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 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 the singlemode distributed feedback and the multimode Fabry-Perot, 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. On solution is to use uncooled singlemode lasers separated by a small wavelength spacing, 120 nm for example, as popularized for the coarse WDM grid.