Relentless traffic growth leads to a ROADM rethink
Technology briefing: ROADMs
Lumentum has developed an optical switch to enable reconfigurable optical add-drop multiplexers (ROADMs) to cope with the traffic growth expected over the next decade.
The company’s MxN wavelength-selective switch (WSS) will replace the existing multicast switch used in colourless, directionless and contentionless ROADMs. The Lumentum TrueFlex 8x24 twin switch will enable networking nodes of 400-terabit capacity.
“This second-generation switch is what will take us into the 100 gigabaud and super-channel era of network scalability,” says Brandon Collings, CTO of Lumentum.
ROADMs
ROADMs sit at the mesh nodes in an optical network. Their function is to pass lightpaths destined for other nodes in the network - referred to as optical bypass - and enable the adding and dropping of wavelengths at the node. Such add/drops may be rerouted traffic or provisioned new services.
As network traffic continues to grow, so do the degrees of a ROADM and the ports of its sub-systems. The degree of a ROADM is defined to the number of connections or fibre pairs it can support. In the diagram, a ROADM of degree three is shown.
A multicast switch-based 3-degree CDC ROADM. Source Lumentum.
It is rare to encounter more than five or six fibre routes leaving any given mesh node in a network, says Lumentum. “But in those fibre routes there is typically a large number of fibres - 64 or 128,” says Collings. “Operators deploy a conduit of fibre between cities.”
When the C-band fills up, an operator will light another fibre pair, taking up another of the ROADM’s degrees. ROADMs built today have 16 degrees. And since a fibre’s C-band can occupy some 30 terabits of data, this is how 400-terabit mesh nodes will be achieved.
“That is a pretty big node but that is the end [of life] capacity,” says Collings. “I don’t think you will find a 400-terabit node today but we build our networks so that they get there, five to eight years from when they are deployed.”
This raises another issue: the length of time it takes for any generational change of a ROADM design to take hold in the network.
“When a new approach comes along, it takes a couple of years for everyone to figure out how they will use it,” says Collings. Then, once a decision is made, it takes another two years to deploy followed by five to eight years before the ROADM node is filled.
“Nothing happens quickly in this business,” says Collings. “But the upside, from a business point of view, is that as things are designed in, they have a long deployment cycle.”
Lumentum illustrates the point with its own products.
The company is seeing growing demand for its dual TrueFlex WSS deployed in route-and-select ROADM architectures. “But we are still seeing growth on the older broadcast-and-select architectures underpinned by singe 1x9 WSSes,” says James Goodchild, director, product line management for wavelength management products at Lumentum.
CDC ROADMs
A colourless, directionless and contentionless (CDC) ROADM uses a twin multicast switch for the wavelength add and drop functions. The input fibre to each degree’s WSS is connected to the output path WSS of each of the ROADM’s other degrees. The input WSS also connects to the drop multicast switch (see diagram above).
Using a WSS on the input path means that only wavelengths of interest are routed to the WSS’ output ports. Hence the ROADM’s reference as a route-and-select architecture.
Using a 1xN splitter array instead of a WSS for the input path results in a broadcast-and-select ROADM. Here, the input fibre’s wavelengths are broadcast to all the N output ports. The high optical loss associated with the splitters is the main reason why CDC ROADM designs have transitioned to the WSS-based route-and-select architecture.
This second-generation switch is what will take us into the 100 gigabaud and super-channel era of network scalability
However, there is still an optical loss issue to be contended with, introduced by the add or drop multicast switch. Accordingly, along with the twin multicast switch are two arrays of erbium-doped fibre amplifiers (EDFAs). One EDFA array is on the drop ports to the MxN multicast switch and the second amplifier array boosts the outputs of the add-path multicast switch before their transmission into the network.
The MxN multicast switch comprises 1xN splitter arrays, N being the number of add-drop ports, and Mx1 selection switches where M is the number of directions the ROADM supports. A typical multicast switch is 8x16: eight being the ROADM’s number of directions and 16 the drop-port count.
Each of the N splitter arrays sends the signals on a drop port to all the Mx1 selection switches where each one pulls off the channel to be dropped. Having a selection switch at each of the multicast switch’s N drop ports is what enables contentionless operation, the avoidance of a collision when the same wavelength is droppedat a node from different degree directions.
MxN switch
Lumentum’s decision to develop the MxN switch to replace the multicast switch follows its study to understand how optical transmission networks will evolve with continual traffic growth.
One development is the adoption of higher-baud-rate, higher-capacity coherent transmissions that require wider channel widths. A 400-gigabit wavelength requires a 75GHz channel compared to the standard 50GHz fixed grid used for 100- and 200-gigabit transmissions. Future transmission speeds of 800 gigabits will use two such channels or 150GHz of spectrum, while a 1 terabit signal is expected to occupy 300GHz of fibre spectrum. “This is how we anticipate coherent transmission evolving,” says Collings.
