Analysis of radiation loss in grating-assisted codirectional couplers

In this paper, we present a new accurate numerical method, named leaky mode propagation method, to analyze the radiation loss in grating-assisted codirectional couplers. The complex propagation constant and electromagnetic field distribution of the normal modes are evaluated by using a space harmonic field expansion derived from the Floquet theorem. In order to prove the accuracy of our method, a comparison between our results and those calculated by the most used method, i.e., the transfer matrix method, is carried out in terms of power attenuation coefficient and total radiation loss. Quantitative and qualitative discrepancies between the two methods are widely discussed. In particular, an oscillatory variation of radiation loss with grating depth and gap thickness in contrast to the monotonic behavior predicted by the transfer matrix method has been found