Oclaro demonstrates flexible rate coherent pluggable module

• The CFP2 coherent optical module operates at 100 and 200 Gig
• Samples are already with customers, with general availability in the first half of 2015
• Oclaro to also make more CFP2 100GBASE-LR4 products

The CFP2 is not just used in metro/ regional networks but also in long-haul applications

Robert Blum

The advent of a pluggable CFP2, capable of multi-rate long-distance optical transmission, has moved a step closer with a demonstration by Oclaro. The optical transmission specialist showed a CFP2 transmitting data at 200 Gigabits-per-second.

The coherent analogue module demonstration, where the DSP-ASIC resides alongside rather than within the CFP2, took place at ECOC 2014 held in September at Cannes. Oclaro showcased the CFP2 to potential customers in March, at OFC 2014, but then the line side module supported 100 Gig only.

"What has been somewhat surprising to us is that the CFP2 is not just used in metro/ regional networks but also in long-haul applications," says Robert Blum, director of strategic marketing at Oclaro. "We are also seeing quite significant interest in data centre interconnect, where you want to get 400 Gig between sites using two CFP2s and two DSPs." Oclaro says that the typical distances are from 200km to 1,000km.

The CFP2 achieves 200 Gig using polarisation multiplexing, 16-quadrature amplitude modulation  (PM-16-QAM) while working alongside ClariPhy's merchant DSP-ASIC. ClariPhy announced at ECOC that it is now shipping its 200 Gig LightSpeed-II CL20010 coherent system-on-chip, implemented using a 28nm CMOS process.

"One of the beauties of an analogue CFP2 is that it works with a variety of DSPs," says Blum. Other merchant coherent DSPs are becoming available, while leading long-haul optical equipment vendors have their own custom coherent DSPs.

Oclaro's CFP2, even when operating at 200 Gig, falls within the 12W module's power rating. "One of the things you need to have for 200 Gig is a linear modulator driver, and such drivers consume slightly more power [200mW] than limiting modulator drivers [used for 100 Gig only]," says Blum.

Oclaro will offer two CFP2 line-side variants, one with linear drivers and one using limiting ones. The limiting driver CFP2 will be used for 100 Gig only whereas the linear driver CFP2 supports 100 Gig PM-QPSK and 200 Gig PM-16-QAM schemes. "Some customers prefer the simplicity of a limiting interface; for the linear interface you have to do more calibration or set-up," says Blum. "Linear also allows you to do pre-emphasis of the signal path, from the DSP all the way to the modulator." Pre-emphasis is used to compensate for signal path impairments.

By consuming under 12W, up to eight line-side CFP2 interfaces can fit on a line card, says Blum, who also stresses the CFP2 has a 0dBm output power at 200 Gig. Achieving such an output power level means the 200 Gig signal is on a par with 100 Gig wavelengths. "When you launch a 200 Gig signal, you want to make sure that there is not a big difference between signals," says Blum.

To achieve the higher output power, the micro integrable tunable laser assembly (micro-iTLA) includes a semiconductor optical amplifier (SOA) with the laser, while SOAs are also added to the Mach–Zehnder modulator chip. "That allows us to compensate for some of the [optical] losses," says Blum.

Customers received first CFP2 samples in May, with the module currently at the design validation stage. Oclaro expects volume shipments to begin in the first half of 2015.

100 Gig and the data centre

Oclaro also announced at ECOC that it has expanded manufacturing capacity for its CFP2-based 100GBASE-LR4 10km-reach module.

One reason for the flurry of activity around 100 Gig mid-reach interfaces that span 500m-2km in the data centre is that the 100GBASE-LR4 module is relatively expensive. Oclaro itself has said it will support the PSM-4, CWDM4 and CLR4 Alliance mid-reach 100 Gig interfaces. So why is Oclaro expanding manufacturing of its CFP2-based 100GBASE-LR4?

It is about being pragmatic and finding the most cost-effective solution for a given problem

"There is no clear good solution to get 100 Gig over 500m or 2km right now," says Blum. "CFP2 is here, it is a mature technology and we have made improvements both in performance and cost."

Oclaro has improved its EML design such that the laser needs less cooling, reducing overall power dissipation. The accompanying electronic functions such as clock data recovery have also been redesigned using one IC instead of two such that the CFP2 -LR4's overall power consumption is below 8W.   

Demand has been so significant, says Blum, that the company has been unable to meet customer demand. Oclaro expects that towards year-end, it will have increased its CFP2 100GBASE-LR4 manufacturing capacity by 50 percent compared to six months earlier.   

"It is about being pragmatic and finding the most cost-effective solution for a given problem," says Blum. "There are other [module] variants that are of interest [to us], such as the CWDM4 MSA that offers a cost-effective way to get to 2km."