In order to obstruct the core mode effictively and extract the cladding mode in a long-period fiber grating(LPFG),a new core mode blocker(CMB) is proposed,which is fabricated by magnetic fluid.Based on the theory of mode coupling and a single mode fiber to multimode fiber network,the total coupling efficiency from HE13 cladding mode and LP01 core mode to the multimode fiber is investigated,and the core mode blocker radius is optimized.According to the results of theory analysis,a core mode block...