Approaches for Mitigating Polarization-Induced Fading in Optical Vector Network Analyzer for Long-Length SDM Fiber Characterization

Comprehensive component characterization is essential to unlocking the full potential of fiber systems in multiple divisions of space, providing insights into performance limitations and driving their development and deployment in real-world applications. Space-division multiplexing (SDM) systems require characterization tools capable of simultaneously measuring the properties of all modes and cores to evaluate impairments and understand interactions. The optical vector network analyzer (OVNA) fully characterizes all linear parameters of such fibers over a broad wavelength range in a single fast scan. However, characterizing kilometer-scale fibers necessitates pathlength matching between the reference and measurement arms by adding a reference fiber to maintain coherence, allowing lowspeed photo-detectors and digitizers to be used. Wavelength-dependent polarization rotation in the reference fiber results in fading in the captured interference pattern when the signals are recombined, severely distorting the measurements and the derived linear parameters of the device-under-test (DUT), such as insertion loss (IL). In this work, we investigate the impact of polarization-induced fading and present two optimized OVNA configurations incorporating automatic polarization control and Faraday rotator mirror techniques to mitigate fading by stabilizing the reference arm polarization across the sweep range. These methods are validated by characterizing a 10 km, 7-core uncoupled multi-core fiber, demonstrating their effectiveness in suppressing fading and eliminating distortions in IL measurements. This results in a more accurate measurement of the true fiber characteristics.