Fiber based ultrashort pulse laser systems at ultrahigh average power levels
We report on the generation of 830 W compressed average power at 78 MHz pulse repetition frequency and 640 fs pulse duration. We discuss further power scaling including the issue of transversal spatial hole burning. Therefore, we describe a low-nonlinearity fiber design capable of producing fundamental mode radiation at ultra high average powers from short length (range of 1m) and large mode field diameter (>50µm) fibers. In conventional large mode area fiber most of the core is typically uniformly doped. As a consequence gain factors for the fundamental mode and the next higher order modes are comparable. Furthermore, the fundamental mode extracts inversion only in the central part of the core according to its intensity profile, leading at high pump and signal power levels to high and unused inversion density with a strong overlap with higher order transversal modes. In experiments this leads to a threshold-like onset of mode instability, originating from mode competition. Finally, this effect avoids further power scaling. The presented fiber features an optimized doping profile to prefer the amplification of the fundamental mode. In addition non-extracted inversion is minimized avoiding the issue of transversal spatial hole burning. As a consequence ultrafast fiber laser systems with novel performance are in reach, i.e. systems delivering simultaneously >1GW peak power and >1kW average power. In a first iteration a ROD-type fiber with 60µm MFD and 1.7m length was used in a CPA system to produce pump power limited 355 W of average power at 1 MHz.