Kotura demonstrates a 100 Gigabit QSFP

Kotura has announced a 100 Gigabit QSFP with a reach of 2km.  

“QSFP will be the long-term winner at 100 Gig; the same way QSFP has been a high volume winner at 40 Gig”

Arlon Martin, Kotura

The device is aimed at plugging the gap between vertical-cavity surface-emitting laser (VCSEL) -based 100GBASE-SR10 designs that have span 100m, and the CFP-based 100GBASE-LR4 that has a 10km reach.

“It is aimed at the intermediate space, which the IEEE is looking at a new standard for," says Arlon Martin, vice president of marketing at Kotura.

The device is similar to Luxtera's 100 Gigabit-per-second (Gbps) QSFP, also detailed at the OFC/NFOEC 2013 exhibition, and is targeting the same switch applications in the data centre. “Where we differ is our ability to do wavelength-division multiplexing (WDM) on a chip,” says Martin. Kotura also uses third-party electronics such as laser drivers and transimpedance amplifiers (TIA) whereas Luxtera develops and integrates its own.

The Kotura QSFP uses four wavelengths, each at 25Gbps, that operate around 1550nm. “We picked 1550nm because that is where a lot of the WDM applications are," says Martin. “There are also some customers that want more than four channels.” The company says it is also doing development work at 1310nm.

Although Kotura's implementation doesn't adhere to an IEEE standard - the standard is still work in progress - Martin points out that the 10x10 MSA is also not an IEEE standard, yet is probably the best selling client-side 100Gbps interface.

Optical component and module vendors including Avago Technologies, Finisar, Oclaro, Oplink, Fujitsu Optical Components and NeoPhotonics all announced CFP2 module products at OFC/NFOEC 2013. The CFP2 is the next pluggable form factor on the CFP MSA roadmap and is approximately half the size of the CFP.

The advent of the CFP2 enables eight 100Gbps pluggable modules on a system's front panel compared to four CFPs. But with the QSFP, up to 24 modules can be fitted while 48 are possible when mounted double sidedly - ’belly-to-belly’ - across the panel. “QSFP will be the long-term winner at 100 Gig; the same way QSFP has been a high volume winner at 40 Gig,” says Martin.

The QSFP uses 28Gbps pins, which is also called the QSFP28, but Kotura refers to it 100Gbps product as a QSFP. The design consumes 3.5W and uses two silicon photonic chips. Kotura says 80 percent of the total power consumption is due to the electronics.

One of the two chips is the silicon transmitter which houses the platform for the four lasers (gain chips) combined as a four-channel array. Each is an external cavity laser where part of the cavity is within the indium phosphide device and the rest in the silicon photonics waveguide. The gain chips are flip-chipped onto the silicon. The transmitter also includes a grating that sets each laser's wavelength, four modulators, and a WDM multiplexer to combine the four wavelengths before transmission on the fibre.

 Kotura's 4x25 Gig transmitter and receiver chips. Source: Kotura
The receiver chip uses a four-channel demultiplexer with each channel fed to a germanium photo-detector. Two chips are used as it is easier to package each as a transmitter optical sub-assembly (TOSA) or receiver optical sub-assembly (ROSA), says Martin.  The 100Gbps QSFP will be generally available in 2014. 

Disruptive system design

The recent Compass-EOS IP router announcement is a welcome development, says Kotura, as it brings the optics inside the system - an example of mid-board optics - as opposed to the front panel. Compass-EOS refers to its novel icPhotonics chip combining a router chip and optics as silicon photonics but in practice it is an integrated optics design. The 168 VCSELs and 168 photodetectors per chip is massively parallel interconnect, says Martin.

“The advantage, from our point of view of silicon photonics, is to do WDM on the same fibre in order to reduce the amount of cabling and interconnect needed,” he says. At 100 Gigabit this reduces the cabling by a factor of four and this will grow with more 25Gbps wavelength channels used to 10x or even 40x eventually.

“What we want to do is transition from the electronics to the optical domain as close to those large switching chips as possible,” says Martin. “Pioneers [like Compass-EOS] demonstrating that style of architecture are to be welcomed."

Kotura says that every company that is building large switching and routing ASICs is looking at various interface options. "We have talked to quite a few of them,” says Martin.

One solution suited to silicon photonics is to place the lasers on the front panel while putting the modulation, detection and WDM devices - packaged using silicon photonics - right next to the ASICs. This way the laser works at the cooler room temperature while the rest of the circuitry can be at the temperature of the chip, says Martin.