PhD defence Xiaoyu Xu

Title: Time Lens for Passive Optical Network


Principal supervisor: Senior Researcher Pengyu Guan, Department of Electrical and Photonics Engineering, DTU, Denmark
Co-supervisor: Senior Researcher Peter David Girouard, Department of Electrical and Photonics Engineering, DTU, Denmark
Co-supervisor: Senior Researcher Peter David Girouard, Department of Electrical and Photonics Engineering, DTU, Denmark

Evaluation Board
Senior Researcher Francesco Da Ros, Department of Electrical and Photonics Engineering, DTU, Denmark
Professor Christophe Peucheret, University of Rennes, France
Professor Oskars Ozolins, RISE Research Institutes of Sweden, Sweden

Master of the Ceremony
Associate Professor Anders Clausen, Department of Electrical and Photonics Engineering, DTU, Denmark

Nowadays, the ever-increasing energy consumption in communication networks receives much research attention. In particular, to cope with the exponentially increasing demands of internet users, the applications of wavelength-division multiplexing (WDM) passive optical network (PON) systems consume more energy. This project aims to investigate the potential of optical signal processing (OSP) techniques to achieve an energy-efficient PON system with a high and scalable capacity.

In this work, the Lens-PON system can accomplish a conversion from a timedivision multiplexing (TDM) signal to multiple WDM signals with a time lens based on the optical Fourier transformation (OFT). This system is an outstanding candidate for realizing energy efficiency. However, there is room for development to fulfill the demands of system capacity in reality. In terms of the numerical, theoretical and experimental study, this thesis explores the limits of system throughput and tries to overcome the obstacle to some extent. A time lens contains two procedures of dispersion and quadratic phase modulation. The latter is completed by a four-wave mixing (FWM) process throughout the entire investigation of this project. First of all, to explore distortions of the WDM signals observed in the experiment, the third order dispersion (TOD) was investigated by employing two types of dispersive elements: standard single mode fiber (SSMF), which has non-negligible TOD and customized Fiber Bragg Gratings (FBGs) with negligible TOD. Second, both the highly nonlinear fiber (HNLF) and the integrated waveguide were implemented in the FWM process. The HNLF was mainly used in this project.

Nevertheless, the TDM-to-WDM conversion using an amorphous silicon waveguide based a time lens was investigated as well. Third, the time lens is transparent to modulation formats. In particular, Pulse Amplitude Modulation (PAM) is prevalent for its simplicity and low cost, two attributes which are favorable for usage in PON. To explore the potential of the data capacity growth using higher order modulation formats, a Lens-PON system combined with PAM4 and PAM8 are demonstrated. As the order of the modulation format was increased, non-negligible nonlinear noise was observed on the WDM signals. After investigating the source of such noise theoretically and numerically, we found that the nonlinear noise stems from the pump noise, which can be improved by applying geometric shaping. This approach was validated to improve the system performance successfully in theoretical and experimental methods. For instance, the bit error rate (BER) performance of 28×1.5 Gb/s PAM8 WDM channels can reach a Hard decision forward error correction (HD-FEC) threshold of ii Abstract 3.8×10−3 after 26 km transmission after geometric shaping. In addition, we derived that the optimized power level distribution based on decision boundary should have an exponential profile theoretically.

To summarize, this work contributes a further step on optical properties investigation of the Lens-PON system. The geometric shaping focuses on reducing the influence of nonlinear noise on the system performance. To further enhance the data rate of the Lens-PON system combined with PAM, lowering the nonlinear noise is a possible way in the future.


ons 05 okt 22
13:30 - 16:30



The defence will take place in Building 303A, auditorium 49 at DTU Lyngby Campus and through zoom.