1-Femtojoule Optical Interconnect Paves the Way for Low-Power, High Performance Chips and eventual exaflop and zettaflop supercomputers

Michael Watts and pals at MIT have designed and built the first photonic modulator that operates at an ultralow 1 femtoJoule power level. “We propose, demonstrate, and characterize the first modulator to achieve simultaneous high-speed (25 Gigabits per second), low voltage (0.5 peak-to-peak Voltage) and efficient 1 femtoJoule per bit error-free operation,” they say.

The new device is a hollow silicon cylinder that acts as a cavity for trapping light waves. It modulates this light thanks to a phenomenon known as the electro-optic effect in which the refractive index of silicon can be changed by modifying the voltage across it.

The modulator solves a number of problems that electronics engineers have been wrestling with. First, it is entirely compatible with the CMOS (complementary metal oxide semiconductor) process used for manufacturing chips and so can be made inside any existing fabrication plant. Previous attempts to make devices of this kind relied on indium which is not compatible with CMOS.

This is for handling of input and output. If processing power can also be brought down to around these levels then zettaflop systems become possible.

Femtojoule operations would mean one watt for a petaflop of processing and 1000 watts for an exaflop and a megawatt for a zettaflop. 100 zettaflop supercomputers would need 100 megawatts of power.

Arxiv – A one femtojoule athermal silicon modulator

The full 23 page paper is here

“The results represent a new paradigm in modulator development,” they say.

At the very least, it should make possible a new generation of powerful chips operating at lower power than ever before.


Silicon photonics has emerged as the leading candidate for implementing ultralow power wavelength division multiplexed communication networks in high-performance computers, yet current components (lasers, modulators, filters, and detectors) consume too much power for the femtojouleclass links that will ultimately be required. Here, we propose, demonstrate, and characterize the first modulator to achieve simultaneous high-speed (25-Gb/s), low voltage (0.5VPP) and efficient 1-fJ/bit error-free operation while maintaining athermal operation. Both the low energy and athermal operation were enabled by a record free-carrier accumulation/depletion response obtained in a vertical p-n junction device that at 250-pm/V (30-GHz/V) is up to ten times larger than prior demonstrations. Over a 7.5{deg}C temperature range, the massive electro-optic response was used to compensate for thermal drift without increasing energy consumption and over a 10{deg}C temperature range, increasing energy consumption by only 2-fJ/bit. The results represent a new paradigm in modulator development, one where thermal compensation is achieved electro-optically.

Getting to one femtojoule per calculation is the key for multi-exaflop processing.

In 2008, nextbigfuture looked at DARPA research that indicated the need for low energy high speed onchip photonic communication for next generation high speed computing

What is the fundamental limit on the energy per bit for an optical link?

We recall that Shannon’s theorem puts the lower limit at kTlog(2), equivalent to 3 zeptoJoules or 3 millionths of femtojoule!

The energy of a 1500 nm photon is 130 zeptoJoules.

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