Transferring data with many colours of light

In a study published in Nature Photonics, researchers at Columbia University have demonstrated an energy-efficient method for transferring large quantities of data over the fibre-optic cables that connect individual nodes in data centres and high-performance computers. This technology improves on previous attempts to transmit multiple signals simultaneously over the same fibre-optic cables.

Data centres and high-performance computers that run artificial intelligence programs, such as large language models, are limited by the computational power of their individual nodes. The amount of data they can transfer among the nodes underlines the ‘bandwidth bottleneck’ that currently limits the performance and scaling of these systems. The nodes in these systems can be separated by more than one kilometre. Since metal wires dissipate electrical signals as heat when transferring data at high speeds, these systems transfer data via fibre-optic cables. Unfortunately, a lot of energy is wasted in the process of converting the electrical data into optical data (and back again) as signals are sent from one node to another.

Instead of using a different laser to generate each wavelength of light, the new chips developed by Columbia University require only a single laser to generate hundreds of distinct wavelengths of light that can simultaneously transfer independent streams of data. The millimetre-scale system employs a technique called wavelength-division multiplexing (WDM) and devices called Kerr frequency combs that take a single colour of light at the input and create many new colours of light at the output. The Kerr frequency combs developed by Michal Lipson, Higgins Rickey Professor of Electrical Engineering, and Alexander Gaeta, Professor of Electrical Engineering, allowed the researchers to send clear signals through separate and precise wavelengths of light, with space in between them.

Senior author Karen Bergman said the researchers recognised that these devices made ideal sources for optical communications, where one can encode independent information channels on each colour of light and propagate them over a single optical fibre. This development could allow systems to transfer more data without using more energy. The researchers miniaturised all of the optical components onto chips a few millimetres on each edge for generating light, encoded them with electrical data, and then converted the optical data back into an electrical signal at the target node. They devised a novel photonic circuit architecture that allows each channel to be individually encoded with data while having minimal interference with neighbouring channels. That means the signals sent in each colour of light don’t become muddled and difficult for the receiver to interpret and convert back into electronic data.

Lead author Anthony Rizzo said that the researchers’ approach is more compact and energy-efficient than comparable approaches. “It is also cheaper and easier to scale since the silicon nitride comb generation chips can be fabricated in standard CMOS foundries used to fabricate microelectronics chips rather than in expensive dedicated III-V foundries,” Rizzo said.

The compact nature of these chips enables them to interface with computer electronics chips, reducing the total energy consumption since the electrical data signals only have to propagate over millimetres of distance rather than tens of centimetres. “What this work shows is a viable path towards both dramatically reducing the system energy consumption while simultaneously increasing the computing power by orders of magnitude, allowing artificial intelligence applications to continue to grow at an exponential rate with minimal environmental impact,” Bergman said.

The researchers also transmitted 16 gigabits per second per wavelength for 32 distinct wavelengths of light for a total single-fibre bandwidth of 512 Gb/s with less than one bit in error out of one trillion transmitted bits of data. The silicon chip transmitting the data measured 4 x 1 mm, while the chip that received the optical signal and converted it into an electrical signal measured 3 x 1 mm — both smaller than a human fingernail.

“While we used 32 wavelength channels in the proof-of-principle demonstration, our architecture can be scaled to accommodate over 100 channels, which is well within the reach of standard Kerr comb designs,” Rizzo said.

Image caption: Photonic transmitter chip mounted on a printed circuit board with electrical and fibre-optic connections. Image credit: Lightwave Research Laboratory/Columbia Engineering.