Writing of Long-Period Gratings in Conventional and Photonic-Crystal Polarization-Maintaining Fibers by CO$_{2}$-Laser Pulses

We report the writing of long-period gratings in conventional and photonic-crystal (PC) polarization-maintaining (PM) fibers by high-frequency CO-laser pulses. We find that the writing efficiency depends on the irradiation direction of the writing beam with respect to the principal axes of the fiber, with the highest efficiency obtained when the irradiation direction is along the slow axis of the fiber. Such orientation dependence is much stronger for a conventional PM fiber than for a PC PM fiber. Using a conventional PM fiber, we demonstrate a grating that shows two clean polarization-splitting rejection bands with a separation of 33.4 nm and a contrast higher than 30 dB.