Narrow linewidth lasers have emerged as a strong candidate for high precision optical metrology. Commercial single-frequency lasers to date achieve sub 100 Hz free-running linewidth at telecommunication wavelengths. While this may be adequate for high frequency measurements, most projects focussing on or involving infrasonic frequencies still require laser stabilisation to reach their target sensitivity.
This talk describes an optical fibre based frequency reference system, designed to precisely measure, and ultimately stabilise the fluctuations in laser frequency. When compared to its free-space, optical cavity counterpart, the fibre-based system provides a flexible and robust architecture, while maintaining its low construction cost. While these systems are typically optimised to track at acoustic frequencies, their stability is often limited by environmental noise at infrasonic frequencies. We use a digital signal processing technique, Digitally Enhanced Homodyne Interferometry (DEHoI), to track and compensate for these effects. Through exploring these advantages, we aim to develop a frequency reference with broadband stability, able to substitute for an optical cavity in field deployable applications and harsh environments.
In this talk, I will discuss the past and current progress towards this goal, including a systematic optical upgrade enabling audio-band sensitivity improvement. I will also discuss future work that is projected to further enhance the performance of the fibre frequency reference.