POLARIZATION CONTROL METHOD FOR SUPPRESSING POLARIZATION MODE DISPERSION INFLUENCE IN OPTICAL-TRANSMISSION SYSTEMS

Recent progress in long-span optical repeater systems indicates that t he polarization-mode dispersion (PMD) influence has become one of the main degradation factors in highspeed and wideband systems. This paper discusses polarization-control methods for suppressing the PMD influe nce for both the coherent FDM system and the IM-DD optical repeater sy stem. A principal-state transmission method, which can avoid PMD influ ence in coherent FDM common polarization control, has been proposed. I n this scheme, FDM light is launched at one of the principal states in the fiber using two polarization controllers, located at both ends of the fiber. The feasibility of this scheme was confirmed through 2.5-G b/s CPFSK, three-channel FDM transmission experiments. For a 150-km lo ng fiber, a 700-GHz optical bandwidth can be used with the principal-s tate transmission method. This bandwidth is about three times wider th an that for conventional common polarization control in a 150-km long fiber. The principal-state transmission method has been modified to ap ply to a long-span optical-repeater transmission system which includes optical isolators. In this method, small frequency modulation was add ed to the signal light to search for the principal state for the total transmission line. The modified method can avoid inter-symbol interfe rence (ISI) degradation due to accumulated PMD in long-span optical fi bers and optical components. Power-penalty-free operations were succes sfully demonstrated with up to one bit time PMD value in 5-Gb/s IM-DD, 1000-km transmission experiments. This method is expected to apply to a long-span undersea optical-repeater transmission system.