Fiber Bragg grating location by a side-scatter technique based on cladding-mode coupling

At wavelengths slightly shorter than the normal resonance for a fiber Bragg grating, radiation can be coupled into cladding and radiation modes that become visible external to the fiber. We describe experimental measurements on a fiber Bragg grating under broadband illumination, showing the axial variation and angular distribution of side-scattered radiation. The scattering signal can locate the grating position subject to an offset in the backscatter direction. The spectrum of scattered radiation is consistent with a published theoretical model of mode coupling by a grating in a finite cladding fiber.