Spectrum design of multimode fiber Bragg gratings based on suppression of mode coupling

The direct femtosecond laser inscription method is very competitive in the fabrication of fiber Bragg gratings, but its refractive index modification region is very limited and leads to low reflectivity and wide spectra of multimode fiber gratings. In this work, the relationship between the refractive index modification region, the coupling coefficient, and the spectrum of the multimode fiber grating is investigated based on the intrinsic mechanism of mode coupling; thus the annular and eccentric elliptic refractive index modification regions are accordingly proposed to suppress mode couplings in the multimode fiber Bragg grating, so as to achieve a highly discrete spectrum and narrow spectrum of the multimode fiber Bragg grating, respectively. This work can provide a solid theoretical foundation for the spectrum design of the multimode fiber Bragg grating, which can be utilized to customize the spectrum according to application scenarios such as optical communication, fiber lasers, filtering, and sensing.