Distributed measurement of gas pressure in the core of hollow-core optical fibres using nonlinear optics

We investigate theoretically and numerically the use of nonlinear optics to measure the spatial distribution of the gas density in the core of antiresonant hollow-core optical fibres. Short pump and signal laser pulses at different wavelengths and with controlled delay are launched into the fibre. Due to their different group velocity, they overlap within a certain region of the fibre where their nonlinear interaction create an idler pulse (via four-wave mixing) and/or the signal pulse experiences a nonlinear phase shift induced by the pump pulse via cross phase modulation. As the optical nonlinearity in these fibres is dominated by the gas content, a measurement of the idler power or signal phase shift at the fibre output thus provides information about the gas density (its pressure) at the pulse overlap position. We discuss the feasibility of the scheme and its dependence on fibre and pulse parameters. We conclude that such distributed gas measurements are possible within current experimental parameters and can potentially measure pressures far below current experimental techniques with high spatial resolution.