The PDL characteristics of the filters have to be dramatically improved to achieve a measurement system being free of polarization shifts due to a movement of the fiber feed lines. Attaching such a device would also reduce the drift of the system due to temperature dependent polarization rotation in the feed lines.
A Fiber-Lyot-Depolarizer or a Fiber-Ring-Depolarizer is such a device being capable to achieve a rest polarization of less than 1 %. The fiber Lyot depolarizer is a device fabricated by splicing two highly birefringent fibers together with their principal axes including an angle of 45°. The length of the fiber is determined by the coherence length of the light source. Using a broadband light source leads to an output state of polarization varying cyclically with wavelength. So over the whole bandwidth every possible state of polarization exists several times what leads to an averaging over the whole spectrum of the source.
A fiber ring polarizer offers a second possibility for depolarizing polychromatic light sources.