Moving to wider channels also benefits the ROADM’s cost. If operators continued to use 50GHz channels, the channel count would grow exponentially with the growth in traffic. In contrast, adopting wider channels means the add-drop port count grows only linearly with traffic. “Using wider channels, the advantage is you don't have to support 600 ports of add-drop in your ROADM networks,” says Collings.
But wider channels means greater amplification demands on the EDFA arrays, an issue that will only worsen over time.
Multicast switch-based designs don’t support the wider channels we know are coming
Losing the amp
Because the power spectral density is constant, the power in a channel increases proportionally with its width. For example, a 75GHz channel has 2dB more power compared to a 50GHz channel spacing, a 150GHz channel 5dB more while a 300GHz channel has an extra 8dB.
The EDFA array is engineered to handle the worst case power requirement that occurs when all 16 optical transceivers into the multicast switch go to the same ROADM degree. Here the EDFA must be able to boost all 16 channels.
For a multicast switch with 16 ports, 22dBm amplification is needed for a 150GHz channel which requires going from an uncooled pump design to a cooled pump one. Equally, 25dBm amplification is needed for 300GHz channels. And as the number of degrees grows, so do the demands on the amplification until no practical amplifier design is possible (see diagram).
The EDFA requirements to compensate for the optical loss of the multicast switch. The complexity of the EDFA design grows with the multicast switch's port count until it becomes insupportable. Source: Lumentum.
“This is not an issue today because we use very modest-sized channels and we engineer our systems to accommodate them,” says Collings. “But if you look forward, you realise they [multicast switch-based designs] don’t support the wider channels we know are coming.”
Using a WSS-based MxN switch solves this issue because, as with the input port WSS of a route-and-select architecture, the switch has a lower optical loss - under 8dB - compared to the 17dB of the splitter-based multicast switch.
The sub-8dB loss is below the threshold where amplification is needed: the optical signal is sufficiently strong at the drop port to be received, as are the added signals for transmission into the network. The resulting removal of the EDFAs simplifies greatly the complexity, size and cost of the CDC ROADM.
“The MxN is a WSS - it’s a router - so it sends all of the light in the direction it is supposed to go,” says Collings. “You can push through the MxN switch channels of any width and of any power because there is no amplifier that needs to be there and be designed appropriately."
The resulting second-generation CDC ROADM design is shown below.
Source: Lumentum
Lumentum's Goodchild says the 8x24 twin implementation of the MxN switch will be available in the first quarter of 2019.
“Certain systems vendors already have access to samples,” says Goodchild.
Further reading
2D WSSes, click here
ROADMs and their evolving amplification needs, click here
ECOC 2015 Review - Part 1
- Several companies announced components for 400 gigabit optical transmission
- NEL announced a 200 gigabit coherent DSP-ASIC
- Lumentum ramps production of its ROADM blades while extending the operating temperature of its tunable SFP+
400 gigabit
Oclaro, Teraxion and NeoPhotonics detailed their latest optical components for 400 gigabit optical transmission using coherent detection.
Oclaro and Teraxion announced 400 gigabit modulators for line-side transmission; Oclaro’s based on lithium niobate and Teraxion’s an indium phosphide one.
NeoPhotonics outlined other components that will be required for higher-speed transmission: indium phosphide-based waveguide photo-detectors for coherent receivers, and ultra-narrow line-width lasers suited for higher order modulation schemes such as dual-polarisation 16-quadrature amplitude modulation (DP-16-QAM) and DP-64-QAM.
There are two common approaches to achieve higher line rates: higher-order modulation schemes such as 16-QAM and 64-QAM, and optics capable of operating at higher signalling rates.
Using 16-QAM doubles the data rate compared to quadrature phase-shift keying (QPSK) modulation that is used at 100 Gig, while 64-QAM doubles the data rate again to 400 gigabit.
Higher-order modulation can use 100 gigabit optics but requires additional signal processing to recover the received data that is inherently closer together. “What this translates to is shorter reaches,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics.
These shorter distances can serve data centre interconnect and metro applications where distances range from sub-100 kilometers to several hundred kilometers. But such schemes do not work for long haul where sensitivity to noise is too great, says Lipscomb.
What we are seeing from our customers and from carriers looking at next-generation wavelength-division multiplexing systems for long haul is that they are starting to design their systems and are getting ready for 400 Gig
Lipscomb highlights the company’s dual integrable tunable laser assembly (iTLAs) with its 50kHz narrow line-width. “That becomes very important for higher-order modulation because the different states are closer together; any phase noise can really hurt the optical signal-to-noise ratio,” he says
The second approach to boost transmission speed is to increase the signalling rate. “Instead of each stream at 32 gigabaud, the next phase will be 42 or 64 gigabaud and we have receivers that can handle those speeds,” says Lipscomb. The use of 42 gigabaud can be seen as an intermediate step to a higher line rate - 300 gigabit – while being less demanding on the optics and electronics than a doubling to 64 gigabaud.
Oclaro’s lithium niobate modulator supports 64 gigabaud. “We have increased the bandwidth beyond 35 GHz with a good spectral response – we don’t have ripples – and we have increased the modulator’s extinction ratio which is important at 16-QAM,” says Robert Blum, Oclaro’s director of strategic marketing.
