COUPLING EFFICIENCY OF DIRECTIONAL FIBER COUPLERS IN THE MIDINFRARED

Coupling efficiency characterization in the mid-IR region (2-10 mu m) of contact couplers made of step index multinode As2Se3 fiber is given . A unique setup based on an FT-IR spectrometer was built to measure t his parameter in several couplers. It is observed that the coupling ef ficiency between the coupled and the direct output arms increases line arly with the interaction length and monotonously with the wavelength in the mid-IR region, A coupling ratio of almost 20% at 10 mu m has be en achieved with an interaction length of tao mm, while the conical in put beam has a 40 degrees opening angle. A theoretical model connectin g the coupling efficiency and the coupler's gap is introduced. A compa rison between this model and the observations yields an effective gap value of about 40 nm.