Monolithically integrated coherent comb de-multiplexer using facetless semiconductor ring lasers

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Dernaika, Mohamad
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University College Cork
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Due to the growing demand for faster Internet connections and the growth of services requiring high bandwidths, the migration of the networks to higher and higher bit rates is inevitable. The surge in data usage due to real time application in the entertainment, business and medical sector has placed tremendous pressure on the current infrastructure and its ability to keep up with the constant traffic increase. Wavelength division multiplexing (WDM) systems, which currently dominate long-haul optical transmission links employ guard bands between channels to avoid interference. This leads to inefficient use of the system bandwidth, which is limited by the erbium doped fibre amplifier (EDFA). A solution to this problem is to use coherent WDM (Co-WDM), or called optical orthogonal frequency division multiplexing (OFDM) to increase the bandwidth efficiency. In Co-WDM or optical OFDM, the interference from adjacent orthogonal channels can be eliminated at the receiver end. Therefore, the spacing between adjacent channels can be reduced without using guard bands. Moreover, the series of individual lasers used in WDM networks can be replaced with a single coherent comb source. However, Co-WDM still requires the use of discrete bulky components, such as narrow band filters to operate as a comb demultiplexer, modulators and multiplexer. To justify the potential move from WDM to Co-WDM a significant reduction in cost, size and production time is necessary. The photonic integrated circuit (PIC) offers an attractive solution to integrate all components on a single chip. The vision of the larger research project was to design and fabricate a fully integrated Co-WDM transceiver. The initial demonstration PIC will comprise an integrated coherent comb generator, a comb filter and an integrated electro-absorption modulator (EAM). However, this work mainly focused on the filter aspect of the PIC to demultiplex the narrowly spaced comb lines (<25 GHz). The filter has two functions: to demultiplex the comb lines, and to selectively amplify the individual comb line through injection locking. The filter is designed to be facet-less so that it can be suitable for integration with other devices. The fabrication process of the PIC employs Standard UV lithography and a regrowth-free process to further reduce the time, cost and complexity making it suitable for mass production. The process of optimizing the facet-less lasers in term of fabrication yield, consistency and filtering performance will be presented.
Monolithic integration , Photonic integration , Coherent optical comb , Ring lasers , Demultiplexing , Optical filter , Injection locking , Single mode laser , Tunable semiconductor laser , Regrowth free , Semiconductor laser , Coherent WDM , Optical OFDM , Elastic networks , Optical communications , Terabit communications
Other, A. N. 2019. Monolithically integrated coherent comb de-multiplexer using facetless semiconductor ring lasers. PhD Thesis, University College Cork.