We have already demonstrated a 400 Gig single-wavelength transmission over 500km using DP-16-QAM and 56 gigabaud
Indium phosphide is now coming to market and will eventually replace lithium niobate because of the advantages of cost and size, says Blum, but lithium niobate continues to lead the way for highest speed, long-reach applications. Oclaro has been delivering its lithium niobate modulator since the third quarter of the year.
Teraxion offers an indium phosphide modulator suited to 400 gigabit. “One of the key differentiators of our modulator is that we have a very high bandwidth such that single-wavelength transmission at 400 Gig is possible,” says Martin Guy, CTO and strategic marketing at Teraxion. “We have already demonstrated a 400 Gig single-wavelength transmission over 500km using DP-16-QAM and 56 gigabaud.”
“What we are seeing from our customers and from carriers looking at next-generation wavelength-division multiplexing systems for long haul is that they are starting to design their systems and are getting ready for 400 Gig,” says Blum.
Teraxion says it is seeing a lot of activity regarding single-wavelength 400 Gig transmission. “We have sampled product to many customers,” says Guy.
NeoPhotonics says the move to higher baud rates is still some way off with regard systems shipments, but that is what people are pursuing for long haul and metro regional.
200 Gig DSP-ASIC
Another key component that will be needed for systems operating at higher transmission speeds is more powerful coherent digital signal processors (DSPs). NTT Electronics (NEL) announced at ECOC that it is now shipping samples of its 200 gigabit DSP-ASIC, implemented using a 20nm CMOS process.
Dubbed the NLD0660, the DSP features a new core that uses soft-decision forward error correction (SD-FEC) that achieves a 12dB net coding gain. Improving the coding gain allows greater spans before optical regeneration or longer overall reach, says NEL. The DSP-ASIC supports several modulation formats: DP-QPSK, DP-8-QAM and DP-16-QAM, for 100 Gig, 150 Gig and 200 Gig rates, respectively. Using two NLD0660s, 400 gigabit coherent transmission is achieved.
NEL announced its first 20nm DSP-ASIC, the lower-power 100 gigabit NLD0640 at OFC 2015 in March. At the same event, ClariPhy demonstrated its own merchant 200 gigabit DSP-ASIC.
Reconfigurable optical add/ drop multiplexers
Lumentum gave an update on its TrueFlex route & select architecture Super Transport Blade, saying it has now been qualified, with custom versions of the line card being manufactured for equipment makers. The Super Transport Blades will be used in next-generation ROADMs for 100 gigabit metro deployments. The Super Transport Blade supports flexible grid, colourless, directionless and contentionless ROADM designs.
“This is the release of the full ROADM degree for next-generation networks, all in a one-slot line card,” says Brandon Collings, CTO of Lumentum. “It is a pretty big milestone; we have been talking about it for years.”
Collings says that the cards are customised to meet an equipment maker’s particular requirements. “But they are generally similar in their core configuration; they all use twin wavelength-selective switches (WSSes), those sort of building blocks.”
This is the release of the full ROADM degree for next-generation networks, all in a one-slot line card. It is a pretty big milestone; we have been talking about it for years
Lumentum also announced 4x4 and 6x6 integrated isolator arrays. “If you look at those ROADMs, there is a huge number of connections inside,” says Collings. The WSSes can be 1x20 and two can be used - a large number of fibres - and at certain points isolators are required. “Using discrete isolators and needing a large number, it becomes quite cumbersome and costly, so we developed a way to connect four or six isolators in a single package,” he says.
A 6x6 isolator array is a six-lane device with six hardwired input/ output pairs, with each input/ output pair having an isolator between them. “It sounds trivial but when you get to that scale, it is truly enabling,” says Collings.
Isolators are needed to keep light from going in the wrong direction. “These things can start to accumulate and can be disruptive just because of the sheer volume of connections that are present,” says Collings.
Tunable transceivers
Lumentum offers a tunable SFP+ module that consumes less than 1.5W while operating over a temperature range of -5C to +70C. At ECOC, the company announced that in early 2016 it will release a tunable SFP+ with an extended temperature range of -5C to +85C.
Further information
Heading off the capacity crunch, click here
For the ECOC Review, Part 1, click here
Verizon readies its metro for next-generation P-OTS
Verizon is preparing its metro network to carry significant amounts of 100 Gigabit traffic and has detailed its next-generation packet-optical transport system (P-OTS) requirements. The operator says technological advances in 100 Gig transmission and new P-OTS platforms - some yet to be announced - will help bring large scale 100 Gig deployments in the metro in the next year or so.
Glenn Wellbrock
The operator says P-OTS will be used for its metro and regional networks for spans of 400-600km. "That is where we have very dense networks," says Glenn Wellbrock, director of optical transport network architecture and design at Verizon. "The amount of 100 Gig is going to be substantially higher than it was in long haul."
