| Mid-infrared (Mid-IR) coherent sources, within 2～3μm spectral region, are of great significance and prospect for applications, such as organic molecular spectroscopy, remote sensing, infrared photoelectrical countermeasure, non-invasive biomedical treatment, free-space or satellites communication, and scientific research.In recent decades, a new cutting-edge technology, transition-metal doped Ⅱ-Ⅵ (Tm2+:Ⅱ-Ⅵ) lasers, leads the development direction of Mid-IR lasers. The laser materials of Cr2+:ZnSe and Cr2+:ZnS crystals are known as "Ti:Sapphire of Mid-IR" due to many advantages, such as the ultra-broad absorption and emission bands, large absorption and emission cross-sections, negligibly low excited state absorption, and suitable for laser-diode pumped operation under room-temperature. Mid-IR lasers based on the Cr2+:ZnSe and Cr2+:ZnS crystals are not only located in "atmospheric window", but also called as "molecular fingerprint". Therefore, researchers around the world are continuously committed to develop the high-performance materials and Mid-IR lasers of Cr2+:ZnSe and Cr2+:ZnS crystals. The remarkable achievements have been attained on the international, especially by the Lawrence Livermore National Laboratory in USA, Sergey B. Mirov at the University of Alabama in Birmingham, Irina T. Sorokina at the Norwegian University of Science and Technology in Trondheim, IPG Photonics and Q-peak corporations in USA. However, few academic research has been done on the materials and Mid-IR lasers of Cr2+:ZnSe and Cr2+:ZnS crystals by Chinese researchers.Given mentioned above, this dissertation closely tracks these cutting-edge technologies and has comprehensive investigated the properties and Mid-IR lasers of Cr2+:ZnSe and Cr2+:ZnS crystals, in order to improve the overall level of Mid-IR solid-state lasers in China. First of all, after a comprehensive research of the structure, energy level and optical characteristic of Cr2+:ZnSe and Cr2+:ZnS crystals, great potential applications on the Mid-IR solid-state lasers have been evaluated. Moreover, passive mode-locking properties of Cr2+:ZnSe and Cr2+:ZnS crystals have been deeply studied at 2μm; and the passively Q-switched and mode-locked Tm:YAL03 lasers have been realized around 2μm with the Cr2+:ZnSe and Cr2+:ZnS saturable absorbers. On the other hand, the high-power continuous-wave, broadly tunable, passively Q-switched Mid-IR lasers based on the Cr2+:ZnSe gain medium have been thoroughly investigated. Firstly, high-power Cr2+:ZnSe continuous-wave lasers have been realized at 2.4μm. And, the pump sources of Cr2+:ZnSe crystal have been explored, including Tm:YAL03 laser and Tm:fiber laser; the absorption percentage of Cr2+:ZnSe crystal to pump laser have been comparative studied. Secondly, tunable Cr2+:ZnSe lasers have been detailedly investigated in Mid-IR region. The basic principle of tunable elements (dispersion prism and birefringence filter) have been analyzed in detail; furthermore, broadly tunable Mid-IR Cr2+:ZnSe lasers have been attained with CaF2 prism and birefringence quartz filter as tunable elements. At last, passively Q-switched Cr2+:ZnSe lasers have been achieved at 2.4μm. The 2D saturable absorption materials of graphene grown on SiC substrate and black phosphorus have been explored and investigated; Mid-IR pulsed Cr2+:ZnSe lasers operated in passively Q-switched regime have been achieved with graphene grown on SiC substrate and BP-based saturable-absorber mirror.The research results of this dissertation have importantly scientific significance and applicable value for exploring Mid-IR lasers directly pumped by LD and expanding technical approaches of Mid-IR coherent sources.
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