Scattered light reduction in Sagnac speed meters with tunable coherence

Sagnac speed meter and ring resonators can be used as high-precision instruments, but they are limited in their sensitivity through scattered light causing nonlinear noise. Here, we experimentally demonstrate a technique called tunable coherence, where the long coherence length of the laser is broken in a controlled way, to suppress the coupling of scattered light in a Sagnac interferometer. We demonstrate a scattered light suppression of 24.2 dB in a Sagnac interferometer and discuss the experimental limitations. Further, we show an analytical discussion on how tunable coherence could be a fundamental solution to light scattering back from optical surfaces into the counterpropagating beam, which is an issue particularly in ring resonators. Published by the American Physical Society 2025