With the need for 100 Gb/s links in data communication, the IEEE 802.3 Ethernet Working Group has included implementations for reach up to 10 km (100GBASE-LR4) or, with tighter tolerances, 30 km (100GBASE-ER4) by using four wavelength channels in single mode fiber, centered at: 231.4 THz (1295.56 nm), 230.6 THz (1300.05 nm), 229.8 THz (1304.58 nm), 229.0 THz (1309.14 nm).
Spectral measurements for passive components
The spectral response of components used in WDM links is a key factor in determining link performance at the physical level. The insertion loss (IL) of passive components influence the signal power budget. The wavelength selectivity of filters used for multiplexing and especially demultiplexing, characterized from traces of IL vs. wavelength with parameters like ripple or flatness in the passband and isolation of wavelength outside the passband, is important for signal stability and avoiding crosstalk. Reflections, parametrized as return loss (RL), can also degrade link performance and should be controlled. Low dependence of these response parameters on the polarization of the optical signal is also needed to avoid fluctuations in power, because the polarization state can change randomly along fiber links. So passive WDM components are typically tested and verified by measuring IL, PDL and often RL across the applicable wavelength range. Using a tunable laser source at the common side of an LR4 multiplexer, for example, allows all four lane ports to be measured simultaneously with synchronized power meters. A block diagram for such measurements is shown below, implemented using the N7700A-100 application software package.
Block diagram for swept-wavelength IL and PDL measurements
Continue reading? You can find the whole application note and others inside the new Keysight Lightwave Catalog 2017.
Look for this application note on page 14.