Polarisation-Multiplexing Ring-Cavity Fibre Laser for Dual-Comb Generation

Abstract

This thesis investigates the development and characterization of a polarization-multiplexing ring-cavity fibre laser designed for dual-comb generation. It explores the underlying physics, practical implementation, and potential applications of this innovative laser system, particularly in LIDAR technology. Dual-frequency comb systems for metrology typically use two optical frequency combs synchronized via complex feedback loops, which can suffer from phase-locking issues and increase system cost and fragility. In contrast, single-cavity dualcomb systems generate two combs with slightly different repetition rates within the same cavity, ensuring mutual coherence and noise cancellation. However, these systems often generate unstable regimes and are generally demonstrated only in laboratories. The objective of this thesis is to design, build, characterize, and optimize a single-cavity polarization multiplexed fibre laser capable of producing dual optical frequency combs with sufficient stability and precision for dual-comb LIDAR. This system aims to simplify the generation process while enhancing the practical applicability of dual-comb technology. Secondary objectives include studying the laser’s intensity dynamics and collaborating with a company to address commercial needs and challenges in LIDAR technology, particularly under harsh environmental conditions. The results demonstrate the successful generation of dual optical frequency combs with minimal drift (1 Hz/hour) and high stability (over 250 hours of operation). The system shows potential for practical applications, achieving sub-millimetre precision in ambiguity ranges of 5m with a fundamental frep = 39.25MHz and a Δfrep = 869Hz. The findings highlight the robustness of the dual-comb regime for high-precision ranging applications while also revealing some precision limitations. Moreover, the thesis visualizes and analyses the build-up and propagation dynamics of the dual-comb in a single cavity, showcasing the two-stage build-up process and proving that the evolution of the energy of the initial spikes conditions the successful separation. Finally, this thesis offers mitigation strategies for commercial LIDAR devices operating under harsh environmental conditions.