Low-linewidth optical comb sources based on gain-switched lasers

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Alexander, Justin K.
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University College Cork
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As the number of internet users continues to grow, along with the increase in bandwidth demanding services, the optical communications network struggles to keep up. Upgrading the optical fibre network is a costly endeavour, therefore research into spectrally efficient communication methods must be performed to combat the year-on-year increase in bandwidth demand. Long-haul communications has typically relied on wavelength division multiplexing (WDM) to transmit multiple optical carriers through a single fibre. Each wavelength carrier must be separated by a guard band to reduce interference between channels. By moving to a coherent set of carriers, these guard bands can be reclaimed as usable bandwidth. Optical superchannels, which consist of several sub-carriers, can be used to increase the information spectral density (Gbps/Hz). An even greater increase in the information spectral density can be achieved by implementing all-optical orthogonal frequency division multiplexing (AO-OFDM). Coherent communications systems, such as AO-OFDM, require a coherent optical comb source as a fundamental component. The work presented in this thesis investigates several device designs for the generation of optical combs based on gain-switched slotted Fabry-Pérot lasers. The semiconductor rate equations were used to analyse the phase noise properties of a gain-switched single mode laser both with and without optical injection-locking. Phase noise was shown to increase with gain-switching, and decrease when the gain-switched laser was injection-locked to a low linewidth master laser. Photonic integration was used to integrate two lasers on a single chip for the purpose of generating a low linewidth optical comb. Optical combs with linewidths in the region of 300 kHz were generated, with a comb line spacing of up to 10 GHz demonstrated. The versatile design of these comb sources show promise for further integration with splitters and modulators to facilitate the development of an AO-OFDM transmitter photonic integrated circuit (PIC). Such a PIC is essential in meeting the future bandwidth demands of the optical communications network.
Photonics , Photonic integration , Optical comb , Gain-switching , Laser , Semiconductor
Alexander, J. K. 2017. Low-linewidth optical comb sources based on gain-switched lasers. PhD Thesis, University College Cork.