Signal Propagation Over Polarization-Maintaining Fibers: Problem and Solutions

Polarization-maintaining (PM) fibers are able to preserve the state of polarization (SOP) of a signal in the fiber reference frame. The SOP follows one of the axes of the fiber defined by the mechanical constraint that has been inserted during manufacturing. This ability is characterized by the polarization extinction ratio, which should be as high as possible. It is known to be very high (often above 50 dB) for the fiber itself. However, the somewhat low polarization extinction ratios (often between 20 and 30 dB) of connections (connectors, splices, ...) induce strong distortions of the signal. The authors first explain why for one PM patch cord, i.e., a fiber and its connectors, with numerical and analytical models. They then extend these models to a patch-cord concatenation. Finally, they show and compare analytically and numerically different solutions to this problem: fiber length reduction, polarizer insertion, high-performing connectors, and axis alternation. The latest solution consisting in alternating the slow and the fast axes from one fiber to the next one is particularly efficient. They have implemented these solutions for the purpose of the source of the "Laser MegaJoule", and it exhibits far better performance