PhD defence of Mostafa Khalil – Enabling Technologies for Coherent Optical Communications

Abstract

In recent years, silicon photonics has emerged as a promising technology for optical communication systems. The integration of silicon photonics with other materials and technologies has opened up new avenues for developing high-performance and cost-effective optical communication systems. This has led to significant research and development efforts in the field, intending to achieve better performance, higher integration, and lower power consumption. In this thesis, we aim to address an intriguing question - what would coherent transceivers look like in the future to serve high-capacity wavelength division multiplexing (WDM) systems?

In the first part of the thesis, we propose a wavelength-selective filter based on waveguide Bragg gratings on the Silicon-on-insulator (SOI) platform. The proposed device not only provides control of the resonance wavelength but can also provide multi-band rejection filters and Fabry-Pérot-like filters. Furthermore, we design and propose 3-cascaded Mach–Zehnder modulators on the SOI platform and investigate the feasibility of generating optical frequency combs with such devices. Among the various optical frequency comb generators, quantum dash mode-locked laser diodes (QD-MLLDs) are potential candidates for future WDM transceivers. QD-MLLDs can emit a broadband of optical carriers with a fixed frequency spacing between each carrier. In the second part of the thesis, we demonstrate the performance of such devices to serve high-capacity coherent optical systems and conduct a thorough performance comparison between QD-MLLDs and a narrow linewidth integrable tunable laser assembly. Moreover, we demonstrate that such devices can be used not only as multi-wavelength sources at the WDM transmitter but also as multi-wavelength local oscillators at the receiver side for coherent detections of WDM signals. Lastly, thanks to the possibility of hybrid integration, we propose a schematic for future coherent optical transceivers using chip-scale optical frequency comb sources, such as QD-MLLDs, and silicon photonic modulators.

In the last part of the thesis, we propose and verify, both analytically and experimentally, innovative encryption techniques to safeguard sensitive data. We propose an encryption technique utilizing a phase modulator at the transmitter side and based on a roundtrip mechanism wherein an authenticated user does not share the secret key with other entities. Then, we introduce a quantum-inspired encryption technique for one-way transmission, which is based on displacing the modulated symbol randomly with random amplitudes and phases. Only the authenticated user would be able to decrypt the signal with the correct secret key after performing synchronization processes. We finally show that the proposed techniques can be used in the existing optical fiber links and are protected against eavesdropping attacks wherein adversaries tap into the fiber link to steal sensitive information.