Technology and theory of high-frequency directly modulated semiconductor lasers.

摘要

The goal of this program was the creation of state-of-the-art high-speed directly modulated semiconductor lasers that were compatible with monolithic integration into optoelectronic integrated circuits (OEICs). To accomplish this, several design tools were developed to examine the material gain, high-frequency characteristics, and the linewidth enhancement factor. In addition, the fabrication technology of short-cavity semiconductor lasers was extended to fabricate dry-etched lasers with single-sided outputs. The result of this project was the creation of the first directly modulated semiconductor laser with a total-internal-reflection corner facet that had a modulation bandwidth of 20 GHz.;Measurements of both corner reflector lasers and standard lasers with two plane mirrors revealed that the modulation response had characteristics that could not be explained by a standard rate equation analysis. Specifically, the square of the resonance frequency of the response did not increase linearly with the applied bias. The underlying physical mechanisms behind this anomaly were investigated with a model that used an analytical density of states and an effective differential gain to explore the role of lattice heating on the direct modulation response. Lattice heating is shown to degrade the modulation response and cause the resonance frequency to saturate with increasing bias. This heating of the active region is due, in large part, to the poor thermal conductivity of the GaAs substrate on which the epitaxial structure is grown. To improve the operating characteristics of semiconductor lasers they were flip chip bonded onto coplanar waveguide (CPW) transmission lines fabricated on diamond. Diamond was chosen because it is an excellent thermal conductor and electrical insulator. Improved performance was seen in the DC and spectral characteristics of the laser, but a corresponding improvement of the high-speed performance of the laser was not seen. Since the DC performance improvement would indicate that the lattice heating was reduced, there are still several physical mechanisms limiting the high speed response. Additional work on understanding these effects should further improve the high-frequency characteristics of these lasers, as well as provide a foundation for improving the modulation bandwidths of long wavelength, telecommunication lasers.