaling are considered.

Beam Propagation Factor of Hollow Higher-order Cosh-Gaussian Beams

Based on the second-order moments definition, we investigate in this paper the beam propagation factor of new mathematical model of Hollow higher-order Cosh-Gaussian (HhCG) beams. Two analytical formulae of the M2-factor of HhCG beams are derived. Moreover, numerical simulations are developed to illustrate the effects of the beams orders n and l, the parameter δ and the beam waist ω0 on the M2-factor. The result shows a more general characteristic of higher-order Cosh-Gaussian, Cosh-Gaussian and the fundamental Gaussian beams can be obtained as specials cases of HhCG beams.

Beam Propagation Factor of a Cosh-Airy Beam

Based on the second-order moments, the analytical expression of the beam propagation factor of a cosh-Airy beam has been derived. The beam propagation factor was determined by the decay factor and the cosh parameter. Because the beam propagation factors in the x- and y-directions of the cosh-Airy beam have the same form, only the beam propagation factor in the x- direction was selected as the object of numerical calculation and analysis. The effects of the decay factor and the cosh parameter on the beam propagation factor were investigated. When the decay factor was greater than 1, the beam propagation factor first increased and then decreased with the increase of the cosh parameter, and finally, tended to a minimum value. Under the condition that the decay factor was less than 1, the beam propagation factor always increased with the increase of the cosh parameter. As the decay factor increased, the beam propagation factor decreased and tended to a minimum value. Finally, the effects of the decay factor and the cosh parameter on the squares of the beam waist and the divergence were analyzed in more detail.