Asymmetric transverse-load characteristics and polarization dependence of long-period fiber gratings written by a focused CO₂ laser

Abstract

Asymmetric transverse-load characteristics and the polarization dependence of long-period fiber gratings (LPFGs) written by high-frequency CO₂ laser pulses are investigated in detail. It is demonstrated that the resonant wavelength is dependent on the direction of the applied force and on the polarization state of the input light; however, the coupling strength is independent of these parameters. When a transverse load is applied along different orientations of the LPFG, the resonant wavelength may be shifted toward the longer wavelength, the shorter wavelength, or hardly shifted, whereas the absolute value of peak transmission attenuation is linearly decreased with an increase of the applied transverse load, with almost no sensitivity to the load direction. These unique transverse-load characteristics and the polarization dependence are due to the load-induced birefringence that leads to the rotation of optical principal axes in the LPFG.