Silicon Integrated Nanophotonics       


 photo TYMON BARWICZ photo photoWilliam M. J. Green photo Swetha Kamlapurkar photoJason S. Orcutt photo Jonathan E. (Jon) Proesel photo Jessie C. Rosenberg photoChi Xiong photo

Silicon Integrated Nanophotonics - 2007 Ultra-compact optical modulator

On December 6, 2007 IBM have announced an ultra-compact and low-power silicon optical modulator, which performs the task of converting an electrical input signals into pulses of light. This device is a critical component in our work toward wiring a chip with light rather than copper wires. The optical modulator performs the function of converting a digital electrical signal carried on a wire, into a series of light pulses, carried on a silicon nanophotonic waveguide. The modulator is capable of transmitting optical data at a rate of 10 billion bits per second (10 Giga bits per second).

The report on this work, entitled "Ultra-compact, low RF power, 10 Gb/s silicon Mach-Zehnder modulator" by William M. J. Green, Michael J. Rooks, Lidija Sekaric, and Yurii A. Vlasov of IBM’s T.J. Watson Research Center in Yorktown Heights, N.Y. is published in Volume 15 of the journal Optics Express. This work was partially supported by the Defense Advanced Research Projects Agency (DARPA) through the Defense Sciences Office program "Slowing, Storing and Processing Light".

Movie illustrating principle of operation

Low-power ultra-compact 10Gb/s silicon modulator

Video explanation
1. First, an input laser beam (marked by red color) is delivered to the optical modulator. The optical modulator (black box with IBM logo) is basically a very fast “shutter” which controls whether the input laser is blocked or transmitted to the output waveguide.

2. When a digital electrical pulse (a “1” bit marked by yellow) arrives from the left at the modulator, a short pulse of light is allowed to pass through at the optical output on the right.

3. When there is no electrical pulse at the modulator (a “0” bit), the modulator blocks light from passing through at the optical output.

4. In this way, the device “modulates” the intensity of the input laser beam, and the modulator converts a stream of digital bits (“1”s and “0”s) from electrical input pulses into pulses of light.

Image Gallery. Click to enlarge

Cross-section of the modulator

Modulator cross-section

The optical modulator uses “silicon nanophotonic waveguides,” to control the flow of light on a silicon chip. The waveguides are made of tiny silicon strips (marked by purple color) with dimensions 200 times smaller than the diameter of a human hair, in a silicon-on-insulator SOI wafer.

Optical mode profile in the modulator

 Optical mode profile in the modulator

Light is strongly confined within the silicon nanophotonic waveguide as shown by the colored concentric ellipses overlaid with the waveguide image. The strong confinement of light allows the IBM modulator to be dramatically scaled down in size.

Carriers injection into the modulator

Carriers injection into the modulator

Figures captions

Digital electrical signals are applied to the p+-i-n+ doped silicon nanophotonic waveguide through the electrodes (marked by gold color). Electrical charges (holes – green particles; electrons – red particles) are injected into the waveguide and change the optical properties of silicon, which is used to perform the modulation function.

Rationale and metrics

For on-chip interconnect applications, these modulators need to have three very important characteristics. Because hundreds or even thousands of such modulators will be needed on a single chip, they must first have a very small size, and second, require little electrical power to do their job. Finally, the optical modulators must be temperature insensitive, because they will have to operate within the chip multi-processor, in which the temperature can change dramatically around “hot spots” which move around on the chip depending upon the exact operating conditions. While silicon modulators with one of these three characteristics have been demonstrated previously, IBM researchers have recently made a device which simultaneously satisfies all three design points.

Just like fiber optic networks have enabled the rapid expansion of the Internet by enabling users to exchange huge amounts of data from anywhere in the world, a related technology known as “silicon nanophotonics” is bringing similar capabilities to the level of the computer chip. Using silicon optical waveguides, or nanometer-sized “light pipes,” integrated on the same piece of silicon material as the chip multi-processor, the huge amount of data which needs to be passed back and forth between all the cores can be carried by pulses on a beam of laser light, using much less total power than is used by electrical signals. Therefore, much of the electrical wiring within the multi-processor can potentially be replaced with a network of silicon waveguides, which will act as the “nervous system” of future on-chip supercomputers. While this may sound like science fiction, industrial and academic researchers throughout the world are working on this as a promising approach to solving the interconnect bottleneck.

One of the key components needed for any such optical network is a silicon optical modulator, which has the job of transferring high-speed electrical signals traveling on wires into pulses of laser light, traveling along a silicon waveguide.

Additional information

2014 OFC Executive Forum presentation

2012 IEDM postdeadline paper

2012 CLEO Plenary talk

2012 IEEE Comm. Mag., Silicon Nanophotonics Beyond 100G

2011 IBM R&D Journal: Technologies for Exascale systems

2010 SEMICON Talk: CMOS Nanophotonics for Exascale

2008 ECOC Tutorial: On-Chip Si Nanophotonics