Mid-Infrared Adaptive Fiber Micro-Lenses for Inline Beam Shaping

Mid-infrared (mid-IR) light sources and integrated photonic devices are enabling technologies for applications in numerous sectors including sensing, material processing and medical diagnostics. However, crucial devices are lacking for large-scale photonic integration in the mid-IR range. In this work, we report on the design and fabrication of mid-IR integrated fiber micro-lenses based on fluorozirconate fibers. Models of the fiber micro-lenses were built both based on the ABCD-matrix method and the finite element method, showing the possibility of beam manipulation including collimation and focusing, with proper sets of parameters. Micro-lenses based on single-mode and core-less fluorozirconate fibers were fabricated using a high-precision fiber micro-processing workstation, with a process optimized for fluoride glasses to avoid surface crystallization. The micro-lenses are characterized using a 2.8-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mu }$</tex-math></inline-formula>m single-mode fiber laser, showing good agreement with the simulations. In particular, collimating micro-lenses reduce the divergence angle of the single mode output from 6.72<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> down to 1.73<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>; and focusing micro-lenses show equivalent focal lengths of 0.98mm and 1.61 mm. The micro-lenses have an excess loss lower than 0.14 dB. These mid-IR integrated fiber micro-lenses should pave ways for interconnects and adaptive beam control at a microscopic scale, enabling further integration of mid-IR photonic devices.