Femtosecond laser fabricated fiber Bragg grating in microfiber for refractive index sensing.

Fiber Bragg grating (FBG) is fabricated in the microfiber by the use of femtosecond laser pulse irradiation. Such a grating can be directly exposed to the surrounding medium without etching or thinning treatment of the fiber, thus possessing high refractive index (RI) sensitivity while maintaining superior reliability. The grating in the microfiber may have a number of propagation modes in its transmission spectrum, depending on the fiber diameter, and the higher order of mode has larger RI sensitivity. The RI sensitivity also depends on the fiber diameter and a smaller diameter corresponds to a large sensitivity. The maximum sensitivity obtained is approximately 231.4 nm per refractive index unit at the refractive index value of approximately 1.44 when the fiber diameter is approximately 2 microm. The FBG fabricated in the microfiber has high potential in various types of optical fiber sensor applications.