Directional multimode fiber couplers in the mid-infrared

Construction and characterization of the first multimode fiber couplers in the mid-IR region (2 to 10 μ) are reported. These couplers are made of step-index As₂Se₃ chalcogenide glass fibers whose cores were brought to a contact along a certain distance. Far-field measurements using selective angular excitation indicated a typical coupling ratio of 15% (8.2 dB) from couplers made of 550-μm core diameter fibers with a 10-cm interaction length. The excess insertion loss was less than 0.2 dB. Strong mode mixing caused by the coupling mechanism is observed, resulting in a significant broadening of the radiation distribution.