Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers

Reconfigurable optical add/drop multiplexers (ROADMs) based on 1 X 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.

I. Introduction

Dense wavelength division multiplexed (DWDM) systems have become the standard technology in high capacity telecommunication networks ranging in distance from a few kilometers to thousands of kilometers. Commercial DWDM systems use wavelengths modulated at 2.5, 10, or 40 Gb/s covering the C and L band spectral range with typical frequency spacing between channels of 100 GHz or 50 GHz. These wavelengths are added and dropped from the multi-wavelength network fiber at geographical locations where the optical information is converted to electronic format for processing and switching. Such network nodes, known as optical add/drop multiplexer (OADM) nodes use optical filters to add/drop the desired wavelengths, while passing through all other wavelengths towards other OADM nodes in the network. Over the past few years, there has been a growing trend to add wavelength reconfigurability to OADM nodes (ROADMs), thereby allowing remote changes to the add/drop traffic pattern across the network [1].