Femtosecond Laser Plane-by-Plane Inscription of Bragg Gratings in Sapphire Fiber

We report on a cylindrical telescope assisted femtosecond laser direct writing technology for fabricating plane-by-plane Bragg gratings in sapphire fibers. This approach could effectively extend the width of refractive index modulation (RIM) regions. Meanwhile, we propose the spherical aberration to elongate the RIM regions. Such expanded RIM regions lead to the improvement of the spectral characteristics of sapphire fiber Bragg gratings (SFBGs). The effects of the pulse energy and the number of pulses on morphology of RIM regions were studied, after optimizing these parameters, an enlarged RIM region was induced into a sapphire fiber with a diameter of 100 μm. The maximums of the length and the width of RIM region are 25 μm and 15 μm, respectively. Note that this RIM region could be used to cover roughly 50% of fundamental mode field distribution, increasing the reflectivity of the fundamental mode. An SFBG formed by these RIM regions exhibits an enhanced reflectivity of 6.34% and a reduced −3 dB bandwidth (full width at half-maximum [FWHM]) of 3.43 nm. The fabrication time for such an SFBG only requires ∼20 s. Moreover, the high temperature response of this SFBG was studied, and the results showed it can withstand a high temperature of 1612 °C. Such an SFBG can be developed for temperature measurement in many fields, i.e., boilers, reactor cores, and aviation engines.