Multi-layer, offset-coupled sapphire fiber Bragg gratings for high-temperature measurements.

We demonstrate the fabrication of multi-layer sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser line-by-line scanning technique. This multi-layer grating structure enlarges the index modulation area and can effectively increase the reflectivity of SFBGs. The spectral characteristics of multi-layer SFBGs with different layer quantities and spacings were studied. A double-layer SFBG with a layer spacing of 5 μm exhibits a reflectivity of 34.1%, which is much higher than that of a single-layer SFBG (i.e., ∼6.3%). Moreover, the higher-order modes of multi-layer SFBG could be suppressed by offset coupling, leading to a reduced SFBG bandwidth (full width at half-maximum [FWHM]) of 1.32 nm. In addition, the high-temperature response of the multi-layer, offset-coupled SFBGs was also studied. The SFBG can withstand a high temperature of 1612°C and exhibits a sensitivity of 45.2 pm/°C at high temperatures. Such multi-layer, offset-coupled SFBGs could be developed for high-temperature sensing in metallurgical, chemical, and aviation industries.