Finisar demonstrates its first silicon photonics transceiver

• Finisar unveiled its first silicon photonics-based product, a 400-gigabit QSFP-DD DR4 module, at the recent ECOC event.

• The company also showed transceiver technology that simplifies the setting up of dense wavelength-division multiplexing (DWDM) links.

• Two 200-gigabit QSFP56 client-side modules and an extended reach 30km 400-gigabit eLR8 were also demonstrated by Finisar. 

• A 64-gigabaud integrated tunable transmitter and receiver assembly (ITTRA) was used to send a 400-gigabit coherent wavelength.  

Finisar is bringing to market its first silicon photonics-based optical module. 

Christian UrricarietThe 400GBASE-DR4 is an IEEE 500m-reach 400-gigabit parallel fibre standard based on four fibres, each carrying a 100-gigabit 4-level pulse amplitude modulation (PAM-4) signal. Finisar’s DR4 is integrated into a QSFP-DD module. 

“The DR4 is the 400-gigabit interface that most of the hyperscale cloud players are interested in first,” says Christian Urricariet, senior director of global marketing at Finisar.

The company demonstrated the module at the recent European Conference on Optical Communication (ECOC), held in Rome.   

Silicon photonics-based DR4

The DR4 is an integrated design, says Finisar, comprising modulators and photo-detectors as well as modulator drivers and the trans-impedance amplifiers (TIAs).  

Finisar chose silicon photonics for the DR4 after undertaking an extensive technology study. Silicon photonics emerged as ‘a clear winner’ in terms of cost and performance for photonic designs made up of similar functions in parallel, such as the four-channel DR4. Silicon photonics manufacturing is also scalable, making it ideal for high-volume designs.

The DR4 is the 400-gigabit interface that most of the hyperscale cloud players are interested in first

The DR4 can also be used in a breakout mode to interface to four 100GBASE-DR modules. Also referred to as the DR1, the 100GBASE-DR fits within an SFP-DD or a QSFP28 module. 

The DR4-DR1 combination can link four servers, each using a 100-gigabit link, to a 400-gigabit port on a top-of-rack or mid-row switch. The top-of-rack 400-gigabit DR4 can also connect to a leaf switch with multiple 100-gigabit ports. “The DR4 can be used ‘top-of-rack down’ [to servers] or ‘top-of-rack up’ [to leaf switches],” says Urricariet. “This is similar to what people are doing with the [100-gigabit parallel fibre] PSM4.”

400-gigabit eLR8

Finisar also showcased an extended reach version of the IEEE 400GBASE-LR8 standard.

Dubbed the eLR8, the QSFP-DD module is a technology demonstrator not a product that extends the reach of the LR8 from 10km to 30km.  

Finisar already has an LR8 product in a CFP8 pluggable module and is moving the design to the smaller QSFP-DD. The LR8 is an eight-wavelength duplex interface where each wavelength carries a 50-gigabit PAM-4 signal. 

“The 400GBASE-LR8 is a low-risk approach to achieving a 400-gigabit duplex single-mode link in the short term,” says Urricariet. “You don’t have to wait for 100-gigabit PAM-4 [ICs] to be manufactured in high volume.”

Urricariet says the IEEE is considering developing an extended LR8 standard with a 40km reach but such distances could also be addressed using inexpensive coherent technology. 

Finisar’s design achieves the extended range using the same components as its LR8 module - directly modulated DFB lasers and PIN photodetectors. “There is plenty of margin with that [LR8 design],” says Urricariet. This suggests Finisar picked the best performing DFBs and PINs for the eLR8 design.

The QSFP-DD 10km LR8 design is sampling now, with general availability from the first half of 2019. 

Flextune

Configuring DWDM links can be likened to two groups of people separated in a wood at night. Each individual has a flashlight and is tasked with finding a counterpart from the second group, a process repeated until everyone is paired.

Setting up DWDM links is comparable to telling each individual the exact path to take to find their counterpart. The Flextune technology that Finisar has developed can be viewed as giving each individual the confidence to stride out - sweeping their flashlights as they go - till they find a counterpart.

Currently, setting up a DWDM link requires coordination between a field engineer and network operations staff. Each tunable transceiver that is plugged into a port is told which wavelength to tune to. The system itself may tell the transceiver the wavelength to use or a field engineer programs each transceiver before it is plugged into the platform. 

