Origins of Spectral Changes in Fiber Bragg Gratings Due to Macrobending

The effect of macrobending on fiber Bragg gratings (FBGs) in standard and specialist use single-mode fibers has been investigated for bend diameters down to 6 mm. The bend dependence of various spectral properties of FBGs, including center wavelength, were found to depend upon fiber type. A maximum centre wavelength shift of 0.39 nm was measured for a FBG written in low bend loss fiber bent to a 6 mm diameter. To investigate the cause of the observed wavelength shift, and the dependence on fiber design, a number of theoretical models have been applied to the problem. The results of modeling suggest that bend-induced changes in mode structure may explain the observed differences between fiber types, while wavefront distortion might be the cause of the general shift to shorter FBG center wavelengths when the bend diameter decreases.