Regrowth-free monolithic vertical integration of passive and active waveguides

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Duggan, Shane P.
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University College Cork
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Data usage continues to rise exponentially with user demand, and the bandwidth of optical communications is reaching its limit. Spectrally efficient advanced modulation formats are being used to increase the data rate within the existing bandwidth. Mach-Zehnder Modulators achieve high spectral density by modulating both the phase and amplitude of light. At the moment, laser light sources and Mach-Zehnder Modulators are built discretely and packaged and coupled thereafter. Photonic Integrated Circuits (PICs) place multiple photonic components onto single chips, with the benefit of compactness resulting in lower cost through simpler coupling, lower power consumption, and volume manufacture. The difficulty faced by photonic integration is that different components require different material for their different functions, and so cannot be easily placed onto the same material substrate wafer. Existing methods use regrowth techniques to selectively replace material around the chip, but regrowth is expensive and time consuming. This work vertically integrated a passive waveguide above an active waveguide using regrowth-free monolithic integration. The passive and active waveguides are designed for Mach-Zehnder modulator and laser material. The waveguides were processed on an InP platform, and consisted of AlGaInAs alloys. To our knowledge this is the first time a passive waveguide has been vertically integrated above an active waveguide for III-V semiconductors, and the first time two AlGaInAs waveguides have been vertically integrated, using monolithic regrowth-free techniques on InP. Vertical coupling was performed through lateral tapers, and an isolation barrier between the stacked waveguides allows the vertical integration of already optimised discrete components without modification. As the isolation barrier deliberately reduces the coupling between the waveguides away from the transition region, a technique using a third passive waveguide in the barrier was used to increase the vertical coupling while maintaining isolation. A regrowth-free vertical monolithic PIC was conceived, designed, grown, fabricated and tested to prove its operation. The two vertically stacked waveguides provide a solution for integrating active and passive material, so that a laser and Mach-Zehnder Modulator can be monolithically integrated to create a low cost advanced modulator to meet today’s ever increasing data demands.
Monolithic integration , Regrowth-free integration , Photonic integration , Lasers , Waveguides , Photonic integrated circuits , Optoelectronics
Duggan, S. P. 2019. Regrowth-free monolithic vertical integration of passive and active waveguides. PhD Thesis, University College Cork.