Fabrication of an antiresonant hollow-core fibre for mid-infrared nonlinear Raman conversion in nitrogen pumped at 2μm

We report the fabrication of an antiresonant silica hollow-core fibre (HCF) specially designed as a platform for emission at 4 μm via stimulated Raman scattering (SRS) in nitrogen gas (N₂) when pumped at 2.06 μm. The fabrication was realised using a two-step stack, fuse and draw process. The HCF microstructure is composed of six non-touching nested capillaries (attached in each corner of a hexagonal jacket) surrounding a 70 μm diameter core. The HCF propagation losses measured at 2 and 4 μm wavelength are below 0.3 dB/m, and the light is propagating in the core with reasonably good beam quality at both wavelengths. A high power 2.06 μm laser source, delivering 25 ns pulses with energy up to 6 mJ, was coupled to the fibre in a free-space configuration. When filled with air at atmospheric pressure, the HCF showed remarkably high damage threshold, being able to transmit more than 35 W of pump power without damage. However, when filled with high pressure N₂, the fibre was damaged at lower power. We show evidences suggesting that laser-induced optical breakdown of the gas is responsible for damages observed.