Mode add/drop multiplexers of LP₀₂ and LP₀₃ modes with two parallel combinative long-period fiber gratings

Two parallel combinative long-period fiber gratings (LPFGs) can convert the fundamental core mode LP(01) in a single-mode fiber (SMF) into one desired higher order core mode LP(0m) in a few-mode fiber (FMF), in the process of which one specific cladding mode acts as a medium coupled from one fiber to another. Different LP(0m) modes can be obtained by controlling the grating period of LPFG in FMF to meet the phase matching condition. In this article we focus on the design and analyses of LP(02) and LP(03) mode add / drop multiplexers (MADMs). This device has some advantages of facile and good scalability, and particularly, of eliminating coupling interferences for the ahead multiplexed modes by the posterior MADMs or couplers. Furthermore, the conversion rate of mode power theoretically can approach as much as 98%and the 3dB bandwidth can reach 10nm or more.