High-efficiency, double-tapered 3 × 1 mid-infrared tellurite fiber combiner with brightness recovery capability

Scaling power in mid-infrared (MIR) multimode fiber combiners is often limited by beam-brightness degradation caused by the unavoidable excitation of higher-order modes (HOMs). To circumvent this limitation, we report what we believe is the first demonstration of a 3 × 1 multimode tellurite fiber combiner employing an optimized double-tapered (DT) architecture. Unlike conventional designs, this device incorporates a secondary tapering stage that functions as a spatial mode filter to recover beam quality. The fabricated combiner exhibits a high single-port transmission efficiency of 86.1% at 1960nm. In a multi-wavelength demonstration covering the 2-3 µm band (1960, 2120, and 2770 nm), a total output power of ∼3.6 W was achieved with an overall efficiency of 80.1%. This efficiency is primarily limited by hydroxyl-induced absorption at longer wavelengths, which significantly outweighs the low fabrication excess loss (∼0.5 dB at 1960nm). Crucially, the DT design recovers the beam quality factor Mx/y2 from 15.9/13.9 (single-taper baseline) to 11.6/10.2. This enhancement yields a calculated brightness recovery ratio (BRR) of ∼1.8, validating the device as a compact and robust solution for brightness-conserving MIR all-fiber laser systems.