Effect of irradiation symmetry of CO2laser on mode coupling in long-period gratings inscribed in photonic crystal fiber

Abstract

Long-period gratings (LPG) are inscribed in endlessly single mode (ESM) photonic crystal fiber (PCF) and conventional single mode fiber (SMF) with symmetric and asymmetric CO 2 laser irradiation. Transmission measurements and near field imaging indicate that symmetric index perturbation induced by laser irradiation with the aid of a 120° gold-coated reflecting mirror results in LP 0n symmetric mode coupling, while asymmetric irradiation without using the mirror leads to LP 1n asymmetric mode coupling for both fiber types. Symmetric irradiation yields far more reproducible LPG in PCF than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPG inscribed in SMF due to the isotropy of its all-solid cladding structure. The removal of the outer solid cladding of PCFLPG results in a significant change in its transmission characteristics, which provides rich information on the refractive index perturbation and mode field distribution in the outer solid cladding of PCF-LPG.