Precision optical interferometry measures the optical phase at the interferometer output to infer information on time, length and frequency. To extract the phase information, frequently used techniques are IQ-demodulation and the phased lock loop (PLL). PLLs directly measure relative phase error between a local oscillator and the signal, while continuously updating the local oscillator to maintain a small relative phase error – _an example of a closed-loop measurement. To have reliable, linear phase tracking, this phase error must remain small, necessitating all sizable phase dynamics be captured by the bandwidth of the PLL control loop to avoid discontinuities in the readout. This bandwidth limit imposes a restriction on the applicability of PLLs especially when signal dynamics are faster than the control bandwidth achieved. 

IQ demodulation is an open-loop measurement and relies on the arctan operation to recover the phase but removing the requirement for closed-loop control. The main disadvantage of this method is the discontinuity of the arctan operation, which makes the system highly susceptible to noise around the discontinuity point. This non-linearity makes it crucial to track at high frequencies to ensure good phase fidelity and to avoid phase discontinuities. 

In this talk, we discuss an alternative open-loop phasemeter. It aims to reduce the dynamic range and signal bandwidth required to measure the phase, rejecting common mode noise before the necessary non-linear operations in phase recovery. By performing this common-mode rejection prior to phase recovery, the signal dynamics to be tracked by the arctan operation are reduced. It allows for improved phase measurement recovery from highly dynamic optical interferometers.