The quiet period of silicon photonics

Michael Hochberg discusses his book on silicon photonics and the status of the technology. Hochberg is director of R&D at Coriant's Advanced Technology Group. Previously he has been an Associate Professor at the University of Delaware and at the National University of Singapore. He was also a director at the Optoelectronic Systems Integration in Silicon (OpSIS) foundry, and was a co-founder of silicon photonics start-up, Luxtera.

Part 2: An R&D perspective

If you are going to write a book on silicon photonics, you might as well make it different. That is the goal of Michael Hochberg and co-author Lukas Chrostowski, who have published a book on the topic.

Michael HochbergHochberg says there is no shortage of excellent theoretical textbooks and titles that survey the latest silicon photonics research. Instead, the authors set themselves the goal of creating a design manual to help spur a new generation of designers.

The book aims to provide designers with all the necessary tools and know-how to develop silicon photonics circuits without needing to be specialists in optics.

“One of the limiting factors in terms of the growth and success of the field is how quickly can we breed up more and more designers,” says Hochberg.

The book - Silicon Photonics Design: From Devices to Systems - starts by exploring the main silicon photonics building blocks, from optical waveguides and grating couplers to modulators, photo-detectors and lasers. The book then addresses putting the parts together, with chapters on tools, fabrication, testing and packaging before finishing with system design examples. 

The numerical tools used in the book are mostly based on the finite-difference time-domain method, what the authors describe as the typical workhorse in silicon photonics design. Hochberg admits that the systems software tools, in contrast, are less mature: “It is a moving target that will change year to year.”

Myths 

Hochberg is also a co-author of a Nature Photonics’ paper, published in 2012, that debunks some of the myths regarding silicon photonics. “We wrote the myths paper after seeing an upswing in the ratio of hype-to-results going on,” says Hochberg.

He says part of the problem was that people were claiming silicon photonics was going to solve problems that it was plainly unsuited to address, for example integrating photonics with cutting-edge ultra-scale sub-micron electronics, for instance at 16 nm and 28 nm nodes. “That is not a practical solution for any near term problem,” says Hochberg.

More recent events, such as Intel’s announcement in February that it is delaying the commercial introduction of its silicon photonics products, highlights how bringing the technology to market is a significant engineering challenge.  Instead, we are in a quiet period for silicon photonics, he says.  Companies are getting into serious product mode, where they stop publishing and start focussing on building a product.

Moreover, these products - what he refers to as second-generation silicon photonics designs - are increasingly sophisticated with more functions or channels placed on the chip. “It is the standard story of almost any technology in silicon,” he says. “Silicon wins when you can do more stuff on a single chip.”

Silicon photonics and III-V 

Hochberg stresses that while it is an understandable desire, it is very hard to compare the performance of silicon photonics as a whole with traditional optical components using III-V compounds. The issue being that silicon photonics comprises many different platforms where designers have made tradeoffs. The same applies to III-V compounds where there are hundreds of processes aimed at thousands of different products.  “It is very hard to compare them in a generic way,” he says.

“The great advantage silicon photonics gives you is access to first-rate fabrication infrastructure,” says Hochberg. Silicon photonics offers 8- and 12-inch wafers, high volume foundries, high process control, the ability to ramp to high volumes and achieve high yields of complex-structure designs with hundreds, even thousands of components on-chip.  

In contrast, III-V materials such as indium phosphide and gallium arsenide offer higher mobilities - electrons and holes move faster - and, unlike silicon, can straightforwardly emit light.

“The downside is that III-V foundries use technology processes that silicon stopped using 20 to 30 years ago,” says Hochberg. Wafers that are 2-, 3- or 4-inch in diameter, lithography that is ten times coarser than is used for silicon, process controls that are less advanced, and less automation. 

If you are going to design a complex chip with lots of different components that require a predictable relationship with each other, this is where silicon tends to beat III-Vs, he says.

But the claim of large silicon wafers and huge volumes is what silicon photonics proponents have been promoting for years, and which has fed some of the false expectation associated with the emerging technology, says one industry analyst. 

Hochberg counters by highlighting two trends that play in silicon photonics’ favour.

One is the well-known one of optics slowly replacing copper. This has been going on for 40 to 50 years, he says, in long haul, then in metro and now linking equipment in the data centre. “This will continue for shorter and shorter distances and then, at some point, stop,” he says. That said, Hochberg stresses that there are other applications for silicon photonics besides data communications.

“Just because you run out of opportunities at shorter and shorter reach at some point in the distant future, doesn't mean that the field collapses,” he says. “There's a lot of other cool stuff being done in silicon photonics these days with serious commercial potential.” Example applications include medical and remote sensing.

Once you can do something in silicon and do it adequately well, it tends to displace everything else from the majority of the market

The second trend he highlights is that silicon ends up dominating fields, not necessarily because it is the best choice in terms of performance but because it ends up being so cheap in scale. “Once you can do something in silicon and do it adequately well, it tends to displace everything else from the majority of the market.”

There are up-front costs of getting silicon photonics into a CMOS fab so companies have to be judicious in choosing the applications they tackle. “But once the infrastructure gets going to make a new application, the speed with which the industry can scale is just mind-blowing,” he said.

At Coriant, Hochberg leads a team that is doing advanced R&D. “We are doing advanced research with the goal to develop new technology that may eventually make its way into product.”

Does that include silicon photonics? “There is certainly an interest in silicon photonics; it is one of the things we are exploring,” says Hochberg.  

Further reading:

Book: Michael Hochberg and Lukas Chrostowski, Silicon Photonics Design: From Devices to Systems, Cambridge University Press, 2015

Paper:  Myths and rumours of silicon photonics, Nature Photonics, Vol 6, April 2012.