In the mid-infrared spectral range of 2–5 μm, ultrafast laser sources are indispensable for a number of scientific and industrial applications. In these applications, some unique properties of mid-infrared light are utilized, such as molecular overtone and combined tone absorption for sensitive gas detection, minimal atmospheric attenuation for efficient free-space optical communication, phase-matching extension in nonlinear optical processes for high-order harmonic generation, and non-invasive molecular vibration spectroscopy for biomedical imaging. However, the generation of high-power, tunable mid-infrared lasers is hindered by the complex spectral phase of supercontinuum sources, the demanding resonator design of optical parametric oscillators, the limited tuning range of rare-earth-doped fiber lasers, and the power limitations of intrapulse difference-frequency generation. To cope with these challenges, this study employs a difference-frequency generation (DFG) scheme in which a high-power dual-wavelength ultrafast fiber laser system is utilized. The system comprises an Er-doped fiber laser operating at 1556 nm and a Yb-doped fiber amplifier extending the spectrum to 1030 nm. The 1.03-μm pump pulses are amplified to 31.5 W with a pulse energy value of 0.95 μJ and a duration of 260 fs, while the 1.55-μm signal pulses are amplified to 4.6 W, featuring 136 nJ in energy and 290 fs in width. A key innovation is the spectral broadening of the signal pulses via the SESS (SPM-Enabled Spectral Selection) technique in dispersion-shifted fiber, achieving tunable sidebands from 1.3 to 1.9 μm with average power values of 200–400 mW.The DFG process occurs in a 3-mm fan-out PPLN crystal, where the pump and signal pulses are temporally synchronized and focused into 200-μm spots. By solving the three-wave coupling equations with the split-step Fourier method, we reveal that the idle light energy exhibits linear, exponential, and saturation regimes with respect to pump energy and signal energy. Experimental optimization of the pulse delay between the pump beam and signal beam enhances the idle light energy, achieving a central wavelength of 3.06 μm with 3.06-W average power and 92-nJ pulse energy at a 33.3-MHz repetition rate. Moreover, by tuning the signal wavelength from 1.3 to 1.9 μm and adjusting the PPLN poling period, we generate tunable mid-infrared radiation across 2–5 μm, maintaining average power above 1 W throughout the range. At a specific wavelength like 3.28 μm, the output power reaches 1.87 W, with the power gradually decreasing towards longer wavelengths due to crystal phase-matching limitations.The physical significance of these results is profound. The high-power, broadly tunable mid-infrared source can realize high-sensitivity gas detection with an accuracy of a few parts per billion. Real-time combustion diagnostics can be carried out through simultaneous multi-species monitoring, and desktop harmonics can be generated for attosecond pulse synthesis. Furthermore, this study elucidates the nonlinear energy transfer mechanisms in PPLN crystals, providing some rules for designing future high-power mid-infrared systems. The experimental demonstration not only advances the power frontier of this spectral region but also establishes a robust platform for exploring various cutting-edge scientific and industrial applications.
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