Power Scaling of Single‐Frequency Fiber Laser at 3.5 µm Mid‐Infrared Region Toward Ten‐Watt Level

Single‐frequency lasers in 3–5 µm region are of interest for coherent detection and precision spectroscopy applications. Fiber lasers offer rich frequency selection schemes, but the single‐frequency laser power has been limited to hundred‐milliwatt‐level in the mid‐infrared region due to the immature fiber devices. Here, we directly address this problem by developing a master‐oscillator power‐amplifier (MOPA) tailored for a high‐power single‐frequency laser in the 3.5 µm waveband. Numerical modeling is performed to analyze the power behavior of the MOPA and determine the routine for efficient power amplification with a low‐power single‐frequency laser seed. By optimizing the gain fiber length, 976 nm pump power, and pumping scheme to balance laser efficiency and thermal effects, the laser produces 8.6 W single‐frequency output at 3470 nm, with a slope efficiency of 23.1% relative to absorbed 1990 nm pump power. To our knowledge, this represents the first demonstration of fiber MOPA at 3.5 µm, and also delivers the highest single‐frequency output power ever achieved from mid‐infrared fiber lasers. Furthermore, the modeling also reveals the potential of this MOPA to generate ten‐watt‐level output at >3.7 µm long wavelength region.