Reconfigurable optical add/drop multiplexers (ROADMs) based on 1$\,\times\,$N wavelength-selective switches (WSS) are evolving to support DWDM networks with higher capacity and increased flexibility in wavelength routing. Different WSS technologies can be employed to provide colorless and steerable functionality for ring, or meshed architectures. Improvements in specifications of WSS modules operating on the 50 GHz wavelength grid have enabled 40 Gb/s transmission rates through extensive ROADM networks. The same ROADMs are also expected to support 100 Gb/s transmission in the near future. In parallel, development of lower-cost WSS technologies is allowing ROADMs to expand into edge networks. In all these network applications, propagation through multiple ROADMs generates transmission penalties for the DWDM channels, which need to be factored into the network design. Such OSNR or Q factor penalties can be induced by passband narrowing, imperfect isolation across the signal bandwidth, insertion loss, PDL, and other effects. The impact of these impairments depend on the transmitter and receiver types (e.g., data rate and modulation format), and on the WSS characteristics (e.g., insertion loss, passband width, shape, isolation magnitude and isolation stopband). Key transmission impairments such as bandpass narrowing, crosstalk, insertion loss, and PDL are estimated based on experiments and numerical simulations for common data rates and modulation formats. Implications of temporal fluctuations during power setting throughout a ROADM network are also discussed.
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