Reconfigurable optical add/drop multiplexers (ROADMs) play a key role in optical transport networks. As shown in Figure 1, they can be used to switch wavelengths from one DWDM line port (ROADM degree) to another. Add/drop ports are also used to connect wavelengths from DWDM transceivers (transponders, muxponders, DWDM pluggables in a router, etc.) to particular ROADM degrees. They have been widely adopted by network operators due to benefits including faster wavelength provisioning, simplified planning, extended reach with wavelength power optimization, and the option for optical restoration schemes that increase network availability.
Figure 1: Four-degree ROADM with add/drop ports
However, ROADMs also create several challenges for coherent transmission. In a previous blog, I discussed how ICE6 can help with terrestrial networks challenged by high-loss spans. In this second blog, I will explain how ICE6 can also help with high ROADM cascade.
Metro/Regional Networks and ROADM Cascade
ROADM cascade describes the number of ROADM nodes the wavelength passes through on its A-to-Z journey across the network. In a metro/regional network, most if not all nodes will be ROADM-based. Based on Infinera studies, for North American metro networks, 5% of wavelengths will pass through nine ROADM nodes, and the worst 1% will pass through 14 to 16 ROADM nodes. Some wavelengths may even pass through more than 20 ROADM nodes. For Europe, the equivalent figures are seven 7 ROADM nodes for the top 5% and 10 ROADM nodes for the top 1%.
On the other hand, in a long-haul network, ROADM nodes will typically only be present at key intercity sites, with multiple in-line amplifiers at the intermediate sites, typically every 60 to 80 km, on the links between cities. For this reason, despite the longer distances in long-haul networks, wavelengths may actually pass through far fewer ROADM nodes in a long-haul network than in a large meshed metro/regional network.
ROADM Cascade Challenge 1: WSS Passband and Filter Narrowing
The passband of a flexible-grid ROADM can typically be assigned in 12.5 or 6.25 GHz increments. Older fixed-grid ROADMs typically have passbands significantly smaller than the nominal channel grid might imply. For example, a typical legacy 50 GHz wavelength-selective switch (WSS) might have an actual passband of 46 GHz, while an older legacy 100 GHz WSS will typically have an actual passband in the 40 to 50 GHz range.
Figure 2: Filter narrowing
However, as the wavelength passes through multiple WSSs, any differences in width, center wavelength, or shape of the passband will cause the aggregate passband to shrink, an effect commonly referred to as “filter narrowing.” Filter narrowing reduces the maximum wavelength baud rate that can be supported even by high-performance flexible-grid ROADMs to below that of the nominal passband, as shown in Figure 2. For example, after a cascade of 10 flexible-grid WSSs, each configured for a 100 GHz passband, the actual passband will typically be under 90 GHz, limiting the baud rate to approximately 85 Gbaud once wavelength roll-off in the 5% to 10% range is considered.
ROADM Cascade Challenge 2: Polarization-dependent Loss
Polarization-dependent loss (PDL) occurs because of asymmetries that result in higher losses (or lower gains) on one polarization and lower losses (or higher gains) on the other polarization. For example, in optical fibers, one polarization incurs higher loss due to being more impacted by the cladding than the other polarization. However, most of the PDL in a terrestrial network is a result of WSS asymmetries. For example, a single WSS contributes PDL equivalent to approximately 10,000 km of fiber. PDL is also challenging for coherent communications because it cannot be compensated for in the DSP.
ROADM Cascade Challenge 3: ROADM Loss, Amplification, and Noise
The various components of a ROADM incur loss. The loss of each WSS is typically in the 2 to 8 dB range, with 6 dB being a common figure. Other sources of loss inside the ROADM include splitters, filters, taps for monitoring, and the add/drop components. The total loss in a ROADM is likely to be in the 10 to 20 dB range, equivalent to a 40- to 80-km fiber span. This loss requires amplification that adds amplified spontaneous emission (ASE) noise. This noise accumulates as the wavelength traverses the network, resulting in a low optical signal-to-noise ratio (OSNR) that makes the signal hard to decode at the receive end.
ICE6 and High ROADM Cascade
ICE6 supports multiple features to address passband and filter narrowing challenges. These include a tuneable baud rate from 31 to 100+ Gbaud, a tight roll-off that keeps the spectral width of the wavelength close to the baud rate, super-channels, and a shared wavelocker that enables the lasers of two wavelengths to drift in tandem. With dynamic bandwidth allocation (DBA), the data rate on each Nyquist subcarrier can be set individually with a different bits/symbol setting for probabilistic constellation shaping (PCS). This enables a lower data rate with higher tolerances on the outer subcarriers, which typically experience higher filter narrowing penalties, and a higher data rate on the inner subcarriers.
In addition to digital compensation of the distortion aspect of PDL (different power levels on each polarization) in the DSP, ICE6 further mitigates PDL by digitally controlling the combined X+Y polarization properties of each subcarrier at the transmit end in order to minimize the impact of asymmetries. Finally, ICE6 supports multiple features that enable high tolerance to the additional noise that comes from compensating for losses inside the ROADM with amplification. These include long-codeword PCS and high-gain forward error correction.
Figure 3: Key ICE6 Features for High ROADM Cascade
Together these features enable network operators to boost spectral efficiency and fiber capacity while reducing cost per bit, footprint, and power consumption, even on metro/regional networks with wavelengths that have to pass through up to 20+ ROADM nodes. To learn more about how ICE6 and ICE6 Turbo can address challenging terrestrial conditions, which also include high-loss spans, G.655 fiber with low chromatic dispersion, and aerial fiber in regions with frequent lightning strikes, download the Infinera application note, “ICE6 for Challenging Terrestrial Conditions.”