IBM demos a 100 Gigabit silicon photonics transceiver

IBM has demonstrated a 100 gigabit transceiver using silicon photonics technology, its most complex design unveiled to date. The 100 gigabit design is not a product but a technology demonstrator, and IBM says it will not offer branded transceivers to the marketplace.

“It is a demonstration vehicle illustrating the complex design capabilities of the technology and the functionality of the optical and electrical components,” says Will Green, manager of IBM’s silicon integrated nano-photonics group. 

Will Green

IBM has been developing silicon photonics technology for over a decade, starting with building-block optical functions based on silicon, to its current monolithic system-on-chip technology that includes design tools, testing and packaging technologies.

Now this technology is nearing commercialisation.   

“We do plan to have the technology available for use within IBM’s systems but also within the larger market; large-volume applications such as the data centre and hyper-scale data centres in particular,” says Green. 

IBM is already working with companies developing their own optical component designs using its technology and design tools. “These are tools that circuit designers are familiar with, such that they do not need to have an in-depth knowledge of photonics in order to build, for example, an optical transceiver,” says Green.  

We do plan to have the technology available for use within IBM’s systems but also within the larger market

100 gig demonstrator

IBM refers to its silicon photonics technology as CMOS-integrated nano-photonics. CMOS-integrated refers to the technology’s monolithic nature that combines CMOS electronics with photonics on one substrate. Nano-photonics highlights the dimensions of the feature sizes used.   

IBM is rare among the silicon photonics community in combining electronics and photonics on one chip; other players implement photonics and electronics on separate dies before combining the two. What is not included is the laser which is externally attached using fibre.

The platform supports 25 gigabit speeds as well as wavelength division multiplexing. Originally, IBM started with 90 nm CMOS using bulk silicon before transferring to a silicon-on-insulator (SOI) substrate. An SOI wafer is ideal for creating optical waveguides that confine light using the large refractive index difference between silicon and silicon dioxide. However, to make the electrical devices run at 25 gigabit, the resulting transistor gate length ended up being closer to a 65 nm CMOS process.   

Source: IBM Corporation.

IBM's optical waveguides are sub-micron, having dimensions of a few hundred nanometers. This is the middle ground, says Green, trading off the density of smaller-dimensioned waveguides with larger, micron-plus ones that deliver low propagation loss.   

Also used are sub-wavelength optical 'metamaterial' structures that transition between the refractive index of the fibre and that of the optical waveguide to achieve a good match between the two. “These very tiny sub-wavelength structures are made using lithography near the limits of what is available,” says Green. “We are engineering the optical properties of the waveguide in order to achieve a low insertion loss when bringing the fibre onto the chip.”  The single mode fibre to the chip is attached using passive alignment.

The 100 gigabit transceiver demonstrator uses four 25 gigabit coarse wavelengths around 1310 nm.  The technology is suited to implement the CWDM4 MSA.

The whole technology is available to be commercialised by any chip manufacturer

“We are working with four wavelengths today but in the same way as telecom uses many wavelengths, we can follow a similar path,” says Green.

The chip design features transmitter electronics - a series of amplifiers that boost the voltage to drive the Mach-Zehnder Interferometer modulators - and a multiplexer to combine the four wavelengths onto the fibre while the receiver circuitry includes a demultiplexer, four photo-detectors and trans-impedance amplifiers and limiting amplifiers, says Green. What is lacking to make the 100 gigabit transceiver functional is a micro-controller, feedback loops to control the temperature of key circuits, and the circuitry to interface to standard electrical input/ output.  

Green highlights how the bill of materials of a chip is only a fraction of the total cost since assembly and testing must also be included.  

“We reduce the cost of assembly through automated passive optical alignment and the introduction of custom structures onto the wafer,” he says. “We believe we can make an impact on the cost structure of the optical transceiver and where this technology needs to be to access the data centre.” IBM has also developed a way to test the transceiver chips at the wafer level. 

Green admits that its CMOS-integrated nanophotonics process will not scale beyond 25 gigabit as the 90-65 nm CMOS is not able to implement faster serial rates. But IBM has already shown an optical implementation of the PAM-4 modulation scheme that doubles a link's rate to 50 gigabit.    

Meanwhile, IBM’s process design kit (PDK) is already with customers. A PDK includes documents and data files that describe the fabrication process and enable a user to complete a design. A PDK includes a fab’s process parameters, mask layout instructions, and the library of silicon photonics components; grating couplers, waveguides, modulators and the like [1].  

“They [customers] have used the design kit provided by IBM but have built their own designs,” says Green. “And now they are testing hardware.”

IBM is keen that its silicon photonics technology will be licensed and used by circuit design houses. "Houses that bring their own IP [intellectual property], use the enablement tools and manufacture at a site that is licensing the technology from IBM,” says Green. "The whole technology is available to be commercialised by any chip manufacturer.”

Reference

[1] Silicon Photonics Design: From Devices to Systems, Lukas Chrostowski and Michael Hochberg, Cambridge University Press, 2015. Click here