THEORETICAL DESCRIPTION OF FIBER BRAGG REFLECTORS PREPARED BY FRESNELDIFFRACTION IMAGES

The theory of Fresnel diffraction images is applied to Bragg-grating f ormation in a germanium-doped silica fiber. Fresnel diffraction images arm from the near-field diffraction at a periodic mask. The diffracti on images are calculated as a function of the propagation distance for several mask configurations. The average of the diffraction-image int ensities is calculated for a single longitudinal repetitive interval, and it is shown that the period of the resulting average intensity fie ld is twice that of the original mask period. In some cases the Period ic mask can be predicted for a desired average intensity field by calc ulation of the magnitude of its Fourier coefficients.