Beam quality analysis and optimization for 10 kW-level spectral beam combination system

Abstract Filter-based spectral beam combining provides a promising approach for power scaling and brightness enhancement, especially for broad-linewidth fiber lasers. In this paper, we have demonstrated the beam combining experiments of two high power broad-linewidth fiber lasers using an edge filter as the combining element and achieved 10.25 kW combined power with an efficiency of 97%. Due to the thermal lens effect, the combined beam quality shows obvious signs of deterioration, which occurs in all cases of high power spectral beam combination system, including grating-based and prism-based schemes. The surface deformation and the thermal behavior are monitored in real-time so as to research the mechanism of the combined beam quality deterioration during the combining process. The primary reason for the decline of beam quality could be attributed to the thermal induced change in the refractive index, which not only results in an inhomogeneous wave-front aberration but causes the different focal waists of the incident channels. To counterbalance these aberrations, an air-cooling device and a passively thermally compensated optical system based on CaF2 windows are integrated into the system. In this case, the thermal lensing effect is minimized and the beam quality deterioration factor (BQD) of the combined output beam has been improved 52% in horizontal direction and 101% in vertical direction, showing the great potential of the filter-based approach to achieve a combined beam with high power and excellent beam quality.