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A team of researchers at UC Santa Barbara, led by electrical and computer engineering Professor John Bowers, has given the world, a glance into the future. The team has developed mode-locked quantum dot lasers on silicon. It’s a technology that can, not only massively increase the data transmission capacity of data centers, telecommunications companies and network hardware products to come, but also do so, with high stability, low noise and the energy efficiency of silicon photonics.
The level of data traffic in the world is going up very, very fast. And according to Bowers, the co-author of a paper on the new technology in the journal Optica, the transmission and data capacity of state-of-the-art telecommunications infrastructure must double roughly every two years to sustain high levels of performance. That means that even now, technology companies such as Intel and Cisco have to set their sights on the hardware of 2024 and beyond to stay competitive.
Enter the Bowers Group’s high-channel-count, 20 GHz, passively mode-locked quantum dot laser, directly grown — for the first time, to the group’s knowledge — on a silicon substrate. With a proven 4.1 terabit-per-second transmission capacity, it leaps an estimated full decade ahead from today’s best commercial standard for data transmission, which is currently reaching for 400 gigabits per second on Ethernet.
The technology is the latest high-performance candidate in an established technique called wavelength-division-multiplexing (WDM), which transmits numerous parallel signals over a single optical fiber using different wavelengths (colors). It has made possible the streaming and rapid data transfer we have come to rely on for our communications, entertainment and commerce.
The Bowers Group’s new technology takes advantage of several advances in telecommunications, photonics and materials with its quantum dot laser — a tiny, micron-sized light source — that can emit a broad range of light wavelengths over which data can be transmitted. According to Songtao Liu, a postdoctoral researcher in the Bowers Group and lead author of the paper, the team wants more coherent wavelengths generated in one cheap light source. Quantum dots can offer wide gain spectrum, and that’s why they can achieve a lot of channels. Their quantum dot laser produces 64 channels, spaced at 20 GHz, and can be utilized as a transmitter to boost the system capacity. The laser is passively ‘mode-locked’ — a technique that generates coherent optical ‘combs’ with fixed-channel spacing — to prevent noise from wavelength competition in the laser cavity and stabilize data transmission.
This technology represents a significant advance in the field of silicon electronic and photonic integrated circuits, in which the primary goal is to create components that use light (photons) and waveguides — unparalleled for data capacity and transmission speed as well as energy efficiency — alongside and even instead of electrons and wires. Silicon is a good material for the quality of light it can guide and preserve, and for the ease and low cost of its large-scale manufacture. However, it’s not so good for generating light.
According to Liu, in reference to the ideal electronic structural property for light-emitting solids, if one wants to generate light efficiently, one would want a direct band-gap semiconductor. Silicon is an indirect band-gap semiconductor. The Bowers Group’s quantum dot laser, grown on silicon molecule-by-molecule at UC Santa Barbara’s nanofabrication facilities, is a structure that takes advantage of the electronic properties of several semiconductor materials for performance and function (including their direct band-gaps), in addition to silicon’s own well-known optical and manufacturing benefits.
This quantum dot laser, and component like it, is expected to become the norm in telecommunications and data processing, as technology companies seek ways to improve their data capacity and transmission speeds. Data centers are now buying large amounts of silicon photonic transceivers in comparison to nothing before, Bowers points out.
Since Bowers a decade ago demonstrated the world’s first hybrid silicon laser (an effort in conjunction with Intel), the silicon photonics world has continued to create higher efficiency, higher performance technology while maintaining as small a footprint as possible, with an eye on mass production. The quantum dot laser on silicon, Bowers and Liu say, is state-of-the-art technology that delivers the superior performance that will be sought for future devices.
Research on the quantum-dot project was also conducted by Xinru Wu, Daehwan Jung, Justin Norman, MJ Kennedy, Hon K. Tsang and Arthur C. Gossard at UC Santa Barbara.