Rate equations and transient phenomena in semiconductor lasers

Abstract

This contribution attempts to review certain resonance effects which occur in the dynamic behaviour of semiconductor lasers. To study these effects theoretically, a rate equation approach is used for single-mode operation in the region of lasing threshold.

The two basic rate equations are given and their transient solutions discussed. The existence of two time constants in these equations, viz. the electron lifetimeτ _e and the photon lifetimeτ _e; gives rise to a characteristic resonance frequency in the GHz region. This resonance manifests itself in transient ‘spiking’ effects, in quantum noise phenomena, and in high-frequency modulation experiments. In a modified form the resonance frequency may also be studied in lasers with external cavities and in double-diode configurations (or, equivalently, conventional devices with non-uniform excitation along the cavity length).

In the latter two examples mentioned above, the resonance is excited by optical feedback of the laser radiation into the active medium via a ‘lossy’ or insufficiently inverted region. In the ‘spiking’ oscillations commonly observed at the commencement of laser operation, the initial ‘population overshoot’ is the cause of the resonance. For the case of quantum noise it is the requirement that the photon and electron populations have integer values which supplies the driving force-a true quantum effect. High-frequency modulation experiments directly reveal the same resonance frequency where a strong maximum in modulation intensity occurs.