The spectral band of 2-20 μm is defined as the mid-infrared (MIR) band, which is within the atmospheric transmission window and eye-safe range and plays an important role in environmental monitoring, optoelectronic countermeasures, lidar, and surgical operations. For example, MIR band lasers cover the absorption bands of many gas molecules, such as H₂O, CO₂, NH₃, and O₃, which can be employed in environmental monitoring and differential absorption lidars. Additionally, due to the large absorption coefficient and shallow penetration depth of 2-20 μm lasers in biological tissues, they are widely adopted in laser medical surgery. Currently, there are two major methods for generating MIR ultrafast lasers. The first is to directly generate MIR lasers based on the energy level structure of the gain medium itself, such as solid lasers, gas lasers, fiber lasers, and quantum cascade lasers (QCLs). The other is to adopt nonlinear frequency conversion technology to generate MIR ultrafast lasers, such as supercontinuum light generation, optical parametric processes, difference frequency, and four-wave mixing. QCLs are a new type of semiconductor laser, which enables semiconductor lasers to operate in the MIR band. The working principles of traditional semiconductor lasers and QCLs are shown in Fig. 1. QCLs are characterized by compact structure, easy wavelength tuning, and long lifetime. However, their manufacturing process is complex and expensive, and features high environmental requirements and poor beam quality, thus limiting its application range. Its application is limited to long-distance remote sensing detection and optoelectronic countermeasures. The core technology of mid-infrared solid-state lasers is to employ laser crystals as gain media, to dope impurities in the crystals to change their energy level structure, and finally to achieve light amplification via the cavity to produce MIR laser output. Among them, the main doped impurities are rare earth ions (Tm³⁺, Ho³⁺, Er³⁺) and transition metals (Cr²⁺, Fe²⁺). As shown in Fig. 2, it shows the energy level transition of rare earth doped ions. The band generated by MIR ultrafast solid-state lasers is in the range of 2-5 μm. MIR solid-state lasers have the characteristics of high conversion efficiency and high stability, but due to the immature material processing and preparation process of laser gain media, the laser will produce thermal effects and quantum losses during the oscillation process, with limited output wavelength. MIR solid-state lasers realized by doping transition metals into crystals have advantages in outputting high-power pulses while operation at high temperatures will reduce the luminescence life of the doped crystals. MIR lasers generated based on nonlinear frequency conversion technology mainly utilize nonlinear effects in nonlinear crystals. These include difference frequency generation (DFG), optical parametric amplification (OPA), optical parametric generation (OPG), and optical parametric oscillation (OPO) (Fig. 3). Among them, DFG combined with a high-stability near-infrared ultrafast fiber laser as a pump and signal source is more conducive to the miniaturization and high stability of MIR ultrafast lasers. Additionally, the intra-pulse difference frequency generation (IP-DFG) technology based on a short-wavelength ultrafast fiber laser source is a simple nonlinear frequency conversion method. We describe the generation of MIR lasers, the basic principles and research progress of IP-DFG technology, and the application of ultrafast MIR lasers. Finally, the future development and application prospects of the IP-DFG system are presented.

The IP-DFG process (Fig. 9) mainly employs an ultra-wide light source that can cover the range of both signal and pump light sources to directly complete the difference frequency process in the nonlinear crystal to generate MIR laser pulses. If the spectral width of the incident light source is not wide enough, a nonlinear compression scheme must be added before the difference frequency stage. The wide spectrum light source is mainly obtained by spectral broadening in positive dispersion fiber, soliton self-compression in negative dispersion fiber, Kerr lens mode-locked direct output, and other methods. Subsequently, we summarize the nonlinear coefficient, transparency range, thermal conductivity, damage threshold, and other physical and nonlinear optical properties of commonly adopted nonlinear crystals, such as PPLN, GaSe, OP-GaP, CSP, and LGS. Then, we analyze both the detailed properties of different crystals and research the progress of IP-DFG based on these crystals to generate corresponding ultrafast MIR lasers in detail. Finally, the advantages and applications of IP-DFG technology based on ultrafast fiber lasers as the light source are summarized. Meanwhile, we expect to adopt ultrafast fiber lasers as the light source based on the cascaded IP-DFG technology to control the new MIR few-cycle pulse waveform by adjusting the carrier-envelope phase (CEP) shift frequency of the driving pulse.

IP-DFG system based on fiber lasers as the driving light source has a compact structure, high stability, and low cost. As high-energy ultrashort pulse fiber laser technology becomes increasingly mature, it is very helpful to enhance the nonlinear effect. Therefore, utilizing a wide-spectrum femtosecond laser as a driving source is an important direction for the future development of this field. Additionally, the MIR light source generated by IP-DFG in an ultrafast fiber laser features a wide spectrum, high signal-to-noise ratio, strong coherence, and large pulse energy, with irreplaceable special applications in environmental detection, medical treatment, and industrial processing. With further exploration and research by scientific researchers, the spectrum of MIR lasers will be wider, with narrower pulse, higher peak power, and broader application prospects.