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