Efficient mid-infrared light coupling into a single-mode optical fiber

For the first time, to the best of our knowledge, a mode-selective photonic lantern is demonstrated as an efficient, transverse offset-tolerant coupling mechanism in a single-mode optical fiber. The principle relies on the selective excitation of the fundamental propagation mode at the multi-mode end of the photonic lantern. It is, after the transition, unambiguously guided in one of the single-mode fibers at the other end, i.e., in the single mode fiber having the largest normalized frequency V. To validate this concept, a photonic lantern based on indium fluoride optical fibers has been designed. A prototype has been fabricated and tested at the wavelength λ = 3.34 µm, demonstrating excellent transverse offset tolerance. The experiment is in good agreement with the simulations confirming the feasibility of the device. The operation of the proposed mode-selective photonic lantern is strongly promising for various applications, including laser pigtailing and coupling between anti-resonant hollow-core fibers and standard single-mode fibers.