Optimized grating coupler designs for integrated photonics

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Zagaglia, Luca
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University College Cork
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A wide range of expertise is enclosed in the expression: “Photonics Packaging” with the common aim of interfacing a Photonic Integrated Circuit (PIC), either optically, electrically, thermally, and mechanically, with the surrounding environment. Thus, a multi-physics approach is essential to tackle the many challenges that Photonics Packaging poses. In this context, the design process is a crucial and vital step to overcome the majority of the issues that can potentially arise during the packaging assembly procedure. As a consequence, a single person cannot manage all the aspects behind the design process, but a multidisciplinary team needs to work together engineering and optimizing the different types of connections. The current thesis work is oriented on the optical packaging branch with particular focus on the design of the optical connections needed to deliver, in an efficiently and controlled manner, the light signal from a specific external light source or waveguide into the PIC. This work aims to show the importance of the design process in photonics packaging, and how it can be carefully exploited and tailored to optimize coupling schemes to obtain high efficient and packaging compatible optical connectors, which can constitute the building-blocks of future photonic devises. In this context, my research deals with the optimization of complex grating couplers for SOI platforms to couple light from a specific coupling scheme and it is divided in 4 sections. First, Chapter 2 presents in details the customized design routine, developed during my PhD, based on a Particle Swarm Optimization, an iterative algorithm, implemented using a commercial Finite Different Time Domain (FDTD) software. Then, in Chapter 3, the routine is exploited and tested to optimize the structural features of non-uniform grating coupler designs, characterized by a non-constant pitch. The aim is boosting their Coupling-Efficiency (CE) under a horizontal fibre coupling scheme, which is of particular interest in photonics packaging. The optimal designs were then fabricated in Cornerstone, the Silicon photonics foundry of the University of Southampton, and eventually packaged and tested at Tyndall National Institute. High CE values, up to 83% at 1550nm, are demonstrated and the results are shown to be in excellent agreement with the computational predictions. Due to the high efficiency, these designs were requested by the foundry as part of their official Process Design Kit, which is now on offer. In Chapter 4, the first experimental and FDTD comparative analysis of the multiwavelength response, in terms of bandwidth and asymmetry of the CE curve, is conducted and reported for such optimized non-uniform grating couplers. Here, the bandwidth is shown to be inversely proportional to the dimension of the impinging mode field diameter, which affects, together with the energy dispersion curve of each grating structure, its CE curve. Grating coupling is not only a suitable packaging solution for fiber-to-PIC coupling, but also for direct laser-to-PIC coupling. Chapter 5 shows how millimetre-scale FDTD simulations can be used to carefully design a Micro Optical Bench, made of a micro ball-lens and a micro prism. The Optics are used to reshape the laser emission making it compatible with a single mode fiber emission, thus suitable for grating coupling. Here, a uniform grating coupler is used to couple the impinging electromagnetic field. A laser-to-PIC performance penalty of only 1dB, with respect to the fiber-to-PIC coupling scheme, is shown under perfect alignment conditions. The origin of the higher loss is carefully analysed suggesting that 0.6dB are due to back-reflection from the optics interfaces, and 0.4dB due to spherical aberrations. Moreover, a detailed analysis of the manufacturing and alignment tolerances is conducted demonstrating their compatibility with current standard packaging processes.
Grating coupling , Photonics packaging , Integrated photonics , Silicon photonics
Zagaglia, L. 2021. Optimized grating coupler designs for integrated photonics. PhD Thesis, University College Cork.