PhD Thesis Final Defense to be held on January 29, 2020, at 13:00

Photo Credit: Nikolaos Iliadis

The examination is open to anyone who wishes to attend (Multimedia Room, Central Library of NTUA).

Thesis Title: Coherent optical transceivers with dynamic spectral characteristics for flexible optical networks

Abstract:Today's society may well be seen as the "information society", which is characterized by the relentless need to serve higher-capacity telecommunication networks to meet users' needs to the fullest extent possible for the unimpeded use of broadband services (applications of social media , high definition television, online gaming, etc.). Until now, the demands of these applications for increasing use of the available bandwidth of networks are largely met by previous generation optical networks. Although the conventional techniques in the field of optical communications that concerned all the stages of generating and transmitting data streams used in Legacy optical networks had dominated the past years, the truth is that in the coming years they will not be able to cope the ever-growing bandwidth demand for the new broadband services of the modern era. Recent forecasts for broadband interconnectivity indicate that over the five-year period 2016-2021 the annual growth rate of global telecommunications traffic will be around 24% reaching a total of 3.3 Zettabytes in 2021.
The ever-increasing demand for as much bandwidth as possible is not the only factor triggering the need for a new era to emerge in the implementations of modern optical networks. An equally important factor is the dynamic and volatile profile of data traffic that characterizes modern telecommunication networks. The emergence of cloud computing, the exponential growth of mobile applications and the relentless demand for high-definition video services have instigated this massive explosion generating at the same time a more dynamic and volatile data traffic profile, rendering the past network deployments scarce and inadequate to satisfy the new capacity demand. In addition, the explosion observed in the increase in the number of mobile devices such as smart phones and tablets feeds yet another change in the Internet access way, once more the data traffic from these wireless devices that has an inherently dynamic profile prevails as to the commitment of network resources compared to wired communications devices.
Therefore, weighing all the above factors, Telecom providers are oriented towards introducing more and more dynamically the concept of flexibility in terms of the modulation schemes that an optical transceiver will handle and how it will be distributed the available bandwidth. The previous generation conventional grid networks so far tend to be abandoned by the trend which is widely adopted in high-capacity networks to require dynamic adaptation of the available bandwidth of networks to lead to flexible optical network (EON) implementations that will can be controlled remotely by software defined networking (SDN).
Within these research frameworks, the European research community, outlining the above-mentioned requirements and needs, and following modern trends that require flexible telecommunications networks, announces collaborative research projects. Through cooperative research programs, it is possible for organizations such as universities, research centers and companies to collaborate and interact by exchanging know-how in the field of their specialization in order to meet the needs of the content defined by the research community. This PhD dissertation was conducted in the framework of the European research program SPIRIT (Software-defined energy-efficient photonic transceivers introducing intelligence and dynamics in terabit super-channels for flexible optical networks), a partnership of specialized collaborators with a proven track record in the field of optical network architecture and applications , software-defined networking (SDN), design and evaluation of flexible optical systems, digital signal processing (DSP), photon integration and packaging. The SPIRIT team of partners was set up in such a way as to achieve a perfect balance between partners from the academic and industrial sectors. The consortium of the SPIRIT program consists of the Photonic Communications Laboratory of the National Technical University of Athens, one of the leading universities in the field of optical signal processing and communication systems design (ICCS / NTUA), as well as two of the largest and most recognized European institutes (Fraunhofer-HHI) and the design of electronic systems and opto-electronic integration (IMEC). In addition, the SPIRIT team of partners includes a European silicon manufacturing institute (AMO), a specialist with long experience in photon integration and opto-packaging (TEO), one of the world's largest suppliers of telecommunication equipment and one of the largest providers of Telecommunication Networks in Southern Europe (OTE).
An essential and cornerstone of the SPIRIT program's objectives was the Photonics Communications Laboratory (PCRL) of the NTUA, which was the coordinator of the project, exploiting its long experience in coordinating corresponding major European programs. The PCRL's role was not limited only to the coordination of the project but also contributed decisively to the architecture of the proposed solution of the flexible optical transmitter with the capability of coherent transmission and data acquisition in cooperation with the other partners of the project. The proposed solution aimed at 50% reduction in both energy consumption and the cost of producing channel transmitters for 1 Tbit / s [5.22] as well as 85% reduction of the chip's footprint.. In addition, PCRL has been actively involved in the design of the couplers (rings, couplers, waveguides, MZM) that formed the backbone of the silicon chip of the SPIRIT optical transmitter, parameter setting, simulation and final evaluation of opto-electronic elements such as drivers, segmented modulator and MUX / DEMUX to evaluate their performance and redesign them where necessary. Furthermore, a portfolio of digital signal processing algorithms (DSPs) has been developed at PCRL to evaluate the reception and demodulation of coherent signals. Finally, PCRL actively participated in the experimental evaluation of the flexible optical transmitter and the MUX / DEMUX prototype both in a laboratory environment and in an actual data transmission and reception environment (Ericcson Italia).
