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When a laser pump beam of sufficient intensity is incident on a Raman-activemedium such as hydrogen gas, a strong Stokes signal, red-shifted by the Ramantransition frequency {\Omega}$_R$, is generated. This is accompanied by thecreation of a "coherence wave" of synchronized molecular oscillations withwavevector {\Delta}{\beta} determined by the optical dispersion. Within itslifetime, this coherence wave can be used to shift by {\Omega}$_R$ thefrequency of a third "mixing" signal, provided phase-matching is satisfied,i.e., {\Delta}{\beta} is matched. Conventionally this can be arranged usingnon-collinear beams or higher-order waveguide modes. Here we report collinearphase-matched frequency shifting of an arbitrary mixing signal using only thefundamental LP$_{01}$ modes of a hydrogen-filled hollow-core PCF. This is madepossible by the S-shaped dispersion curve that occurs around thepressure-tunable zero dispersion point. Phase-matched frequency shifting by 125THz is possible from the UV to the near-IR. Long interaction lengths and tightmodal confinement reduce the peak intensities required, allowing conversionefficiencies in excess of 70%. The system is of great interest in coherentanti-Stokes Raman spectroscopy and for wavelength-conversion of broadband lasersources.

Broadband-tunable LP$_{01}$ mode frequency shifting by Raman coherence waves in H$_2$-filled hollow-core PCF