PhD Thesis Final Defense to be held on December 17, 2018 at 13:30

Katopodis Thesis Image

Thesis Title: Polymer technology for optical modulators and node design for flexible and high-speed optical interconnects for intra- and inter- data centers.

The examination is open to anyone who wishes to attend (Room 1.1.31, Old ECE Building).


In today's world, use of telecommunications as well as the sector itself are inextricably linked to the everyday life of man, aiming at improving his own life. To this end, recent advances in smart devices, mobile voice and data services, cloud data, large file transfer and the Internet of Things have contributed to an exponential increase in traffic requirements globally, resulting in a tripling of global IP traffic between 2015 and 2021. Consequently, these requirements have led to an increasing use of the available bandwidth of networks, in order to fully meet the increasing needs by using only optical networks, which are the only solution to serve such requirements. And this is because the satisfaction of the capacity requirements depends on two factors. Firstly, there is a dependence on the capacity of the interfaces for communication and transfer of data within the data centers, and secondly on their transfer between remote data centers, taking into account efficiency as well as low cost.

Based on the current data, interfaces within data centers use 10 and 40 Gb/s interfaces with on-off-keying (OOK) format and wavelength division multiplexing (WDM), while Ethernet switches will adopt connections 100 Gb/s, making it necessary to use high-speed optical technologies. In this context, the objective should focus on the development of integrated optical solutions that will allow simple, direct switching from 10 Gb/s data rates to 40 Gb/s and ultimately to 100 Gb/s or even higher speeds 400 Gb/s), while reducing the physical size of the devices and power consumption to fully meet current requirements.

In this general picture, it should be taken into account that peak Internet traffic times are growing faster than average online traffic, suggesting that network capacity requirements are evolving into ever greater and more dynamic, resulting in the weight in the core network dropping through the input/output elements (edge switches), which correspond to the interfaces between data centers. In order for data of such size to be accommodated, state-of-the-art edge switches are now equipped with multiple interfaces, each with 10, 40, 100 or even 400 Gb/s of total capacity. However, the increasing number of services very soon leads to data flows that can reach transmission rates of up to 1 Tb/s, so these interfaces need to be upgraded and support similar capacities to keep pace. Consequently, the current applications based on 100 Gb/s dual-polarization Quadrature Phase Shift Keying (DP-QPSK) seem already obsolete. This dictates that the next transition from 100 Gb/s to 400 Gb/s and further to 1 Tb/s Quadrature Amplitude Modulation (QAM) based optical line cards is imperative for the immediate future, also taking into account the flexibility in managing this capacity in order to simplify network design and optimize the sharing of valuable resources such as fiber optic lines and network optical interfaces. So far, the flexibility of the optical transceivers (Tx/Rx interfaces) has been taken into account mainly in the context of point-to-point optical connections in a flexible grid environment, and in the case of the application of single and multiple carriers as the ability of the transceivers to adjust the transmission rate, modulation format, configuration format, transmission reach available and wavelength allocation of the transmitted signals. The factors currently hindering the development of ultra-high-speed transceivers of the order of 100 Gb/s and multi-flow transmitters with terabit capacities is the absence of a flexible combination of integration platforms that can make available high-performance photonic and electronic circuits for ultra-high speed with gigabit and terabit capacity.

In accordance with the above-mentioned requirements and needs, this PhD dissertation is divided into two parts, namely high-speed optical interfaces employing optical transmitters inside data centers as well as the corresponding flexible for interconnections between data centers, both based on the technology of the optical polymers. Optical polymers are a new and promising material technology for optical communications, presenting optical structures both active with extremely high electro-optical response (EO-response) for the development of Mach-Zehnder interferometers and passive capabilities to implement complex photonic routing networks to support multiflow transceiver circuits. Consequently, the first part of this dissertation focuses on the use and properties of the polymeric electro-optical platform for the development of interferometric transmitters capable of modulating input signals with TM mode of up to 100 Gb/s as well as in the study of modulation technology using NRZ-OOK format.

This particular optical polymer takes precedence over the other integration platforms as it exhibits an extremely fast electro-optical response with a 3-dB bandwidth > 65 GHz for optical Mach-Zehnder modulation structures. Accordingly, based on the hybrid integration of a distributed feedback laser structure inverted by 90 degrees with the EO polymer Mach-Zehnder modulator, an integrated transmitter is implemented in combination with the use of a 100 Gb/s electrical data multiplexing and driving circuit on the InP-DHBT platform. This integrated optical transmitter was designed and built within the framework of the European research project POLYSYS, aiming to upgrade and increase data communication system capacity by providing integration technology for simple 100 Gb/s rack-to-rack and chip-to-chip connectivity. Its evaluation was achieved through laboratory experiments at the National Technical University of Athens with NRZ-OOK modulation format demonstrating error

The second part of this thesis is related to the demonstration and development of flexible optical transceivers and the flexible optical nodes with terabit capacity and flexibility in the definition of optical flows, wavelength allocation, number of subcarriers, polarization sliceability and m-QAM modulation format targeting elastic optical networks and more specifically metropolitan and core segments. The implementation of flexible multiflow optical transceivers is based on hybrid integration of the passive polymer low loss integration platform (PolyBoard) for the development of complex optical routing circuits with the InP photonic technology and is combined with EO IQ-MZ modulators and high-speed electronic circuits in InP-DHBT in order to produce multiple optical flows up to 64 Gbaud. These optical flows can transfer data to a variable wavelength and type of m-QAM modulation (m = 4, 16, 64) depending on the transmission distance and the volume of signal data, further increasing the total capacity through selection of the polarization, using a developed software control in labview and FPGA. For first time this capability of choosing the type of polarization (single or dual) is given by a flexible multiflow transceiver, achieving a doubling of the final capacity.

At the same time, based on the flexible multiflow transceivers, a colorless and directionless optical node was designed and developed to fully exploit the capability of multiple flows, which implemented using wavelength selective switches. This system was evaluated in laboratory conditions demonstrating dynamic operation with m-QAM modulation format up to 28 Gbaud and error-free coherent transmission over 100 km of standard single-mode fiber.

Supervisor: Avramopoulos Hercules, Professor

PhD student: Katopodis Vasilis