the least number of degrees of freedom for a polarization controller in each stage of pmd compensator
Automatically controlled polarization controllers (PC) are essential integral parts in an automatic compensator for PMD. It is suggested by K. Karsson et al. and afterward widely believed that we need to adjust only two waveplate PC (two degrees of freedom, DOF) for each stage PMD compensator to complete PMD compensation, because it is believed that only two of degrees of freedom for each PC are needed to complete the state of polarization (SOP) transformation from any input state into any other state covering the entire Poincar Sphere. This paper will take two types of commercial available PC as examples to show theoretically and experimentally that except for the reset-free problem it is necessary to adjust at least three instead of two waveplates in order to transform any input state into any other state covering the entire Poincar Sphere. Therefore we can achieve complete PMD compensation at least using 3 DOF instead 2 PC in each stage PMD compensator.
degree of freedom polarization controller state of polarization