This project will investigate a new approach for generating very high continuous-wave output power from fibre lasers in the visible wavelength regime with the ambition of demonstrating levels of performance in terms of power, efficiency and wavelength flexibility that go well beyond the capabilities of the current state-of-the-art. The work is motivated mainly by the growing demands of laser-based manufacturing, and particularly additive manufacturing (3D printing), for high power laser sources in the visible band due to significantly greater absorption in important metals, such as copper. Our approach is based on the use of a novel scheme for internal nonlinear frequency conversion that can be used with lasers (such as cladding-pumped fibre lasers) that have significant (i.e. non-negligible) cavity losses without detrimentally affecting the performance. The aim is to show that this scheme is compatible with kilowatt-class power generation in the green and that it offers the flexibility to address the needs for high power levels at other wavelengths in the visible band. The improved performance will directly benefit applications in laser-based manufacturing where enhanced absorption at visible wavelengths is crucial, as well as many other applications (e.g. bio-medical imaging, DNA sequencing, optical pumping, display technologies). The idea behind this proposal is the subject of a granted patent, which has been licensed by the University of Southampton to SPI Lasers. SPI is a leading UK laser company (and spin-out from the University) and one of the world leaders in the manufacture of high power fibre lasers. Thus, the project will involve very close collaboration with SPI Lasers, building on the latest developments in optical fibres and fibre laser technology at the Optoelectronics Research Centre (ORC). The project, if successful, will pave the way for the next generation of high power visible laser sources, benefiting a wide range of applications.