Physical properties of optical fiber sidetap grating filters: Free-space model

This paper presents theoretical and experimental results on the wavelength properties of sidetap grating filters written in single-mode optical fibers . A simple theoretical model based on Fraunhofer diffraction and antenna th eory is used, modified to take into account the longitudinal variation of t he incident field, Expressions are derived for the filter attenuation and b andwidth, and good agreement is demonstrated with experiment. The results s how that the shape of the attenuation spectrum does not change with the fil ter length or index modulation, but that the magnitude of the attenuation i ncreases with both parameters. The results also show that the effect of dec reasing the fiber core-cladding index difference is to increase the peak at tenuation and reduce the filter bandwidth. The existence of a strict longit udinal phase-matching condition is shown to cause asymmetry in the shape of the spectrum. Comparison with the properties of bulk sidetap gratings show s there to be a shift in the peak wavelength and apodization of the filter sidelobes, A simple diagram for the design of sidetap filters is presented for the first time.