Polarisation multiplexing ring-cavity fibre laser for dual-comb generation

TL;DR

This thesis develops a stable, single-cavity polarisation-multiplexed fibre laser capable of generating dual optical frequency combs with high stability and precision, suitable for advanced LIDAR applications in harsh environments.

Contribution

It introduces a novel polarisation multiplexing ring-cavity fibre laser design that simplifies dual-comb generation, enhancing stability and practical applicability for LIDAR technology.

Findings

Achieved stable dual-comb generation with minimal drift (1 Hz/hour)

Operated continuously over 250 hours with high stability

Demonstrated sub-millimetre ranging precision in practical tests

Abstract

This thesis presents the development and characterisation of a polarisation-multiplexing ring-cavity fibre laser for dual-comb generation. It explores the underlying physics, implementation, and potential uses of this innovative laser system, especially in LIDAR technology. Conventional dual-frequency comb systems for metrology use two optical frequency combs synchronised via complex feedback loops, which suffer from phase-locking issues and raise system cost and fragility. In contrast, single-cavity dual comb systems generate two combs with slightly different repetition rates in the same cavity, ensuring mutual coherence and noise cancellation. However, these systems often display unstable regimes and are typically demonstrated only in laboratories. This thesis aims to design, build, characterise, and optimise a single-cavity polarisation multiplexed fibre laser that produces dual optical frequency combs with enough stability and precision for dual-comb LIDAR. The system simplifies generation while enhancing applicability. Secondary goals include analysing the laser intensity dynamics and collaborating with a company to address commercial needs and challenges in LIDAR, particularly under harsh conditions. Results show stable dual-comb generation with minimal drift (1 Hz per hour) and operation over 250 hours. The system achieves sub-millimetre precision in 5-metre ambiguity ranges with a fundamental frep of 39.25 MHz and Delta frep of 869 Hz. The findings confirm the regime's robustness for high-precision ranging while exposing some limits. The thesis also analyses build-up and propagation dynamics in a single cavity, showing a two-stage process and how initial spike energy evolution affects successful separation. Finally, it offers mitigation strategies for LIDAR systems in harsh environments.