Linewidth modelling and measurements of single mode-VCSELs

Linewidth modelling and measurements of single mode-VCSELs. The vertical-cavity surface-emitting laser (VCSEL) is used for many applications and is today produced in large volumes for optical interconnects in data centres, gas sensors, and smart-phones. Some of these applications require the VCSEL to be single-mode, in other words to achieve a single spectral emission line. The linewidth, which is related to the phase noise and coherence of the laser, represents the width of such emission. It depends on the design of the single-mode VCSEL and on the operating conditions. This thesis work covers the analysis of VCSEL lasers up to the linewidth developed at Chalmers University of Technology. Hereby a theoretical study of the physics of the strained Quantum wells forming the active regions of the device is presented. The used code is one of the constituents of the VCSEL electro-opto-thermal numerical simulator VENUS that has been revised and modified to include a strained structure and simulate different QW too. More precisely the calculation of the electronic band structure and the optical properties of the nano-structure are covered and in depth explained in this thesis work. Strain is included in the calculation of the band structure based on the kp method by implementing a Conduction Band Offset model to retrieve the changes in the band structures of the material composing the wells. The next step involves the assembling of 8x8 Hamiltonian with strain related terms which is later block diagonalized to obtain a 4x4 Hamiltonian. The implemented kp method also includes the so called Burt–Foreman ordering that avoids spurious solution in the QW band gap. Moreover the parameters of the well have all been changed and adjusted to have results very similar with the experiments. To perform such comparisons the previously calculated band structure and sub-bands are used to extract quantities such as the spontaneous emission, refractive index change, photolumiscence peak, and gain. The whole description is completed with the inclusion of Many body effects related to the Coulomb interactions between charge carriers and re-normalization of the gap. Then the performed measurements at Chalmers are shown and explained. In this framework different type of VCSELs are taken into consideration and characterized in many ways: L-I-V, Spectrum, Near field imaging of the Spontaneous emission and linewidth are collected for all of them together with an in depth explanation of the used set-up. The whole data is sampled for different temperatures to see how the devices operate in different ambient conditions. In conclusion the experimental and theoretical normalized linewidth of the VCSELs are obtained. With the letter calculated by using the values from the model and through an appropriate fitting with the experiment. The theoretical part of the thesis has been carried out at Politecnico di Torino and CNR-IEIIT, while the experiments have been performed at Chalmers in the department of Photonics.