Continuing the series on lasers. In the first part, I went fast over plenty of concepts and left others on the side because they were not necessary to get a global understanding of lasers. Here, I want to get a little deeper in some specific concepts. I want to talk more in details about the gain medium of a laser and why population inversion is a vital concept. For now, I have been relatively simple on the gain medium characteristics… I have considered a simple material with two atomic energy levels, and I have been assuming that the transition is …