Fabrication and Characterization of Femtosecond Laser Inscribed Long-Period Fiber Grating in Few-Mode Fiber

We experimentally investigated the fabrication and characterization of long-period fiber grating in few-mode fiber (FMF-LPFGs) by using the femtosecond laser direct writing technique. Thanks to the high flexibility and repeatability of this technique, stable FMF-LPFGs fabrication technology has been developed. In addition, the properties of FMF-LPFGs have also been characterized. The coupling of LP₀₁- LP₁₁ and LP₀₁- cladding mode LP₁₃ were investigated by both the transmission spectrum and the mode patterns. The temperature, strain and twist sensitivities of the resonance dips were calibrated by experiments, and the sensitivity difference between the dips was explained. By using their disparate responses to external temperature and strain, a cross coefficient matrix was established and dual-parameter discrimination determination is realized. This work provides a thoroughly theoretical and experimental investigation on the fabrication and characterization of FMF-LPFGs based on femtosecond laser direct writing technique. It is believed that the flexible and efficient LPFGs fabrication technique will benefit a lot in the future for developing various functional specialty optical fiber devices and sensors.