Time-delay Interferometry
Time-delay interferometry (TDI) is a post-processing technique to reduce the overwhelming laser frequency noise in the LISA phasemeter measurements.
Due to the split interferometry design of LISA each of the interferometers on the spacecraft interferes light coming from two different lasers. As a result, the inter-spacecraft interferometers which are sensitive to gravitational waves are swamped by laser frequency noise. To overcome this, we apply TDI and form virtual equal arm Michelson interferometers which drastically reduces the impact of laser frequency noise. This procedure requires knowledge of the inter-satellite ranges to meter precision. To relate the three Michelson combinations (each centred in one of the spacecraft) we need to synchronize the satellite clocks to milliseconds.
In a recent publication, we show that TDI can be carried out in arbitrary time reference frames. The time reference frames of relevance in LISA are the clock times of the three spacecraft and a common time like the Barycentric Coordinate Time (TCB). Traditionally, all LISA measurements were synchronized to TCB before passing them through TDI. We demonstrate that the sensitive clock synchronization step can be moved after TDI which simplifies the noise reduction pipeline and also makes it more robust.
In the future, we want to study TDI in more realistic LISA setups including e.g. offset locked lasers. Here, we have the choice among six different locking topologies which all have different characteristics and produce varying levels of laser frequency noise residuals after TDI. Additionally, we aim to extract an estimate of the absolute laser frequency of the primary laser to aid the calibration of the instrument.
