Linewidth measurement using Self-Heterodyne interferometric method based on suppressing 1/f noise of delay fiber

In this work, we address the critical limitation of the 1/f noise from long delay fibers in conventional delayed self-heterodyne interferometry (DSHI), which leads to overestimated and inconsistent integral linewidth measurements. Through theoretical modeling and experimental validation, we first clarify how the thermal noise in the delay fiber contaminates the phase-noise spectrum of the laser under test (LUT). We then propose and demonstrate a novel Mach–Zehnder interferometer (MZI) configuration to directly quantify the influence of fiber noise by isolating it from the differential phase noise of the LUT. A novel method that uses the normalized instantaneous phase noise, which is independent of the fiber length, is then introduced, to accurately determine the integral linewidth. This approach overcomes the long‑standing accuracy issue in conventional DSHI, offering a more reliable method for laser-linewidth characterization.