Modes of periodically segmented waveguides

Modal properties of periodically segmented waveguides (PSW) are analyzed by using the beam-propagation method and the coupled-mode theory. The modal effective indices in a PSW are shown to be equal to those of a continuous waveguide with an averaged Δn. As a result, guided modes spread out, particularly those that are not strongly guided. The radiation loss is shown to oscillate with most of the waveguide parameters. In practical terms, this loss can be made negligible even for long periods. The results can be applied to the design of novel waveguide devices such as two-dimensional mode tapers and frequency-selective couplers.