Single and low order mode interrogation of a multimode sapphire fibre Bragg grating sensor with tapered fibres

Multimode sapphire fibre Bragg gratings (SFBG) made with an ultrafast Ti:sapphire 800 nm laser and a phase mask were probed using a tapered single mode fibre of different taper diameters to produce single and low order mode reflection/transmission responses. A configuration made of an input single mode tapered fibre and multimode silica fibre used for output coupling was also tested and has delivered a filtered multimode transmission spectrum. The tapered coupling improved the spectral resolution of the SFBG. Such improvements facilitate the utilization of the SFBG as a high temperature sensor. Wavelength shifts of the single mode response were monitored as a function of temperature up to 1500 °C with no detectable degradation in the grating strength or hysteresis in the Bragg resonance.