Enhanced high temperature properties of overexposed FBG fabricated by femtosecond laser.

• High temperature characteristics of overexposed and unexposed FBGs are compared. • Overexposed FBGs shown better high temperature stability than the unexposed FBGs. • Overexposed FBGs can withstand a high temperature of up to 1000 °C. • The wavelength hysteresis and spectral repeatability is improved by air cooling treatment. The high temperature properties of fiber Bragg gratings (FBGs) fabricated by overexposure using femtosecond laser phase mask technology are investigated. It is found that the overexposed FBG (FBG-II) has exhibited a temperature resistance of up to1000 °C and a reflection intensity of over 12 dB after air cooling treatment at 1000 °C for 5 h, as well as showing better high temperature stability than the unexposed gratings (FBG-I). Moreover, the temperature stability of FBG-II can be enhanced obviously through the rapid air cooling treatment after high temperature annealing, as well as the higher the air cooling rate, the more significant the effect. In addition, the wavelength hysteresis and spectral repeatability of the FBG-IIs can be significantly improved through the method. This research essentially provides a new approach to enhance the thermal properties of FBG and shows great potential in a harsh environment. [ABSTRACT FROM AUTHOR]