Mid-infrared (mid-IR) laser sources are now becoming enabling tools for cutting-edge applications, including greenhouse gas sensing[1,2], medical diagnosis, and security and defense. Many molecules and molecular functional groups experience vibrational absorption in the mid-IR region. Indeed, spectroscopic applications would benefit from scaling the optical frequency comb to the mid-IR region, leading to numerous absorption lines being recorded with unprecedented accuracy and resolution[5,6]. Moreover, ultrafast pulses with a stable carrier–envelope phase permit an exhaustive understanding of molecular structure and dynamics[7,8], and they enable the soft X-ray generation to be scaled on a tabletop system[9,10]. Over the last decade, steady progress has been made in the field of ultrafast mid-IR frequency comb generation. There is a wide array of innovative solutions to generate coherent mid-IR laser sources, with novel gain media, quantum cascade lasers and micro-resonators, and supercontinuum generation in waveguides and fibers[14,15]. Compared with these approaches, detecting and controlling the offset frequency of the mid-IR sources is a challenge, which is the prerequisite for frequency comb spectroscopy. A more direct approach is to employ nonlinear frequency conversion of ultrashort pulses in the visible or near-infrared (NIR) regime to generate coherent mid-IR sources. Among the different nonlinear processes, difference frequency generation (DFG) with signal and pump from the same oscillator offers several advantages for a mid-IR frequency comb system, as it allows us to use compact and well-developed fiber laser technology, and also to achieve ultrashort pulses and intrinsic carrier–envelope phase stability, which reduce the complexity and improve the quality of the long-term performance[16–19]. In DFG systems, the mid-IR spectrumd coverage depends on the NIR spectrum and the transmissivity of the crystal. Limited by the gain bandwidth of laser materials, the general NIR sources cannot directly emit pulses with such a broad spectrum. To achieve a broad mid-IR spectrum, highly nonlinear fiber (HNLF) is widely applied in DFG systems for NIR spectrum broadening. The oscillators split the output laser into two beams and nonlinearly broaden the NIR laser in HNLF[16–20]. By phase matching, the signal and pump pulses are focused into a crystal to generate an idler pulse in the mid-IR region. The latest representative work demonstrated a high-power mid-IR femtosecond source with a broad spectrum from 1.6 to 10.2 μm by DFG. With such a broad spectrum, the high-power mid-IR femtosecond source will be a powerful tool for multispecies trace gas detection.