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\section{Introduction}  Coupled-mode theory is concerned with coupling spatial modes of differing polarizations, distribution, or both. To understand codirectional coupling it is useful to have an understand of background material that builds to codirectional coupling. First, consider coupling normal modes in a single waveguide that is affected by a perturbation. Such case is single-waveguide mode coupling. The perturbation in question is spatially dependent and is represented as $\Delta P(r)$, a perturbing polarization. Consider the following Maxwell's equations \[ \nabla \times E=i \omega \mu_0 H \\ \nabla \times H=-i \omega \epsilon E-i \omega \Delta P\] Consider two sets of fields $(E_1,H_1)$ and $(E_2,H_2)$, they satisfy the Lorentz reciprocity theorem give by $\nabla \cdot \left( E_1 \times H_2^{*} +E_2^{*} \times H_1\right)=- i \omega \left( E_1 \cdot \Delta P_2^{*}-E_2^{*} \cdot \Delta P_1 \right)$   For $\Delta P_1=\Delta P$ and $\Delta P_2=0$ and integrating over the result for the cross section of the waveguide in question, we get \[ \sum_{\nu} \frac{ \partial }{\partial z} A_{\nu}(z)e^{i\left(\beta_{\nu}-\beta_{\mu}\right)} z=i \omega e^{-i \beta_{\nu} z} \int_{- \infty}^{\infty} \int_{- \infty}^{\infty} E_{\mu}^{*} \cdot \Delta P dxdy \] Evoking orthonormality, we can get the coupled-mode equation \[ \pm \frac{ \partial A_{\nu} }{\partial z} =i \omega e^{-i \beta_{\mu} z} \int_{- \infty}^{\infty} \int_{- \infty}^{\infty} E_{\nu}^{*} \cdot \Delta P dxdy \] The plus sign indicates forward propagating modes when $B_{\nu} > 0$ and the minus sign indicates a backward propagating mode with $B_{\nu} < 0$ \\  Many applications are concerned with the coupling between two modes. This coupling between two modes can be within the same waveguide or can be coupled between two parallel waveguides. For a system where we are interested in coupling two modes for either the parallel waveguides case or within the same waveguide, the two modes are described by two amplitudes A and B. The coupled equations are given by $ \pm \frac{ \partial A}{\partial z}=i \kappa_aa A+ i\kappa_{ab} B e^{i(\beta_b-\beta_a)z$