Size-unlimited sapphire single-crystal fiber growth and the anisotropic & size-dependent mechanical and thermometry performance

Sapphire fibers are becoming a research hotspot for high-temperature sensing in extreme environments due to their high melting point, wide transmission band and superior thermal stability. Nevertheless, the preparation of high-quality, few-mode, large-length sapphire fibers is still a major challenge for further application. Here, we have successfully fabricated high-quality sapphire fibers with a minimum diameter of ∼16 μm and a maximum length of more than 50 m using a state-of-the-art laser-heated pedestal growth system, which, to the best of our knowledge, is the size record for single-crystal fibers. Besides, the anisotropic growth behavior and mechanical properties were investigated in detail, demonstrating that c-oriented sapphire fibers present higher high-temperature tensile strength and superior creep resistance compared with a- and m-oriented sapphire fibers. Further reasearch demonstrates that the tensile strength increases dramatically with decreasing diameter, with the 30 μm-diameter sapphire fibers achieving a tensile strength of more than 8000 MPa and a maximum strain over 20 000 με, both of which are more than twice those of conventional sapphire fibers. Furthermore, a fiber Bragg grating was fabricated within a 30 μm-diameter sapphire fiber for the first time by the femtosecond laser line-by-line scanning method, exhibiting few-mode and stable spectral response in the range of 20–1600 °C with a maximum sensitivity of 40.45 pm °C⁻¹ at 1600 °C. This work provides a feasible approach for the preparation of sapphire fibers without size limitation, and demonstrates huge potential of ultra-fine sapphire fibers for applications in harsh environments and strain sensing.