In general, there exist many degenerate modes in a waveguide. Some are coupled together while others are not. Prove that the mode coupling does not take place between the two degenerate $\mathrm{TE}_{m n}$ and $\mathrm{TM}_{m n}$ modes in a rectangular waveguide: that is, prove that the power-interaction terms
$$
P_{\mathrm{TE}}^{\mathrm{TM}}=\frac{1}{2} \iint_S \mathbf{E}_{\mathrm{TE}_{\operatorname{man}}} \times \mathbf{H}_{\mathrm{TM}_{\operatorname{men}}}^* \cdot d S=0
$$
and
$$
P_{\mathrm{TM}}^{\mathrm{TE}}=\frac{1}{2} \iint_S E_{\mathrm{TM}_{\mathrm{men}}} \times \mathbf{H}_{\mathrm{TE}}^* * \cdot d S=0
$$