Spatial Radiation Patterns of Second-Order Fiber Bragg Gratings

The spatial radiation characteristics of second-order fiber Bragg gratings (FBGs) are experimentally investigated. A second-order FBG fabricated in photosensitive fiber PS-1500 is characterized through transmission, reflection, and radiation measurements. Three directional radiation peaks with narrow beam divergences are observed in both resonant and non-resonant wavelength regions. At the resonant wavelength, the radiation angles are measured to be 49.5°, 92.7°, and 135.9°. The surface-normal radiation near 90° forms a narrow directional beam. In addition to the expected surface-normal radiation, forward- and backward-radiation peaks are observed and are associated with the transmitted and reflected waves, respectively.