Recently, optical planar waveguides have been utilized by many researchers for various integrated optical sensor and filter applications [1–3]. Especially long-period waveguide gratings (LPWGs) have attracted significant interest for potential applications to wide tunable filters and refractive-index sensors [4–6]. A number of researchers have reported theoretical and numerical analyses on the planar waveguide modes and the core-clad mode coupling in LPWGs [7–9]. The mode coupling conditions between the core and clad waveguides depend on the effective indices of both the waveguides, the grating period, and the clad mode numbers. In practical applications, some buffer layers may also be needed to release the temperature-dependent strain and polarization sensitivity between the substrate and the core waveguides [3]. Thus, when we have many complicated waveguide layers, it is not easy to identify the clad mode numbers clearly, and we may need a new theory that is adaptable to the new structures. Waveguide theory is fundamentally well-established, and many references are available [10–15]. In principle, the guiding modes can be determined by solving the field-transfer matrix in thin films numerically for general dielectric planar waveguides. Among the references, both Ruschin et al. [12] and Lit et al. [13] used simplified equations that were transformed from the matrix. The simplified equations not only enhanced the numerical calculations but also enabled them to characterize the mode properties. However, Lit et al. [13] used numerous parameters in their calculations, which were not easy to follow, and Ruschin et al. [12] treated all layers as guiding layers, which was not true for some guiding modes. In addition, all the existing methods lack a clear statement of how to determine an accurate guiding