Broadband-tunable LP01 mode frequency shifting by Raman coherence waves in a H2-filled hollow-core photonic crystal fiber
Abstract
When a laser pump beam of sufficient intensity is incident on a Raman-active medium such as hydrogen gas, a strong Stokes signal, redshifted by the Raman transition frequency, is generated. This is accompanied by the creation of a “coherence wave” of synchronized molecular oscillations with wave vector Δβ determined by the optical dispersion. Within its lifetime, this coherence wave can be used to shift by ΩR the frequency of a third “mixing” signal, provided phase matching is satisfied, i.e., Δβ is matched. Conventionally, this can be arranged using noncollinear beams or higher-order waveguide modes. Here we report the collinear phase-matched frequency shifting of an arbitrary mixing signal using only the fundamental LP01 modes of a hydrogen-filled hollow-core photonic crystal fiber. This is made possible by the S-shaped dispersion curve that occurs around the pressure-tunable zero dispersion point. Phase-matched frequency shifting by 125 THz is possible from the UV to the near IR. Long interaction lengths and tight modal confinement reduce the peak intensities required, allowing conversion efficiencies in excess of 70%. The system is of great interest in coherent anti-Stokes Raman spectroscopy and for wavelength conversion of broadband laser sources
