Raman-assisted three-wave mixing of non-phase-matched waves in optical fibres: application to wide-range frequency conversion

Abstract

We analyse theoretically and experimentally the Raman-assisted parametric coupling between non-phase-matched waves propagating in normally dispersive single-mode fibres. We perform a careful analysis of the wave-coupling behaviour, which shows that scalar and vector three-wave mixing (TWM) interactions induce a relatively small periodic power flow between a central-frequency pump at frequency ω 0 and a pair of up-shifted (anti-Stokes) and down-shifted (Stokes) sidebands at frequencies ω 0+Ω and ω 0−Ω , respectively. For sufficiently high pump powers, the stimulated Raman scattering enters into play, causing a unilateral transfer of energy from higher to lower frequency waves. This energy transfer destroys the spatial periodicity of the parametric energy-exchange process. As a result, parametric seeding and subsequent Raman amplification of a Stokes idler wave is achieved by mixing a strong pump with a weak anti-Stokes signal. This Raman-induced Stokes power-gain enhancement leads to efficient anti-Stokes → Stokes frequency conversion, with frequency detunings which can be relatively large (typically, from 7 to 30 THz), even for very short parametric coherence. Raman-assisted TWM thus overcomes the strict spectral limitation usually imposed by the phase-matching condition, leading to broadband frequency conversion processes that are inaccessible with a pure parametric interaction.