Beat interference penalty in optical duplex transmission

The near- and far-end source spectra in optical full duplex systems can heterodyne, producing a high level of beat interference noise in the receiver bandwidth. This is called coherent common-channel crosstalk, the penalty from which is found in addition to that from incoherent near-end crosstalk (NEXT) quantified in an earlier publication. We find most directly modulated high-chirp laser systems, such as those using single-mode distributed feedback lasers or multimode Fabry-Perot (FP) lasers, are relatively immune to coherent NEXT for speeds up to 100 Mb/s. In the transform limit, however, which occurs at high bit rates or low chirp, the maximum allowable NEXT must be decreased by as much as 20 dB, compared to the incoherent case. One solution is to use uncooled single-mode lasers separated by a small wavelength spacing (20 nm, for example) as popularized for the coarse wavelength division multiplexing (WDM) grid.