A review of 2.1-μm Tm/Ho doped solid-state lasers: From continuous wavelength to nanosecond-pulse emission

The 2.1 μm laser, characterized by high absorption coefficients in the atmospheric window, is important in remote sensing and medical treatment. In pulsed operation, Ho3+ and Tm3+ solid-state lasers have a higher gain than Raman lasers, semiconductor lasers, optical parametric oscillators, fiber lasers, and several other methods. In recent years, there has been research on solid-state laser systems doped with Ho3+ and Tm3+, as well as the development of host-doped Ho3+ and Tm3+. The principles of generating a 2.1 μm laser with singly doped Ho3+, singly doped Tm3+, and co-doped Ho3+ and Tm3+ systems are described. The advantages and disadvantages of the three methods are discussed in detail below. The single-doped Ho3+ laser can produce a high-energy laser, while the doped Tm3+ and co-doped Ho3+ and Tm3+ systems have a simple structure. The properties of the host materials determine the upper limits of the laser parameters achievable in the system. This study summarizes the physicochemical properties of different host materials as well as the unique advantages of their output in the 2.1 μm band. In recent years, there has been research on solid-state laser systems doped with Ho3+ and Tm3+, as well as the development of host-doped Ho3+ and Tm3+.