Researchers create 40-channel optical comms link

A collaboration of US researchers has demonstrated a silicon-based optical communication link that combines two multiplexing technologies to create 40 optical data channels that can simultaneously move data.

Described in the journal Optics Letters, the chip-scale optical interconnect can transmit about 400 GB of data per second — the equivalent of about 100,000 streaming movies. This could improve data-intensive internet applications from video streaming services to high-capacity transactions for the stock market.

“As demands to move more information across the internet continue to grow, we need new technologies to push data rates further,” said Peter Delfyett, who led the research team from the University of Central Florida’s College of Optics and Photonics (CREOL). “Because optical interconnects can move more data than their electronic counterparts, our work could enable better and faster data processing in the data centres that form the backbone of the internet.”

The optical communication link achieves 40 channels by combining a frequency comb light source based on a new photonic crystal resonator developed by the US National Institute of Standards and Technology (NIST) with an optimised mode-division multiplexer designed by the researchers at Stanford University. Each channel can be used to carry information, much like different stereo channels, or frequencies, transmit different music stations.

“We show that these new frequency combs can be used in fully integrated optical interconnects,” said Chinmay Shirpurkar, co-first author of the paper, also from CREOL. “All the photonic components were made from silicon-based material, which demonstrates the potential for making optical information handling devices from low-cost, easy-to-manufacture optical interconnects.”

In addition to improving internet data transmission, the new technology could also be used to make faster optical computers that could provide the high levels of computing power needed for artificial intelligence, machine learning, large-scale emulation and other applications.

Using multiple light dimensions

The researchers created the optical link using tantalum pentoxide (Ta₂O₅) waveguides on a silicon substrate fabricated into a ring with a nanopatterned oscillation on the inner wall. The resulting photonic crystal micro-ring resonator turns a laser input into 10 different wavelengths. They also designed and optimised a mode-division multiplexer that transforms each wavelength into four new beams that each have different shapes. Adding this spatial dimension enables a fourfold increase in data capacity, creating the 40 channels.

The ring-shaped photonic crystal resonator (left) features a nanopattern inside (right) that splits a selected resonant mode for comb generation. Images taken with scanning electron microscopy. Image credit: Su-Peng Yu, NIST.

Once the data is encoded onto each beam shape and each beam colour, the light is recombined back into a single beam and transmitted to its destination. At the final destination, the wavelengths and beam shapes are separated so that each channel can be received and detected independently, without interference from the other transmitted channels.

“An advantage of our link is that the photonic crystal resonator enables easier soliton generation and a flatter comb spectrum than those demonstrated with conventional ring resonators,” said co-first author Jizhao Zang, from NIST. “These features are beneficial for optical data links.”

Better performance with inverse design

To optimise the mode division multiplexer, the researchers used a computational nanophotonic design approach called photonic inverse-design. This method provides a more efficient way to explore a full range of possible designs while offering smaller footprints, better efficiencies and new functionalities.

“The photonic inverse-design approach makes our link highly customisable to meet the needs of specific applications,” said co-first author Kiyoul Yang, from Stanford.

Tests of the new device matched well with simulations and showed that the channels exhibited a low crosstalk of less than -20 dB. Using less than -10 dBm of received optical receiver power, the link performed error-free data transmission in 34 out of the 40 channels using a PRBS31 pattern, a standard used to test high-speed circuits under stress.

The researchers are now working to further improve the device by incorporating photonic crystal micro-ring resonators that produce more wavelengths or by using more complex beam shapes. Commercialising these devices would require the full integration of a transmitter and receiver chip with high bandwidth, low power consumption and a small footprint. This could enable the next generation of optical interconnects for use in data-centre networks.

Open-source code for the photonic optimisation software used in the paper is available at https://github.com/stanfordnqp/spins-b.

Top image caption: The researchers designed and optimised a mode-division multiplexer that transforms each of the 10 wavelengths into four new beams that each have different shapes. Image credit: Kiyoul Yang, Stanford University.

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