Toward single-growth monolithically integrated electro-absorption modulated lasers

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Mulcahy, Jack
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University College Cork
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Every year the demand for bandwidth is growing exponentially due to the emergence of data-intensive services such as high-definition video streaming, cloud-based computing, and machine-to-machine communication. This rapid expansion is primarily driven by the extensive deployment of fibre-based optical communication networks. Consequently, there is an increasing need for photonic components to meet the requirements of these networks, which are expanding both in geographical coverage and terminal density. To satisfy this demand, the photonics industry must enhance its production capabilities and adopt more efficient fabrication processes. A crucial aspect of streamlining fabrication involves eliminating slow and costly processes. In photonics fabrication, epitaxial regrowth and advanced lithography steps are typically time-consuming and expensive, making them prime targets for process optimisation. Moreover, the integrated electronics approach provides valuable insights by enabling the monolithic integration of multiple photonic components fabricated simultaneously. This integration technique allows for the creation of highly complex circuits while reducing overall fabrication complexity. This research focuses on a key component at the heart of photonic circuits: the tunable single-mode laser. The aim is to contribute to the development of components that can be fabricated without the need for regrowth or advanced lithography. Additionally, the study emphasises the importance of monolithic integration, specifically with electro-absorption modulators (EAMs). By integrating EAMs with tunable lasers, the resulting devices can offer enhanced functionality and performance, leading to more efficient and compact photonic systems. The issue at hand, however is the varied epitaxial requirements of lasers and EAMs, which provides a noted barrier to a monolithic, regrowth-free integration process. This thesis aims to advance the development of single-growth monolithically integrated externally modulated lasers (EMLs) based on electro-absorption modulators (EAMs). The design of quantum well structures is explored, revealing the significance of introducing an imbalance in the position of the quantum wells to optimise the transit times of carriers in EAMs, thus maximising the bandwidth. Simulation studies on epitaxial structures led to the identification of an optimal material that balances the performance of lasers and EAMs, providing an ideal platform for EML fabrication. Different laser designs are investigated, including slotted Fabry P\'erot lasers and snails, with a focus on achieving a redshifted single-mode laser. Simulation models are developed to predict laser reflectivity and spectral output, which were verified through fabrication and testing. The optimal laser design for integrated EMLs was determined through critical evaluation with a laser being produced with $>$ \SI{40}{dB} SMSR and a tuning range of \SI{60}{nm}. A high-speed process for fabricating EAMs is developed, featuring optimised lithographic mask layers for the isolation of contact pads and metal bridges to reduce parasitic capacitance. The resulting EAMs exhibited a predicted bandwidth of approximately \SI{80}{GHz}. Drawing upon the knowledge gained from laser and EAM simulation, fabrication, and characterisation, a new high-speed process for EMLs is devised. The o-band lasers and EAMs were designed based on optimal principles determined in previous chapters. The fabricated single-mode lasers were successfully matched to simulated models. Further analysis identified potential avenues for improving future EML fabrication yields. In summary, this thesis provides valuable insights and tools for the creation of single-growth monolithically integrated electro-absorption modulated lasers. The journey spans from material design to device outputs, with the aim of enabling readers to replicate and enhance the development of EMLs.
EAM , Laser , Photonics
Mulcahy, J. 2023. Toward single-growth monolithically integrated electro-absorption modulated lasers. PhD Thesis, University College Cork.
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