Synthesis and characterization of spinel MgAl₂O₄ thin film as sapphire optical fiber cladding for high temperature applications

Spinel MgAl₂O₄ thin films have been coated on ϕ75-μm single crystal sapphire fibers and sapphire wafers by low-temperature (700 °C) firing of (Mg^{2 +},Al³⁺)-chelated polymeric precursor films. The MgAl₂O₄ coating was demonstrated to be chemically stable on sapphire substrate at temperatures as high as 1200 °C. No evidences of solid state reactions or ion diffusion between the MgAl₂O₄ and sapphire layers were found by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy examinations for the samples annealed at 1200 °C for 672 h. The refractive indices of the MgAl₂O₄ films fired at 1000 °C and 1200 °C were close to the index of MgAl₂O₄ single crystal but slightly smaller than that of the sapphire fiber. The MgAl₂O₄ thin film-coated sapphire fiber exhibited a dramatically reduced numerical aperture and improved preservation of transmitting light intensity as compared to the uncoated fiber. Results of this study demonstrate that the MgAl₂O₄ thin film is suitable for sapphire fiber cladding for high temperature applications. - Highlights: • Spinel MgAl₂O₄ films are synthesized at low-temperature; • Spinel films are demonstrated for high temperature sapphire fiber cladding; • The spinel-cladded sapphire fiber is chemically stable up to 1200 °C; • The stability is based on the MgAl₂O₄-α-Al₂O₃ phase equilibrium behavior