Lasers are essentially quantum devices. The process of stimulated emission is a quantum phenomenon. Stimulated emission is essential to the generation of spatially and spectrally coherent radiation, which is also quantum phenomena. Thus, albeit initially a macroscopic device, the laser emits radiation that is intrinsically quantum in character. In this chapter, we consider a first and essential step in the creation of a laser: laser excitation.
Here, we consider the process of laser excitation and emission, in a gain medium, from a semi classical and quantum perspective. In Chapter 9 we examine the optics phenomena applicable to generate spatially and spectrally coherent radiation.