Tian, F. ; Kaňka, Jiří ; Zou, B. ; Chiang, K.S. ; Du, H. Effect of irradiation symmetry of CO2 laser on mode coupling in long-period gratings inscribed in photonic crystal fiber. In Udd, E.; Pickrell, G.; Du, H.H. (ed.). FIBER OPTIC SENSORS AND APPLICATIONS X (Proceedings of SPIE 8722) BELLINGHAM : SPIE, (2013)