The construction of an emitter or detector ensures the interaction betweenmatter and light. Maxwell’s equations and the quantum theory furnish the details of the interactions, while the so-called rate equations provide the best summary. These equations represent a key result for optoelectronic devices by describing relatively complicated physical phenomena (covered in detail in the last part of this book). We primarily focus on the laser but show how the equations apply to the light emitting diode (LED) and the laser amplifier, both of which come from the laser geometry but with the appropriate output facets. The rate equations describe how the gain, pump, feedback, and output coupler

mechanisms affect the carrier and photon concentration in a device. The rate equations manifest the matter-light interaction through the gain term. The gain represents the mechanisms for stimulated emission and stimulated absorption which both require an incident photon field to operate. Later chapters will develop the quantum mechanics of this type of emission and absorption. The photon rate equation describes the effects of the output coupler and feedback mechanism through a relaxation term incorporating the cavity lifetime. The rate equations provide a wealth of information and have great predictive power.

These equations can determine the bandwidth, the threshold current, the emitted optical power versus bias current, and the noise content of the beam. This chapter introduces the simplest rate equations and relates its parameters to the physical construction. A great amount of engineering physics must be included from later chapters to make accurate models of the construction.