Fiber based ultrashort pulse laser systems at ultrahigh average power levels
2010
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.
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