At present, approaches to 8-12 μm LWIR laser mainly include direct radiation from gain media represented by carbon dioxide (CO₂) lasers and semiconductor quantum cascade lasers (QCLs) and nonlinear optical techniques represented by second-order nonlinear frequency conversion. CO₂ lasers have been one of the most mature coherent radiation sources for the LWIR band since the invention of the first CO₂ laser in 1964. However, their output wavelengths are limited to the spectral lines of 9.2-9.8 μm and 10.1-11 μm. In addition, since CO₂ lasers usually need to be supported by a large cooling system, the overall size of the device is huge, which greatly limits the application range of CO₂ lasers. QCLs feature a broad emission spectrum (3.5-160 μm) with a relatively narrow linewidth and favorable wavelength tunability. However, due to the limited depth of their quantum wells, QCLs offer low efficiency in the 8-12 μm band and consequently fail to achieve high-power and high-pulse energy operation. Besides, they are difficult to design and entail a relatively high manufacturing cost.Although 8-12 μm LWIR lasing has already been achieved with gas and semiconductor as gain media, no mature method of LWIR lasing by directly pumping crystalline gain media is obtained so far due to the restriction of the intrinsic emission spectra of the currently available crystals. As the most mature and most widely used method, nonlinear frequency conversion is an effective approach to 8-12 μm lasing. Notably, solid-state lasers based on second-order nonlinear frequency conversion techniques break through the predicament that crystalline gain media cannot directly achieve LWIR lasing. Furthermore, compared with CO₂ lasers and QCLs, all-solid-state lasers based on nonlinear frequency conversion techniques have the characteristics of excellent wavelength tunability and power scalability. The diversities of the available pump parameters (wavelength, width, energy, power, etc.) and emerging nonlinear optical crystals provide LWIR lasers based on nonlinear frequency conversion with a broader development space towards but not limited to ultrashort pulse width, high repetition rate, wide wavelength tuning range, high energy, and high power. This paper reviews the working mechanisms and research progress of LWIR lasers based on second-order nonlinear frequency conversion to provide a reference for the personnel engaged in the research and development of lasers.

The 8-12 μm long-wave infrared (LWIR) laser, which is within the atmospheric transmission window and the eye-safe range and demonstrates a higher transmittance in atmospheric media (Fig. 1), has critical applications in various fields, such as directed infrared countermeasures, environmental monitoring, lidar, and surgery. For example, the laser in this LWIR band plays an important role in environmental monitoring and differential absorption lidar because this band covers the fundamental absorption bands of many gas molecules, such as H₂O, CO₂, NH3, and O₃. In terms of medical treatment, the 8-12 μm LWIR laser, with a large absorption coefficient and a shallow penetration depth in water and other components of biological tissues, serves as a unique and effective tool in biological tissue treatment. In addition, high-energy 8-12 μm LWIR lasers are in high demand in the field of defense.

Specifically, the working principles and characteristics of the second-order nonlinear frequency conversion techniques, including optical parametric generation (OPG), optical parametric oscillation (OPO), difference frequency generation (DFG), and optical parametric amplification (OPA), are described (Fig. 3). Subsequently, the physical and nonlinear optical properties, including nonlinear coefficient, transparency range, thermal conductivity, and damage threshold, of commonly used nonlinear crystals, such as ZnGeP2, BaGa4Se7, CdSe, GaSe, LiGaS2, orientation-patterned GaAs, and orientation-patterned GaP, are summarized (Table 1). Then, the detailed properties of different crystals and the output characteristics of the corresponding LWIR laser based on the crystals are analyzed. The research progress analysis shows that LWIR lasers based on second-order nonlinear frequency conversion have achieved femtosecond, picosecond, and nanosecond output in pulse width and repetition rates ranging from several hertz to megahertz. However, due to the low inherent quantum conversion efficiency of nonlinear frequency conversion towards the LWIR band (pumped by 1-3 μm near- and mid-infrared lasers), the output energy of the LWIR lasers is mainly at the microjoule and millijoule levels at present (Fig. 10). Finally, the opportunities and challenges for LWIR lasers based on second-order frequency conversion techniques are discussed, and the potential method of LWIR lasing via Raman conversion based on the third-order nonlinear effect and its prospect are presented.

Crystalline LWIR lasers based on second-order nonlinear frequency conversion techniques have made outstanding achievements in ultrashort pulse width, high repetition rate, wide wavelength tuning range, and high peak power. The improvement of crystal growth technique, the emergence of new types of nonlinear optical crystals, and the development of currently available crystals with higher optical quality and larger volume crystals pave the way for the further improvement of the power and conversion efficiencies of LWIR lasers. In addition to the above reviewed second-order nonlinear frequency conversion techniques, diamond Raman lasers (based on the third-order nonlinear optical effect) with an extremely wide spectral transmission range and an extremely high thermal conductivity are considered a promising way of wavelength conversion from short-wave to long-wave.