Research interest: This thesis deals with the interconnection (coupling) problem between a silicon photonic integrated circuit (chip) and the outside world, (i. e., a single mode fiber). This is one of the most important problems that the silicon integrated optics scientific community is facing nowadays. While very high quality silicon photonic integrated circuits can be realized using standard CMOS fabrication tools, the interface with the optical fiber remains the most important source of loss, due to the large difference in mode size between the single mode fiber and the waveguides on the photonic integrated circuits. Addressing the issues is therefore very important to apply silicon photonic integrated circuits in a practical application.
Objectives: The aim of this work is so to tackle this fiber-chip coupling interface problem, with an emphasis on the packaging. Thus, the main objectives are: 1) study, modelling and design optimization of efficient coupling techniques between optical fibers and silicon photonic integrated circuits, 2) fabrication and experimental demonstration of the obtained designs, 3) assembling and packaging of some of the fabricated coupling prototypes.
Methodology: This work develops along two investigation lines, in accordance with the two main strategies that can be found in literature, namely diffractive grating coupler structures (the fiber couples vertically onto the circuit surface) and inverted taper structures (horizontal fiber coupling onto the circuit end). The manuscript starts with an introduction (chapter one), including the motivation and research context, the definition of the fiber-chip coupling problem in silicon photonics, as well as the objectives and organization of the thesis. The state-of-the art of the two coupling strategies to be investigated is therefore included in chapter two. Each strategy (grating coupler and inverted taper) is then extensively discussed in a separate chapter (chapters three and four, respectively), including the device operation principle, the design and fabrication strategy followed, as well as experimental testing results of fabricated designs, in order to cross-check their validity. In chapter five, the packaging solutions based on both fiber-chip coupling techniques are discussed, where also briefly the state-of-the-art of silicon photonic packaging is reviewed. Hereto, the main focus is multichannel silicon photonic integrated circuits and their packaging based on commercial available components. In last chapter, the achieved results are summarized and future perspectives are discussed.
Obtained results: Both in the case of diffractive grating couplers and in the case of inverted taper structures, important advances are made over the stateof-the-art. Concerning diffractive gratings, two types of structures have been demonstrated. On one hand, grating couplers suitable for conventional silicon waveguides have been achieved. On the other hand, diffractive grating structures are shown to work on horizontal slot waveguide structures for the first time, which are very promising for nonlinear optics applications. With regard to the coupling via inverted taper, a novel inverted taper-based structure is experimentally demonstrated. Using this structure, important advances are made in the packaging of optical fibers with the silicon waveguide circuit. Its innovative integration with V-groove structures is presented as a means to passively align arrays of fibers to a photonic integrated circuit. Also, packaging of fiber arrays using diffractive grating couplers is studied, resulting in a prototype of small form factor package.


Abstract of the Doctor thesis “Addressing Fiber-to-Chip Coupling Issues in Silicon Photonics” 
of D. Jose Vicente Galán Conejos
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