All-Fiber Passive Alignment-Free Depolarizers Capable of Depolarizing Narrow Linewidth Signals

We report on two fully-fiberized, input polarization independent depolarizer configurations that do not need any alignment once fabricated. This makes them suitable for real-world applications. The first configuration uses polarization-maintaining (PM) fibers and relatively few components. The second configuration uses standard (non-PM) single mode fibers to introduce the large time delays needed to enable narrow-linewidth signal depolarization, albeit at the cost of a slightly increased component count and insertion loss. We theoretically analyze the depolarization performance of the proposed configurations as a function of various depolarizer parameters. Subsequently, we design the depolarizers considering the imperfections of standard commercially available components. Finally, we fabricate prototypes of both configurations, characterize their depolarization performance using sources of variable linewidth, and compare the experimental results with theoretical analysis. The fabricated depolarizers are shown to be capable of the depolarization of 20 MHz and 1 MHz linewidth signals with residual degree of polarization below 4%.