Monolithically integrated tuneable slotted Fabry-Pérot laser, wavelength monitor and modulator

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Ramaswamy, Prasanna
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University College Cork
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To aid the ever-increasing demand for data bandwidth, tuneable lasers will be a key requirement in transmitter modules of next-generation optical communication networks for data transmission in access networks. The lasers need to be wavelength-stabilised and cost is a critical factor since they will be widely used in consumer products. Currently used tuneable lasers in the downstream direction in access networks are too expensive to be also used widely in consumer products (upstream), and current designs for integrated wavelength monitors use passive waveguides made using spatially selective bandgap engineering techniques which are not cost-effective and make integration with a laser challenging. This thesis demonstrates a low-cost tuneable transmitter module using a slotted Fabry-Pérot laser, monolithically integrated with a modulator, wavelength monitor and a semiconductor optical amplifier (SOA). The laser is based on a 3-section slotted Fabry-Pérot design and has a discrete tuning range of about 20nm which covers half the C-band. The wavelength can be further fine-tuned by adjusting the temperature between 16-22⁰C with a sensitivity of approximately 0.1nm/⁰C. The modulator is an absorbing section with Ground-Signal-Ground (GSG) pads that is capable of delivering data rates of up to 10 Gb/s with a peak-to-peak driving voltage of 1V using Non-return-to-zero on-off-keying (NRZ-OOK) modulation. Error free transmission was achieved for distances up to 50km of single mode fibre. The module also incorporates an integrated semiconductor optical amplifier (SOA) which provides an optical gain control of up to 20dB with a drive current of 30mA. The wavelength monitor has a linear working range of approximately 25nm and can provide a sensitivity of 13-15GHz (~0.1nm). It employs the same epitaxial structure as the tuneable laser, thus requiring no regrowth, making monolithic integration possible. Along with the fact that the slots required for tuning of the laser are fabricated in the same step as the ridge waveguide, presenting no additional complexity during the fabrication process, makes the whole transmitter module low-cost and compatible for use in Optical Network Units (ONUs) in the consumers’ premises in a Fibre-to-the-Home (FTTH) scenario in access networks. The thesis also explores the challenges of integration such as electrical, optical and thermal crosstalk between the various active components within a single device and suggests design considerations that will help mitigate this crosstalk. For future work, the performance of the modulator section could be enhanced by the use of quantum well intermixing and also reducing its pad capacitance by introducing high-k dielectrics. After fabrication, the common substrate could be stripped off from underneath the entire device and the various components could be ‘printed’ separately on other substrates and optically connected via passive waveguides using novel transfer-printing technology, which could help reduce crosstalk further by eliminating substrate coupling.
Laser , Telecommunication , Tuneable , Crosstalk , Wavelength monitor
Ramaswamy, P. 2019. Monolithically integrated tuneable slotted Fabry-Pérot laser, wavelength monitor and modulator. PhD Thesis, University College Cork.
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