Butt-coupling of 4.5 μm quantum cascade lasers to silica hollow core anti-resonant fibers

We report on experimental characterization of butt-coupling between a quantum cascade laser operating around the wavelength of 4.5 μm and a silica hollow core anti-resonant fiber. The used fiber has a single ring of non-touching capillaries surrounding a 42.5 m dimeter air core and a mid-infrared transmission window in the wavelength range from 2.8 m to 4.7 m. A lens-less butt-coupling interconnection is established and coupling efficiency of 22% is achieved with over 100 mW of power at the fiber output. Bending losses in this test-bed system are verified depending on radius with the loss remaining below 1 dB/m for 20 mm loops, while bend losses exceeding 10 dB/m are observed when the fiber is bent down to a 5 mm radius. The presented coupling dramatically improves the output beam quality of the quantum cascade laser with a measured NA = 0.45. The beam at the fiber output has a circular shape, measured M2=1.04 and uniform numerical aperture of NA = 0.13. Together with the hollow core fiber geometry, the proposed system would be suitable for simple implementations of absorption spectroscopy of popular atmospheric agents, like N2O.