Abstract
Orthogonal polarization launch has been proposed as a way of decreasing nonlinear propagation effects in long-haul WDM systems.1 Orthogonal launch means that adjacent WDM channels are launched into the transmission line with an orthogonal state of polarization (SOP), thus experiencing less overlap and nonlinear interference with each other. For two channels with orthogonal SOP, four-wave-mixing is eliminated and cross-phase-modulation is cut into the half as compared to parallel SOP. However, even though adjacent channels are launched with an orthogonal SOP, the PMD in the transmission link decreases the degree of orthogonality along the transmission, thus reducing the benefits from orthogonal launch.2–4 On the other hand, submarine WDM systems are usually constructed with high-quality optical fibers with a very low PMD. We have previously shown, through numerical simulations, that the PMD in a transoceanic WDM system can be sufficiently low to maintain the orthogonality of adjacent channels enough for orthogonal launch to be beneficial.2 In Kovsh et al.,2 our simulations showed that in particular WDM systems with a small channel spacing ben efit from orthogonal launch as the orthogonality of adjacent channels is less disturbed by the PMD for small channel spacings. To continue our studies on orthogonal launch, this work presents an investigation of the advantages of orthogonal launch in a 6,500 km straight-line WDM transmission experiment. We compare the transmission performance of orthogonal launch with that of parallel launch, which means that adjacent channels are launched with a parallel SOP. To fairly estimate the benefits from reduced nonlin-earities accomplished by orthogonal launch, we carry out pre-emphasis measurements to opti-mize the optical power per channel for each launch state. Our experimental results are also confirmed through numerical simulations.
© 2002 Optical Society of America
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