We remove some ambiguities associated with the coupled mode description of light propagation in fiber Bragg gratings (FBG's). We show, in particular, that different methods employed in the literature lead to physically distinct results. The significant distinctions are discussed both for interferometric systems and intensity-only spectral measurements. Analysis of the reflection spectrum of a suitably designed double-grating structure is shown to result in a novel form of spectral hole, similar to the well-known effect derived from discontinuous phase gratings.
Martin McCall, "On the Application of Coupled Mode Theory for Modeling Fiber Bragg Gratings," J. Lightwave Technol. 18, 236- (2000)