  Ultrashort high-energy pulses at wavelengths longer than 1 $\mu$m are
nowadays desired for a vast variety of applications in ultrafast and
strong-field physics. To date, the main answer to the wavelength tunability for
energetic, broadband pulses still relies on optical parametric amplification
(OPA), which often requires multiple and complex stages, may feature imperfect
beam quality and has limited conversion efficiency into one of the amplified
waves. In this work, we present a completely different strategy to realize an
energy-efficient and scalable laser frequency shifter. This relies on the
continuous red shift provided by stimulated Raman scattering (SRS) over a long
propagation distance in nitrogen-filled hollow core fibers (HCF). We show a
continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with
conversion efficiency higher than 70% and high beam quality. The highly
asymmetric spectral broadening, arising from the spatiotemporal nonlinear
interplay between high-order modes of the HCF, can be readily employed to
generate pulses (~20 fs) significantly shorter than the pump ones (~200 fs)
with high beam quality, and the pulse energy can further be scaled up to tens
of millijoules. We envision that this technique, coupled with the emerging
high-power Yb laser technology, has the potential to answer the increasing
demand for energetic multi-TW few-cycle sources tunable in the near-IR.

Optics