Fast Algorithm to Find the Minimal Polarization Change Points on a Fiber-Optic Stretcher Using One Polarization Rotator and a Polarimeter

Fiber stretchers are used to stabilize the effective length of optical fiber links, but if they are not carefully calibrated, they can produce polarization changes and instabilities, which interact with the polarization mode dispersion (PMD) of the fiber link producing phase changes on the transmitted signals. Those can be minimized if the input polarization is aligned to one of the two orthogonal polarization states of the fiber stretcher, which show minimum polarization change across the dynamic range of the stretcher. This paper presents a noniterative method to find these eigenstates, dubbed “sweet spots,” based on the Jones matrix Eigen-analysis known for measuring PMD of optical fibers, and its application to the line length correction system of the Atacama Large Millimeter/submillimeter Array (ALMA). The paper reviews the theoretical relationship between the sweet spots and the principal states of polarization of the fiber in the stretcher. The proposed method was found to be more accurate and faster than the iterative optimization method originally used for this system.