Silicon Nanophotonics

4/10/2013 9:07:33 AM

Fiber optic connections enable high-speed connections inside silicon chips, and they could be available in the not too distant future too

Fiber optics have had a major impact on TV broadcasts, broadband speeds and even hi-fi cables, but soon we could also be effectively putting them inside silicon chips. Bert Jan Offrein, manager of the photonics research group at IBM’s Zurich lab, and his colleagues, have built a working optical electrical interface, commonly known as nanophotonics, directly in silicon.

In basic terms, future chips could feature tiny optical interconnects rather than the standard, metal electrical connections used today, resulting in much more bandwidth. In fact, Offrein’s team has made a variety of nanophotonics components - wavelength division multiplexers, modulators, detectors-all integrated with standard CMOS circuitry.

Offrein and I sit down to discuss this breakthrough, the result of a decade of research, and he immediately fingers copper as the major bottleneck in developing high speed computers. 'As you scale a system’s performance, you have to scale it at all levels,’ he says. 'That means the processor, the memory and the bandwidth-everything. We’re close to the limits of what we can do using electrical interconnects.’

IBM s new CMOS Integrated Silicon Nanophotonics chip technology integrates electrical and optical devices on the same piece of silicon

There’s only so much you can do to boost the bandwidth of electrical interconnects. You can increase the data rate, but the losses become larger. You can put in more lines, but then you increase crosstalk (interference).

The problem is that propagation delay -the ratio of the distance a signal has to travel, and the speed at which the signal travels through a particular medium - in copper wires is fixed by the physical properties of the metal. The speed of a signal moving through copper is somewhere around two thirds of the speed of light, give or take a little. It isn’t exactly slow, but it isn’t a patch on optical fibers, where everything moves at light speed.

Everything ought to be running over optical fiber, he argues, so why do we still depend on wires7 That’s simple: copper is so conductive that, when we started playing with electricity, it was the best possible material to use, and that was still the case when we started making electrons dance around inside computers. Of course, it has weaknesses, such as the signal strength fading with distance, and its irritating sensitivity to heat, but we’ve found ways to accommodate and design around such quirks.

'Optics is much better: you use one fiber over very large distances. With increases in bandwidth requirements, it makes sense to use optics over shorter distances, and we’re at the point where this is the case. But it’s a lot of hassle to implement an optical system,’ says Offrein, 'because you have lots of electrical data. You have to convert it at every stage, so you need lots of components and fibers.’ And that’s bulky and expensive.

Cross-sectional view of a90nm IBM silicon nanophotonics chip, which combines optical and electrical circuits

Now, IBM’s Zurich lab has succeeded in integrating the optical connections directly in silicon. At a stroke, you get rid of all that extra processing, raise the density and lower the cost of optical connections. What’s more, this isn’t contained in a research lab any more.

By making a few additions to the production process, the 90nm chip has been made in a proper industrial fab, with a standard process.

The trick, he tells us, was realizing that, because silicon is transparent to infrared light, you can get to work making a waveguide on a chip. 'In a silicon layer, we can etch to get a very thin stripe-sub-micrometer. Then, because silicon has a high refractive index, we can transmit light through that stripe,’ he says.

Offrein says the resulting silicon photonics chip performs all required functions except light generation. For that job, external lasers are assembled onto the silicon chip and the light is coupled into the silicon waveguides. The conversion of an electrical signal to an optical signal is performed by electro-optical modulators in the silicon, a step that allows an astonishing flow of data from the chip into the optical fiber. 'The laser is continuously generating light and the modulator acts as a switch, transmitting and blocking light depending on the electrical input,’ he adds.

'What’s special in I BM’s technology is that the electrical functions required to drive the modulator are integrated into the same silicon chip together with the photonics. Each wavelength is modulated with a 25Gb/sec signal, by combining several wavelengths using a wavelength division multiplexer (WDM), and the aggregate bandwidth of the transceiver increases accordingly. For example, it would increase to 100Gb/secin the case of four wavelengths.’

And the amount of data these chips handle could, in principle, go higher. Offrein explains that the team is currently limiting the chip to either four, eight or 12 wavelengths (300Gb/ sec), so there’s enough distance between each wavelength to avoid confusion. Variables such as temperature changes can affect the sensitivity of the instrument,’ he points out, as 'wavelength can drift with temperature - and you don’t want that to affect your data’.

In the receiver, the WDM signals are split apart using a WDM Demultiplexer and then converted back to electrical signals using integrated detectors based on germanium. The small electrical signals originating from the detectors are then amplified using electronics integrated into the same chip.

But why only 90nm? It turns out that the size of the waveguide is determined by the refractive index of silicon and glass. With a single mode and one state of light, this total device is in the order of half a micron. With a part of the device being so large, relative to the process size, there’s no benefit in going smaller, and 90nm costs are lower than 22nm.

The result is single-chip optical communications transceivers that can be conventionally manufactured, and Offrein isn’t shy about hinting at commercial application in the not too distant future. The short-term goal is to be able to move large volumes of data quickly between chips in servers, data centers and supercomputers. At the highest possible end, Offrein mentions that the technology could be applied to the signal processing workload in the Dome project; part of the detailed planning for the international Square Kilometer Array radio telescope.

On its completion in 2024, the SKA will be the largest and most sensitive radio telescope on Earth, and astronomers will use it to peer back 13 billion years to the early days of the universe, where they hope to observe the evolution of early galaxies. The quantity of data it will gather is so large that new techniques will be needed to process it without blowing the power budget of a small nation. Dome is a five-year collaboration with the Netherlands Institute for Radio Astronomy to develop the high-speed computing systems for this job.

Inside a silicon photonics chip-the blue optical waveguides transmit high-speed optical signaIs, while the yellow copper wires carry high-speed electrical signals

'The technology clearly has potential to scale, because it’s silicon,’ he adds. 'It will find uses in high-end applications and low-end applications, such as commodity consumer apps, because it decreases the cost of optics,’ he says. 'You’ll use it without noticing, just like you use a massive data center or super-computer every ti me you Google something today.’

Offrein stresses the importance of this technology being made in a real working foundry. Unlike many tech breakthroughs, this is based on familiar materials. IBM says it’s already working with several lab partners, and is looking for more. Although he speaks with the reserve of an academic unwilling to commit to much, Offrein reiterates that this is on the road to commercialization already, and should be out in the commercial world in 'another few years’.

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