Scalable and Reconfigurable Optical Switches: From Network Design to System Application

Abstract

The exponential growth in internet traffic causes increasing demands on network switching technologies, both in the communication networks and datacentre networks. The optical switch has been the primary research topic in recent years since it shows great potential for meeting the requirements of large scalability, flexibility, energy efficiency and high bandwidth. The current optical switch fabric using Microelectromechanical systems (MEMs) technology can achieve up to 1000 ports but is limited by the low tolerance to external disturbance and complex control panels. With the development of InP and silicon integrated platforms, mature optical components with thermo-optics, electro-optics and semiconductor optical amplifiers (SOAs) become promising candidates for the large scalable switch fabric with a more compact design. However, the SEs with binary status require balanced non-blocking switching architectures to form the planar switching fabrics or networks. Mach-Zehnder interferometers (MZIs), Micro-ring Resonators (MRRs) and SOAs can provide the on-off switching states and thus are widely applied in the switch design. However, challenges on the MZI SEs still exist to realise the switches with the high port count, low insertion loss and crosstalk ratio, whereas switches built by MRR SEs have limitations on extinction ratio, insertion loss and narrow bandwidth. SOAs give optical amplification for loss compensation; however, they also induce noise and distortion. In the cascaded switching architecture, noise and distortion accumulate and degrade the signal performances. The study focuses on the design factors that affect the characterization of MZI, MRR, and SOA SEs for the purpose of designing large port count optical switching networks. To meet the design requirements, the MZI SEs need coupling coefficients between 0.45-0.55 and an imbalanced loss of less than 2dB caused by phase arms, while MRR SEs need to meet the critical coupling condition for perfect add-drop configuration. Therefore, this thesis studies the feasibility of the 1024-port optical switching network design used in data centre networks. InP-based dilated hybrid MZI and SOA switches are applied to achieve fast switching on nanoseceond scales. Over 15dB input power dynamic range (IPDR) with a power penalty below 2dB is assessed. The switching scheme of 4096-port is for the first time proposed with the performance assessment, indicating the IPDR of 12dB with a power penalty of less than 3dB. It has been recognised that the InP-based switch has a large footprint and thus is limited for scaling up the port count further. Silicon photonics, with its low-cost, compatible and compact platform, becomes attractive for scalable switch design. However, it can only support the passive components that induce excess losses. Hybrid integration technology subsequently gives the possible solutions by providing switching functionalities and the amplification for on-chip losses simultaneously. We, therefore, propose the 8×8 gain-integrated silicon switch fabric using adiabatic couplers, in which the low-loss MZI SEs on silicon platforms provide crosstalk isolation, whereas SOAs are implemented to give the additional gain. Detailed physical simulations are executed to verify and assess the performance of the switches with the formation of 64×64 switch fabric. To further reduce the excess loss induced by adiabatic couplers between InP and silicon platforms, the coupling strategies are also studied in terms of fabrication deviation and coupling misalignment. Another alternative for building switch fabric is MRR SEs because they have more compact volumes and fewer control signals. However, it is challenging to design the MRR SEs in the O-band because of high waveguide bending loss and more compact volumes. We introduce the extra degree of freedom by applying racetrack resonators and bent waveguides. The switch-and-select architecture is used to construct the monolithic switch fabric on the chip, and the design space is explored to obtain the applicable configurations. Experiment demonstrations illustrate the designed racetrack-based MRR chip cannot provide switching functionality, whereas the bent waveguide-based MRR switch has a low loss ranging from 5-20dB and an average crosstalk ratio of below -40dB. A 3dB passband of 43.6GHz gives promising solutions for high-performance switching applications in optical networks. In order to demonstrate the system implementation, the switch-and-select MRR switch fabric is applied in the optical fronthaul network as switching application scenarios. An envisaged converged optical network with the combination of digital radio-over-fibre (DRoF) and data service is proposed, where the silicon-based switch-and-select MRR switch fabric provides multiple functionalities such as wavelength-selective switching, multicast switching and space switching. The proposed switching network consolidates fixed and wireless services between C-RAN and PON architectures, experimentally demonstrating power penalties below 1.5dB and RF dynamic ranges of over 40dB with a bandwidth of 8Gb/s. The 14dB optical link budget enables scaling up the port count to more than 100. The studies in this thesis, therefore, illustrate the great potential of optical switches for future scalable and reconfigurable switching network design and diverse optical network applications.