Polarization equalization in optical vector network analysis for SDM fiber characterization

Precise characterization of space division multiplexing fibers is a key requirement for their further development and deployment in the field. Optical vector network analysis allows to measure all linear parameters of such fibers in a single, fast acquisition over a large bandwidth. As the reference arm of the optical vector network analyzer must approximately match the length of the measured fiber, additional fiber must be introduced when measuring longer fibers. This results in the polarization of the reference arm varying with wavelength over the sweep range employed in the measurement. The resulting fading of the recorded interference signal produces severe distortions of the measurement and hence must be compensated for. This work introduces a simple equalization technique to compensate for this fading, based on introducing a polarizer in the reference path and recording a fraction of the polarized reference as a correction signal. Applying the inverse of the correction signal to the recorded signal, the impact of the polarization fading is almost entirely removed, as are the corresponding measurement distortions. The proposed technique is validated in the measurement of a 39-core few-mode multi-core fiber, significantly improving measurement of wavelength dependent insertion loss.