ROADMs and their evolving amplification needs

Technology briefing: ROADMs and amplifiers

Oclaro announced an add/drop routing platform at the recent OFC/NFOEC show. The company explains how the platform is driving new arrayed amplifier and pumping requirements.  

A ROADM comprising amplification, line-interfaces, add/ drop routing and transponders. Source: Oclaro

Agile optical networking is at least a decade-old aspiration of the telcos. Such networks promise operational flexibility and must be scalable to accommodate the relentless annual growth in network traffic. Now, technologies such as coherent optical transmission and reconfigurable optical add/drop multiplexers (ROADMs) have reached a maturity to enable the agile, mesh vision.    

Coherent optical transmission at 100 Gigabit-per-second (Gbps) has become the base currency for long-haul networks and is moving to the metro. Meanwhile, ROADMs now have such attributes as colourless, directionless and contentionless (CDC). ROADMs are also being future-proofed to support flexible grid, where wavelengths of varying bandwidths are placed across the fibre's spectrum without adhering to a rigid grid.

Colourless and directionless refer to the ROADM's ability to transmit or drop any light path from any direction or degree at any network interface port. Contentionless adds further flexibility by supporting same-colour light paths at an add or a drop.

"You can't add and drop in existing architectures the same colour [light paths at the same wavelength] in different directions, or add the same colour from a given transponder bank," says Bimal Nayar, director, product marketing at Oclaro's optical network solutions business unit. "This is prompting interest in contentionless functionality."

The challenge for optical component makers is to develop cost-effective coherent and CDC-flexgrid ROADM technologies for agile networks. Operators want a core infrastructure with components and functionality that provide an upgrade path beyond 100 Gigabit coherent yet are sufficiently compact and low-power to minimise their operational expenditure.

ROADM architectures

ROADMs sit at the nodes of a mesh network. Four-degree nodes - the node's degree defined as the number of connections or fibre pairs it supports - are common while eight-degree is considered large. 

The ROADM passes through light paths destined for other nodes - known as optical bypass - as well as adds or drops wavelengths at the node. Such add/drops can be rerouted traffic or provisioned new services. 

Several components make up a ROADM: amplification, line-interfaces, add/drop routing and transponders (see diagram, above). 

"With the move to high bit-rate systems, there is a need for low-noise amplification," says Nayar. "This is driving interest in Raman and Raman-EDFA (Erbium-doped fibre amplifier) hybrid amplification." 

The line interface cards are used for incoming and outgoing signals in the different directions. Two architectures can be used: broadcast-and-select and route-and select.

With broadcast-and-select, incoming channels are routed in the various directions using a passive splitter that in effect makes copies the incoming signal. To route signals in the outgoing direction, a 1xN wavelength-selective switch (WSS) is used. "This configuration works best for low node-degree applications, when you have fewer connections, because the splitter losses are manageable," says Nayar.

For higher-degree node applications, the optical loss using splitters is a barrier. As a result, a WSS is also used for the incoming signals, resulting in the route-and-select architecture.

Signals from the line interface cards connect to the routing platform for the add/drop operations. "Because you have signals from any direction, you need not a 1xN WSS but an LxM one," says Nayar. "But these are complex to design because you need more than one switching plane." Such large LxM WSSes are in development but remain at the R&D stage.

Instead, a multicast switch can be used. These typically are sized 8x12 or 8x16 and are constructed using splitters and switches, either spliced or planar lightwave circuit (PLC) based .

"Because the multicast switch is using splitters, it has high loss," says Nayar. "That loss drives the need for amplification."

Add/drop platform

With an 8-degree-node CDC ROADM design, signals enter and exit from eight different directions. Some of these signals pass through the ROADM in transit to other nodes while others have channels added or dropped.

In the Oclaro design, an 8x16 multicast switch is used. "Using this [multicast switch] approach you are sharing the transponder bank [between the directions]," says Nayar.

The 8-degree node showing the add/drop with two 8x16 multicast switches and the 16-transponder bank. Source: Oclaro

A particular channel is dropped at one of the switch's eight input ports and is amplified before being broadcast to all 16, 1x8 switches interfaced to the 16 transponders.

It is the 16, 1x8 switches that enable contentionless operation where the same 'coloured' channel is dropped to more than one coherent transponder. "In a traditional architecture there would only be one 'red' channel for example dropped as otherwise there would be [wavelength] contention," says Nayar.

The issue, says Oclaro, is that as more and more directions are supported, greater amplification is needed. "This is a concern for some, as amplifiers are associated with extra cost," says Nayar.

The amplifiers for the add/drop thus need to be compact and ideally uncooled. By not needing a thermo-electrical cooler, for example, the design is cheaper and consumes less power.  

The design also needs to be future-proofed. The 8x16 add/ drop architecture supports 16 channels. If a 50GHz grid is used, the amplifier needs to deliver the pump power for a 16x50GHz or 800GHz bandwidth. But the adoption of flexible grid and super-channels, the channel bandwidths will be wider. "The amplifier pumps should be scalable," says Nayar. "As you move to super-channels, you want pumps that are able to deliver the pump power you need to amplify, say, 16 super-channels."

This has resulted in an industry debate among vendors as to the best amplifier pumping scheme for add/drop designs that support CDC and flexible grid. 

EDFA pump approaches

Two schemes are being considered. One option is to use one high-power pump coupled to variable pump splitters that provides the required pumping to all the amplifiers. The other proposal is to use discrete, multiple pumps with a pump used for each EDFA. 

Source: Oclaro

In the first arrangement, the high-powered pump is followed by a variable ratio pump splitter module. The need to set different power levels at each amplifier is due to the different possible drop scenarios; one drop port may include all the channels that are fed to the 16 transponders, or each of the eight amplifiers may have two only. In the first case, all the pump power needs to go to the one amplifier; in the second the power is divided equally across all eight.

Oclaro says that while the high-power pump/ pump-splitter architecture looks more elegant, it has drawbacks. One is the pump splitter introduces an insertion loss of 2-3dB, resulting in the pump having to have twice the power solely to overcome the insertion loss.

The pump splitter is also controlled using a complex algorithm to set the required individual amp power levels. The splitter, being PLC-based, has a relatively slow switching time - some 1 millisecond. Yet transients that need to be suppressed can have durations of around 50 to 100 microseconds. This requires the addition of fast variable optical attenuators (VOAs) to the design that introduce their own insertion losses.

"This means that you need pumps in excess of 500mW, maybe even 750mW," says Nayar. "And these high-power pumps need to be temperature controlled." The PLC switches of the pump splitter are also temperature controlled.

The individual pump-per-amp approach, in contrast, in the form of arrayed amplifiers, is more appealing to implement and is the approach Oclaro is pursuing. These can be eight discrete pumps or four uncooled dual-chip pumps, for the 8-degree 8x16 multicast add/drop example, with each power level individually controlled.

Source: Oclaro

Oclaro says that the economics favour the pump-per-amp architecture. Pumps are coming down in price due to the dramatic price erosion associated with growing volumes. In contrast, the pump split module is a specialist, lower volume device.

"We have been looking at the cost, the reliability and the form factor and have come to the conclusion that a discrete pumping solution is the better approach," says Nayar. "We have looked at some line card examples and we find that we can do, depending on a customer’s requirements, an amplified multicast switch that could be in a single slot."