Challenges in component testing for optical systems

We discuss several examples that have been obtained from testing components in system-like conditions from our point of view as a component maker. First, we examine the conventional bidirectional transmission using interleaved signals for two directions within same band. One advantage of the bidirectional interleaving transmission, over separate band bidirectional transmission is the reduction of the four wave mixing phenomenon, which occurs between narrowly spaced copropagating adjacent channels, and is mitigated due to the copropagating channel space doubling. In both uni- and bidirectional cases, separate paths and amplifiers for the traffic in the two directions are used at the amplification nodes. This, in turn, is not cost effective, and if not properly filtered could easily form a loop for Rayleigh backscattering. This limitation is almost entirely removed with the development of a birefringent interleaver filter, which can be combined with a non-reciprocal birefringent filter to deliver near-complete blockage of backreflected channels. The resulting components (i.e., bidirectional isolator, bidirectional circulator, four-port interleaver) enable the use of standard unidirectional optical components to create bidirectional networks that deliver twice the spectral efficiency of unidirectional networks. We also illustrate some of the technical challenges that face the designer of 40 Gbit/s systems but can only be overcome with new types of components, such as tunable dispersion compensators and low dispersion components