Experimental investigation of the influence of the refractive index of the intermediate planar layer on the properties of a polished type single-mode fibre coupler

We present a detailed experimental investigation of the influence of the refractive index and thickness of a waveguiding planar layer placed between two fibres in a polished type single-mode optical fibre coupler. Experiments were performed for different distances between the fibres. The investigation not only revealed some new possibilities for the practical use of such coupler, but provided a reliable quantitative database for future theoretical modelling of this complex waveguiding structure.