Design and simulation of femtosecond laser modulated cladding few-mode sapphire fibre for ultra-high-temperature sensing

Abstract

Conventional silica-based optical fibre sensors cannot withstand ultra-high-temperature conditions for key-parameter monitoring in hypersonic vehicles and aerospace propulsion systems. Sapphire fibre possesses outstanding high-temperature stability and serves as a promising waveguide for ultra-high-temperature fibre sensors. Nevertheless, the absence of conventional cladding triggers severe multimode excitation and mode coupling, degrading demodulation accuracy and hindering its practical engineering deployment. To mitigate this limitation, this paper proposes a few-mode sapphire fibre with femtosecond-laser-fabricated modulated cladding, whose structural parameters are optimised via finite-element simulation. We investigate how refractive-index variation, modulation-unit geometry and unit spacing govern fundamental-mode transmission and mode confinement. Simulation results demonstrate that the negative-index modulation array inside a 125 μm sapphire fibre delivers effective high-order-mode suppression at Δn = −0.008 and unit spacing d = 5.0 μm. Under optimised conditions at the 1.55 μm communication band, the fundamental-mode energy fraction exceeds 0.95, suppressing multimode-interference noise. This design maintains the native mechanical and thermal performance of monolithic sapphire with good process robustness. It offers a feasible few-mode design route and theoretical support for fabricating high-stability sapphire-fibre sensors intended for ultra-high-temperature intelligent monitoring systems.