The diversity of partners in the SPIRIT consortium was deliberately chosen to reflect the multi-science of the individual milestones set as objectives of the program. HHI had an excellent collaboration with PCRL and IMEC to define specifications, design of individual optoelectronic elements, and finally build IQ SEMZM with a maximum resolution of 5 bits per I / Q arm that is the heart of the flexible optical transmitter [5.10]. In addition, HHI contributed substantially to the design of the drivers manufactured by IMEC, exploiting its long experience in CMOS electronics [5.17]. In addition, AMO is a company that offered its integration expertise on an SOI board of silicon structures that will act as thermoregulatory filters of variable spectral content and wavelength filters. The packaging of the flexible optical transmitter, which includes all opto-electronic interconnections, optoelectronic board development, thermal simulations for the individual elements as well as fiber placement will be taken over by TEO. In addition, TEI - Ericcson provided its facilities and equipment for conducting experiments to transmit DWDM information under real 40 Gb / s conditions and actively participated in the determination of the scenarios assessed. Finally, OTE, with its long course as one of the leading telecommunication providers, participated in the determination of the parameters to be evaluated regarding the scenarios that were implemented both for the assessment of the prototypes in a laboratory environment and in a field environment.
Within the framework of cooperation between SPIRIT partners, the PhD thesis was divided into two parts, namely the experimental evaluation of the MUX / DEMUX with the capability of flexible adaptation of its spectral content as well as the characterization of the innovative IQ SEMZM capable of creating multi- level signals formed both in width and phase without the presence of an external DAC.
In the first part of this dissertation there is presented an innovative multiplexer /de- multiplexer on a silicon platform (SOI) for flexible mesh applications, which will play a decisive role in the implementation of the flexible transmitter. The entire step chain began with the design and simulations of the flexible filtering elements based on 2nd order MRs (micro-racetracks) with built-in MZI between the two micro-ring cavities and the subsequent experimental evaluation of the performance of these elements. This experimental evaluation was initially carried out in an array of flexible filtering elements in chip scale, reaching the crown of this dissertation, which was the experimental characterization of the packaged MUX / DEMUX. Each sub-MR of the 2nd order micro-racetrack structure can be thermally tuned and thus its wavelength resonance can be individually controlled. Moreover, the design of the filtering element as is already mentioned incorporates a symmetric Mach-Zehnder Interferometer between the two sub-MRs that serves as a variable splitting ratio optical coupler. The MZI is equipped with thermo-optical phase shifters in each branch, allowing the regulation of the power coupled between the two resonant cavities and thus making the bandwidth reconfigurability of the structure feasible. More specifically, the capability of the flexible filtering elements to dynamically alter both their bandwidth and their real-time resonant wavelength using a micro-heater has been evaluated to make them suitable for use in flexible mesh network applications [4.4] (necessary variation between 12.5 GHz and 35 GHz).
In the second part of this PhD dissertation, a thorough analysis of the function and characteristics of an IQ InP segmented modulator has been completed on a silicon platform together with a low energy demand electronic CMOS technology driver. The IQ segmented modulator is the backbone of the flexible optical transmitter performing an innovative feature of this optical-DAC. In the majority of cases, IQ modulators capable of generating QAM signals are combined with high-speed DACs and the use of digital signal processing algorithms (DSPs), which contributes to both the overall size of the transmitter and its energy consumption, making them inefficient for applications where the aforementioned parameters are considered critical [5.19-5.20]. Increased voltage requirements for integrated circuits (ICs) contribute significantly to increasing the energy consumption of today's transceivers, with this figure expected to rise above 50% in the next generation [ 5.21] [5.18]. Therefore, there is a clear need to reduce total energy consumption and the concomitant use of DACs. The IQ SEMZM modulator presented in the thesis has adopted an innovative design model using an array of low energy consumption drivers that, with appropriate fittings, lead to reduced driving voltage (Vp) requirements. Additionally, the proposed structure of the conformational optical transmitter does not use a digital to analogue (DAC) electrical converter, thus performing the operation of a 5-bit optical-DAC per I / Q arm. Additionally, mature CMOS electronics technology is used, characterized by the need for low drive voltage on the electrodes of the segmented modulator as well as low power consumption. In the final part of this dissertation are presented the results of the experimental evaluation of the flexible optical transmitter including BER measurements and eye diagram of the multi-level signals (modulated both in amplitude and in phase) generated by the flexible optical transmitter with CMOS drivers of both 1st and 2nd generation. System performance evaluation of the SPIRIT’s single transmitter was carried out in a testbed developed by ICCS/NTUA in collaboration with researchers from HHI and IMEC.

Supervisor: Hercules Avramopoulos, Professor

PhD student: Nikolaos Iliadis