Xue Yan-Ru, Tian Peng-Fei, Jin Wa, Zhao Neng, Jin Yun, Bi Wei-Hong. Superimposed long period gratings based mode converter in few-mode fiberJ. Acta Physica Sinica, 2019, 68(5): 054204. DOI: 10.7498/aps.68.20181674