Dominance of backward stimulated Raman scattering in gas-filled hollow-core photonic crystal fibers

TL;DR

This paper demonstrates efficient noise-seeded backward stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fibers, revealing unique dynamics that favor backward emission over forward, with high conversion efficiencies and potential for novel laser designs.

Contribution

First unambiguous observation of noise-seeded backward stimulated Raman scattering in gas-filled hollow-core fibers, highlighting the dynamics that enhance backward emission and efficiency.

Findings

Backward Stokes signal exceeds forward in strength at high powers.

Quantum conversion efficiencies over 40%, reaching 65% with seeding.

Backward process exhibits unique temporal dynamics favoring input-end growth.

Abstract

Backward stimulated Raman scattering in gases provides a promising route to compression and amplification of a Stokes seed-pulse by counter-propagating against a pump-pulse, as has been already demonstrated in various platforms, mainly in free-space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable with, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532-nm-pump. The efficiency increases to 65% when the backward process is seeded by a small amount of back-reflected forward-generated Stokes light. At high pump powers the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forward-propagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fine-period Bragg grating) to grow in strength towards the input end of the fiber. A good understanding of this process, together with the rapid development of novel anti-resonant-guiding hollow-core fibers, may lead to improved designs of efficient gas-based Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the mid-infrared.