Lecture
This lecture introduces a four-level system as an extension of the three-level system, focusing on the efficiency of power requirement. The instructor explains the benefits of a four-level system over a three-level system, deriving a formula for calculating laser power in continuous wave lasers. The lecture covers the concept of inversion population, the development of rate equations for a four-level system, and the calculation of gain saturation and power necessary to reach threshold. The instructor also discusses the relationship between input power and laser output power, deriving equations for gain and output intensity. Various observations and factors affecting output power are explored, along with the assumptions used in deriving the expressions.