The laser output end of the rod (the right side of the rod in Fig. 1) is assumed to be AR coated at the laser wavelength and may be reflective at the pump wavelength, allowing the pump light to be effectively double passed. The distinguishing feature of quasi-three level systems compared to classic four level laser systems from the pump excitation point of view is they are strongly pumped. Strong pumping is necessary to overcome the deleterious effects of ground state re absorption present because the terminal laser Stark level in a quasi-three level system lies in the ground state manifold and so contains a thermalized fraction of the population pres- ent in the ground state. Depending on the detailed spec- troscopy of the laser ion, some level of saturation pumping is generally required if these systems are to be operated efficiently. In a previous publication, Fan addressed optimizing the efficiency and stored energy in quasi-three level lasers using an active mirror configuration [ 61. These active mirror systems consist of thin slabs that typically doped at 10 to 100 times higher concentration than the systems under consideration in this manuscript [ 71. This high doping is required for the efficient absorption of the pump as it is passed through the thin slab. The active mirror configuration is also fundamentally dif- ferent than the architecture studied here in that the pump beam is unconfined in those systems. Because the pump is unconfined, the active mirror architecture
is