Coupling efficiency of directional fiber couplers in the mid-infrared

Coupling efficiency characterization in the mid-IR region (2-10 μm) of contact couplers made of step index multimode As₂Se₃ fiber is given. A unique setup based on an FT-IR spectrometer was built to measure this parameter in several couplers. It is observed that the coupling efficiency between the coupled and the direct output arms increases linearly with the interaction length and monotonously with the wavelength in the mid-IR region. A coupling ratio of almost 20% at 10 μm has been achieved with an interaction length of 120 mm, while the conical input beam has a 40° opening angle. A theoretical model connecting the coupling efficiency and the coupler's gap is introduced. A comparison between this model and the observations yields an effective gap value of about 40 nm.