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

• 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.

Spinel MgAl2O4 thin films have been coated on ϕ75-μm single crystal sapphire fibers and sapphire wafers by low-temperature (700 °C) firing of (Mg2 +,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.