Improvement in Direct Modulation Speed by Enhanced Differential Optical Gain and Quantum Confinement

Chapter 2 describes the basic intensity modulation dynamics of semiconductor lasers in general and in a most fundamental way — by proper bookkeeping of electrons and photons flowing in and out of the laser active region; the basic result is encapsulated in the very simple formula (2.1). This result, first published in 1983 [15] is of fundamental importance in studies of direct modulation properties of semiconductor lasers.

In particular, the hitherto unclear role of differential optical gain was clearly captured, which pointed to the fact that the direct modulation speed of laser diodes can be improved by engineering the gain medium material properties. However, due to the fact that the differential gain is a basic property of the gain material. The explicit dependence of the relaxation oscillation frequency on differential gain cannot be easily verified experimentally, since comparing different material systems may involve a multitude of factors. As an affirmative verification of the validity of this relationship, Lau et al. [26] measured the modulation bandwidth of the same laser diode at various low temperatures in order to increase the differential gain of the device while keeping other material parameters and device structures unchanged and, as such clearly demonstrated this effect. This result was further corroborated by an elegant experiment by Newkirk and Vahala [34]. These results will be illustrated in Sects. 5.1.1 and 5.1.2.