Combining cw-seeding with highly nonlinear fibers in a broadly tunable femtosecond optical parametric amplifier at 42 MHz.

We report on a precisely tunable and highly stable femtosecond oscillator-pumped optical parametric amplifier at a 41.7 MHz repetition rate for spectroscopic applications. A novel concept based on cw-seeding of a first amplification stage with subsequent spectral broadening and shaping, followed by two further amplification stages, allows for precise sub-nanometer and gap-free tuning from 1.35 to 1.75 μm and 2.55 to 4.5 μm. Excellent spectral stability is demonstrated with deviations of less than 0.008% rms central wavelength and 1.6% rms bandwidth over 1 h. Spectral shaping of the seed pulse allows precise adjustment of both the bandwidth and the pulse duration over a broad range at a given central wavelength. Transform-limited pulses nearly as short as 107 fs are achieved. More than half a Watt of average power in the near- and more than 200 mW in the mid-infrared with power fluctuations less than 0.6% rms over 1 h provide an excellent basis for spectroscopic experiments. The pulse-to-pulse power fluctuations are as small as 1.8%. Further, we demonstrate for the first time, to the best of our knowledge, that by using hollow-core capillaries with highly nonlinear liquids as a host medium for self-phase modulation, the signal tuning range can be extended and covers the region from 1.4 μm up to the point of degeneracy at 2.07 μm. Hence, the idler covers 2.07 to 4.0 μm.