ZBLAN Optical Fiber Couplers for the Mid-Infrared

A single-mode optical fiber coupler is demonstrated from ZBLAN fiber with a coupling ratio of 61%/39% at a wavelength of 2.7 $\mu$m. Single-modedness is evidenced from the absence of modal interference in the transmission spectrum. Coupling to the cross-port is limited by the onset of crystallization during the tapering process, which fosters signal loss through scattering.