Equally, the transceiver output fibre must be connected to the right optical multiplexer and demultiplexer (mux-demux) port, as do the transceivers at the link’s other end. 

The result is a time-consuming process that is prone to human error.   

With Flextune, the tunable transceivers are plugged into the equipment’s ports and connected to the mux-demux’s ports. “It does not matter which port,” says Urricariet. “The transceivers search for each other and self-configure to the right wavelength.”

Each Flextune-enabled transceiver operates independently of the transceiver at the other end; there is no master-slave arrangement, says Urricariet, although a master-slave arrangement can be used if requested. 

The mux-demux must also be a blocking architecture for Flextune to work. “If the mux-demux does not block the other wavelengths on each port, then you have a mess,” says Urricariet. With such a mux-demux, the channels scanned are blocked until the transceiver’s output is passed to the right channel. Once the link is established, the two transceivers set permanently to that wavelength. 

“It [the process] happens at both ends simultaneously and on all the ports,” says Urricariet. “The basic technique can self-tune up to 96 [DWDM] channels in around five minutes.”    

Being able to tune independently of the host equipment means that the Flextune-enabled transceivers can also be sold directly to operators and plugged into any of their equipment.   

Urricariet says Flextune promises welcome operational savings given DWDM’s increasing adoption in the access network with developments such as 5G fronthaul. 

The basic technique can self-tune up to 96 [DWDM] channels in around five minutes    

Flextune will also be used for metro and data centre interconnect applications, as well as connecting Remote PHY nodes being deployed in cable networks. “The Remote PHY is also a big focus for this type of feature,” says Urricariet.

Finisar demonstrated Flextune with its 10-gigabit tunable SFP+ modules that are now sampling. Flextune will also be adopted for its 25-gigabit SFP+ that will  sample ‘very soon’, followed by coherent modules.  

“We do have a CFP2-ACO module in production and other coherent products on our roadmap,” says Urricariet. “We will be looking to implement Flextune technology in these products as well.” 

Google has started deployments of 2x200GbE

200 Gigabit Ethernet: a growing interim solution 

Finisar also demonstrated two 200-gigabit modules. The QSFP56 implements the 2km FR4 specification. The 200-gigabit FR4 uses four coarse WDM (CWDM) wavelengths, each carrying a 50-gigabit PAM-4 signal.

Finisar has previously said it will develop 200-gigabit modules for the large-scale data centres interested in the technology as an interim solution before 400-gigabit modules ramp. Such an intermediate market for “one hyperscaler and maybe two” is sufficient to justify making 200-gigabit modules, says Urricariet.

Market research firm LightCounting has increased its forecast for 200 Gigabit Ethernet (GbE) modules due to interest from Facebook. 

A presentation by Facebook at ECOC suggested that 400 GbE is far from being ready, says Vladimir Kozlov, CEO of LightCounting. “It looks like 200GbE is being considered now, but Facebook may change its mind again,” says Kozlov. “In the meantime, Google has started deployments of 2x200GbE [in an OSFP module] as planned.”

As with the 400-gigabit eLR8, Finisar also demonstrated an extended reach version of the 200-gigabit FR4 to achieve a 10km reach. “This is not to be confused with the 10km 200-gigabit LR4 that is a LAN-WDM grid based design,” says Urricariet. “The extended FR4 uses a CWDM grid.” 

ITTRA

At OFC 2018 in March, Finisar unveiled its 32-gigabaud (Gbaud) integrated tunable transmitter and receiver assembly (ITTRA) that combines the optics and electronics required for an analogue coherent optics interface. 

The ITTRA comprises a tunable laser, an optical amplifier, modulators, modulator drivers, coherent mixers, a photo-detector array and the accompanying TIAs. All the components of the 32Gbaud ITTRA are integrated within a gold box that is 70 percent smaller than the size of a CFP2 module. The integrated assembly also has a power consumption below 7.5W.

At ECOC, the company demonstrated its second ITTRA design operating at 64Gbaud to transmit a 400-gigabit wavelength using 16-ary quadrature amplitude modulation (16-QAM). Finisar would not detail the power consumption of the 64Gbaud ITTRA. 

“The doubling of the speed to 64Gbaud will enable 400-gigabit DCO modules as well as 400ZR,” says Urricariet. Digital coherent optics (DCO) refers to coherent modules that integrate the optics and the coherent digital signal processor (DSP). 

Samples and production of the 64Gbaud ITTRA are due in 2019.