Characterization of Long-Period Fiber Gratings Written by CO $_{2}$ Laser in Twisted Single-Mode Fibers

The rejection band of a long-period fiber grating written in a heavily twisted single-mode fiber by a CO₂ laser can split into two, when the twist applied to the fiber is removed after the writing of the grating. We attribute the wavelength-splitting effect to the generation of a rotary frozen-in torsion strain along the fiber in the writing process. The wavelength split increases with the twist rate and the effect is independent of the polarization state of light. We present a simple expression to estimate the wavelength split, which agrees reasonably well with the experimental results. We also measure the temperature and torsion characteristics of the grating. Such a grating could find applications as an optical filter or a temperature-insensitive torsion sensor.