  The serrodyne principle enables shifting the frequency of an electromagnetic
signal by applying a linear phase ramp in the time domain [1]. This phenomenon
has been exploited to frequency-shift signals in the radiofrequency (RF),
microwave and optical region of the electromagnetic spectrum over ranges of up
to a few GHz e.g. to analyse the Doppler shift of RF signals, for noise
suppression and frequency stabilization [2-9]. Here, we employ this principle
to shift the center frequency of high power femtosecond laser pulses over a
range of several THz with the help of a nonlinear multi-pass cell. We
demonstrate our method experimentally by shifting the central wavelength of a
state-of-the-art 75 W frequency comb laser from 1030\,nm to 1060\,nm and to
1000\,nm. Furthermore, we experimentally show that this wavelength shifting
technique supports coherence characteristics at the few Hz-level while
improving the temporal pulse quality. The technique is generally applicable to
wide parameter ranges and different laser systems, enabling efficient
wavelength conversion of high-power lasers to spectral regions beyond the gain
bandwidth of available laser platforms.
