We present an experimental study of the coupling of light from a waveguide into a submicron size optical transmission line. We demonstrate that the coupling efficiency depends upon the polarization direction in the waveguide. On the condition when a quasi TEM mode is launched in the transmission line we observe a substantial enhancement of the coupling efficiency. We demonstrate that the coupling efficiency also depends on the mismatch of the impedances of waveguide and transmission line, that

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... s in turn determined by their respective geometries. We have made a prediction of the condition of the maximum coupling efficiency through the approach of impedance matching. We believe these results can be of importance in the development of new concepts of probes for scanning near-field optical microscopy.