Investigation of tunnel radiation coupling into diffused $LiNbO_3$ waveguides

An analysis was made of the tunnel excitation of diffused waveguides with the aim of optimization of the coupling of $H$ and $E$ waves. The optimal profile of the gap between a prism and a waveguide was calculated for the case of “weak” $(\Delta n\ll n)$ waveguides. A study was made of the dependence of the tunnel emission coefficient on the refractive index of the prism. A method was developed for determining the shape of the gap between the prism and the waveguide. A high efficiency of tunnel excitation of the $H$ waves ($90\%$) and $E$ waves ($65\%$) was achieved experimentally.