Quantum cascade lasers (QCLs) are new unipolar semiconductor devices based on the principle of electron inter-band transition and phonon-assisted oscillation tunneling in the semiconductor quantum well. Different from the electron-hole recombination stimulated emission mechanism of traditional p-n junction semiconductor lasers, only electrons participate in the QCL stimulated emission process. The selection of the lasing wavelength can be achieved through energy band tailoring of the potential well and barrier in the active region. QCL has led the revolution in semiconductor laser theory, mid-infrared and THz semiconductor light sources. It is an ideal light source for trace gas monitoring and free space communications. It has significant application prospects in the fields of public security, national security, environment and medical science. The quantum cascade laser (QCL) is a mid-infrared band monopole light source based on electronic transitions between subbands. Its working principle is completely different from that of ordinary semiconductor lasers. Its lasing scheme is to use the separated electronic states caused by the quantum confinement effect in the semiconductor heterojunction thin layer perpendicular to the nanometer-thickness to produce particle number inversion between these excited states. The active area of ??the laser is coupled by A multi-level series connection of quantum wells (usually more than 500 layers) is used to achieve multi-photon output from single electron injection. The emergence of quantum cascade lasers pioneered the use of wide bandgap materials to develop mid- and far-infrared semiconductor lasers, setting a new milestone in the development history of mid- and far-infrared semiconductor lasers. In 1994, Federico Capasso and his colleagues Zhuo Yihe and others took the lead in inventing the quantum cascade laser at Bell Labs. This is regarded as a revolution in the field of semiconductor lasers. In 2000, the research team of Chinese scientist Li Aizhen (currently an academician of the National Academy of Sciences) was the first in Asia to develop a semiconductor quantum cascade laser with a wavelength of 5 to 8 microns, thus making China join the ranks of countries that have mastered the development technology of such lasers.