Parity-Time Symmetry in Lasers and Optoelectronic Oscillators

Abstract

Parity-time (PT) symmetry, originally proposed in quantum mechanics, has been extended to photonics and microwave photonics, offering a new framework to study the interplay between gain, loss, and coupling coefficients for single-mode selection in lasers and optoelectronic oscillators (OEOs).

This thesis focuses on the use of PT symmetry for mode selection in fiber ring lasers and OEOs. Five PT-symmetric architectures are proposed and experimentally demonstrated in this work to address challenges such as structural complexity, limited stability, poor tunability, and relatively low sidemode suppression ratios (SMSRs) in conventional PT-symmetric lasers and OEOs.

Chapter 1 introduces the background of PT symmetry, tracing its origin in quantum mechanics and its extension into the broad field of physical waves. It further reviews the principles of lasers and OEOs, providing the motivation for employing PT symmetry to simplify architectures and improve performance. Building on this motivation, Chapter 2 establishes the theoretical framework by developing coupled-mode and temporal coupled-mode theory, from which PT-symmetric Hamiltonians and the operation principles of single-longitudinal-mode (SLM) lasers and OEOs are derived.

Guided by this theoretical basis, Chapter 3 demonstrates a PT-symmetric fiber ring laser realized in a single physical loop. By emulating two counter-propagating subloops with balanced gain and loss in a polarization-dependent Sagnac structure, stable SLM lasing with sub-kilohertz linewidths and high SMSR is achieved. Extending the single-loop PT system concept from optics to microwave photonics, Chapter 4 reports the first PT-symmetric OEO in a single physical loop, enabling frequency-tunable single-mode oscillations from 2 to 12 GHz with excellent phase noise performance, thereby eliminating the need for ultra-narrowband RF filters.

While these single-loop designs prove the feasibility of PT symmetry for mode selection, enhanced performance can be achieved by combining PT symmetry with the Vernier effect. Chapter 5 introduces a dual-loop PT-symmetric fiber laser with a rational loop-length ratio, which drastically reduces mode density and improves mode selectivity. Experiments confirm single-mode lasing with a record SMSR of 53.2 dB and sub-kilohertz linewidths. Building on the same principle, Chapter 6 presents a dual-loop PT-symmetric OEO with a rational loop-length ratio, where the joint action of PT symmetry and the Vernier effect results in stable single-mode oscillations tunable from 2 to 12 GHz, with an SMSR of 40.7 dB and superior phase noise characteristics.

In general, PT-symmetric mode selection enables stable single-frequency oscillation, but inherently limits the generation of multiple frequencies. However, as demonstrated in Chapter 7, the combination of PT symmetry and injection locking enables stable dual-frequency single-mode oscillation in an OEO. The two output frequencies are 8.15 GHz and 9.35 GHz, with corresponding SMSRs of 17.2 dB and 22.3 dB, respectively.

Finally, Chapter 8 summarizes the research findings and highlights future directions, including the integration of PT symmetry into photonic circuits, and exceptional point (EP)-enhanced sensing technologies.