Enabling technology for suppressing nonlinear interchannel crosstalk in DWDM transoceanic systems

Abstract

Summary form only given. Nonlinear interchannel crosstalk may result in significant performance degradation in transoceanic dense wavelength division multiplexing (DWDM) transmission systems at high bit rates. Three nonlinear mechanisms are mainly responsible for the signal distortion: self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), and the latter two mechanisms cause nonlinear interchannel crosstalk. Several techniques for suppressing nonlinear interchannel crosstalk were introduced. Using large effective area fibers permits reducing the value of signal intensity for the given channel power. Dispersion management can reduce the propagation distance over which closely spaced channels overlap while ensure that the total dispersion returns to zero at the end of the system. Transmission format were also utilized to tolerant large fiber nonlinearity. The chirped return-to-zero (CRZ) transmission format suppresses nonlinear interaction by broadening the spectrum of optical signal. Orthogonal polarization launch (OPL) between adjacent channels can eliminate FWM and cut XPM by one half. Relative polarization of adjacent channels determines the strength of nonlinear interaction between them. The nonlinear crosstalk is the largest for parallel relative state of polarization (SOP) and smallest for the orthogonal relative SOP. For the orthogonal relative SOP channels FWM is negligible while XPM is only one half of the one in parallel case. Thus it is desirable to launch and keep the orthogonal relative SOP between adjacent channels through the transmission. However the OPL is difficult to realize since real transoceanic systems deploy single-mode fiber from transponders to the input of wet plant, and cannot maintain polarization state. According to our knowledge, all the above techniques except OPL have been realized in full capacity real transoceanic systems. To solve this problem, we use an active polarization controlling method, and use a polarization beam splitter after the wavelength multiplexer to generate feedback signals. Optical signals after transponders pass through a single-mode fiber, and the polarization controllers can generate appropriate birefringence such that the relative SOP between adjacent channels after the wavelength multiplexer is orthogonal. Our simulations and 6,500 Km strait line WDM transmission experiments showed about 1 dB of Q-factor improvement when employing orthogonal launch.