Millijoule-level nanosecond pulse generation beyond 3 μm enabled by an Er ³⁺ /Dy ³⁺ co-doped fluoride fiber master oscillator power amplifier (MOPA)

High-energy nanosecond pulses at wavelengths beyond 2.5 m in the mid-infrared (mid-IR) region are of significant interest for applications such as polymer processing, minimally invasive surgery, laser ranging, and infrared countermeasures.While rare-earthdoped fluoride fibers provide a compact and robust platform for mid-IR pulse generation, achieving millijoule (mJ)-level nanosecond pulses beyond 3 m remains challenging.In this work, we demonstrate high-energy nanosecond pulse generation at 3.17 m using a master oscillator power amplifier based on a 980 nm diode-pumped Er/Dy co-doped fluoride fiber.Seeded by an actively Q-switched oscillator operating at a 1 kHz repetition rate, the system delivers 0.68 mJ of pulse energy with a pulse width of 132 ns and a peak power of 4.8 kW, while maintaining single-transverse-mode operation (M 2 =1.2-1.3).To the best of our knowledge, this work represents the first report on mJ-level nanosecond pulse generation beyond 3 m from a fiber-based system.