Lumentum on ROADM growth, ZR+, and 800G

CTO interview: Brandon Collings

• The ROADM market is experiencing a period of sustained growth  
• The Open ROADM MSA continues to advance and expand its scope
• ZR+ coherent modules will support some interoperability to avoid becoming siloed but optical performance differentiation remains key 

Source: Lumentum

Brandon Collings gave a Market Focus talk at the recent ECOC show in Dublin, where he explained why it is a good time to be in the reconfigurable optical add-drop multiplexer (ROADM) business. 

“Quantities are growing substantially and it is not one reason but a multitude of reasons,” says Collings. The CTO of Lumentum reckons the growth started some 18-24 months ago.  

ROADM markets 

Lumentum highlights three factors fuelling the demand for ROADM components.

The first is the emergence of markets such as China and India that previously did not use ROADMs.

“China has pretty universally adopted ROADMs going forward,” says Collings. Previously, Optical Transport Network (OTN) point-to-point links and large OTN switches have been used. But ongoing traffic growth means this solution alone is not sustainable, both in terms of the switch capacity and the number of optical transceivers required. 

“The bandwidth needed for these OTN switches is scaling beyond the rational use of optical-electrical-optical (OEO) node configuration,” says Collings. “You need 50 to 300 terabits of OTN [switch capacity] surrounded by the equivalent amount of optical transceivers, and that is not economical.”

The Chinese service providers have adopted a hybrid ROADM and OTN network architecture. The ROADMs perform optical bypass - passing on lightpaths destined for other nodes in the network - to reduce the optical transceivers and OTN switch capacity needed. 

The network operators in India, in contrast, are using ROADMs to cope with the many fibre cuts they experience. The ROADMs are used to restore the network by rerouting traffic around the faults.     

A second market magnifier is how modern ROADM networks use more wavelength-selective switches (WSSes). Both colourless and directionless (CD) ROADMs, and colourless, directionless and contentionless (CDC) ROADMs use more WSSes per node (see diagram above). 

Such ROADMs also use more advanced WSS designs. Using an MxN WSS for the multicast switch in a route-and-select CDC ROADM, for example, delivers system benefits especially when adding and dropping wider optical channels that are starting to be used. Collings says Lumentum’s own MxN WSS is now close to volume manufacturing.

The third factor fuelling ROADM growth is the ongoing demand for more capacity. “Every time you fill a fibre, you typically use another degree in your [ROADM] node and light up a second fibre to grow capacity,” says Collings.  

Operators with limited fibre are exploiting the fibre’s spectrum by using the C-band and L-band to grow capacity. This, too, requires more WSSes per node. 

“All of these growth factors are happening simultaneously,” says Collings.

Open ROADM MSA

Lumentum is also a member of the Open ROADM multi-source agreement (MSA) that has created a disaggregated design to enable interoperability between systems vendors’ ROADMs.

AT&T is deploying Open ROADM systems in its metro networks while the MSA members have begun work on Revision 6.0 of the standard. 

“Open ROADM is maturing and increasing its span of interest,” says Collings.

At first glance, Lumentum’s membership is surprising given it supplies ROADM building-blocks to vendors that make the ROADM systems. Moreover, the Open ROADM standard views a ROADM as an enclosed system. 

“The Open ROADM has set certain boundaries where it defines interfaces so that vendor A can talk to vendor B,” says Collings. “And it has set that boundary pretty much at the complete ROADM node.” 

Yet Lumentum is an MSA member because part of the software involved in controlling the ROADM is within the node. “It is not just a hardware solution, it is hardware and a significant software solution to supply into that,” says Collings. 

Pluggable optics is also a part of the Open ROADM MSA, another reason for Lumentum’s interest. “There is a general discussion about potentially making a boundary condition around pluggable optics as well,” he says.

Collings says the MSA continues to build the ecosystem and the management system to help others use Open ROADM, not just AT&T.

400ZR, OpenZR+ and ZR+

As a supplier of coherent optics and line-side modules, Lumentum is interested in the OIF’s 400ZR standard and what is referred to as ZR+. 

ZR+ offers an extended set of features and enhance optical performance. Both 400ZR and ZR+ will be implemented using QSFP-DD and OSFP pluggable modules.

The 400ZR specification has been developed for a specific purpose: to deliver 400 Gigabit Ethernet for distances of at least 80km for data centre interconnect applications. But 400ZR is not suited for more demanding metro mesh and longer-distance metro-regional applications. 

This is what ZR+ aims to address. However, ZR+, unlike 400ZR, is not a standard and is a broad term.

At ECOC, Acacia Communications and NTT Electronics detailed interoperability between their coherent DSPs using what they call ‘OpenZR+’. OpenZR+ uses Ethernet traffic like 400ZR but also supports the additional data rates of 100, 200 and 300 Gigabit Ethernet. OpenZR+ also borrows from the OpenROADM specification to enable module interoperability between vendors for data centre interconnect applications with reaches beyond 120km.

But ZR+ encompasses differentiated coherent designs that support 400 gigabits in a compact pluggable but also lower transmission rates that trade capacity for reach.

“So, yes, both classes of ‘ZR+’ are emerging,” says Collings. 

OpenZR+ seeks interoperability in compact pluggables, as well as higher power, higher performance modes less focused on interoperability, while ZR+ includes proprietary, higher-power solutions. “That [ZR+] is an area where distance and capacity equal money, in terms of savings and value,” says Collings. “That is going to be an area of differentiation, as it has always been for coherent interfaces.”

Collings favours some standardisation around ZR+, to enable interchangeability among module vendors and avoid the creation of a siloed market. 

“But I don’t think we are going to find ZR+ interfaces defined for interoperability because you will find yourself walking back on that differentiation in terms of value that the network operators are looking to extract,” says Collings. “They need every bit of distance they can get.”

Network operators want compact, cost-effective solutions that do ‘even more stuff’ than they are used to. “400ZR checks that box but for bigger, broader networks, operators want the same thing,” says Collings.

There is a continuum of possibilities here, he says: “It is high value from a network operator point of view and it’s a technology challenge for the likes of us and the [DSP] chip vendors.”

800G Pluggable MSA

Lumentum also recently joined the 800G Pluggable MSA that was announced at the CIOE show, held in Shenzhen in September.

“Like any client interface where Lumentum is a supplier of the underlying [laser] chips - whether DMLs, EMLs or VCSELs - we feel it is pretty important for us to be in the definition setting of the interface,” says Collings. “We want the interface to be aligned optimally to what the chip can do.”

Lumentum announced last year that it is exiting the client-side module business and therefore will be less involved in the module aspects of the interface work.   

“Having moved out of the [client-side] module business, we’re finding an awful lot of customers interested in engaging with us on the chip level, much more than before,” says Collings. 

Further information

For an Optical Connections article about OpenZR+, co-authored by Acacia, NTT Electronics, Lumentum, Juniper Networks and Fujitsu Optical Components, click here