Analysis of coupling strength apodization among the three co-directional guided modes in optical period dielectric waveguides

We describe and analyze the coupling strength apodization effect among the three co-directional guided modes using the supermodes theory. The propagation condition is related to the two phase matching distribution induced by two long period co-spatial gratings. It is seen that one input mode can be converted into other two modes with one mode having two peaks at its transmission spectrum. Through adopting suitable apodization functions and adjusting the maximum coupling apodization strength from 100 to 1500, the frequency space between the two peaks in one converted mode can be tuned nearly linearly from 336 GHz to 5400 GHz. Moreover, the other converted mode which has one peak and maximum output power around the central frequency can be used in the transmission operation mode. This kind of design can form the basis for a new generation of optical filtering and mode converting.