Verizon announced in April that it had selected Fujitsu and Coriant for a 100 Gig metro upgrade. The operator has already deployed Fujitsu's FlashWave 9500 and the Coriant 7100 (formerly Tellabs 7100) P-OTS platforms. "The announcement [in April] is to put 100 Gig channels in that embedded base," says Wellbrock.
The operator has 4,000 reconfigurable optical add/ drop multiplexers (ROADMs) across its metro networks worldwide and all support 100 Gig channels. But the networks are not tailored for high-speed transmission and hence the cost of 100 Gig remains high. For example, dispersion compensation fibre, and Erbium-doped fibre amplifiers (EDFA) rather than hybrid EDFA-Raman are used for the existing links. "It [the network] is not optimised for 100 Gig but will support it, and we are using [100 Gig] on an as-needed basis," says Wellbrock.
The metro platform will be similar to those used for Verizon's 100 Gig long-haul in that it will be coherent-based and use advanced, colourless, directionless, contentionless and flexible-grid ROADMs. "But all in a package that fits in the metro, with a much lower cost, better density and not such a long reach," says Wellbrock.
The amount of 100 Gig is going to be substantially higher than it was in long haul
One development that will reduce system cost is the advent of the CFP2-based line-side optical module; another is the emergence of third- or fourth-generation coherent DSP-ASICs. "We are getting to the point where we feel it is ready for the metro," says Wellbrock. "Can we get it to be cost-competitive? We feel that a lot of the platforms are coming along."
The latest P-OTS platforms feature enhanced packet capabilities, supporting carrier Ethernet, multi-protocol label switching - transport profile (MPLS-TP), and high-capacity packet and Optical Transport Network (OTN) switching. Recently announced P-OTS platforms suited to Verizon's metro request-for-proposal include Cisco Systems' Network Convergence System (NCS) 4000 and Coriant's mTera. Verizon says it expects other vendors to introduce platforms in the next year.
Verizon still has over 250,000 SONET elements in its network. Many are small and reside in the access network but SONET also exists in its metro and regional networks. The operator is keen to replace the legacy technology but with such a huge number of installed network elements, this will not happen overnight.
Verizon's strategy is to terminate the aggregated SONET traffic at its edge central offices so that it only has to deal with large Ethernet and OTN flows at the network node. "We plan to terminate the SONET, peel out the packets and send them in a packet-optimised fashion," says Wellbrock. In effect, SONET is to be stopped from an infrastructure point of view, he says, by converting the traffic for transport over OTN and Ethernet.
SDN and multi-layer optimisation
The P-OTS platform, with its integrated functionality spanning layer-0 to layer-2, will have a role in multi-layer optimisation. The goal of multi-layer optimisation is to transport services on the most suitable networking layer, typically the lowest, most economical layer possible. Software-defined networking (SDN) will be used to oversee such multi-layer optimisation.
However, P-OTS, unlike servers used in the data centre, are specialist rather than generic platforms. "Optical stuff is not generic hardware," says Wellbrock. Each P-OTS platform is vendor-proprietary. What can be done, he says, is to use 'domain controllers'. Each vendor's platform will have its own domain controller, above which will sit the SDN controller. Using this arrangement, the vendor's own portion of the network can be operated generically by an SDN controller, while benefitting from the particular attributes of each vendor's platform using the domain controller.
There is always frustration; we always want to move faster than things are coming about
Verizon's view is that there will be a hierarchy of domain and SDN controllers."We assume there are going to be multiple layers of abstraction for SDN," says Wellbrock. There will be no one, overriding controller with knowledge of all the networking layers: from layer-0 to layer-3. Even layer-0 - the optical layer - has become dynamic with the addition of colourless, directionless, contentionless and flexible-grid ROADM features, says Wellbrock.
Instead, as part of these abstraction layers, there will be one domain that will control all the transport, and another that is all-IP. Some software element above these controllers will then inform the optical and IP domains how best to implement service tasks such as interconnecting two data centres, for example. The transport controller will then inform each layer its particular task. "Now I want layer-0 to do that, and that is my Ciena box; I need layer-1 to do this and that happens to be a Cyan box; and we need MPLS transport to do this, and that could be Juniper," says Wellbrock, pointing out that in this example, three vendor-domains are involved, each with its own domain controller.
Is Verizon happy with the SDN progress being made by the P-OTS vendors?
"There is always frustration; we always want to move faster than things are coming about," says Wellbrock. "The issue, though, is that there is nothing I see that is a showstopper."
OFC 2014 product round-up - Part 1
Part 1: Line-side technologies
Technologies for 100 Gigabit were prominent at this year's OFC conference and exhibition held in San Francisco earlier this month.
The transition to smaller pluggable modules – client-side CFP2, CFP4 and QSFP28 interfaces - was one 100 Gig trend, another was the first 100 Gig pluggable modules for metro and metro-regional networks. Acacia Communications detailed its low-power AC-100 CFP, while Oclaro demonstrated coherent optics in the smaller CFP2 pluggable module.
To fit within the CFP2, Oclaro has developed a transmitter that combines two tunable lasers (one being for the coherent receiver) and an indium phosphide modulator, and a micro intradyne coherent receiver (micro ICR).
