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Optical module trends: A conversation with Finisar  

Finisar demonstrated recently a raft of new products that address emerging optical module developments. These include: 

  • A compact coherent integrated tunable transmitter and receiver assembly 
  • 400GBASE-FR8 and -LR8 QSFP-DD pluggable modules and a QSFP-DD active optical cable 
  • A QSFP28 100-gigabit serial FR interface 
  • 50-gigabit SFP56 SR and LR modules

Rafik Ward, Finisar’s general manager of optical interconnects, explains the technologies and their uses.


Compact coherent 

Finisar is sampling a compact integrated assembly that supports 100-gigabit and 200-gigabit coherent transmission. 

The integrated tunable transmitter and receiver assembly (ITTRA), to give it its full title, includes the optics and electronics needed for an analogue coherent optics interface. 

The 32-gigabaud ITTRA includes a tunable laser, optical amplifier, modulators, modulator drivers, coherent mixers, a photo-detector array and the accompanying trans-impedance amplifiers, all within a gold box. “An entire analogue coherent module in a footprint that is 70 percent smaller than the size of a CFP2 module,” says Ward. The ITTRA's power consumption is below 7.5W.

Rafik WardFinisar says the ITTRA is smaller than the equivalent integrated coherent transmitter-receiver optical sub-assembly (IC-TROSA) design being developed by the Optical Internetworking Forum (OIF).

“We potentially could take this device and enable it to work in that [IC-TROSA] footprint,” says Ward.

Using the ITTRA enables higher-density coherent line cards and frees up space within an optical module for the coherent digital signal processor (DSP) for a CFP2 Digital Coherent Optics (CFP2-DCO) design.        

Ward says the CFP2 is a candidate for a 400-gigabit coherent pluggable module along with the QSFP-DD and OSFP form factors. “All have their pros and cons based on such fundamental things as the size of the form factor and power dissipation,” says Ward. 

But given coherent DSPs implemented in 7nm CMOS required for 400 gigabit are not yet available, the 100 and 200-gigabit CFP2 remains the module of choice for coherent pluggable interfaces. 


The demonstration of the ITTRA implementing a 200-gigabit link using 16-QAM at OFC 2018. Source: Finisar


400 gigabits 

Finisar also demonstrated its first 400-gigabit QSFP-DD pluggable module products based on the IEEE standards: the 2km 400GBASE-FR8 and the 10km 400GBASE-LR8. The company also unveiled a QSFP-DD active optical cable to link equipment up to 70m apart.

The two QSFP-DD pluggable modules use eight 50-gigabit PAM-4 electrical signal inputs that are modulated onto eight lasers whose outputs are multiplexed and sent over a single fibre. Finisar chose to implement the IEEE standards as its first QSFP-DD products as they are low-power and lower risk 400-gigabit solutions.

The alternative 2km 400-gigabit design, developed by the 100 Lambda MSA, is the 400G-FR4 that uses four 100-gigabit optical lanes. “This has some risk elements to it such as the [PAM-4] DSP and making 100-gigabit serial lambdas work,” says Ward. “We think the -LR8 and -FR8 are complementary and could enable a fast time-to-market for people looking at these kinds of interfaces.”

The QSFP-DD active optical cable may have a reach of 70m but typical connections are 20m. Finisar uses its VCSEL technology to implement the 400-gigabit interface. At the OFC show in March, Finisar demonstrated the cable working with a Cisco high-density port count 1 rack-unit switch.     


I sometimes get asked by customers what is the best way to get to higher-density 100 gigabit. I point to the 400-gigabit DR4. 



Finisar also showed it 2km QSFP28 optical module with a single wavelength 100-gigabit PAM-4 output. The QSFP28 FR takes four 25 gigabit-per-second electrical interfaces and passes them through a gearbox chip to form a 50-gigabaud PAM-4 signal that is used to modulate the laser. 

The QSFP28 FR is expected to eventually replace the CWDM4 that uses four 25-gigabit wavelengths multiplexed onto a single fibre. “The end-game is to get a 100-gigabit serial module,” says Ward. “This module represents the first generation of that.”

Finisar is also planning a 500m QSFP28 DR. The QSFP28 DR and FR will work with the 500m IEEE 400GBASE-DR4 that has four outputs, each a fibre carrying a 100-gigabit PAM-4 signal, with the -DR4 outputs interfacing with up to four FR or DR modules.

“I sometimes get asked by customers what is the best way to get to higher-density 100 gigabit,” says Ward. “I point to the 400 gigabit DR4, even though we call it a 400-gigabit part, it is also a 4x100-gigabit DR solution.”

Ward says that the 500m reach of the DR is sufficient for the vast majority of links in the data centre. 


SFP56 SR and LR

Finisar has also demonstrated two SFP56 modules: a short reach (SR) version that has a reach of 100m over OM4 multi-mode fibre and the 10km LR single-mode interface. The SR is VSCEL-based while the LR uses a directly-modulated distributed feedback laser.

The SFP is deployed widely at speeds up to and including 10 gigabits while the 25-gigabit SFP shipments are starting to ramp. The SFP56 is the next-generation SFP module with a 50-gigabit electrical input and a 50-gigabit PAM-4 optical output.

The SFP56 will be used for several applications, says Finisar. These include linking servers to switches, connecting switches in enterprise applications, and 5G wireless applications.

Finisar says its 50 and 100 gigabit-per-lane products will likely be released throughout 2019, in line with the industry. “The 8-channel devices will likely come out at least a few quarters before the 4-channel devices,” says Ward.

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