AbstractIt is well known that grating structures with subgratings separated by small identical gaps (for example, quarter‐wavelength gaps) create a transmission window in the middle of the reflectivity spectrum. In this paper, we analyze structures with mirror symmetry with respect to their centers, but with gaps at both ends that continuously decrease toward their centers. These structures exhibit several dips in the central lobe of the reflectivity spectrum, with spectral properties (linewidths, spectral spacing between dips, etc.) dependent upon the design parameters of the subgratings (length, period, and magnitude of the modulation of the core refractive index). In a special case, the spectral separation between two dips in the main lobe can be controlled by adequately changing these parameters. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 223–229, 2001.