Study of vertical coupled-cavity laser structures

This thesis addresses general aspects of the operation of a coupled-cavity vertical surface-emitting laser (CC-VCSEL) which consists of two active cavities optically coupled and independently electrically pumped. The first two chapters of the thesis deal with a general introduction to the VCSEL field, including some historical aspects and a summary of the theory of semiconductor lasers. We also provide with an introduction in the theory of semiconductor lasers with special emphasis on VCSELs. In the following three chapters we present our experimental characterization results on CC-VCSELs and their theoretical interpretation. Two models are developed which together provide a powerful toolbox for the design and characterization of CC-VCSELs. The lumped-mirror model has as a starting point only the optical characteristics of the CC-VCSEL structure. Considering these input parameters, i.e. the reflectivity of the coupling mirror and the detuning between the two cavities, the model provides two important results: the position of the resonant modes of the structure and the confinement coefficients that characterize the degree of localization of each mode in each cavity respectively. The lumped-mirror model is a reliable way to determine these parameters, which are specific to coupled-cavity VCSELs, as these results are in a good agreement with the ones provided by the well-established calculation procedure based on the matrix transfer method. The rate-equation model completes the previously exposed lumped-mirror model considering also the electrical injection, independent for each cavity. The operation of the coupled-cavity device is described starting from simple, intuitive arguments, developing the rate-equations for both the number of carriers in each cavity, and the number of photons in each of the two optical modes. Analyzing the resulting system in steady-state conditions, we identify the different working regimes of the device, in an excellent agreement with the experimental data. We introduce a notion specific to the CC-VCSEL, i.e. the "double-threshold point" where both optical modes start lasing. In analogy with the classical one-cavity device, we show that, in the case of the CC-VCSEL, the complete clamping of carrier densities and gain coefficients takes place only in the regime where the device emits two modes simultaneously, i.e. the "dual-wavelength lasing mode", while in the regime where the device lases on only one mode, i.e. the "single-wavelength lasing mode", the carrier densities in the two cavities, and consequently the gain coefficients, may vary, the only constraint being the need to satisfy the threshold equation for the corresponding lasing mode. In the characterization of the CC-VCSEL, we also address the polarization behavior under special bias conditions. We investigate the state of polarization of the light emitted from the top cavity under direct bias while the bottom cavity is reverse biased. Also, the effect of the reverse bias of one cavity on the light output from the other cavity is examined. For this purpose, the quantum confined Stark effect is evaluated and we show how the modification of the absorption as a result of the reverse applied electric field is integrated in the rate-equation model. The field of multisection optoelectronic devices is already well-established if we refer to edge-emitting lasers (EELs) where new functionalities and performance improvements have been achieved by using the concept of multicavity devices. At present, the application of these concepts to multicavity VCSELs as described in this work is just emerging but should prove equally interesting and challenging. The coupled-cavity VCSEL structure not only exhibits an interesting and rich physics making it interesting from the academic point of view, but we believe that CC-VCSELs capable of wavelength selection and/or wavelength tuning will be indispensable component for the emerging dense wavelength multiplexing data communication systems, while an emerging application of polarization switching is the data routing in optical data communication systems, therefore attributing to the CC-VCSEL device an important role in the photonics revolution.