<title>Ultradense bidirectional optical transmission</title>

Abstract

Present efforts towards construction of large capacity long-haul links are focused on either increase of channel data rate or a very dense channel plans. The latter is of particular interest since it enables spectrally efficient WDM links simply by multiplying the channel count of existing transmission lines. Unfortunately, very dense channel spacing necessarily leads to increased four-wave (FWM) mixing penalty. FWM penalty can be decreased in straightforward manner by lowering signal launch power. Indeed, high spectral efficiency optical link operating with 25GHz spaced signals has been recently demonstrated using hybrid Raman/EDFA amplification architecture: FWM process is minimized by operating in quasilinear transmission regime1. A different approach that does not require deployment of Raman amplifiers relays on combining two closely spaced counterpropagating interleaved channel grids.2 We will describe fundamental design issues encountered in such dense bidirectional optical links. While bidirectional link potentially doubles deployed capacity over single strand of fiber, its realization is made more difficult by host of qualitatively new problems, commonly not seen in conventional unidirectional lines. We describe latest efforts in constructing very dense bidirectional long-haul links over standard and non-zero dispersion shifted fiber.