Having 100 Gig coherent optics in a CFP2 will enable equipment makers to double the 100 Gig line ports on their platforms. The optics will also support polarisation multiplexed, 16-quadrature amplitude modulation (PM-16-QAM) and hence 200 Gig transmission. However, given the CFP2's limited power profile, the coherent DSP-ASIC will need to reside on the line card, external to the module. Oclaro says samples of its 'analogue CFP2' will be with customers from the second quarter of the year.
The same coherent optics will also be used for Oclaro's 100 Gig coherent CFP module. "If you combine the [transmitter and micro ICR] optics, you get the CFP2, and the power target is 12W," says Robert Blum, director, product management, 40 and 100 Gig line-side modules at Oclaro. "Combining the optics with a [coherent] DSP in the CFP, the power target is 32W, the highest CFP [power] class."
Oclaro's 100 Gig CFP will be available by year-end, coinciding with a new generation of merchant coherent DSP-ASIC designs. ClariPhy Communications is sampling its LightSpeed-II devices, implemented using a 28nm CMOS process, while NTT Electronics (NEL) is developing its next-generation DSP-ASIC, expected to use an even more advanced CMOS process.
Integrating the DSP chip and optics in a CFP simplifies a line card design and adds flexibility: the same CFP port can be used for line-side or client-side modules. But given that the coherent optics consumes 12W, the next-generation DSP-ASIC must consume no more than 18W typically, with the remaining 2W to accommodate the physical layer ICs, if the CFP's maximum power profile is not to be exceeded.
Acacia Communications' AC-100 coherent CFP module uses the company's DSP-ASIC and photonic integrated circuit (PIC) implemented using silicon photonics. The resulting 100 Gig CFP consumes 24-26W, well within the CFP's maximum 32W.
Meanwhile, Fujitsu Optical Components demonstrated all the components needed to make a 100 Gig coherent CFP, using its indium phosphide modulator to generate a 100 Gig polarisation multiplexed, quadrature phase-shift keying (PM-QPSK) signal, and a micro ICR.
Considering that the 5x7-inch Optical Internetworking Forum (OIF) multi-source agreement (MSA) 100 Gig transponder for long-haul consumes some 80W, with the DSP-ASIC alone consuming over half that, the advent of the coherent CFP and analogue CFP2 highlights the industry’s recent progress in shrinking the size and power consumption of coherent optics.
"Our focus long term is the CFP2. It is where we think the market is going to go in the next two years."
Ferris Lipscomb, NeoPhotonics
Coherent components
NeoPhotonics detailed an integrated coherent transmitter that combines a narrow-linewidth tunable laser and a PM-QPSK modulator in one package. The device joins NeoPhotonics' micro Integrable Tunable Laser Assembly (ITLA) and micro ICR that have already been announced. "These are the next generation, smaller form factor coherent optical components," says Ferris Lipscomb, vice president of marketing at NeoPhotonics. The transmitter supports PM-QPSK and PM-16-QAM such that it can be used for 100, 200 and even as an element to enable 400 Gig transmission.
The device is suited for line card, OIF MSA modules, and pluggable CFP and CFP2 designs. "Our focus long term is the CFP2," says Lipscomb. "It is where we think the market is going to go in the next two years." NeoPhotonics says its coherent devices has been sampling to customers and will be generally available in the second half of the year.
Oclaro announced that its micro ITLA, first detailed at ECOC 2013, now supports a flexible grid; the tunable laser's wavelength can be set independent of the ITU Grid spanning the C-band. Such a capability is required for advanced optical networks based on flexible-grid ROADMs and spectrally-efficient super-channels. "The flexible-grid micro ITLA gives peace of mind [to operators] even if it is not widely used yet," says Oclaro's Blum. The technology used for the micro ITLA is also used for Oclaro's CFP and CFP2 line side modules.
Fujitsu Optical Components announced a lithium niobate modulator that supports 100 Gig PM-QPSK and 400 Gig PM-xQAM signals. The new modulator has the same drive voltage as its existing 100 Gig lithium niobate modulator but is half the size. The company also announced an accompanying ICR that also supports 100 and 400 Gig transmissions in core networks. The company says both devices will be available from July.
Sumitomo Electric Industries detailed its micro ITLA at OFC. The micro ITLA uses a narrower line width laser and reduces power consumption by a fifth. The company also showcased a micro ICR that supports 100 and 200 Gig transmissions, and an indium phosphide based Mach-Zehnder modulator that is smaller and has lower power than a lithium niobate-based version.
Avago Technologies announced its micro ICR at OFC, a demonstration of Avago's broad component portfolio following its acquisition of CyOptics. Finisar was another company that showcased a new portfolio of high-speed optical components following its acquisition of u2t Photonics. These include indium phosphide-based Mach-Zehnder modulators and 100 Gig receivers and photodetectors.
