Monolithic integration of photonic devices for use in a regrowth-free coherent WDM transmitter

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Kelly, Niall P.
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University College Cork
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In this communication age, consumer internet traffic continues to grow at an exponential rate year on year. As a result, networks need to be continually upgraded to keep up with ever increasing bandwidth demands. Diverse research is currently being undertaken at a global level to produce cost effective solutions to maximize network performance. One such area focuses on the development of photonic integrated circuits (PICs), striving to replicated the same compact design and low power consumption achieved in the electronics industry. However, photonic components are more complex and diverse than their electrical equivalents, such as transistors. As these components can have large footprints, involve multiple electrical contacts and require different material properties for optimal performance, the best approach is not obvious when cost is considered. While platforms such as heterogenous integration and monolithic regrowth have produced PICs with advance functionality, they rely on complex fabrication processes which increase production time and cost. As a result, this thesis proposes a monolithic regrowth-free design for a coherent WDM transmitter which requires less sophisticated fabrication techniques and would therefore be more cost effective to manufacture than alternative methods. The work began with the development of suitable processes for the fabrication of DC and high-speed components associated with the transmitter. Utilizing the associated composite hard mask process, the integration of core components required for the coherent WDM transmitter was undertaken. The monolithic integration of a 1x2 multimode interference coupler, two slotted Fabry-Perot (SFP) lasers, two electroabsorption modulators (EAMs) and a star coupler was demonstrated using regrowth free epitaxy and UV contact lithography. The feasibility of integrating an SFP laser with an EAM by means of injection locking was also investigated, resulting in the production of a 2.5 Gbps eye diagram. It was shown that the high-speed performance of these PICs could be improved by using more advanced modulator designs. As a result, this thesis concludes with an investigation of high speed modulators with the aim of to increase the data rate of the developed PICs. An integratable electroabsorption modulator with a 3 dB bandwidth of 17.5 GHz and a corresponding 12.5 Gbps eye diagram was realized.
Kelly, N. 2017. Monolithic integration of photonic devices for use in a regrowth-free coherent WDM transmitter. PhD Thesis, University College Cork.
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