PhD Oral Exam - Xiaoran Xie, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The adoption of millimeter-wave (mmWave) frequencies in 5G and beyond enables high-capacity, low-latency communication but introduces severe propagation loss. Radio-over-fibre (RoF) offers a cost-effective fronthaul solution by transmitting RF signals directly over fibre, reducing complexity and power consumption at the base station. This thesis investigates quantum-dash mode-locked lasers (QD-MLLs) and quantum-dash semiconductor optical amplifiers (QD-SOAs) as key devices for mmWave RoF (mmWoF) links.

This thesis investigates the use of QD-MLLs as multi-wavelength optical sources for mmWoF fronthaul transmission. By generating tens of highly coherent optical comb lines, QD-MLLs enable heterodyne detection to upconvert baseband and intermediate-frequency signals into the mmWave domain with low phase noise.The fundamental mmWoF link was first optimized through architectural improvements, and two enhanced configurations were experimentally demonstrated. The first configuration added a delay line and multiple polarization controllers, while the second configuration will modulate two optical carriers simultaneously. Two configurations can both significantly reduce error vector magnitude (EVM) compared with baseline systems reported in prior work.

Building on this foundation, four distinct frequency-multiplexing techniques employing QD-MLLs were proposed and experimentally validated. By using multiple coherent comb lines simultaneously, they have different advantages and disadvantages. Technique 1 is the most straightforward and robust in terms of architecture and implementation simplicity, yet it delivers the lowest spectra efficiency and EVM performance, which up to 8.5%. Technique 2 improves the EVM performance to 8.0% and has better spectral efficiency compared with Technique 1, but at the cost of requiring an I/Q modulator, which increases system complexity. Technique 3 further enhances performance and spectral efficiency while maintaining simplicity comparable to Technique 1, as it does not require an I/Q modulator. The EVM performance of Technique 3 is up to 7.6%. Technique 4 achieves the best results in terms of EVM performance and eliminates crosstalk, which is as low as 6.7%. However, it demands more complex hardware, including both an I/Q modulator and a wave shaper or specialized filter. The performance of all four techniques is further verified in a system with up to 50 km of single mode fibre, the EVM performance is consistently below 9.6% even after 50 km of fibre transmission.

This thesis also presents a comprehensive investigation of QD semiconductor optical amplifier (QD-SOA) and their application in mmWoF fronthaul systems. The static performance of QD-SOAs was systematically characterized, including amplified spontaneous emission, gain spectra, optical signal to noise ratio, and noise figure (NF), under varying temperatures, bias currents, and input optical powers. In addition, QD-SOAs with different numbers of stacked QD layers (3, 5, 8, and 12) were compared in detail. The results highlighted distinct trade-offs: an 8-layer QD-SOA provided the highest chip gain (26.4 dB at 1550 nm), the 3-layer device offered a broad bandwidth of 90 nm and low NF of 5.7 dB, while the 12-layer SOA extended operation into the L-band. These measurements establish, for the first time, a comprehensive performance map of QD-SOAs relevant to communication systems.

To evaluate system-level performance, QD-SOAs were integrated into mmWoF fronthaul links to replace erbium-doped fibre amplifiers (EDFAs). Both single-channel and frequency-multiplexed experiments were carried out. It was demonstrated that QD-SOAs can successfully replace EDFAs without degrading performance, with EVM values consistently below the 5G limits specified by 3GPP. The capability of QD-SOAs to suppress crosstalk in multiplexed configurations was also experimentally verified. Moreover, packaged 8-layer QD-SOAs were employed to enable more practical system integration. These demonstrations confirm the feasibility of QD-SOAs as compact, energy-efficient, and integrable optical amplifiers for 5G fronthaul transmission, representing a significant step toward replacing bulky EDFAs with semiconductor-based solutions.

Overall, the results demonstrate that QD-MLLs and QD-SOAs are compact, energy-efficient, and integrable alternatives to conventional solutions, offering strong potential for scalable mmWoF fronthaul networks in 5G and beyond.