Polarization-mode coupling in birefringent fiber gratings.

Polarization-mode coupling in birefringent fiber gratings is analyzed. The general expression for coupling coefficient components is also derived. It indicates that the polarization-mode coupling between any two linearly polarized (LP) core modes is possible by appropriately adjusting the grating parameters such as the grating tilt angle, the grating length, the orientation of the grating plane, the grating period, the birefringence, and the birefringent axis. It is analytically found that the complete LP01x-to-LP01y mode coupling and LP01x-to-LP11y mode coupling occur when fiber is pressed periodically. The LP01x-to-LP11y mode coupling in the linearly birefringent gratings created by pressing a two-mode fiber with a groove plate with a period of 80 microm at a tilt angle between 81.5 degrees and 83.5 degrees has also been experimentally demonstrated. The resonant LP01x-to-LP11y mode coupling in the birefringent gratings and their experimental transmission spectra were reasonably well predicted by the coupled-mode analysis.