Long-period gratings inscribed in photonic crystal fiber by symmetric CO2 laser irradiation.

Long-period gratings (LPGs) inscribed in endlessly single mode (ESM) photonic crystal fibers (PCFs) with symmetric and asymmetric CO2 laser irradiation are investigated both numerically and experimentally. Parallel results from conventional single mode fibers (SMFs) are presented for comparison. Theoretical predictions, transmission measurements, and near-field imaging indicate that, regardless of the fiber type, 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. Our results show that, because of the azimuthally anisotropic hexagonal cladding structure, symmetric irradiation yields far more reproducible LPGs in PCFs than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPGs inscribed in SMFs due to the isotropy of its all-solid cladding structure.