Study Of Diode-Pumped Yb～(3+) Heat Capacity Laser's Feasibility

The Solid-state heat-capacity laser (SSHCL) has been the focus for its peculiar operation mode (heat-capacity operation), high-energy output and excellent beam quality. Quite different from the process in single-shot and steady state operation, in the heat capacity mode, no cooling is present as lasing proceeds and the waste heat is stored in the active medium itself. There are many Yb³⁺ and Nd³⁺ doped crystals suiting for laser diode pump. Nd³⁺ is used in heat-capacity laser because of its four-level system. But Yb³⁺ is not used, for its thermal population is effected by temperature in the quasi-three-level system.Laser crystals' thermal and spectroscopic properties are compared in the paper. YAG is the good choice in the Yb³⁺ doped crystals of small heat capacity laser. Measurements are reported of absorption spectra of Yb:YAG with different Yb³⁺ concentrations at temperatures of the range:300K-420K. The emission cross section and radiative lifetime are calculated. With the increases of temperature, it is found that spectroscopic properties of low doped Yb:YAG change few. Considering the spectroscopic properties, the temperature dependence of reverse population, heat load and optical efficiency and the temperature dependence of gain coefficient, it is predicted that diode-pumped Yb³⁺ heat capacity laser is feasible in 273K-350K. And comparing diverse pumping configuration, the end-pumping is chosen in Yb:YAG heat capacity laser.By using the CW plane wave rate equation in steady state, the dependence of the output laser power on the temperature is analyzed. Finally by properly simplifying rate equations of Yb:YAG heat-capacity laser, the expression of its output energy is derived in QCW operation. Then the optimum reflectivity of output mirror is discussed.