Power Scaling Approach for Visible Lasers Based on Multi-Axial Mode External Resonant Second Harmonic Generation

Advanced material processing of a wide range of materials has increasingly relied on the use of laser sources. Near-infrared fibre lasers emitting around 1 μm have proven particularly efficient for techniques requiring both high power and high beam quality. However, the renewed market for clean energy sources has increased the demand for non-ferrous materials such as copper, which exhibits high reflectivity at this wavelength range. A laser emitting in the visible regime is better suited for this purpose owing to stronger absorption [1]. Even though the benefits of high-power lasers emitting in the green spectrum are well known for processes like copper welding, only a few systems have achieved the power levels in the range of interest. Lasers capable of reaching kW levels have been achieved through single pass second-harmonic generation (SHG) [2], [3], but the conversion efficiency of this technique is usually low (< 40 %). On the other hand, efficiencies surpassing 90 % were achieved through external resonant SHG even at few Watts. This process requires narrow-linewidth near-IR sources, but their power scalability in fibre amplifiers has been hampered due to the onset of Stimulated Brillouin Scattering (SBS). Consequently, the power levels are limited to few hundreds of Watts using external resonant SHG [4].