Abstract

Waveguide gratings are spatially modulated structures possessing a higher average refractive index than the adjacent layers on either side. Under certain conditions that are given in the limit of small modulation by elementary slab waveguide theory, coupling of external diffracted waves with the characteristic modes of the waveguide takes place. This produces sharp resonances where efficient switching of light energy among waves occurs. This effect may be useful for filtering. For small modulation, the calculated linewidths are found to be extremely narrow. Larger linewidths are desirable for many potential applications as well as for experimental studies of these resonance phenomena. These can be achieved by increasing the modulation strength of the waveguide grating. In this paper calculated results for the linewidths and line shapes of experimentally realizable waveguide grating filters are presented. Calculations for planar sinusoidal gratings with modulation amplitude up to 10% of the average permittivity show that linewidths of the order of 100 pm in wavelength can be obtained with well shaped symmetric lines. Increasing the modulation further tends to corrupt the shape of the line. Calculated spectral linewidths and shapes are presented for both planar and surface-relief waveguide grating structures.

© 1992 Optical Society of America