Universality of Coherent Raman Gain Suppression in Gas-Filled Broadband-Guiding Photonic Crystal Fibers

TL;DR

This paper demonstrates that Raman gain suppression is a universal phenomenon in gas-filled hollow-core fibers, significantly affecting the efficiency of fiber-based Raman devices even at high pump powers.

Contribution

It reveals the universality of Raman gain suppression in gas-filled fibers and its impact on nonlinear Raman processes at high powers.

Findings

Raman gain suppression occurs universally in gas-filled hollow-core fibers.

Intermodal stimulated Raman scattering dominates at high pump powers.

Implications for fiber-based Raman devices in the ultraviolet are significant.

Abstract

As shown in the early 1960s, the gain in stimulated Raman scattering (SRS) is drastically suppressed when the rate of creation of phonons (via pump-to-Stokes conversion) is exactly balanced by the rate of phonon annihilation (via pump-to-anti-Stokes conversion). This occurs when the phonon coherence waves synchronized vibrations of a large population of molecules have identical propagation constants for both processes, i.e., they are phase-velocity matched. As recently demonstrated, hydrogen-filled photonic crystal fiber pumped in the vicinity of its zero dispersion wavelength provides an ideal system for observing this effect. Here we report that Raman gain suppression is actually a universal feature of SRS in gas-filled hollow-core fibers, and that it can strongly impair SRS even when the dephasing rate is high, particularly at high pump powers when it is normally assumed that nonlinear processes become more (not less) efficient. This counterintuitive result means that intermodal stimulated Raman scattering (for example between LP01 and LP11 core modes) begins to dominate at high power levels. The results reported have important implications for fiber-based Raman shifters, amplifiers or frequency combs, especially for operation in the ultraviolet, where the Raman gain is much higher.