PhD defence of Yuxuan Xie – Applications of Quantum-Dash Mode-Locked Laser in Microwave Photonics
Abstract
Microwave photonics (MWP) is an interdisciplinary field that merges the principles of microwave engineering and photonics, aiming to enhance the performance and capabilities of microwave systems. Benefiting from the broad bandwidth of electro-optical modulators (EOMs) and photodiodes (PDs), the MWP system can achieve much higher operation frequency than conventional digital signal processing (DSP) systems. Moreover, a quantum dash (QDash) mode-locked laser (MLL) and a programable optical filter (waveshaper) together offers a set of stable, programable and reconfigurable optical taps. Compared to other optical frequency comb (OFC) sources, such as ring resonator and cascaded EOMs, the motivation of using QDash MLL is its flat comb spectrum, reasonable free spectral range (FSR), and a large number of comb lines. The flat comb spectrum can maximize the efficiency of comb shaping, more comb lines offer more programmability and flexibility, and reasonable free spectral range enable the operation frequency range for RF and mmWave signals. This thesis delves into the innovative applications of MWP, focusing on the development and optimization of MWP filters, reconfigurable instantaneous frequency measurement (IFM) systems, arbitrary waveform generation (AWG) systems, phased array antennas (PAAs), and photonic analog-to-digital converters (ADCs).
The first part of the thesis introduces the fundamental concepts and structures of MWP filters, including coherent and digital finite impulse response (FIR) filters. Coherent MWP filters, though capable of achieving high center frequencies, face challenges in narrow transition bands. Digital FIR-based MWP filters, however, offer programmability and reconfigurability, allowing for the design of arbitrarily shaped filters, thus broadening their applicability. A significant contribution of this work is the demonstration of a reconfigurable IFM system with linear frequency response utilizing MWP filters based on a QDash MLL. The QDash MLL, with its flat comb spectrum and sufficient OFC lines, proves ideal for comb shaping, enabling the system to achieve a maximum frequency range of 20 GHz and improved measurement accuracy of a root mean square error 30-42 MHz through an amplitude comparison function system.
The thesis also explores a programmable MWP-AWG system, leveraging the impulse response of MWP filters to generate various waveforms with adjustable characteristics. This system showcases the flexibility and precision of MWP technologies in waveform generation, including rectangular, triangular and sine burst waveforms. In the realm of PAAs, the research demonstrates the use of a QDash MLL as an OFC source to achieve discretely tunable steering angles and beam scanning capabilities in the range of -90~90 degree (half circle) and 0~360 degree (full circle). The system benefits from uniform comb spacing and fiber dispersion, providing effective beamforming and scanning functions. Finally, the thesis investigates photonic ADCs for RF and millimeter-wave signals, presenting time-interleaved and heterodyne photonic ADCs. These ADCs exhibit enhanced effective number of bits (ENOB) up to 11 across their operating frequency ranges and demonstrate their potential for advanced and real-time signal processing in future communication systems, including 5G and beyond.
This comprehensive study underscores the transformative potential of MWP technologies, offering high-frequency operation, reconfigurability, and advanced signal processing capabilities. The findings and developments presented in this thesis pave the way for more efficient, flexible, and powerful communication systems, highlighting the promising future of microwave photonics in the evolution of photonic signal processing technologies.