Intrinsic Sapphire Fiber Scattering Array Interferometer Based on Point-by-Point Laser Inscription for High-Temperature Sensing

We present a multiplexable intrinsic sapphire fiber interferometric sensor using an array of scattering points with longitudinal periodicity to generate spectral interference with coherent scattering. The sensor, a scattering array interferometer (SAI), is fabricated by femtosecond (fs) laser point-by-point inscription technique. Parallel fabrication and thermal annealing are shown to be critical in producing detectable and reliable interference signals in sapphire fibers, where a signal-to-noise ratio (SNR) up to 20 dB can be achieved. The SAI signals can be interpreted from the point of view of both interferometers and fiber gratings. High-order harmonics of the SAI signal are studied to examine the coherence properties of sapphire fibers in comparison with silica multimode fibers (MMFs). Three cascaded SAIs with different pitches are fabricated on a single sapphire fiber to demonstrate multipoint temperature sensing. With a low-cost, fully multimode interrogator that uses a visible-band LED and a charge-coupled device (CCD)-based compact spectrometer, reliable multipoint temperature sensing up to 1500 °C is demonstrated.