Research Progress of High-Power Ho: YAG Lasers and Its Application for Pumping Mid-Far-Infrared Nonlinear Frequency Conversion in ZGP, BGSe and CdSe Crystals

Significance The 2 mu m, mid-infrared (MIR, 3-5 mu m) and far-infrared (FIR, 8-12 mu m) bands are located in the atmospheric transmission windows and the safety ranges of human eyes. At the same time, the vibration absorption spectral peaks of many gas atoms and molecules are also located in this band. Therefore, these three bands have important application value in the fields of spectroscopy, remote sensing, communications, earth atmosphere monitoring, and optoelectronic countermeasures. At present, the methods used to obtain the mid-far infrared lasers have shown the characteristics of many types and obvious advantages and disadvantages, among which the nonlinear frequency conversion (NFC) technologies possess the advantages of relatively high average power and conversion efficiency, compact structure, and tunable wavelength. Thus, it is the best choice to achieve mid-far infrared lasers. Progress National Key Laboratory of Tunable Laser Technology in Harbin Institute of Technology has carried out researches on the NFC technologies based on the studies in high-power Ho: YAG lasers. For 2 mu m lasers, a Ho: YAG Q-switched laser was successfully developed with the highest average output power of 231 W and a pulse repetition rate (PRR) of 10 kHz in 2018 (Figs. 3-5) . In order to meet the requirements of higher power mid-far infrared NFC for pump sources, the Ho: YAG Q-switched laser with the highest average output power of 312 W (Figs. 6 and 7) and a PRR of 20 kHz has also been achieved as expected. In addition, the Ho: YAG Q-switched laser with the highest single pulse energy of 28 mJ and a PRR of 1 kHz has also been successfully used as a pump source for NFC (Figs. 1 and 2) . With the development of high-power Ho : YAG lasers, the optical NFC technology has also shined in the realization of high-power mid-far infrared lasers. First of all, the research on NFC based on zinc germanium phosphate (ZnGeP2, ZGP) crystals has yielded fruitful results. In 2019, through the ZGP master oscillator power amplifier (MOPA) system, the lasers at center wavelengths of 3.92 mu m and 4.57 mu m with the highest average output power of 102 W were achieved (Figs. 8 and 9) . In 2018, the ZGP cascaded OPA structure was adopted to increase the conversion efficiency from pump light to idler light by reusing the OPO output signal light (Figs. 13 and 14) . In 2021, the total output power of the ZGP MOPA system with the highest average output power of 161 W, the PRR of 20 kHz, and center wavelengths of 3.89 mu m and 4.55 mu m was enhanced by 61 yo through the ZGP crystal (Figs. 10 and 11) . In the same year, the laser with the highest average output power of 3.5 W, a PRR of 10 kHz, and the center wavelength of 9.8 mu m using type-I phase-matching ZGP crystal was obtained. Meanwhile, using type-II phase-matching ZGP crystal, the output characteristics of the upper limit band of the output wavelength of this crystal were explored (Fig. 12) . While the ZGP crystal keeps creating new records for higher output powers in the MIR and FIR bands, the emerging optical NFC-crystal BaGa4Se7 (BGSe) has made outstanding achievements in the MIR laser field. In 2020, the lasers with up to a 5.12 W average output power, a PRR of 1 kHz, and the center wavelengths of 3.9 mu m and 4.2 mu m was achieved from the type-I phase-matching BGSe OPO, which refreshed the crystal highest output power. At the same time, the MIR laser with high power and high beam quality was obtained (Figs. 15 and 16) . In the same year, the MIR laser with a full width at half maximum (FWHM) of only 8 nm and a center wavelength of 3.94 mu m was successfully prepared using a type-II phase-matching BGSe OPO followed by a ZGP OPA. The highest average output power was 4.35 W with a PRR of 1 kHz (Fig. 17, Fig. 18, and Table 2) . In the 10-12 mu m band, the optical NFC crystal cadmium selenide (CdSe) has also achieved fruitful results. In 2020, through continuous light seed injection, it is for the first time to obtain an average output power exceeding 1 W at 10.1 mu m from a CdSe crystal (Figs. 19-21) . Conclusions and prospects In this article, the advantages and disadvantages of different methods to obtain mid-far infrared lasers as well as the latest research results are first introduced. Second, a detailed introduction of the latest research results on the optical nonlinear frequency conversion technologies in Harbin Institute of Technology is illustrated, including the acousto-optic modulated Ho: YAG Q-switched lasers with different pulse repetition rates. Third, the Ho: YAG lasers with different pulse repetition rates and the latest research results in high-power mid-infrared and far-infrared lasers with ZGP, BGSe and CdSe crystals in the nearest three years are also introduced. Finally, with the development of the ultrashort pulsed 2 mu m laser, high power ultrashort mid-far-infrared lasers is prospected to become a research hotspot in next few years.