Tunable SFP+
Both JDSU and Oclaro detailed their latest 10 Gigabit tunable SFP+ optical modules. Moving the tunable laser design from an XFP to the SFP+ has been a challenge, meeting the SFP+'s smaller dimensions and 1.5W power consumption.
Oclaro's latest tunable SFP+ now meets the 1.5W SFP+ specification. Oclaro says that to achieve the specification, it produced a more compact integrated laser Mach-Zehnder chip. Oclaro demonstrated the tunable SFP+ operating at 85oC. Beta samples of the tunable SFP+ are being shipped and the module will soon undergo qualification.
JDSU has had a tunable SFP+ product for over a year but its power consumption is 2W. The SFP+ length is also elongated by 4mm to fit the tunable laser. Now, JDSU has announced a revised design that no longer needs the extra 4mm and achieves a power consumption of 1.6W. "We will achieve the 1.5W specification in the near future," says Brandon Collings, JDSU's CTO for communications and commercial optical products.
"The reason why there is a lot of talk about hybrid EDFA-Raman in the industry is that it works very well with coherent."
Rafik Ward, Finisar
Pump lasers and hybrid amplifiers
JDSU also announced pump laser designs. The motivation for these latest pump products is the more demanding link budgets required for 100 Gig-and-greater transmission speeds while still achieving long-distance reaches.
JDSU announced Raman pump lasers for hybrid EDFA-Raman amplifiers. These are more power-efficient and cover the Raman pump wavelengths required, says JDSU: a 600mW output between 1425-1470nm and 550mW at 1470-1495nm. The company has also detailed higher-power 980nm pumps for EDFAs. "More power is almost always a good thing as it allows you a lot more design freedom and performance in your amp," says Collings.
Finisar demonstrated a hybrid EDFA-Raman amplifier for the first time. The hybrid amp is capable of spanning 220km and has a 44dB link loss. "The reason why there is a lot of talk about hybrid EDFA-Raman in the industry is that it works very well with coherent," says Rafik Ward, vice president of marketing at Finisar. Amplifier span distances of 80km are commonly used but the purpose of the demonstration was to showcase the product's capability, says Ward.
WSSes and multicast switches
NeoPhotonics has announced a modular multicast switch that allows an operator to grow a ROADM's node according to demand. The multicast switch is used to add colourless, directionless and contentionless (CDC) attributes to the ROADM. "You can have any wavelength [colourless] from any direction come out at any port [directionless]," says Lipscomb. "And if you have two identical wavelengths coming from different directions, you can drop them through the same switch [contentionless]."
Lipscomb cites as an example an 8-degree ROADM node, with each direction fibre carrying 100 dense WDM channels. Even if only a quarter of the channels are dropped, that is 200 channels, he says: "What we are announcing is a modular multicast switch; you can start with 4 channels and 4 drops and keep adding modular line cards as needed to add more drop ports and more directions."
NeoPhotonics modular multicast switches include such dimensions as 4x4, 4x16 and 8x16. "Carriers don't want to limit their future deployment but they also don't want to spend a lot of money now because they might want to drop 100 channels later," says Lipscomb.
JDSU announced its second-generation twin 1x20 wavelength-selective switch (WSS) that fits on a single-slot card. The twin WSS is used for advanced flexible-grid CDC-ROADM nodes.
The latest twin 1x20 WSS has the same functionality as JDSU's current twin 1x20 WSS that has been available for a year but which occupies two chassis slots. "It has the same capability but is considerably smaller," says Collings.
Indeed, the twin WSS is sufficiently compact that other functions can be added to the card such as amplification, optical power monitoring and optical service channels, communication channels between nodes used for such tasks as provisioning, power management and firmware updates.
For the OFC 2014 product round-up - Part 2, click here
Infinera introduces flexible grid 500G super-channel ROADM
An example showing the impact of a 500G super-channel ROADM node. Source: Infinera
"The FlexROADM will open up the Tier-1 operators in a way Infinera has not been able to do before," says Dana Cooperson, vice president, network infrastructure at market research firm, Ovum. "The DTN-X was necessary but not sufficient; the ROADM is the last piece."
The FlexROADM is claimed to deliver two industry firsts: it can add and drop flexible-grid-based 500 Gig super-channels, and uses the Internet Engineering Task Force’s (IETF) spectrum switched optical networks (SSON).
"SSON is the next generation of WSON [Wavelength Switched Optical Network control plane], except it manages spectrum," says Ron Kline, principal analyst, network infrastructure also at Ovum.
The DTN-X platform combines Infinera's 500 Gig photonic integrated circuits and OTN (Optical Transport Network) switching. With the FlexROADM, Infinera has added switching at the optical layer in 500 Gig increments. Infinera can now offer enhanced multi-layer network optimisation with the combination of electrical and optical switching.
"Optical bypass before was manual using patch cords, now operators can reconfigure with the FlexROADM," says Kline. "It also provides new optical restoration capabilities that Infinera did not have."
The FlexROADM supports up to nine degrees, and is available in colourless, colourless and directionless, and full colourless, directionless and contentionless (CDC) versions.
