Coupled-Cavity Vertical-Cavity Surface-Emitting Lasers for High-speed Data Communication

Student thesis: Doctoral Thesis

Abstract

In the thesis, we present a theoretical and experimental study on high speed Electro Optically Modulated Coupled-Cavity Vertical-Cavity Surface-Emitting Lasers. The work first presents an overview of CC-VCSELs. We focus on devices that, due to EO modulation by the reverse biased cavity, allow generation of ultra-high data rates not limited by the carrier dynamics and with much reduced chirp. In the work, we give full description of the models and techniques developed by us to simulate and optimize the electrical modulation speed of EOM CC-VCSELs. Our optical designs are based on the devices reaching record modulation speeds presented in the literature. We investigate the spectral and polarization characteristics of a CC-VCSEL and show, that by careful detuning of the optical cavities a strong polarization and wavelength detuning is achievable. Hence, a CC-VCSEL might be used as a polarization and wavelength switching device. We then focus on electrical bandwidth optimization of an electro-optically modulated CC-VCSEL with traditional lumped electrodes. We find that the mesa capacitance and the polyimide capacitance and series resistance are the most influencing parameters on high speed electrical performance. In order to enhance the -3dB bandwidth the design steps should be: reduction of the contact pad area and modulator mesa radius together with increasing the modulator cavity length, as well as implementing a pnp structure type. Based on the model, we suggest a realistic structure that is theoretically able to achieve about 100 GHz electrical modulation speed. In the next part of the thesis we propose a novel design of a EOM CC-VCSEL with traveling wave electrode configuration. This particular design of the modulator section, allows to overcome the lumped RC time constant limiting the electrical cut-off frequency. We implement a segmented transmission line structure and carry out equivalent circuit analysis. We show that the optimized structures are theoretically able to achieve electrical modulation speed limited only by the carrier extraction time from the modulator section and at the same time, having lower than -20 dB reflections to the voltage source. We present two different designs of TW EOM CC-VCSELs that are theoretically able to reach ? 330 GHz modulation speed. Finally, we present experimental results on CC-VCSELs on wafer characterization. We investigate forward and reverse bias conditions and spectral properties of the devices. We show that based on the current aperture size, the CC-VCSELs can be assigned into four distinct groups experiencing similar modal threshold behavior. We improve rate equation analysis under steady state CW operation conditions including the temperature dependence. We show, experimentally and theoretically longitudinal mode switching. Our simulations show very good agreement with the experimental results.
Date of Award9 Jul 2013
Original languageEnglish
SupervisorKrassimir Panayotov (Promotor), Hugo Thienpont (Promotor), Roger Vounckx (Jury), Rik Pintelon (Jury), Erik Stijns (Jury), Johan Stiens (Jury), G. Almuneau (Jury) & R. Schatz (Jury)

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