ASE Noise Loading for Terrestrial WDM Applications
Primarily used on subsea networks, noise loading is a method that fills unused channels with ASE (amplified spontaneous emission) noise, simulating a fully filled WDM system. Recently, there’s been some industry interest in using ASE noise loading on terrestrial networks, in addition to traditional subsea applications. Historically, terrestrial WDM systems relied on embedded dynamic power management algorithms to ensure constant per channel power levels, as well as flat gain response across the operating band. Both methods, dynamic power management and ASE noise loading, work equally well, without significant performance differences between the two approaches.
In order to achieve optimal network performance, WDM and ILA nodes actively monitor and adjust wavelength power levels and spectral “flatness” across the C-band, or combined C+L bands. The dynamic power management algorithms run constantly in the background, monitoring per channel power levels, and making amplifier adjustments to maintain constant per channel power levels and flat gain response as wavelengths are added or deleted from the network.
For terrestrial networks, WDM node dynamic power management algorithms perform exceptionally well and have been the primary optical power control method for over 20 years.
Subsea Network Applications
Subsea systems use an alternative optical power management technique, due to the fact that subsea equipment resides under hundreds to thousands of meters of sea water. Repairing subsea cables or repeaters (ILAs) can cost over $1 million dollars (USD) for each repair. In addition, it takes weeks to dispatch a maintenance repair ship to a remote part of the ocean, haul the fiber cable from the ocean floor, and make any necessary repairs.
As a result of the high repair costs and long repair times, subsea “wet plant” equipment is designed with high levels of redundancy, along with the lowest possible failure rate (FIT). One way to lower the failure rate is by using simpler amplifier designs incorporating fewer components. The fewer the components in an amplifier, the fewer things that can fail.
The dynamic power management algorithms used on terrestrial WDM networks are software or firmware control algorithms. However, these algorithms rely on additional hardware built into amplifiers for monitoring optical power levels, as well as for controlling and adjusting amplifier pump lasers and output power. On undersea repeaters (ILAs), these additional components would slightly increase the unit’s failure rate. While the slight increase in FIT rate is insignificant on terrestrial nodes, where a failed card can easily be replaced, on repeater nodes that sit under 2,000 meters of water, any increase in FIT rates is avoided due to the potentially expensive repair bills. As a result, subsea systems utilize ASE noise loading as a simpler, less complex alternative to “dynamic” power management algorithms to monitor and manage optical power levels.
ASE Noise Loading
Noise loading is a technique where optical amplified spontaneous emission (ASE) noise fills all unused channels, as shown in figure 1. The network operates at full capacity with all channels occupied, either with actual traffic carrying wavelengths or with ASE noise channels. As new channels are added to the network, the noise channels are simply replaced by the “live” traffic carrying wavelengths. To the WDM network, the system appears to be operating fully filled at all times.
Figure 1) ASE Noise Loading on unused channels
Incorporating ASE noise loading simplifies optical power management algorithms, since these control algorithms no longer need to be “dynamic”, adjusting for changes in the network as channels are added or deleted. The power management algorithms can be static, only needing adjustment during initial provisioning.
On subsea systems, the ASE noise loading is part of overall the submarine line terminating equipment (SLTE) – the ROADM node installed in the cable landing station. With ASE noise injected on unused channels, the underwater repeaters can incorporate simpler amplifier designs with fewer components, lowering the risks of high repair costs and long repair times.
Optical nonlinearities such as cross-phase modulation (XPM), four wave mixing (FWM) and self phase modulation (SPM) are unwanted wavelength interferences and distortions. Nonlinearities result in a small reduction in overall OSNR performance, slightly reducing a network’s capacity and optical reach. The amount of the nonlinearity penalty depends on several factors, including fiber type, number of active channels, channel spacing, route distance, and wavelength power levels. To ensure networks operate with their designed wavelength capacity and optical reach, vendor WDM simulation tools calculate the nonlinearity penalty and include it their overall OSNR budget for a network design.
Since ASE noise loading fills all unused channels, the theory is that noise loaded systems provide static, known, operating performance, including all nonlinear penalties, that doesn’t vary as channels are added or deleted. On terrestrial WDM networks, dynamic power management algorithms provide per channel optical power and tilt management, while vendor WDM simulation tools calculate the OSNR budget and nonlinear penalties, ensuring network performance, regardless of the number of traffic carrying wavelengths. Both approaches work equally well and result in the approximately same performance and OSNR budgets.
Optical power management on C+L WDM networks is a bit more complex, due to the presence of stimulated Raman scattering (SRS) affecting both C and L-bands. SRS is a nonlinear effect that causes optical power to shift from shorter wavelengths to longer wavelengths, resulting in wavelength tilt. This effect is present in C-band only WDM systems, as well as C+L WDM systems.
Figure 2) SRS induced wavelength tilt
On C+L systems, dynamic power management algorithms operate across both C and L bands, adjusting the wavelength powers and SRS tilt compensation in both C-band amplifiers and L-band amplifiers to ensure a flat wavelength spectrum across both bands, as shown in figure 2.
Figure 3) C+L dynamic power management
On noise loaded systems, SRS tilt compensation is still required, but the amount of tilt compensation doesn’t vary, since ASE noise loading fills all unused channels, simulating a fully loaded network. However, using ASE noise loading for SRS tilt compensation typically requires deployment of both C-band ASE noise sources and L-band ASE noise sources as part of the initial deployment – even if a carrier initially only uses the C-band capacity. As a result, the initial network costs can be slightly higher when using ASE noise loading.
There is some industry misinformation suggesting ASE noise loading improves optical restoration times in WDM networks, which is not accurate or correct. Noise loading does not have any impact on optical restoration times – on well designed WDM nodes.
After a network outage, ROADM nodes carefully control and manage the restoration of dropped channels to prevent power transients from affecting other traffic carrying wavelengths. In well designed WDM equipment, typical restoration times range from 5 – 15s, depending on the size of the network and number of channels being restored. ROADM nodes incorporating well designed optical restoration algorithms incur no impact in restoration times, regardless of whether the network utilizes ASE noise loading or not.
If a network experiences exceptionally long restoration times, it may indicate an underlying problem with a specific vendor’s optical restoration algorithms. While ASE noise loading may indeed improve (or mask) a specific vendor’s product “issue”, there’s no generic industry benefit of ASE noise loading with regards to restoration speed.
ASE Noise Loading – Carrier’s Choice
WDM nodes actively manage their systems to ensure constant per channel power levels and flat gain response across all wavelengths. Traditionally, terrestrial networks have utilized dynamic power management algorithms, running within each ROADM node, to monitor and maintain optimal power levels and flat gain response.
Some industry vendors offer dynamic power management algorithms that operate seamlessly across both C+L WDM bands. As an alternative approach, ASE noise loading can be used on C+L WDM networks. Power and SRS tilt management are still required on ASE noise loaded systems, but those adjustments can be “static”.
There are no significant performance differences between using either dynamic power management algorithms or noise loading on WDM systems. Fortunately, there are industry vendors supporting both dynamic power management and ASE noise loading options on their WDM systems, so carriers have the freedom to choose their network deployment preference.