"The debate about contentionless continues," says Kline. "It is safe to assume that for the majority of applications flexible grid, colourless and directionless will be the high runner." Contentionless will be used by the big carriers, he says, but in certain locations only.
Infinera says the line system announced will support up to 24 Terabit-per-second (Tbps) when it ships in September. The maximum long-haul capacity using its current PM-QPSK super-channels is 9.5Tbps per fibre pair.
"In the future when we enable metro-reach super-channels using PM-16-QAM, they will support 24 Terabit-per-second per fibre pair using the line system we are announcing," says Geoff Bennett, director, solutions and technology at Infinera.
Bennett says the data rate and the spectral efficiency for a given sub-carrier can be varied depending on the reach required. The spacing between sub-carriers that make up a super-channel also can be varied depending on reach. Many different transmission possibilities exist, says Bennett, but to explain the concept, he cites two examples.
The 24Tbps capacity with PM-16-QAM modulation uses pulse shaping at the transmitter to achieve 'Nyquist DWDM' channel spacing, the spacing between channels that approximates the baud rate, says Bennett.
"At this time we are not disclosing the details of the channel spacing, or the number of sub-carriers used by our future line modules," says Bennett. "But the total super-channel spectral width is the equivalent of 200GHz if you are transmitting a one Terabit super-channel, for example." This equates to a spectral efficiency of 5b/s/Hz, and using 16-QAM, the reach achieved will be 600-700km.
"The system we have just launched is designed to operate in long-haul networks and uses PM-QPSK," says Bennett. "For an ultra long-haul reach requirement of 4,500km, the super-channel comprises ten sub-carriers; a total of 500 Gbps over a spectral width of 250 GHz." These line cards are available now, he says.
Infinera continues to make steady market progress, according to Ovum. The company is in the top 10 system vendors globally, while in backbone and 100 Gigabit, Infinera is fourth.
Amplifiers come to the fore to tackle agile network challenges
The growing sophistication of high-speed optical transmission based on 100 Gigabit-plus lightpaths and advanced ROADMs is rekindling interest in amplifier design.
Raman is a signature of the spread of 100 Gig but also the desire of being upgradable to higher bit rates
Per Hansen, II-VI
For the last decade, amplifier designers have been tasked with reducing the cost of Erbium-doped fibre amplifiers (EDFAs). "Now there is a need for new solutions that are more expensive," says Daryl Inniss, vice president and practice leader, components at market research firm, Ovum. "It is no longer just cost-cutting."
Higher output power amplifiers are needed to boost 100 Gig-plus signals that have less energy. Such amplifiers must also counter greater losses incurred by sophisticated colourless, directionless and contentionless (CDC) ROADM nodes. System vendors also require more power-efficient and compact amplifiers to maximise the chassis slots available for revenue-generating 100 Gig transponders.
Such requirements have created interest in all amplifier types, not just EDFAs but hybrid EDFA-Raman and Raman amplifiers.
"Improving the optical signal-to-noise ratio (OSNR) is of paramount consideration to enable higher capacity and reach for 100 Gig-plus lambdas," says Madhu Krishnaswamy, director, product line management at JDSU. "Raman amplification is becoming increasingly critical to delivering this OSNR improvement, largely in long haul."
Other developments include micro-amplifiers that boost single channels, and arrayed amplifiers used with ROADM nodes. These developments are also driving optical components: power-efficient, integrated pump lasers are needed for such higher-power amplifiers.
Operators' requirements span all three amplifier classes: EDFA, hybrid EDFA-Raman and all-Raman, says Anuj Malik, manager, solutions marketing at Infinera: "Some networks require a high OSNR and use hybrid amplifiers but some networks are prone to fibre cuts and hence avoid hybrid as fibre splices can cause more problems with Raman."
Raman differs from EDFA in several ways. Raman has a lower power efficiency, the optical pump power needed to pump an amplifier to achieve a certain gain and output power. This requires higher power to be launched into a Raman amplifier, raising safety issues for staff and equipment. The high launch power requires a sound connection between the Raman pump source and the fibre to avoid equipment being damaged, hence Infinera's reference to fibre splices.
Yet if Raman has a lower power efficiency, it has notable benefits when compared to an EDFA.
An EDFA performs lumped amplification, boosting the signal at distinct points in the network, every 80km commonly. Raman amplifies the signal as it travels down the fibre.
"With Raman amplification the gain is out in the fibre span, and Raman delivers a lower equivalent noise figure - a big advantage," says Per Hansen, head of product marketing, amplifier business unit at II-VI." The company II-VI acquired Oclaro's amplifier business in November 2013.
An amplifier's noise figure is a measure of performance in the network. All amplifiers introduce noise so that the input signal-to-noise ratio divided by the output signal-to-noise ratio is always greater than one. "Raman gives you a significantly better noise figure, an improvement in the range of 3 to 5dB," says Hansen.
EDFA designs continue to progress alongside the growing interest in hybrid and all-Raman. JDSU says that higher output power EDFAs, greater than 24dBm, are increasingly relevant for 96-plus channel systems that support super-channels and flexible grid ROADMs in the metro and long haul.
