Differential phase recovery from correlated long-baseline fiber interferometers using a DFB diode source

We demonstrate real-time differential phase tracking between two correlated long-baseline interferometric systems where the coherence length of the optical source is significantly shorter than each individual interferometer arm mismatch. Using a 5 km Mach-Zehnder interferometer driven by a 100 kHz linewidth diode laser with a coherence length of ∼ 640 m, the coherent differential phase between the two interferometers is recovered without cycle-slip errors over the measurement period, despite the absence of individual phase coherence in either interferometer. We compare this with individual phasemeters, which experience multiple cycle-slip events in individual interferometer measurements and the computed differential, establishing a unique operating regime for interferometric sensing in which correlated measurements enable phase recovery in a regime where individual interferometer outputs are not independently trackable. We achieve a broadband differential phase noise floor of 10^-4 rad/√(Hz), and observe tunable signal fading effects related to fringe visibility reduction inversely proportional to the recovered signal bandwidth, and propose an initial model to describe this effect.