  Based on the interplay between photoionization and Raman effect in gas-filled
photonic crystal fibers, we propose a new optical device to control
frequency-conversion of ultrashort pulses. By tuning the input-pulse energy,
the output spectrum can be either down-converted, up-converted, or even
frequency-shift compensated. For low input energies, Raman effect is dominant
and leads to a redshift that increases linearly during propagation. For larger
pulse energies, photoionization starts to take over the frequency conversion
process, and induces a strong blueshift. We have found also that the
fiber-output pressure can provide an additional degree of freedom to control
the spectrum shift.