"Switchable-gain EDFAs to optimise the noise figure over a wider dynamic range of operation is another element enhancing overall system OSNR," says Krishnaswamy. "This is also common for metro and long haul."
Hybrid amplification combines the best characteristics of EDFA and Raman. In a hybrid, Raman is the first amplification stage where noise figure performance is most important, while the EDFA, with its power efficiency, is used as the second stage, boosting the signal to a higher level.
According to Finisar, 100 Gig uses the same receiver OSNR as 10 Gig transmissions. However, the transmission power per channel at 100 Gig is reduced, from 0 to 1dBm at 10 Gig to -2 to -3dBm at 100 Gig, due to non-linearity transmission issues. "Immediately you lose a few dBs in the OSNR," says Uri Ghera, CTO of the optical amplifier products at Finisar.
An overwhelming portion of WANs worldwide have adopted hybrid EDFA-Raman and this trend is expected to continue for the foreseeable future.
For 400 Gigabit transmission, the weaker signal sent requires the OSNR at the receiver to be 4-10dBm higher, says Ghera: "This is why you need hybrid Raman-EDFA."
Moving to a narrower channel spacing using a flexible grid also places greater demands on amplifiers. "Because of super-channels, if before we were talking about 100 channels [in the C-band], for a channel spacing of 37.5GHz it is more like 130 channels," says Ghera. "If you want the same power per channel, it means higher-output amplifiers."
The spectrum amplified by an EDFA is determined by the fibre. EDFAs amplify the 35nm-wide C-band spanning 1530 to 1565nm, and also the separate L-band at 1570 to 1605nm, if that is used. In contrast, the spectrum amplified by Raman is determined by the pump laser's wavelength. This leads to another benefit of all-Raman: far broader spectrum amplification, 100nm and wider.
Xtera is a proponent of all-Raman amplification. The system vendor has demonstrated 60nm- and even 100nm-wide spectrum amplification, broader than the C and L bands combined.
Xtera conducted trials with Verizon in 2013 using its Nu-Wave Optima platform and Raman operating over a 61nm window. The trials are detailed in Table 1.
Between 15 and 40 Terabits were sent over 4,500km and 1,500km, respectively, using several modulation schemes and super-channel arrangements. In comparison, state-of-the-art 100 Gig-plus systems achieve 16 Terabit typically across the C-band, and are being extended to 20-24 Terabit using closer-spaced channels. Using 16-QAM modulation, the reach achieved is 600km and more.
Table 1: Xtera's Verizon trial results using a 61nm spectrum and all-Raman amplification.
JDSU says hybrid amplification remains the most cost-competitive way to deliver the required OSNR and system capacity, while all-Raman can potentially increase system capacity.
Overall, it is network capacity and reach requirements that drive amplifier choice, says Krishnaswamy: "An overwhelming portion of WANs worldwide have adopted hybrid EDFA-Raman and this trend is expected to continue for the foreseeable future."
Meanwhile, the single channel micro-amp, sits alongside or is integrated within the transmitter. Operators want a transponder that meets various requirements for their reconfigurable networks. "If you look into the numbers, you want to boost the signal early on before it is attenuated," says II-VI's Hansen. "That gives you the best OSNR performance."
"This [single-channel amp] is a type that was rare in old systems," adds Finisar's Ghera. "It is also a market that is growing the fastest for us."
The micro-amp needs to be compact and low power, being alongside the power-hungry 100 Gig coherent transmitter. This is driving uncooled pump laser development and system integration.
Similar design goals apply to arrayed amplifiers that counter losses in ROADM add/ drop cards. "If you have some of the features of colourless, directionless and contentionless, you incur bigger losses in the node but you can make it up with other amps, one of these being arrayed amps," says Hansen.
Arrayed designs can have eight or more amps to support multiple-degree nodes so that achieving a power-efficient, compact design is paramount. Hence II-VI's development of an uncooled dual-chip pump laser integrated in a package. "Having four packages to pump eight amps in a small space that do not require cooling is a huge advantage," says Hansen.
The amplifier design challenges are set to continue.
One, highlighted by Infinera, is expanding amplification to the L-band to double overall capacity. JDSU highlights second-order and third-order Raman designs that use a more complex pump laser arrangement to improve system OSNR. Lowering the noise figure of EDFAs will be another continuing design goal, says JDSU.
II-VI expects further challenges in miniaturising single-channel and arrayed amplifier designs. Finisar also cites the need for more compact designs, citing putting an EDFA in an XFP package as an example.
Another challenge is producing high-power Raman amplifiers that can bridge extremely long spans, 300 to 400km. Such an amplifier must be able to read lots of physical parameters associated with the span and set the line accordingly, said Gheri.
II-VI's Hansen says the adoption of Raman and arrayed amplifiers is a good indicator of the wider deployment of next-generation network architectures. "Raman is a signature of the spread of 100 Gig but also the desire of being upgradable to higher bit rates," he says.
The article first appeared as an OFC 2014 show preview piece