Optical phase measurement is a capability underpinning many forms of precision optical sensing. Continuous tracking of optical phase in interferometers enables macroscopic phenomena to be observed with extraordinary precision. This capability is of particular interest to the field of space-based interferometry, where inter-satellite laser links are used to track changes in displacement between spacecraft separated by hundreds of kilometres to the nanometre level. 

In inter-satellite interferometric missions, such as the Gravity Recovery and Climate Experiment - Follow on (GRACE-FO) mission, real time phase measurement is performed using a phasemeter; a feedback loop utilising a digitally implemented phase-locked loop. A limiting factor in mission design is the minimum optical power at which the phase of the incoming signal can be reliably tracked, as minimum operating optical power constrains parameters such as spacecraft separation and telescope design. To date, the field of low power optical phase tracking has revealed steady improvements in minimum trackable power, culminating in Francis et al. demonstrating phase tracking of a 30fW optical field with a mean time between cycle slips (non-linear errors) of 100 seconds~\cite{francis et al.}. 

This talk discusses modelling, simulation and experimental work that demonstrate, for the first time, optical phase tracking at the sub-femtowatt level, 100 times lower than the previous best observed result with the same performance. As well as improving on prior results, this work addresses the gap in our understanding of how these phase tracking systems behave in the weak-signal regime. The talk will cover a range of topics, including: optimisation of phase sensors for weak field tracking, measurement of femtoWatt level optical fields and how to determine a laser's minimum viable power for phase tracking based on its frequency stability.