Boron nitride coating of sapphire optical fiber for high temperature sensing applications

To avoid the leakage of laser at high temperature oxidative environment, sapphire optical fiber (SOF) was coated with boron nitride (BN) via chemical vapor deposition. The microstructures, mechanical properties and the optical properties of the coatings were characterized by scanning electron microscopy, Raman, optical and tensile tests. Compared with the uncoated fibers, the strength of the coated fibers was dramatically increased almost 3 times at room temperature. However, the tensile strength was decreased to about 500 MPa from about 700 MPa at 1200 °C. Thermal stresses in combination with surface defects seem to be responsible for the strength drop. From the light transmittance performance of the BN coated SOF, BN can work as a total reflection coating to protect SOF from light leakage.