In the mid-infrared spectral range spanning 2-5μm, ultrafast laser sources are indispensable for a multitude of scientific and industrial applications. These applications leverage the unique properties of mid-infrared light, such as molecular overtone and combination 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 has been 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 address these challenges, this study employs a difference-frequency generation (DFG) scheme utilizing a high-power dual-wavelength ultrafast fiber laser system. The system comprises an Er-doped fiber laser operating at 1556nm and a Yb-doped fiber amplifier extending the spectrum to 1030nm. The 1.03μm pump pulses are amplified to 31.5W with a pulse energy of 0.95μJ and a duration of 260fs, while the 1.55μm signal pulses are amplified to 4.6W, featuring 136nJ energy and 290fs width. A key innovation lies in 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 1.9μm with average powers of 200-400mW.
The DFG process occurs in a 3mm fan-out PPLN crystal, where the pump and signal pulses are temporally synchronized and focused to 200μm 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 and signal energies. Experimental optimization of the pulse delay between the pump and signal beams enhances the idle light energy, achieving a central wavelength of 3.06μm with 3.06W average power and 92nJ pulse energy at 33.3MHz repetition rate. Moreover, by tuning the signal wavelength from 1.3 to 1.9μm and adjusting the PPLN poling period, we generate tunable mid-infrared radiation across 2-5μm, maintaining average powers above 1W throughout the range. At specific wavelengths like 3.28μm, the output power reaches 1.87W, 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 enables high-sensitivity gas detection with parts-per-billion precision, real-time combustion diagnostics through simultaneous multi-species monitoring, and table-top high-harmonic generation for attosecond pulse synthesis. Furthermore, the study elucidates the nonlinear energy transfer mechanisms in PPLN crystals, providing design rules for future high-power mid-infrared systems. The experimental demonstration not only pushes the power frontier in this spectral region but also establishes a robust platform for various cutting-edge scientific and industrial applications.