Development and Characterisation of a Fibre Frequency Reference

Abstract

In this thesis, a passive, all optical fibre frequency reference is developed and characterised. The system uses long armlength difference interferometers to measure fluctuations of laser frequency. The phase readout is handled by digital interferometry (DI), which uses spread-spectrum modulation to extract interference signals from the desired range gate with high dynamic range. The frequency stability of the fibre reference is characterised using a differential measurement between two near-identical interferometers. We achieve a relative stability of 0.1 Hz/rt(Hz) above 70 Hz Fourier frequency, which surpasses previous demonstrations of fibre optic references.

Building on prior fibre reference investigations, the new system discussed here is designed around an unbalanced Mach-Zehnder interferometer, removing the impact of first-order Rayleigh backscattering as seen in previous designs. The implementation of DI is modified to enable real-time phase reconstruction instead of at a decimated speed, reducing non-linear errors and improving readout fidelity. The new interferometers are individually enclosed in two updated dual-layer passive isolation chambers.

In system characterisation, we provide the first long-term temperature analysis of the isolation chambers. Their individual time constant is modelled and experimentally measured, at 13.2 hrs and 11.4 hrs respectively. The 14% difference between the two chambers is in alignment with the temperature independence observed in a three-reference optical measurement. We also comprehensively survey the laboratory mechanical profile, and identify the driving source for each mechanical feature in the experimental noise floor.

In addition to temperature and mechanical stability, noise limitations in other Fourier regimes are also identified and characterised. We adapt the Duan fibre thermal noise model for a long armlength interferometer, and experimentally achieve thermo-mechanical noise limited relative stability between 0.4 - 2 Hz. We also develop the first quantitative model for double Rayleigh scattering (DRS) in a fibre interferometer including effects from DI integration and suppression. The modelled contribution from DRS is shown to be in close agreement with the experimental noise floor above 70 Hz Fourier frequencies.

The achieved 0.1 Hz/rt(Hz) frequency stability represents the state-of-the-art performance for fibre references and is comparable with room temperature cavity systems. This makes our system a potential alternative for laser frequency stabilisation at short timescales, particularly in applications where the robustness of fibre systems and their intrinsic optical alignment are important considerations.