Lecture
This lecture covers 3-level and 4-level optical pumping schemes, discussing the concept of population inversion and the conditions required to achieve it. The instructor explains the role of different energy levels in the amplification process and compares the efficiency of three-level and four-level laser systems. Special attention is given to the rapid decay rates and the minimum pumping requirements for each system, highlighting the advantages of four-level systems in achieving population inversion with lower pumping thresholds.