Towards 100 kW fiber laser systems

Fiber lasers are rapidly gaining market shares for industrial applications due to compactness, high electrical-to-optical efficiency, high beam quality and low running cost. However, for directed energy weapons, fiber lasers have until now been unable to meet the requirements of military, where power levels around 100 kW are aimed for through combining of multiple, high beam quality lasers. The key to achieving such high power via beam combined lasers is control of laser line-width of the individual lasers. Fiber lasers are well-known to perform exceptionally well in this respect. However, it becomes increasingly difficult to maintain control as power levels are raised. It is, therefore, crucial to choose the best possible fiber technology for power scaling. Photonic crystal fibers provide a new platform for better control of line-width. This control is achieved through large single-mode core sizes and short fiber lengths – and it comes with an option of welldefined polarization. Compared to conventional fiber lasers, power levels of around a factor of ten higher are expected from photonic crystal fiber based lasers. Power levels up to 8 kW are projected for individual photonic crystal fiber lasers with a line-width of 100 pm suited for spectral beam combining. For coherent beam combining, power levels of more than 2.8 kW per laser are expected with single-frequency and single-polarization performance. In this white paper, we propose to build narrow line-width fiber lasers with output power of 4 kW that are suitable for beam combining to demonstrate the potential of photonic crystal fiber lasers.