Long-period fiber gratings fabricated by use of defocused CO_2 laser beam for polarization-dependent loss enhancement

Abstract

An alternative of the long-period fiber grating (LPFG) fabrication method has been developed in this work by use of defocused CO2 laser pulse scanning, which is achieved by moving the laser beam focus plane slightly away from the fiber cladding surface. By increasing the distance shift from the cladding surface, the area with refractive index change also increases on the side with laser irradiation, which leads to an apparent mode field profile distortion at the resonant wavelength of the LPFG. Such a distortion in the mode field profile significantly increases the polarization-dependent loss of the LPFG. Moreover, for the cascaded LPFGs fabricated by use of this technique, the maximum polarization-dependent loss obtained can be largely increased from 7 to 17.8 dB, with a maximum dip in the wavelength change of 0.45 nm.