High-power mid-infrared laser coupling using tapered graded-index fibers

Conventional free-space lens coupling for mid-infrared laser systems is often limited by bulky optical configurations, alignment sensitivity, poor long-term stability, and inefficient mode-field matching. Here, we propose a compact and robust coupling-and-transmission strategy that combines the self-imaging capability of graded-index fibers with adiabatic fiber tapering. A high-refractive-index chalcogenide glass adhesive is employed for interfacial bonding, replacing conventional anti-reflection coatings. This approach eliminates residual air gaps and mitigates interfacial damage associated with thermal-expansion mismatch under high-power operation. The key tapering parameters are numerically optimized to enable efficient mode-field transformation while maintaining stable beam propagation. Experimentally, the proposed scheme achieves a coupling efficiency of 74.63% at a mid-infrared wavelength of 4.5 μm, with an input power handling capability exceeding 8 W and a stable output power above 6 W, and a well-confined output beam profile is also obtained. This work overcomes the power-handling bottleneck of conventional fiber coupling in the mid-infrared and provides a compact, efficient, and reliable route toward high-power mid-infrared laser delivery for material processing, gas sensing, infrared countermeasures, and integrated photonic systems.