The increased demand for fiber links in metro market networks has made it necessary for AT&T Broadband to build bidirectional DWDM networks in several locations. Furthermore, the collocation of primary and secondary hub ring routes and the need to address third-party requests for baseband digital capacity have required, at times, the transporting of both analog, or quasi-analog signals such as N-QAM (subcarrier multiplexed signals) and baseband digital signals (SONET, Ethernet or ATM) on different wavelengths within the same fiber. In both cases, the effects of crosstalk generated by DWDM passive components and nonlinear fiber effects had to be analyzed carefully in order to prevent severe degradation of the signal. It became important to specify design rules (relative optical power levels) in DWDM networks and DWDM component performance for proper operation of such networks.
This paper reports on the results of the laboratory testing conducted at AT&T Broadband to determine the acceptable levels of crosstalk, and to specify system guidelines to ensure that crosstalk effects remain within tolerable limits. The paper also summarizes basic specification parameters for DWDM passive components for uni-directional and bi-directional links with analog, quasi-analog and digital baseband signals.