Researchers in the US have demonstrated a silicon-based optical communication link with 400 Gbit/s over 40 optical channels
The chip-scale optical interconnect combines a frequency comb light source based on a new photonic crystal resonator developed by the National Institute of Standards and Technology (NIST) with an optimized mode-division multiplexer designed by the researchers at Stanford University.
The work is part of the Photonics in the Package for Extreme Scalability (PIPES) programme run by US research agency DARPA. This aims to use light to vastly improve the digital connectivity of packaged integrated circuits using microcomb-based light sources.
This could enable the next generation of optical interconnects for use in data-centre networks and on-chip, chiplet connections.
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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 ten different wavelengths. They also designed and optimized 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.
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.
To optimize 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 customizable to meet the needs of specific applications,” said researcher Kiyoul Yang from Stanford University.
“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 University of Central Florida College of Optics and Photonics (CREOL) research team. “Because optical interconnects can move more data than their electronic counterparts, our work could enable better and faster data processing in the data centers that form the backbone of the internet.”
“We show that these new frequency combs can be used in fully integrated optical interconnects,” said Chinmay Shirpurkar, co-first author of the paper in Optica with Yang. “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.”
“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.”
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. Commercializing these devices would require the full integration of a transmitter and receiver chip with high bandwidth, low power consumption and a small footprint.
Open-source code for the photonic optimization software used in the paper is available at https://github.com/stanfordnqp/spins-b.
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