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

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

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

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.

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.