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Introduction to BER testing of WDM systems Application note 1299 Wavelength division multiplexing (WDM) is a new and exciting technology for migrating the core optical transmission network to higher bandwidths. The ability to transfer multiple optical carriers over the same span of fiber promises almost unlimited bandwidth. However, the ultimate test for any transmission medium is its bit error ratio (BER) performance. How WDM overlays onto the network If a network hotspot produces capacity shortfall problems, network operators can now opt to deploy a WDM system to quickly expand capacity on existing fiber links. For example, WDM equipment has already been deployed to multiply the capacity of existing STM-16/ OC-48 links--by combining and carrying up to 16 STM-16/OC-48 signals along the existing fiber path. At the same time, upgrading of the existing STM-16/OC-48 line terminal mux to WDM operation is also readily achieved, providing increased bandwidth while retaining existing tributary access and connections. The operation of a WDM system can best be explained by looking at the sub-system level. Figure 1. Simplified WDM system configuration. Figure 2. Example output spectrum for four 2.4 Gb/s laser sources. Schemes are proposed carrying up to 32 Laser sources and remodulators Optical multiplexer STM-16/OC-48 signals. The transmitter of a WDM system produces The multiplexer (mux) couples together Currently, ITU-T draft recommen- laser signals at specific wavelengths and different wavelengths then combines them dations O.mcs specify 43 WDM with a nominal spectral line spacing for transmission into a single mode fiber-- wavelengths. between them (Figure 2). Frequency maintaining the wavelength integrity of