Laser Fundamentals: Gain, Absorption, and Resonators

This lecture covers the fundamental principles of lasers, focusing on the Lorenz model and stimulated emission. It begins with a review of the Lorenz model, discussing the conditions for stimulated emission and the significance of absorption cross sections. The instructor explains how maximum absorption occurs at resonance and derives the absorption cross section formula. The lecture then delves into the concept of Doppler broadening, illustrating its effects on light-matter interactions, particularly in gases like neon. The basic building blocks of a laser are introduced, including the roles of energy input, gain, and the medium containing atoms. The instructor discusses the importance of population inversion and the threshold condition for lasing, emphasizing the balance between gain and losses in the laser cavity. The lecture concludes with practical examples, including the He-Ne laser, and the significance of the Fabry-Perot resonator in achieving laser action, highlighting the relationship between reflectivity, gain, and the free spectral range.