Evanescent field imaging of an optical fiber Bragg grating

We have investigated the evanescent field associated with an optical fiber Bragg grating using the subwavelength imaging properties of scanning near-field optical microscopy. Imaging of either the field distribution within the grating, or the periodic refractive index changes along the grating can be performed by tuning the launched light on or off the grating resonance. These measurements reveal nonuniformity in the resonant standing-wave pattern that occur due to phase errors in the refractive index profile of the grating under study.