what is the best way to achieve population inversion in the following laster system?

Lasers (4.08) X File Options Help One Atom (Absorption and Emission) Multiple Atoms (Lasing) PHET Configure your atoms' electronic energy levels Lamp Control Lifetime Lifetime 2 KIMWwwwwwww x X Energy (ev) Energy Levels Two O Three Options O Lamp View O Mirror Photons Reflectivity (%) Enable mirrors O Beam Laser Power Lower Transiti... Internal 72.0 Display photons emitted from upper Photons Power IIIIII Lasing Dangoil energy state O Wave view Output Power Reset All O View Picture of Actual Laser II ID Helpl