Fabrication of asymmetric long-period gratings in polarization-maintaining fiber with a CO2 laser

We propose the fabrication of the long-period fiber gratings (LPFGs) in a panda polarization-maintaining fiber (PMF) using a two-step CO2 laser writing method. The first laser irradiation was aligned along one of the axes of the PMF and the grating with high contrast can be written after several laser scannings. The second irradiation was performed by rotating the PMF so that the other axis of the fiber aligns toward the laser exposure. New transmission dips can be observed when the first irradiation is along the slow axis and the second one is along the fast axis of the PMF. The stress relaxation effect of the stress-applying parts due to the laser heating was investigated theoretically and experimentally, which is believed to be dominant for the fabrication of the LPFGs in the PMFs by a CO2 laser.