Significance   The laser near 1.6 µm is not only the safe band of human eyes, but also the transmission window in the atmosphere. The high-frequency and high-energy laser close to 1.6 µm can also carry information with high resolution and large amount of data at a longer distances. In recent years, with the improvement of crystal preparation and lens coating technology, the 1.6 µm band laser obtained by directly pumping gain media and frequency conversion technology has greatly improved the parameters such as repetition frequency, energy and beam quality. In this paper, the principles and research progress of 1.6 μm laser generated by erbium-doped crystal direct pumping, optical parametric oscillation and stimulated Raman frequency shift are introduced, the advantages and disadvantages of the above three schemes in 1.6 μm laser are analyzed, and their application prospect in 1.6 μm high-repetition rate and high-energy laser is prospected. The problem of poor output beam quality when high-frequency and high-energy lasers is obtained near 1.6 µm is also analyzed, and several enhancement examples are given. The development prospect of obtaining better beam quality and high-frequency and high-energy lasers by optical parametric oscillation near 1.6 µm is discussed.

  Progress  Firstly, the energy level conversion process of the laser near 1.6 µm directly generated by pumping Er³⁺ doped crystals is given. However, the low absorption efficiency of pump light, the small photon transition cross section, the high number of parasitic lasers in the crystal and the low thermal conductivity of the crystal make the thermal load on the crystal very high. All these reasons limit its application in obtaining high-repetition rate and high-energy lasers at about 1.6 µm band. Then the process of obtaining stokes light by stimulated Raman frequency shift is described. Raman lasers based on conventional Raman gain materials such as BaWO₄, SrWO₄, Ba(NO₃)₂, BaTeMo₂O₉, GdVO₄, YVO₄ and KGd(WO₄)₂ are analysed, as their low Raman gain coefficients and the low thermal conductivity and thermal expansion coefficients of the crystals lead to the inability of these non-linear crystals to obtain high re-frequency, large-energy wavelength band lasers near 1.6 µm. In contrast, the high and low thermal expansion coefficients of diamond and its transparency over a wide wavelength range make up for some shortcomings of traditional Raman crystals, but the Raman frequency shift is only 1 332.3 cm⁻¹, so it is still impossible to convert the existing and technically mature high-power 1 µm band lasers to the 1.6 µm band with second-order Stokes frequency shift. These reasons limit the application of stimulated Raman shifts to obtain high-frequency and high-energy lasers near 1.6 µm. Finally, the OPO technique based on KTA and KTP crystals is presented for application in obtaining a human-safe laser output in the wavelength band near 1.6 µm with wide wavelength tuning, higher beam quality, high heavy frequencies and large energy. Although the spot quality of laser output of OPO technology is poor in the wavelength band near 1.6 µm, it is possible to obtain laser output with high repetition rate, high energy and good beam quality in the wavelength band near 1.6 µm with reasonable resonator design, phase matching method of nonlinear crystal, selection of pump wave shape and pulse width, and use of a Gaussian mirror and a quasi-monolithic 90° image rotation, which is certainly what researchers in OPO technology are working hard to achieve.

  Conclusions and Prospects  The high-frequency, high-energy laser near 1.6 µm is of great significance because it meets the needs of long-distance and high-data transmission without causing unintentional harm to people nearby. The main methods for obtaining lasers in the 1.6 µm band are pump light direct pumping of Er³⁺ doped crystals, SRS and OPO techniques. However, the low absorption efficiency of Er³⁺ crystals, the low thermal conductivity of the gain medium and the short lifetime of the energy level of the crystals make them unable to meet the requirements of high-repetition rate and high energies. The SRS technique is only capable of shifting the 1 µm band to near 1.49 µm due to the low thermal conductivity of the existing Raman medium and the limited Raman frequency shift, while the OPO technique is capable of achieving high-frequency and high-energy output near 1.6 µm by adjusting the parameters of the pump light and resonant cavity with a good nonlinear crystal. Although the beam quality of the output light is not good, laser pulses with good beam quality can be obtained through proper optimization, and there is much room for improvement in the current methods to solve this problem.