Lumentum has started supplying customers with its CFP2-DCO coherent optical module. Operators use the pluggable to add an optical transport capability to equipment.
The company describes the CFP2-DCO as a workhorse; a multi-purpose pluggable for interface requirements ranging from connecting equipment in separate data centres to long-haul optical transmission.
Brandon CollingsThe module works at 100-, 200-, 300- and 400-gigabit line rates.
The pluggable also complies with the OpenROADM multi-source agreement. It thus supports the open Forward Error Correction (oFEC) standard, enabling interoperability with oFEC-compliant coherent modules from other vendors.
“Optical communications is getting more diverse and dynamic with the inclusion of the internet content providers (ICPs) alongside traditional telecom operators,” says Brandon Collings, CTO at Lumentum.
The CFP2-DCO module is being adopted by traditional network equipment makers and by the ICPs who favour more open networking.
CFP2-DCOs modules from vendors support the OIF’s 400ZR standard that links switching and routing equipment in data centres up to 120km apart and more demanding custom optical transmission performance requirements, referred to as ZR+.
So what differentiates Lumentum’s CFP2-DCO from other coherent module makers?
Kevin Affolter, Lumentum’s vice president, strategic marketing for transmission, highlights the company’s experience in making coherent modules using the CFP form factor. Lumentum also makes the indium phosphide optical components used for its modules.
“We are by far the leading vendor of CFP2-ACO modules and that will go on for several years yet,” says Affolter.
Unlike the CFP2-DCO that integrates the optics and the digital signal processor (DSP), the earlier generation CFP2-ACO module includes optics only, with the coherent DSP residing on the line card.
The company also offers a 200-gigabit CFP2-DCO that has been shipping for over 18 months.
As a multi-purpose design, Affolter says some customers want to use the CFP2-DCO primarily at 200 gigabits for its long-haul reach while others want the improved performance of the proprietary 400-gigabit mode and its support of Ethernet and OTN clients.
“Each of the [merchant] DSPs has subtly different features,” says Affolter. “Some of those features are important to protect applications, especially for some of the hyperscalers’ applications.”
Higher baud rates
Lumentum did not make any announcements at the recent OFC virtual conference and show regarding indium phosphide-based coherent components operating at the next symbol rate of 128 gigabaud (GBd). But Collings says work continues in its lab: “This is a direction we are all headed.”
The latest coherent optical components operate at 100GBd, making possible 800-gigabit-per-wavelength transmissions. Moving to a 128GBd symbol rate enables a greater reach for the given transmission speed as well as the prospect of 1.2+ terabit wavelengths.
This means fewer coherent modules are needed to send a given traffic capacity, saving costs. But moving to a higher baud rate does not improve overall spectral density since a higher baud rate signal requires a wider channel.
“We are encountering a fundamental limit set by mother nature around spectral efficiency,” says Collings.
Optical transmission technology continues to follow the familiar formula where the more challenging high-end, high-performance coherent systems start as a line-card technology and then, as it matures, transitions to a more compact pluggable format. This trend will continue, says Collings.
The industry goal remains to scale capacity and reduce the dollars-per-bit cost and that applies to high-end line cards and pluggables. This will be achieved using greater integration and increasing the current baud rate.
“Getting capacity up, driving dollars-per-bit down is now what the game is going to be about for a while,” says Collings.
Whether the industry will go significantly above 128GBd such as 256GBd remains to be seen as this is seen as a technically highly challenging task.
However, the industry continues to demand higher network capacity and lower cost-per-bit. So Collings sees a couple of possible approaches to continue satisfying this demand.
The first is to keep driving down the cost of the 128GBd generations of transceivers, satisfying lower cost-per-bit and expanding capacity by using more and more transceivers.
The second approach is to develop transceivers that integrate multiple optical carriers into a single ‘channel’. A channel here refers to a unit of optical spectrum managed through the ROADM network. This would increase capacity per transceiver and lower the cost-per-bit.
“Both approaches are technical and implementation challenges and it remains to be seen which, or both, will be realised across the industry,” says Collings.
100-gigabit PAM-4 directly modulated laser
At OFC Lumentum announced that its 100-gigabit PAM-4 directly modulated laser (DML), which is being used for 500m applications, now supports the 2km-reach FR single-channel and FR4 four-channel client-side module standards.
This is a normal progression of client-side modules for the data centre where the higher performance externally-modulated laser (EML) for a datacom transceiver is the one paving the way. As the technology matures, the EML is replaced by a DML which is cheaper and has simpler drive and control circuitry.
“We started this [trait] with the -LR4 which was dominated by EMLs,” says Mike Staskus, vice president, product line management, datacom at Lumentum. “The fundamental cost savings of a DML is its smaller chip size, more chips per wafer, and fewer processes, fewer regrowths.”
The company is working on a 200-gigabit EML and a next-generation 100-gigabit DML that promises to be lower cost and possibly uncooled.
Reconfigurable optical add-drop multiplexers (ROADMs)
Lumentum is working to expand its wavelength-selective switches (WSSes) to support the extended C-band, and C- and L-band options as a way to increase transmission capacity.
“We are expanding the overall ROADM portfolio to accommodate extended C-band and more efficient C-band and L-band opportunities to continue to build capacity into ROADM networks,” says Collings. “As spectral efficiency saturation sets in, we are going to need more amplified bandwidth and more fibres, and the C- and L-bands will double fibre capacity.”
The work includes colourless and directionless; colourless, directionless and contentionless, and higher-degree ROADM designs.